Chapter 2 Micro-phytoplankton of the Bay of...
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Chapter 2 Micro-phytoplankton of the Bay of Bengal
Transcript of Chapter 2 Micro-phytoplankton of the Bay of...
Chapter 2Micro-phytoplankton of the
Bay of Bengal
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2A.1. INTRODUCTION
Phytoplankton may be divided into various groups, such as, diatoms,
cyanobacteria, dinoflagellates, cocolithophores and others. These groups are
also known as the ‘plankton functional types’, because, their activity is
intimately linked to the biogeochemical cycles of the marine ecosystem
(Totterdell et al. 1993). Amongst these, the most diverse group of
phytoplankton is the diatoms, with an estimated 200,000 extant species (Mann
1999). It is estimated that diatoms can contribute up to 40% of the total
oceanic primary production (Nelson et al. 1995, Tréguer et al. 1995, Mann
1999, Smetacek 1999), thus contributing significantly to the biogeochemical
cycle of carbon. They also play a significant ecological role by contributing to
the biogeochemical cycles of silicon, nitrogen, phosphorus and iron and tend
to dominate export production (Tréguer et al. 1995, Buesseler 1998, Sarthou
2005). Diatoms such as Rhizosolenia can contribute nitrogen from the deep
waters to the euphotic zone by means of vertical migration and thereby
contribute to the new nitrogen in the ecosystem (Singler and Villareal 2005).
Furthermore, in warm oligotrophic seas in the tropics, symbiosis between
diatoms and endocellular nitrogen fixing cyanobacteria is found and is
estimated to contribute significantly to the biogeochemical cycles of these
waters (Villareal 1989, Carpenter and Janson 2000, Kulkarni et al. 2010). The
tropical oceans are also characterised by the widespread occurrence of the
filamentous cyanobacteria, Trichodesmium. These are often the major
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component of the phytoplankton and contribute significantly to the new
production (Capone et al. 1997).
In view of this, it becomes important to evaluate the spatial and
temporal variation of such groups and/or species in any water body, so as to
understand the variations in biogeochemistry or new production of the given
area. The best approach to this type of study is the microscopic analysis of
water samples to characterise the various groups of the phytoplankton. This
includes the taxonomic elucidation of the phytoplankton community, both,
quantitatively and qualitatively. Another approach is the large-scale mapping
of surface phytoplankton through remote-sensing. Although this technique
might not be as precise as microscopy, the basic functional groups can be
differentiated rather clearly based on their unique water leaving optical
properties (Sathyendranath et al. 2004). These two methods when used
together can further our knowledge about micro-scale and large-scale
variations of the community.
In the Indian Ocean, microscopic observations of the phytoplankton
have been carried out since earlier than 1931 (Devanesen 1942) mainly in
connection with the fisheries of the Malabar Coast (west coast of India) and
the Gulf of Manaar (east coast of India). Even a comparison of the diatoms
observed in the two coasts has been made and it was pondered whether the
same hydrological conditions occurred in both seas. However, many studies
(Longhurst 1998, Prasanna Kumar et al. 2002, Singh and Chaturvedi 2010)
carried out later have reported low productivity in the Bay of Bengal as
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compared to the Arabian Sea. This has been attributed to the intense
stratification of the surface water due to the salinity gradient caused by the
large influx of fresh water (Shetye et al. 1993, Tomczak and Godfrey 1994).
As a result, vertical mixing of nutrient rich deeper waters is restricted.
However, some studies have reported high productivity in some regions of the
Bay of Bengal due to localised upwelling (Shetye et al. 1991) and eddies
(Prasanna Kumar 2004). Studies in this part of the Indian Ocean have
primarily focused on primary production and coastal ecology (Devassy and
Bhattathiri 1981, Devassy et al. 1983, Bhattathiri and Devassy 1981,
Bhattathiri et al. 1980, Radhakrishna et al. 1978, 1982). Subsequent to this,
studies have been carried out by Gomes et al. (2000), Madhupratap et al.
(2003), Jyothibabu et al. (2003), Madhu et al. (2006), Prasanna Kumar et al.
(2004, 2007). So also, in the case of remote sensing, work carried out in the
Bay of Bengal has been dedicated to chlorophyll concentration and primary
productivity (Sarangi et al. 2008, Singh and Chaturvedi 2010). All these
studies give us an idea of how much is there rather than what is there. Very
little information is available on the phytoplankton community such as Paul et
al. (2007), Jyothibabu et al. (2003), Madhu et al. (2006) and most important,
the atlas of diatoms from the Bay of Bengal by Desikachari and Prema (1987).
In the present study, surface water samples were collected at regular
intervals for a period of over four years from three shipping routes in the Bay
of Bengal to analyse the spatial and temporal variation in the phytoplankton
community. This work was carried out in collaboration with the Indian
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Expendable Bathy Thermograph programme wherein samples were collected
by a Ship of Opportunity network which utilizes passenger ships plying
between Chennai and Singapore, Chennai and Port Blair and Port Blair and
Kolkata in the Bay of Bengal. This approach was found suitable as it offered a
temporal resolution of the phytoplankton variability.
The main aim of this study is to elucidate the spatial and temporal
distribution of the dominant phytoplankton groups, viz. diatoms and
Trichodesmium from surface waters in the Bay of Bengal. It was found that
Trichodesmium spp. appeared regularly during the fall and winter in the Bay
of Bengal and made their presence felt during the summer months along the
coast of India. Remote sensing techniques provided an alternative solution for
mapping Trichodesmium on a global scale. Therefore, Trichodesmium, which
formed the major component of the phytoplankton in this study area, was also
mapped using remote sensing tools.
2A.2. MATERIALS AND METHODS
2A.2.1. Study area and sampling
Surface water samples were collected on 15 transects, (12 to 14
samples collected at every longitude in each transect from February 2000 to
September 2002) between Chennai-Singapore (13° 05’ N, 80° 29’ E to 5° 48’
N, 96° 12’ E) and from 8 transects from November 2003 to October 2004
between Chennai and Port Blair (13° 05’ N, 81° E to 11° 28’ N, 92° E) and
Port Blair and Kolkata (12° 03’ N, 93° 14’ E to 21° 05’ N, 93° 14’ E) (10-12
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samples collected at one degree intervals along both routes) (Fig. 2A.1) from
merchant vessels plying between the above mentioned ports. Surface water
samples were collected using a stainless steel bucket from the moving ship.
This collection method was proven to be much less destructive to
phytoplankton when compared with samples taken from the different seawater
Fig. 2.1. Study area. Samples were collected along the Chennai–Singapore, Chennai–Port Blair and Port Blair–Kolkata shipping routes with a one degree interval betweeneach station.
pumps, which tended to break chains and even individual cells (Smetacek et
al. 2002). Since the movement of the ship caused mixing of the water, these
samples represent the upper few metres.
Singapore
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2A.2.2. Microscopic analysis
A settling and siphoning procedure was followed to concentrate
samples from 1000mL to 10 mL. For counting and identification of genera
and species, two 2.5-mL replicates of concentrated samples were transferred
into a culture well and examined microscopically at a magnification of 200–
400 times. Diatoms were enumerated and identified to the lowest possible
taxonomic level. Trichodesmium were counted as individual filaments, and if
in a clump they were standardized into smaller tufts (25 trichomes col-1) and
larger tufts (50 trichomes col-1). Identification was based on Heurck (1896),
Hendey (1964), Desikachary and Prema (1987), Tomas (1997) and Horner
(2002).
2A.2.3. Data analyses
Univariate techniques included calculation of the Shannon-Wiener
diversity index (H’), Margalef’s species richness (d) and Pielou’s evenness
(J’) of the diatom population. One- and two-way analysis of variance (Sokal
and Rohlf 1981) was carried out on the abundance, species richness, diversity
and evenness data to test the significance of spatial and temporal variation.
These analyses were performed on log (X+1) transformed abundance data.
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2A.3 RESULTS
2A.3.1. Distribution of diatoms and Trichodesmium
2A.3.1.1 Chennai – Singapore sector
The Chennai–Singapore (CS) sector was sampled on 15 occasions
over a period of three years. Overall, a total of 142 species belonging to 50
genera were recorded (Table 2.1) with 50 (20 genera) pennate species and 92
(30 genera) centric species. Significant temporal variation (p = 0.0019, two-
way ANOVA) was observed in the distribution of diatoms. However, there
was no significant spatial variation. In the year 2000, three samplings were
carried out during February, August and October. The maximum diatom
abundance, species richness, diversity and evenness were recorded in
February 2000 (Fig. 2A.2a). The maximum diatom recorded during February
(Fig. 2A.2a). Transect wise diatom distribution during this month revealed
high diatom abundance at 86 °E and 94 °E (Fig. 2A.3a). Trichodesmium was
observed at 80 – 84 °E (Fig 2A.6 a). The two most abundant species in this
transect were Trichodesmium and the diatom Proboscia alata (Table 2.2). In
August 2000 the species abundance, richness and diversity were the lowest
(Fig. 2A.2a), this transect was dominated by diatoms Proboscia alata and
Rhizosolenia shrubsolei (Table 2.2). In October 2000, Trichodesmium was
observed to be the most abundant species at 94 °E (962 trichomes L-1, asterisk
in Fig. 2A.6a) and was recorded to be the dominant species during this month
(Table 2.2).
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Table 2. 1. Diatoms recorded in the Chennai-Singapore (CS), Chennai-Port Blair(CP) and Port Blair-Kolkata (PK) sectors during the study period.
Taxa CS CP PK Taxa CS CP PKAchnanthes sp. + Corethron sp. +Amphora spp. + + Coscinodiscus concinus +Asterolampra greveilli + Coscinodiscus granii? +Asterolampra marylandica + + + Coscinodiscus lineatus + + +Asteromphalus brookei + + Coscinodiscus marginatus + + +Asteromphalus elegans + + Coscinodiscus nodulifer +Asteromphalus flabellatus + + Coscinodiscus occuli-irridis +Asteromphalus heptactis + Coscinodiscus radiatus + + +Asteromphalus pettersonii? + Coscinodiscus sp. 2 + + +Asteromphalus spp. + + + Coscinodiscus spp. + + +Asteromphalus weisii + Cyclotella caspia +Bacteriastrum cosmosum + Cyclotella sp. +Bacteriastrum delicatulum + + + Cylindrotheca closterium + + +Bacteriastrum elongatum + Dactyliosolen fragilissimus + + +Bacteriastrum furcatum + + Diploneis crabro +Bacteriastrum hyalinum + + + Diploneis weisflogii +Bacteriastrum spp. + + + Ditylum brightwellii + +Bacteriastrum varians + Ditylum sol +Biddulphia obtusa + Eucampia cornuta +Biddulphia rhombus + Eucampia groenlandica +Campylodiscus sp. + Eucampia zodiacus +Cerataulina bergonii + Eupodiscus radiatus +Cerataulina pelagica + Fragilaria striatula? +Cerataulina sp. + Fragilaria spp. + + +Chaetoceros affinis + + + Fragilariopsis cylindrus +Chaetoceros coarctatus + + Fragilariopsis oceanica +Chaetoceros compressus + Fragilariopsis doliolus + +Chaetoceros concavicornis + Fragilariopsis spp. + +Chaetoceros convolutus + Grammatophora marina +Chaetoceros curvisetus + + Guinardia cylindrus + + +Chaetoceros danicus + + + Guinardia delicatula + + +Chaetoceros decipiens + + + Guinardia flaccida + + +Chaetoceros denticulatus + Guinardia sp. +Chaetoceros diversus + + Guinardia striata + + +Chaetoceros laciniosus + + Gyrosigma sp. +Chaetoceros lorenzianus + Haslea spp. + + +Chaetoceros messanensis + + + Haslea trompii + +Chaetoceros peruvianus + + + Haslea wawrikae + +Chaetoceros radicans + Helicotheca tamesis + + +Chaetoceros simplex + Hemiaulus hauckii + + +Chaetoceros socialis + Hemiaulus indicus +Chaetoceros spp. + + + Hemiaulus membranaceus + + +Chaetoceros tortissimus + + Hemiaulus sinensis + +Climacodium frauenfeldianum + + + Hemiaulus sp. +Corethron criophilum + Hemidiscus hardmanianus +
Contd…
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Taxa CS CP PK Taxa CS CP PK
Lauderia annulata + + Pseudo-nitzschia sp.5 +
Lauderia sp. + Pseudo-nitzschia spp. + +
Leptocylindrus danicus + + Pseudo-nitzschia turgidula +
Leptocylindrus mediterraneus + Pseudosolenia calcar-avis + + +
Leptocylindrus minimus + + + Proboscia alata + + +
Leptocylindrus sp. + Proboscia curvisetus + +
Licmophora paradoxa + Proboscia indica + + +
Lioloma elongatum + Pseudoguinardia recta + +
Lioloma pacificum + + + Pseudo-nitzschia fraudulenta +
Lioloma sp. + + Pseudonitzschia pungens +
Mastogloia rostrata + + + Pseudo-nitzschia seriata + + +
Melosira sulcata + Raphoneis sp. +
Meuniera membranacea + + + Rhizosolenia acuminata + +
Navicula derasa + + Rhizosolenia bergonii + +
Navicula directa + Rhizosolenia castracanei + + +
Navicula distans Rhizosolenia clevei +
Navicula spp. + + + Rhizosolenia crassipes +Navicula transitans f.delicatula + + + Rhizosolenia crassispina +
Navicula vanhoffenii + Rhizosolenia debyana + +
Nitzschia delicatissima + + + Rhizosolenia formosa + + +
Nitzschia longissima + + Rhizosolenia hebetata +
Nitzschia seriata + + + Rhizosolenia imbricata + + +
Nitzschia sigma + + Rhizosolenia robusta + +
Nitzschia sp. + + Rhizosolenia setigera + +
Odontella regia + Rhizosolenia shrubsolei +
Odontella sinensis + + Rhizosolenia sp. + + +
Odontella sp. + Rhizosolenia stylifera +
Phaeodactylum sp. + Rhizosolenia styliformis + + +
Phaeodactylum tricornutum + Schroederella delicatula +
Pinnularia rectangulata + Schroederella schroderei +
Planktoniella sol + + Schroederella sp. +
Pleurosigma angulatum + + Skeletonema costatum + +
Pleurosigma directum + + + Stephanopyxis sp. +
Pleurosigma elongatum + Surirella sp? +
Pleurosigma normanii + + Synedra pulchella +
Pleurosigma sp. + + Synedra? + +
Pseudo-nitzschia sp.1 + Synedropsis hyperborea +
Pseudo-nitzschia sp.2 + + + Synedropsis sp. +
Pseudo-nitzschia sp.3 + + +Thalasionemapseudonitzschioides +
Pseudo-nitzschia sp.4 + Thalassionema bacillare + + +
Table 2.1. …contd.
Contd…
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Taxa CS CP PK Taxa CS CP PK
Thalassionema frauenfeldii + + + Thalassiosira punctigera + + +
Thalassionema javanicum + + Thalassiosira spp. + + +
Thalassionema nitzschioides + + + Thalassiothrix longissima + +
Thalassionema sp. + + Thalassiothrix sp. + +
Thalassiosira eccentrica + Triceratium sp. +
Thalassiosira gravida +
In the year 2001, six samplings were carried out (February, April,
August, September, October and November). The highest abundance was
recorded in August 2001 followed by October and November (Fig. 2A.2b),
whereas minimum abundance was observed in April followed by February.
The most abundant diatom in August and October was P. alata whereas in
November, Thalassionema nitzschioides was abundant. The second dominant
species varied during these three observations (Table 2.2).
In February 2001, Trichodesmium was recorded between 86 and 92 °E (Fig.
2A.6a) ranging between 13 - 633 trichomes L-1 and was the single dominant
species (Table 2.2).
In the year 2002, six samplings were carried out (February, May, June,
July, August, and September). Results obtained during these observations
revealed the highest abundance of diatoms during September and the least in
May followed by February (Fig. 2A.2c). Trichodesmium, followed by
Guinardia flaccida dominated during September (Table 2.2). In May,
Trichodesmium was dominant followed by Thalassionema frauenfeldii.
During February, Climacodium frauenfeldianum and Thalassiosira spp. were
the dominant species (Table 2.2). The abundance recorded in June, July and
Table 2.1. …contd.
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August was almost similar to September (Fig. 2A.2c). However, the spatial
distribution of diatoms along the sector indicated that the abundance in June
and August mainly came from the high diatom counts (523 cells L-1 and 518
cells L-1, respectively) in 84 – 86 °E (Fig. 2A.3a). These were also the highest
abundance recorded throughout the observation period in the CS sector. On
both occasions Proboscia alata was found to be the dominant diatom and
contributed 79% and 75%, respectively, at the above mentioned stations (See
Appendix 1 for abundance values in each sampling station for the respective
month).
Overall, a comparison of the three years revealed low abundance,
species richness, evenness and diversity in August 2000 (Fig. 2A.3a,b,c,d)
whereas, the diatom abundance, species richness, evenness and diversity in
August 2001 and 2002 were comparatively higher (Fig. 2A.3a,b,c,d). A mixed
diatom assemblage was recorded at 88 °E (Appendix 1). A similar trend was
observed in August 2002 at 94 °E, where the increase in diatom abundance
mirrored an increase in the species richness and diversity at the eastern
boundary (94-96 °E, Fig. 2A.3 b, d).
2A.3.1.2 Chennai –Port Blair sector
The Chennai–Port Blair (CP) route was sampled on four occasions
(February, June, July and October 2004). A total of 33 genera (16 pennate and
17 centric) comprising of 86 species (31 pennate and 55 centric species) were
recorded in this sector (Table 2.1). Overall, the highest abundance was
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Fig. 2A.2. Overall diatom abundance (N) (cells L-1), species richness (d), evenness(J′) and species diversity (H′) in (a-c) Chennai–Singapore, (d) Chennai–Port Blairand (e) Port Blair–Kolkata routes.
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recorded in June 2004 and the least abundance was recorded in October 2004
(Fig. 2A.2 d). Leptocylindrus minimus was the dominant diatom contributing
over 20% (Table 2) of the total diatom assemblage. The diatom abundance
and species richness were the highest in June 2004 and lowest in October
2004 (Fig 2A.4 a, b). In February and October 2004, low diatom abundance
were recorded in this sector (Fig. 2A.4 a) and proliferation of Trichodesmium
(Fig 2A.6 c) was observed. The diatom abundance, richness, evenness and
diversity were the lowest in October 2004 at 82 °E (Fig. 2A.4 a-d) and were
low throughout the sector. The diatom abundance was high at 88 °E (Fig.
2A.4 a) in June 2004, whereas, low abundance, richness, evenness and
diversity were recorded at 84 °E (Fig. 2A.4-a-d). At this station, only
Trichodesmium (22 trichomes L-1) and Rhizosolenia imbricata (6 cells L-1)
was recorded (see Appendix 1). Low diatom abundance was recorded
throughout the route in October 2004 (Fig. 2A.4a); this corresponded with the
increase in abundance of Trichodesmium (Fig.2A.6c). A similar trend was
observed in February 2004 with the occurrence of Trichodesmium in this
sector (Fig. 2A.6 c).
2A.3.1.3. Port Blair-Kolkata sector
The Port Blair–Kolkata (PK) route was sampled on four occasions
between November 2003 and October 2004. The diatom abundance ranged
from 5 - 4900 cells L-1 (asterisk in Fig. 2A. 5 a). A total of 104 species
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Fig 2A.3 Spatial and temporal variation along the Chennai – Singapore route in (a)diatom abundance (N, cells L-1), (b) species richness (d), (c) evenness (J′) and (d)species diversity (H′)
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Fig 2A.4. Spatial and temporal variation along the Chennai – Port Blair route in (a)diatom abundance (N, cells L-1), (b) species richness (d), (c) evenness (J′) and (d)species diversity (H′) (the asterisk at 88 °E, Jun ’04 represents 1152 cells L -1).
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Fig 2A.5. Spatial and temporal variation along the Port Blair- Kolkata route in (a)diatom abundance (N, cells L-1), (b) species richness (d), (c) evenness (J′) and (d)species diversity (H′) (the asterisk at 21 °N, Nov ’03 represents 4900 cells L -1)
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Fig 2A.6. Spatial and temporal distribution of Trichodesmium (trichomes L-1) alongthe (a) Chennai – Singapore route (b) Port Blair – Kolkata route and (c) Chennai –Port Blair route. [A common scale was maintained, asterisk * represents countshigher than 500 trichomes L-1].
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belonging to 41 genera were recorded with 38 (17 genera) pennate species and
66 (24 genera) centric species (Table 2.1). The highest diatom abundance was
recorded in November 2003 and the lowest in October 2004 (Fig. 2A.2 e).
Although high abundance was recorded in November 2003, the species
richness and diversity were the highest in February 2004 (Fig 2A.2 e). The
most dominant species in November 2003 was Lauderia annulata
contributing to over 39% of the total abundance (Table 2.2) followed by
Ditylum sol. The diatom abundance in this month was mainly from the
contribution of the diatoms from 21 °N. The diatom abundance in the Port
Blair–Kolkata sector did not show significant spatial or temporal variation.
However there was significant temporal variations in species richness and
diversity (p = 0.0012, p = 0.0022, respectively). In February and July 2004,
the species richness and diversity were high (Fig. 2A.5 b, d) and evenness was
low (Fig. 2A.5 c) throughout the sector except at 14 °N. The diatom species
richness and diversity showed a decrease at this sampling point (14 °N, Fig.
2A.5 b, d) and the presence of Trichodesmium (Fig. 2A.6 b) was recorded.
Similarly, Trichodesmium was recorded at 12 °N in February 2004 and
October 2004 in high numbers (966 and 1045 trichomes L-1, respectively,
asterisks in Fig 2A.6 b) and was the dominant species during these months
(Table 2.2).
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Chennai – SingaporeMonth Taxa % Month Taxa % Month Taxa %Feb '00 Trichodesmium 44.54 Aug '00 Proboscia alata 53.51 Oct '00 Trichodesmium 78.29
Proboscia alata 30.68 Rhizosolenia shrubsolei 14.76 Proboscia alata 6.01Guinardia striata 4.33 Guinardia striata 3.69 Pseudo-nitzschia sp.1 3.02Navicula spp. 4.14 Pseudo-nitzschia sp.1 3.69 Navicula spp. 2.99Climacodium frauenfeldianum 2.58 Navicula transitans f. delicatula 3.59 Haslea spp. 2.06
Feb '01 Trichodesmium 86.18 Apr '01 Trichodesmium 84.50 Aug '01 Proboscia alata 19.63Chaetoceros affinis 2.17 Thalassionema nitzschioides 2.96 Cylindrotheca closterium 15.10Proboscia alata 1.39 Navicula spp. 2.62 Thalassionema nitzschioides 12.31Pseudo-nitzschia sp.3 1.09 Navicula transitans f. delicatula 1.93 Pseudo-nitzschia seriata 12.22Melosira sulcata 1.03 Phaeodactylum tricornutum 1.24 Navicula spp. 9.74
Sep '01 Trichodesmium 30.11 Oct '01 Proboscia alata 18.68 Nov '01 Thalassionema nitzschioides 10.01Thalassionema nitzschioides 13.81 Chaetoceros messanensis 10.24 Thalassiosira spp. 8.91Thalassiosira spp. 12.43 Nitzschia seriata 9.30 Rhizosolenia hebetata 8.48Coscinodiscus marginatus 4.14 Thalassiothrix longissima 6.79 Nitzschia seriata 8.11Pseudosolenia calcar-avis 3.87 Melosira sulcata 5.05 Pseudo-nitzschia sp.3 6.08
Feb '02 Climacodium frauenfeldianum 13.99 May '02 Trichodesmium 20.94 Jun '02 Proboscia alata 45.82Thalassiosira spp. 8.68 Thalassionema frauenfeldii 16.86 Trichodesmium 27.86Proboscia alata 7.23 Thalassiosira spp. 8.88 Guinardia striata 6.17Guinardia striata 6.43 Coscinodiscus marginatus 7.34 Nitzschia spp. 2.87Thalassionema frauenfeldii 5.95 Nitzschia seriata 2.81 Thalassionema frauenfeldii 2.20
Jul '02 Trichodesmium 36.72 Aug '02 Trichodesmium 43.26 Sep '02 Trichodesmium 19.66Chaetoceros spp. 5.52 Proboscia alata 35.72 Guinardia flaccida 7.47Thalassionema frauenfeldii 4.05 Guinardia striata 4.79 Guinardia striata 6.16Guinardia striata 3.78 Thalassionema frauenfeldii 2.37 Nitzschia delicatissima 5.72Chaetoceros messanensis 3.12 Nitzschia spp. 1.87 Pseudosolenia calcar-avis 5.65
Table 2.2. Percentage of dominant (five most abundant) species during each sampled months in the Chennai-Singapore sector
26
Table 2 …contd. Percentage of dominant (five most abundant) species during eachsampling month in the Chennai-Port Blair and Port Blair-Kolkata sectors
Chennai - Port Blair Port Blair - KolkataMonth Taxa % Month Taxa %Feb '04 Trichodesmium 33.37 Nov '03 Lauderia annulata 39.48
Nitzschia delicatissima 10.67 Ditylum sol 12.94Navicula spp. 3.85 Chaetoceros curvisetus 5.29Nitzschia seriata 3.35 Bacteriastrum hyalinum 3.78Guinardia flaccida 3.10 Chaetoceros messanensis 3.71
Jun '04 Leptocylindrus minimus 20.66 Feb '04 Trichodesmium 45.77Guinardia cylindrus 17.27 Nitzschia delicatissima 15.35Proboscia alata 11.88 Guinardia striata 3.14Trichodesmium 5.99 Proboscia alata 3.07Hemiaulus membranaceus 3.59 Cylindrotheca closterium 3.00
Jul '04 Coscinodiscus sp. 2 19.01 Jul '04 Guinardia striata 36.41Thalassiosira spp. 16.20 Thalassiosira spp. 10.87Coscinodiscus marginatus 11.62 Thalassionema frauenfeldii 6.62Trichodesmium 8.45 Thalassionema nitzschioides 4.02Fragilariopsis doliolus 5.28 Coscinodiscus sp. 2 3.78
Oct '04 Trichodesmium 56.34 Oct '04 Trichodesmium 88.67Thalassiosira spp. 8.10 Thalassiosira spp. 3.14Fragilariopsis spp. 4.93 Thalassionema sp. 1.12Climacodium frauenfeldianum 3.52 Coscinodiscus spp. 0.90Fragilariopsis doliolus 2.82 Chaetoceros danicus 0.45
2A.3.2. Comparison of the three sectors of the Bay of Bengal
Overall, a total of 183 diatom species were recorded throughout the
study period (Table 2.1) in all three routes (142 diatom species in the CS
sector, 86 species in the CP sector and 104 species in the PK sector). The
average diatom abundance in the PK sector was the highest (Table 2.3).
However, the species diversity was the highest in the CP sector followed by
the CS sector and least in the PK sector (Table 2.3). Observations revealed
that the micro- phytoplankton community in these three sectors was diatom-
dominated. However, during certain months (October 2000, February 2001
and April 2001 in the Chennai – Singapore sector and October 2004 in the
27
Table 2.3. Average values (±stdev) along the Chennai – Singapore (CS), Chennai–Port Blair (CP) and Port Blair–Kolkata (PK) sectors.
CS CP PKTrichodesmium(Trichomes L-1) 64(±14.46) 23(±58.57) 86(±252.96)
Diatoms (cells L-1) 95(±38.21) 79(±19.26) 212(±96.27)
Margalef's speciesrichness (d) 2.66(±0.49) 2.07(±0.59) 1.87(±0.60)
Pielou's evenness (J') 0.80(±0.08) 0.89(±0.01) 0.85(±0.04)
Shannon-Weinersdivesity (H') 1.78(±0.19) 1.81(±0.41) 1.69(±0.32)
Port Blair Kolkata and Chennai–Port Blair routes), Trichodesmium replaced
diatoms and dominated the micro-phytoplankton community (Table 2.2).
The dominance pattern in the Chennai – Singapore route, mainly
alternated between Proboscia alata and Trichodesmium (Table 2.2) except in
November 2001 and February 2002 where Climacodium frauenfeldianum and
Thalassionema nitzschioides dominated (Table 2.2) respectively. In the Port
Blair–Kolkata and Chennai–Port Blair routes, the dominant diatom species
varied with months, yet a clear pattern of Trichodesmium dominating mainly
during February and October (Table 2.2) was observed. Trichodesmium was
among the dominant species in nine out of fifteen months in the Chennai–
Singapore route and two out of four months in the Port Blair–Kolkata and
Chennai–Port Blair routes.
28
2A.4. DISCUSSION
The Bay of Bengal (BOB) is a small ocean basin (Longhurst 1998)
known for its unique features such as the strong stratification and low saline
waters due to the large volumes of freshwater input from rivers and rains. The
large amounts of fresh water (ca. 1.6-1012 m3 yr-1; Subramanian 1993)
discharged into the Bay give it an estuarine characteristic with low surface
salinities over a large area. Thus, excess precipitation and river runoff over
evaporation lead to very low salinities and highly stratified upper layers
(Shetye et al. 1993). This hampers the exchange processes between the
atmosphere, surface and deeper water layers that consequently affect the
biological and biogeochemical processes (Ittekkot et al. 2003).
It was observed in this study, that the diatom abundance pattern in the
southern Bay of Bengal (Chennai–Singapore sector) in the year 2000 was
different from 2001 and 2002. A decreasing trend in the abundance from pre-
monsoon (February) to post monsoon (October) was observed in the year
2000 (Fig. 2A.2 b,c) whereas, an increase in abundance was observed in the
following two years. The northern Bay of Bengal (Port Blair–Kolkata sector)
also displayed a decrease in abundance from pre-monsoon to post monsoon
(Fig.2A.2e). A comparison of the rainfall data indicated that there was a
negative correlation of diatoms with rainfall (Fig. 2A.7 a-j) however it was not
significant (two-way ANOVA, ns) (rainfall data was obtained from global
monthly merged precipitation analysis available at
http://precip.gsfc.nasa.gov/). Generally the monsoons have a variable effect on
29
Rainfall (mm d-1) Diatom (cells L-1)
1
10
100
1000
Feb 00 Aug 00 Oct 00
(b)
0123456789
Feb 00 Aug 00 Oct 00
(a)
1
10
100
1000
Feb 01 Apr 01 Aug 01 Sep 01 Oct 01 Nov 01
(d)
1
10
100
1000
Feb 02 May 02 Jun 02 Jul 02 Aug 02 Sep 02
(f)
0123456789
Feb 02 May 02 Jun 02 Jul 02 Aug 02 Sep 02
(e)
1
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Nov'03 Feb'04 Jul'04 Oct'04
(j)
0123456789
Nov'03 Feb'04 Jul'04 Oct'04
(i)
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Feb'04 Jun'04 Jul'04 Oct'04
(h)
0123456789
Feb'04 Jun'04 Jul'04 Oct'04
(g)
0123456789
Feb 01 Apr 01 Aug 01 Sep 01 Oct 01 Nov 01
(c)
Fig. 2A.7. Comparison between rainfall (mm d-1) data and diatom abundance (cellsL-1) in (a-f) Chennai–Singapore during the years (a, b) 2000, (c, d) 2001 and (e, f)2002, (g, h) Chennai–Port Blair and (i, j) Port Blair–Kolkata routes.
30
the distribution and abundance of phytoplankton. It has been reported that the
loads of nutrients brought in by run off during monsoon is not utilised due to
increase in turbidity and cloud cover (Madhu et al. 2006).
The average abundance of diatoms in the three sectors was calculated
(Table 2.3) to find out the variation in the biomass of each sector. The PK
sector was numerically abundant whereas the CS was most species rich,
however, species diversity was the highest in CP. Hillebrand and Sommer
(2000) have suggested that species diversity indices are appropriate measures
of local changes in micro-algal assemblages and allows to evaluate their
response to the environment. In view of this, it is worthwhile to note that this
sector comes under the influence of cold core eddies (Prasanna Kumar et al.
2004), which pumps nutrients from the deep and increases the productivity of
the region. These eddies are generally observed during the summer monsoon
(June–August) and occur from off the coast of Chennai to about 88 °E. Their
positions, however, are variable from year to year (Gopalan et al. 2000). The
high abundance recorded at 88 °E (Fig. 2A. 4a) comprising of mixed diatoms
dominated by Leptocylindrus minimus may perhaps be a result of such eddy
pumping. Leptocylindrus minimus is a neritic species and it was shown that
this species was capable of sustaining higher growth rates than oceanic
species in an iron fertilization experiment carried out in an oligotrophic
subarctic Pacific gyre (Tsuda et al. 2005).
The sequence of dominance and the repeatability of certain species in
the Bay of Bengal reflect the hydrographical settings of the Bay of Bengal.
31
The phytoplankton community showed clear seasonal succession (Table 2.2)
in the appearance of diatoms and Trichodesmium. It was observed that
Trichodesmium were abundant during February and October in the Bay of
Bengal (Fig. 2A.6 a).
Characteristic features of some of the dominant diatoms from the Bay of
Bengal.
While the Bay of Bengal supports a diverse diatom community, a few
species such as Proboscia alata, Climacodium frauenfeldianum,
Leptocylindrus minimus, Lauderia annulata, Thalassiosira spp., Guinardia
striata and Thalassionema nitzschioides including the cyanobacterium
Trichodesmium tend to be dominant during the different seasons in the three
sectors (discussion on Trichodesmium will follow in section 2B). All these
species exhibit characteristic features that enable growth in the generally low
nutrient and stratified surface waters of the Bay.
Proboscia alata is widespread in the boreal and sub-tropical waters.
However, it is now considered as a diatom with cosmopolitan distribution
(Tomas 1997). This species occurred at most times in the CS sector (Table 2).
It has been indicated that Proboscia alata proliferated against the background
of the sharp seasonal pycnocline and the deficiency in nutrients in the upper
mixed layer (Sukhanova et al. 2006). This hydrological feature is
characteristic of the Bay of Bengal. Based on the experimental consumption
of nitrogen, Goering and Iverson (1981) arrived at the conclusion that, in
32
distinction from other diatom species, Proboscia alata was capable of
utilizing ammonium and urea as a nitrogen source. In view of this, it is
worthwhile to note that the nitrogen fixed by Trichodesmium is released as
ammonia and dissolved organic nitrogen (Mulholland et al. 2004). This may
perhaps indicate a syntrophic interaction between the diatom and
Trichodesmium.
Climacodium frauenfeldianum was recorded in the CS sector in
February 2002 as the dominant form. This species was also recorded in
February 2000 as one of the dominant species (Table 2.2). It is an oceanic
species commonly found in tropical and sub-tropical waters (Tomas 1997).
The characteristic feature of this species is the presence of endocellular
cyanobionts (Carpenter and Janson 2000), which are capable of fixing
nitrogen, suggesting symbiosis for nitrogen-fixation in the nitrate-depleted
environment of the Bay of Bengal.
Species like Thalassionema nitzschioides and Guinardia striata have a
cosmopolitan distribution (Tomas 1997). These species were dominant in
November 2001 and July 2004 respectively. These species are known to show
a preference for nutrient-enriched conditions (Revelante and Gilmartin 1980;
1976). In the Bay of Bengal, Thalassionema nitzschioides was observed in
nitrate rich waters where the N:Si raios were >1 (Paul et al. 2008). Another
study carried out on the variations of diatoms off Java (Romero et al. 2009)
also noted that Thalassionema nitzschioides was representative of nutrient-
enriched coastal conditions associated with the monsoons in the Java Sea.
33
Phytoplankton species in a given water mass undergo continuous
changes over time in their dominance and diversity, a process that may be
compared to terrestrial succession. The sequence of dominance and the
repeatability of certain species in the Bay of Bengal reflect the hydrographical
settings of the Bay of Bengal. The phytoplankton community showed clear
seasonal succession (Table 2.2). These species are well adapted to their region
of existence and there appeared to be a succession pattern in the appearance of
diatoms and Trichodesmium. It was observed that Trichodesmium were
abundant during October in the Bay. Subsequently, they were encountered
abundantly during the summer months along the coast of India.
34
2B. Distribution of Trichodesmium
2B.1. INTRODUCTION
Based on the results obtained in section 2A, it was evident that
Trichodesmium formed an important component of the micro-phytoplankton
community of the Bay of Bengal. It was found that Trichodesmium appeared
regularly during the fall and winter in the Bay of Bengal and made their
presence felt during the summer months along the south west coast of India.
This prompted for a more detailed study on the distribution of Trichodesmium
in the BOB. Since Trichodesmium could be followed on a larger scale with
the help of remote sensors, remote-sensing applications were made use of to
study its distribution in the Bay of Bengal. Algorithms to detect
Trichodesmium from space have been developed by Borstad et al. (1992),
Subramaniam et al. (2002) and Westberry et al. (2005).
The first description of Trichodesmium was by Captain Cook in the
1700s in the Australian waters, and ever since, Trichodesmium has awed
mariners and oceanographers with its large blooms spreading over hundreds
of kilometres. Trichodesmium has been recognized as the most important
marine nitrogen-fixing cyanobacteria (diazotrophs), with intriguing features
such as fixing nitrogen in the presence of light without the formation of
heterocysts (site of nitrogen fixation in other cyanobacteria). These
diazotrophs contain proteinaceous vacuoles, or gas vesicles, that provide the
cells with the necessary buoyancy to keep them near the surface (Carpenter
and Price 1977, Capone and Subramaniam 2005). They are widespread in
35
tropical waters (Brongersma-Sanders 1957, Capone and Carpenter 1982) and
are often the major component of the phytoplankton (Carpenter 1983, Capone
et al. 1997), thus making them an important contributor to the global chemical
cycles by new production (Dugdale et al. 1961, Capone et al. 1997).
Phytoplankton absorption could contribute to the seasonal warming of the
surface waters of the water bodies (Sathyendranath et al. 1991).
Trichodesmium, being a major component of phytoplankton in the Bay of
Bengal, could make a significant contribution to the warming of the Bay of
Bengal. Warm waters, calm weather, low or no wind during several days, very
low densities of other micro-organisms and highly transparent waters are
among the conditions required for the formation of the blooms (Carpenter and
Price 1976, Capone et al. 1997, Sellner 1997). Since Trichodesmium can fix
nitrogen, they are not nitrogen limited. Hence it is suggested that their
production may be limited by other elements like iron and phosphorous (Karl
et al. 1992 and 1997, Rueter et al. 1992, Mulholland et al. 2002).
In the Bay of Bengal, Trichodesmium blooms have been recorded since
1942 (Devassy et al. 1978 and references therein Ramamurthy et al. 1972,
Gomes et al. 2000, Jyothibabu et al. 2003). They occur with a well-defined
periodicity and annual rhythm between February and May in the Arabian sea,
and have been described as a phenomenon by Devassy et al. (1978). The
regular timing of the blooms suggests that physical factors such as
stratification, winds and current patterns influence these blooms. However,
elucidation of spatial and temporal distribution was not attempted in any of
36
the earlier investigations. In this study, which is the first of its kind, surface
samples were collected at regular intervals for a period of three years. It was
found in this study that Trichodesmium appeared regularly during the fall and
winter in the Bay of Bengal and make their presence felt during the summer
months along the south west coast of India.
2B. 2. MATERIALS AND METHODS
Collection of samples and the analysis was the same as carried out in
section 2A. To identify probable Trichodesmium patches for the area bounded
by 0˚N - 20˚N and 65˚E - 104˚E, SeaWiFS level 2 images were downloaded
from http://oceancolor.gsfc.nasa.gov and SeaWiFS Data Analysis System
(SeaDAS version 4.9.2). The “L2_flags” tool in SeaDAS was used. Here,
checks are made for different ‘defined’ conditions, and when certain tests and
conditions are met for a given pixel, a flag is set for that pixel for that
condition. In this case the condition indicated is Trichodesmium and the
algorithm “tricho” (http://oceancolor.gsfc.nasa.gov/DOCS/Ocean_Level-
2_Data_Products.pdf), is specific to SeaWiFS (the code can be viewed at
SeaDAS: General Questions, Topic: L2 Flags). The checks used to indicate
the presence of Trichodesmium are areas of warm waters, low wind stress and
water leaving radiances.
Images from SeaWiFS level 2 (MLAC 1km) were studied for each
month and the best swaths were used for the study region (0˚N - 20˚N and
65˚E - 104̊ E). Since SeaWiFS level 2 makes available daily images, 8 days’
37
individual swaths was grouped as a week i.e. 1st -7th as the first week of the
month, 8th -14th as the second and so on and any one good image for that week
was downloaded. This was carried out for all the weeks in the month. A
minimum of 4 images of the area of interest were used for a month. However,
there were some occasions where good images were not available for some
weeks in a month mostly because of distorted images and cloud obstruction.
To display and mask SeaWiFS-L2 products with flags, SeaWiFS Level
2 file was downloaded for the desired day and loaded onto SeaDAS.
Subsequently, the chlorophyll and l2_flags products were selected. The L2
Flags Display from the Utilities menu is used to load and display probable
Trichodesmium patches on the image. On the image window, the cursor was
moved on the points of Trichodesmium appearance marked on the image
display, and the output for the location (Lat/Lon) was noted. These values
were then superimposed on a map to represent month-wise, qualitative
presence of Trichodesmium (Fig. 2B.2 a-l, stars and circles indicate the
months February to May and November to January respectively.
Cloud cover and distorted images were major drawbacks for the region
of interest, especially from June to August (Indian monsoons). Therefore, the
data collated for the months November to May were used. This information
was segregated into two sets i.e., November to January (NDJ) and February to
May (FMAM) based on the changes in the current patterns in the Bay of
Bengal (Cutler and Swallow 1984, Shetye et al. 1993, Tomczak and Godfrey
2001). According to Carpenter et al. (1997) and Capone et al. (1998),
38
Trichodesmium colonies are positively buoyant in the early mornings and if
wind stress is low, rise to the surface but in the afternoon they become
negatively buoyant and sink, and thus may not be detected by satellite. Taking
this into consideration, early morning swaths were used.
2B.3. RESULTS
2B.3.1. In situ and remote sensing measurements
Presence of high numbers of Trichodesmium was recorded on 1st
August 2002 in three sampling stations off Chennai i.e. at 12.83N / 81.00E,
2.42N / 82E, and 11.98E / 83.03N with approximately 330 trichomes L-1 (Fig
2B.1). Satellite images were not available for the same day due to cloud cover,
since Trichodesmium blooms last for a few days cloud free images around the
date of observation were examined, and an image for the 4th of August 2002,
showed Trichodesmium at 13.96N / 80.38E. Also, Trichodesmium was
recorded on 19th October 2000 at 7.5N / 92.02E with ~1800 trichomes L-1
(Fig. 2B.1); there were no cloud-free remotely-sensed images for the entire
week but a swath on 15th October 2000 showed a Trichodesmium patch
between 7.85N - 8.17N / 100.63E – 100.81E. On 18th August 2002 in situ
samples collected at 5.48N / 97.40E showed the presence of Trichodesmium
(~900 trichomes L-1, Fig. 2B.1); remotely-sensed values for the same day
recorded Trichodesmium at 5.42N – 5.30N / 96.29E - 96.57E, the remaining
area was covered with clouds. As the three occurrences of high numbers of
Trichodesmium matched positively with remote-sensed data, three more
39
0
1
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3
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Tric
hom
esL-1
[log(
x+1)
]
Tric
hom
esL-1
[log(
x+1)
]
Longitude (OE)
78 80 82 84 86 88 90 92 94 96 78 80 82 84 86 88 90 92 94 96 78 80 82 84 86 88 90 92 94 96
Oct 00
Aug 00
Feb 00
Feb 01
Apr 01
Aug 01
Sep 01
Oct 01
Nov 01 Sep 02
18 Aug 02
1 Aug 02
Jun 02
May 02
Feb 02
Fig 2B.1 Trichodesmium spp. abundance (trichomes L–1) plotted for the ship-basedobservations in the 15 transects between Chennai - Singapore, with longitude on thex-axis and number of individual filaments (trichomes) on the y-axis
40
Fig 2B.2 Remotely-sensed Trichodesmium occurrences superimposed on a map ofthe region for the period of 1997–2003. (a–f) November to January (circles); (g–l)February to May (stars)
(a)
(j)(d)
(i)(c)
(b) (h)
(g)
(k)(e)
(l)(f)
41
locations with no occurrence of Trichodesmium were randomly selected (5th
February 2002 at 12.99N/81.06E, 6th October 2001 at 11.51N/86.19E and 2nd
August 2001 at 9.46N/92.84E, Fig. 2B.1) and were checked with remotely-
sensed images. These points too matched positively with remotely-sensed
images. Hence, it was assumed that the flag could be used to determine
presence of Trichodesmium in the Bay. Trichodesmium was recorded in the
Bay (4-100 trichomes L-1) in most transects. Using the ship-based observation
as starting points, the temporal and spatial variations of Trichodesmium was
explored using remote sensing for the period 1997 to 2003.
Occurrence of Trichodesmium is predominant in the Bay of Bengal
especially in the winter months (November to January, Fig. 2B.2 a-f). During
summer (February to May, Fig 2B.2 g-l), the occurrence of Trichodesmium is
nearer to the east coast of India and in the eastern Arabian Sea. However, in
the Arabian Sea, the incidences of Trichodesmium are comparatively lesser
than that in the Bay. Trichodesmium occurrences increased over the years in
the winter months from 1998-2001, followed by a decrease in the years 2001-
2002, after which it again increased in the years 2002-2003. The period 1997-
2003 covers two El Niño events: 1997-98 and 2002-03. Minimal spots for
Trichodesmium in November 1997 to January 1998 (Figure 2B.2 a) were
obtained. It should be noted that numbers of images for this period were few
and coverage was nil for the Bay of Bengal. November 2002-January 2003
(Fig. 2B.2f) showed a widespread occurrence of Trichodesmium in the south
central part of the Bay, spreading to the east and west coast in the subsequent
42
months, with the highest occurrence noticed in the east coast of India during
February-May 2003 (Fig 2B.2 l). Data for February–May 1999 (Fig. 2B.2 h)
was different from the other years, with many occurrences of Trichodesmium
in the Bay of Bengal.
2B.4. DISCUSSION
The interesting and intriguing feature of Trichodesmium blooms is the
suddenness with which the blooms appear and disappear (Qasim 1972).
Devassy et al. (1978) suggested two possibilities: Some kind of dormant
resting stage may be present during non-bloom conditions that go unnoticed
or misidentified during analyses; or the organism in the non-bloom season
remains distributed throughout the mixed-layer such that sampling at the
surface under these conditions will show only a few filaments, and then, at the
onset of favourable conditions, the filaments become buoyant and come to the
surface to form blooms. Recent observations using video plankton recorder
(Davis and McGillicuddy 2006), revealed Trichodesmium colonies up to a
depth of 130 m and not just restricted to the surface.
Trichodesmium blooms have been observed on several occasions in
the past in the area of study (Ramamurthy et al. 1972, Devassy et al. 1978,
Gomes et al. 2000, Jyothibabu et al. 2003). In the areas where high numbers
of Trichodesmium were encountered, remote-sensing was used to support the
observations. Since the present observations and the remotely-sensed data
matched fairly well, blooms reported by Jyothibabu et al. (2003) at 10˚ 58'N /
43
81˚ 50'E (Karaikal) on 11th April 2001 and at 19˚ 44'N / 89˚ 04'E (off
Calcutta) on 25th April 2001 were also checked with the remote-sensed data.
The former could not be seen in the remotely-sensed images due to cloud
cover (there were no cloud-free images for the entire week for the region).
The latter was picked up by remote-sensors and showed Trichodesmium
patches close to the area at 20.07N – 20.16N / 86.95 – 87.20E. Since the study
region comes under the influence of monsoon (June to September), the
retrieval of data through remote-sensing was hampered due to cloud cover
over the area. At such times, one has to rely on ground truth data to get a
comprehensive view of Trichodesmium in the Bay of Bengal.
This study shows that Trichodesmium are abundant during the fall and
winter in the Bay (Fig. 2B.2a-f). Subsequently, they were encountered
abundantly during the summer months along the coasts of India (Fig. 2B.2 g-
l). An inter-annual variability in the distribution of Trichodesmium was
observed. The remotely-sensed images show the presence of Trichodesmium
almost round the year in the south-eastern sector. The signatures of
Trichodesmium from ship-based collections (Fig. 2B.1) also indicate their
prevalence in the south-eastern sector and show a west or northwestward
propagation. From the ground data (Fig. 2B.1), it appears as though
Trichodesmium originates and blooms in the south east and then appears to
move north westwards (see Fig. 2A.6a), forming blooms in the western Bay of
Bengal after a few months. Remotely-sensed images also suggest that
Trichodesmium generally proliferates in the south-eastern part of the Bay
44
during the winter months and in the west (west Bay of Bengal and east
Arabian Sea) during summer. Resting cells in phytoplankton are a well known
biological strategy to survive adverse conditions. Such a strategy is not well
reported with Trichodesmium. In light of this, seeding of a new bloom will
have to be dependent on the residual survivors of the previous bloom at very
low densities. Berman-Frank et al. (2004) were tempted to speculate that
programmed cell death pathways in Trichodesmium may regulate
differentiation of selected cells into hormogonia (small group of cells that are
released from a parent filament after death), providing an inocula for the new
bloom upon improved environmental conditions. It was also stated that
hormogonia are observed typically deep in the euphotic zone around or below
1% light level after a large fraction of trichomes decomposed. This highlights
the possibilities of hydrographic settings playing an important role in its
distribution.
In order to elucidate the reasons for prevalence of Trichodesmium in
the Bay, the hydrographic settings of the Bay of Bengal were looked into.
Temperature
There was no correlation between temperature and the abundance of
Trichodesmium in this study. This may be attributed to the temperature range
in the study region (27˚C to 32˚C) being well within the favoured range for its
growth. Another set of observations by Tyrrell et al. (2003) also found a lack
45
of correlation between temperature and Trichodesmium for the Atlantic
Ocean.
Shallow mixed layer depth and stratification of the water column
Abundance of Trichodesmium was found to correlate with shallow
mixed layer depths (Tyrrell et al. 2003) and BOB has a shallow mixed layer
depth of ~30 m (Prasad 2004). The seasonal cycle of the barrier layer is
computed up to 25 m in the Bay of Bengal and eastern equatorial Indian
Ocean in winter (Montegut et al. 2004). The other aspect necessary for the
growth of Trichodesmium is stratification, as this allows the cells to float to
the surface (Carpenter and Price, 1976). The predominance of Trichodesmium
blooms during February to May along the west coast of India is also attributed
to stratification (Devassy et al. 1978). The Bay of Bengal shows stratification
of water almost throughout the year due to the variations in the salinity
brought about by fresh water influx (Shetye et al. 1993). Even the cold
continental winds of the Northeast Monsoon does not cool the surface water
enough to cause an inversion since the overturning is prevented or reduced by
the stable background stratification due to strong salinity gradients (Tomczak
and Godfrey 1994).
Nutrient availability
It is well known that Trichodesmium is generally seen in nutrient poor
waters. Iron and phosphorous limitation are most often suspected to affect the
46
growth of Trichodesmium (Karl et al. 1992 and 1997, Rueter et al. 1992). Low
surface PO43- -P values of 0.1µg-atL-1 have been recorded in the Bay of
Bengal and Andaman sea increasing to a maximum of 2.90 µg-at L-1 at depth
(Rozanov 1964, Kabanova 1964). Likewise, the dissolved nitrate and nitrite
values in the Bay of Bengal are also reported to be low in the surface waters
and increase sharply to 15-20 µg-at L-1 in the thermocline and upto 26 µg-at
L-1 at depth (Rozanov 1964, Kabanova 1964). The dominance of the
Trichodesmium in the Bay of Bengal and the reported low concentration of
PO4 indicate the potential importance of the mechanisms such as phosphorus
mining (Karl et al. 1992). Mulholland et al. 2002 report that organic
phosphate alleviated P limitation and promoted P uptake and growth. In this
context, it is worthwhile to note that Bay of Bengal has higher level of organic
phosphate in the surface waters when compared to inorganic phosphate (Rao
and Rao 1968). Iron has been implicated to be a limiting factor for
Trichodesmium growth. High iron deposition to the surface ocean correlated
to high abundance of Trichodesmium (Tyrrell et al. 2003). The aerosol input
from the subcontinent carrying iron dust might have a role to play in the
proliferation of the organism. While the Arabian Sea receives more
atmospheric input of crustal dust containing non-sea sulphate, nitrate,
phosphate and iron from the Arabian Desert (Patra et al. 2007), the Bay of
Bengal receives aerosols due to anthropogenic activities (pers. comm., Dr.
Dilip Kumar, COD, NIO). Iron from the pollution sources produce labile
forms of Fe (Siefert et al. 1999) and this bio-available Fe may be utilized by
47
Trichodesmium. Thus, the amount of bio-available iron in the two basins
might determine the proliferation of Trichodesmium.
The El Niño years (2002-03) showed the widespread occurrence of
Trichodesmium (Fig. 2B.2f). A rough correspondence between the strength of
Southern Oscillation Index (SOI) and the occurrence of Trichodesmium
blooms was found (Westberry et al. 2006). In this context, Abram et al. (2003)
had reported the occurrence of a red tide spreading over hundreds of
kilometres, which caused the death of coral reefs. They also reported high iron
discharge into the sea as a result of the Indonesian forest fires. There was no
evidence about the identity of the bloom organism and the bloom was thought
to be caused by dinoflagellates. Going by literature, it could have been a
Trichodesmium bloom as it is well known that Trichodesmium is capable of
forming red tides spreading over hundreds of kilometers, and they are also
responsible for the smothering of corals (Bell et al. 1999). In the subsequent
El Niño year (November 2002- January 2003, Fig. 2B.2f), Trichodesmium
was seen in the Bay of Bengal as well as along the coast of India.
Currents
Currents seem to play an important role in the dispersal of
Trichodesmium in the Indian Ocean. Ship drifts in the Bay of Bengal based on
10 day averages compiled by Cutler and Swallow (1984) show a westward
movement of the North Equatorial Current (NEC) from December to March.
Tomczak and Godfrey (2001) describe the NEC as a narrow current of about
48
0.3 m s-1 from Malacca Strait to southern Sri Lanka. They further elaborate on
the evolution of surface currents through the seasons, showing that the NEC is
prominent in March – April. Between September and November the direction
of the East India Coastal Current (EICC) is equator-ward (Shetye et al. 1996)
along the east coast of India. This current turns northwards south of Sri Lanka
and flows into the Arabian Sea (Tomczak and Godfrey 2001). Remote sensed
observations (Dupuoy et al. 2000) show high abundance of Trichodesmium in
the southwest pacific as a recurring phenomenon. Considering the Indonesian
throughflow (Gordon 2001, Wyrtiki 1961) it is possible to deduce dispersion
of trichomes westward by currents causing seeding between December and
February resulting in blooms off the west coast of India between February and
May. During November 2002, extensive blooms were reported off the coast of
southeast Queensland, Australia (Bell et al. 2005) and the signatures were also
seen in the eastern sector of the study area (Fig. 2B.2f) and spread westwards.
Davis and McGillicuddy (2006) in their study observed higher abundance of
Trichodesmium in the anticyclonic eddies and attributed this covariance to
eddy-driven transport of these populations from a distant source region,
indicating that Trichodesmium dispersal is current-driven.
It was also initially opined that colonies can be destroyed by strong
winds (Capone et al. 1997). However, observations by Davis and
McGillicuddy (2006) showed no reduction in colony abundance in a region
affected by hurricanes (wind speed 200 km h-1). In view, of this, it is possible
49
that Trichodesmium can survive in the Bay of Bengal which is cyclone-ridden
during the north-east monsoons.
It can be deduced that the Bay of Bengal is conducive for
Trichodesmium blooms due to its unique hydrography. Further studies to
elucidate this phenomenon with rigorous sampling coupled with remote-
sensing and video profiling of the water column will be of immense use in
understanding the biogeochemistry of the area.
