Journal of Applied Phycology 9: 229–236, 1997. 229c 1997 Kluwer Academic Publishers. Printed in Belgium.

Factors controlling the growth of field populations of Hydrodictyonreticulatum in New Zealand

Julie Hall� & George PayneNational Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand(�Author for correspondence)

Received 5 January 1997; revised 15 May 1997; accepted 17 May 1997

Key words: nutrients, growth rate, Hydrodictyon reticulatum, seasonal changes, temperature, growth prediction

Abstract

Since Hydrodictyon reticulatum was introduced to New Zealand it has spread rapidly and produced persistent annualnuisance growths in areas where nuisance algae had not occurred previously. Field bioassays were conducted at 10sites between August 1993 and February 1995 to evaluate the seasonal growth patterns and the factors controllinggrowth under natural conditions. H. reticulatum exhibited a strong seasonal growth pattern with growth rates up to0.33 doublings d�1 from August to March, a reduction in growth rate in April and little or no growth from May toJuly. The H. reticulatum present in New Zealand has a relatively low requirement for dissolved inorganic nitrogen(DIN) in comparison with other nuisance species, with its growth rate being saturated at 200 mg m�3. This andthe high affinity for DIN as shown by a Ks of 29 mg m�3 have been key factors in the establishment of nuisancegrowths of H. reticulatum in New Zealand.

Introduction

Hydrodictyon reticulatum was first reported in NewZealand in 1987 (Coffey & Miller, 1988) and since thenhas spread rapidly throughout the eastern Bay of Plen-ty and Central North Island region (Figure 1). In NewZealand H. reticulatum has formed nuisance growthsas surface mats over large areas in several economi-cally important lakes during the summer period. Theyear to year consistency of this nuisance problem inNew Zealand is unlike overseas reports where H. retic-ulatum has formed nuisance growths for one summeror at most cause an irregular problem in some lakes(Sorensen, 1950; Corrillion & Geurlesquin, 1977).

Contradictory information exists on the require-ments for H. reticulatum growth. For example Prescott(1970) suggested high pH and Ca were growth require-ments, while Fitzgerald (1981) suggested low pH sup-ported bloom conditions. Moss (1973) suggested thateutrophication rather than high Ca was required forrapid growth. In New Zealand, extensive growth ofH. reticulatum has been observed in low Ca, neutral

to acidic pH and moderately enriched waters (Haweset al., 1991; Hawes & Smith, 1993).

Laboratory studies by Hawes and Smith (1993)using batch cultures showed that a New Zealand iso-late of H. reticulatum had a high affinity for nitrogen incomparison to other nuisance algal species. Filterablereactive phosphorus (FRP) above 5 mg m�3 and NO3-N of 20 mg m�3 was found to support rapid growth.An observational study in mid-summer by Hall andCox (1995) showed no relationship between healthof H. reticulatum populations and dissolved reactivephosphorus or tissue phosphorus concentrations.Therewas however a significant inverse relationship betweenpopulation health and the tissue C:N ratio. C:N ratiosbelow 18 supported growing populations. Dissolvedinorganic nitrogen (NO3-N + NH4-N) concentrationsabove 30 mg m�3 were shown to support growing pop-ulations, which suggests that nitrogen is likely to be themajor limiting nutrient for H. reticulatum populationsin this region of New Zealand.

The study reported here was initiated to establishseasonal growth patterns and to identify the factorscontrolling the growth rate of natural populations of

Article: japh 484 GSB: Pips nr 141398 BIO2KAP

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Figure 1. Location of region and study sites in 1993/94.

H. reticulatum in New Zealand. This is potentially dif-ficult due to the interaction of physical factors such aslight and temperature, chemical factors such as nutri-ent concentrations and changes in biomass due to lossand decay. To reduce these problems, bioassay exper-iments were conducted over short periods with a stan-dard algal inoculum used for each experiment. Thisstudy was undertaken to elucidate the factors control-ling natural nuisance populations of H. reticulatum inNew Zealand.

Methods

Approximately monthly between August 1993 andJuly 1994 (Table 1), field bioassays were conductedat 10 sites in the Waikato and Bay of Plenty regions ofNew Zealand (Figure 1) to measure the growth rate ofH. reticulatum under near natural conditions. A secondset of five field bioassay experiments was conductedfrom December 1994 to February 1995 at nine sitesin Whirinaki Arm of Lake Ohakuri (Figure 2) where

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Table 1. Seasonal variation in growth rates: Growth rates of H.reticulatum averaged over 10 sites, during 1993/94 sampling period.

Growth rate Temperature

doublings d�1 �C

Date Mean Range Mean Range

5/8–19/8/93 0.14 <0.0–0.26 11.1 9.5–13

6/10–20/10/93 0.18 <0.01–0.28 16.2 14.5–19.5

24/11–8/12/93 0.20 0.13–0.28 17.0 13.5–24.5

8/12–22/12/93 0.15 <0.01–0.28 18.2 13.5–22

10/1–24/1/94 0.18 0.10–0.25 21.5 15.0–25.8

10/2–24/2/94 0.21 0.11–0.29 20.6 14.0–25.8

15/3–29/3/94 0.17 0.06–0.33 18.0 16.2–19.5

14/4–28/4/94 0.06 <0.01–0.10 14.3 12.0–16.5

19/5–2/6/94 0.02 <0.01–0.09 10.2 9.4–11.9

16/6–30/6/94* <0.01 <0.01–0.05 7.6 5.0–9.5

14/7–28/7/94* <0.01 <0.01–0.10 7.5 6.0–9.5

� Growth only recorded at site 1.

a significant nutrient gradient existed along the lengthof the Arm, with a new bioassay started every 14 days.At each site three replicate cages, constructed of steelrod with nylon mesh liners were deployed using sty-rofoam floats and anchored with shock chord attachedto a weight. The cages were 0.5� 0.5� 0.5 m anddesigned to float at the water surface. An inoculumsample was added to each cage. For each bioassaythe inoculum material was collected from site 1 (Fig-ure 1), shaken to remove excess water and the amountof material adjusted to give a wet weight of 30 g. Eachinoculum was then stored at the water temperature ofthe pond until the material was needed for inoculation(within 6 h).

At each site, temperature was measured and watersamples were collected for analyses of NO3-N, NH4-N, and PO4-P (FRP) concentrations. Water sampleswere stored on ice and filtered within 24 h throughacid soaked and rinsed Whatman GF/F filters. Sam-ples were then stored frozen until analysed for NO3-N,NH4-N and FRP using a Technicon II Autoanalyser(Downes, 1988). Dissolved inorganic nitrogen (DIN)was calculated as the sum of NO3-N and NH4-N con-centrations.

As a check for inoculum variation between themonthly bioassays, three replicate inocula were placedin a constant temperature room in cages suspendedin fish crates with 90 L hydroponics solution. Thesesamples were maintained at 20 �

� 1 �C, a light inten-sity of 100 �mol m2 s�1 and a light/dark cycle of16:8 h. In addition, three inoculum samples were dried

Figure 2. Location of study sites in 1994/95.

to constant weight and weighed to establish initial dryweights for each sampling period.

After 14 days the H. reticulatum was harvestedfrom the field cages, temperature was measured, andwater samples were collected from each site for nutri-ent analysis as described above. The H. reticulatumsamples were dried at 80 �C, to constant weight,weighed and then the samples were ground for analy-sis of particulate carbon (PC), nitrogen (PN) and phos-phorus (PP). Tissue carbon and nitrogen content weredetermined using a Perkin-Elmer 2400 CHN ElementalAnalyser and phosphorus content was determined col-orimetrically following wet digestion (Downes, 1988).Specific growth rate (GR) was calculated as

GR = ln(B2 + B1)t ln2

where B1 and B2 are biomass (as dry weight) and t isthe length of the bioassay in days.

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Four sites (3–6) in Whirinaki Arm of Lake Ohakuri(Figure 2) used in experiments in 1993/94 were alsoused in 1994/95, enabling direct comparisons betweenthe two years.

Results

Seasonal growth pattern

In the bioassays conducted in 1993/94 between Augustand March growth occurred in 10 out of the 11 sites,with mean growth rates over all sites ranging from0.14 to 0.21 doublings d�1 (Table 1). The overall meangrowth rate was highest in February, while the max-imum growth rate of 0.33 doublings d�1 for a singlesite occurred at site 10 in March. During the August toMarch period the mean temperature ranged from 11.1to 21.5 �C with the highest mean temperature occur-ring in January (Table 1). In April there was significantdecrease in the growth rate to a mean of 0.06 doublingsd�1 with growth occurring at 9 out of the 11 sites. Afurther decrease in growth rate occurred in May witha mean of 0.02 doublings d�1 with growth recordedat only 4 sites. In June and July growth only occurredat site 1. A decrease in mean temperature was alsorecorded during this period from 14.3 �C in Marchto 7.5 �C in July (Table 1). A clear seasonal growthpattern was observed at sites 3, 8 and 10 where DINand FRP concentrations were always above saturation.Growth was relatively consistent from August to Aprilafter which a significant decrease in growth rate wasobserved (Figure 3).

In 1994/95 the mean growth rates recorded fromDecember to March were similar to those in 1993/94and ranged from 0.11 to 0.19 doublings d�1. Growthoccurred at all sites for each bioassay with a maximumgrowth rate of 0.26 doubling d�1 recorded at site 9 inJanuary. The temperatures were also similar to thoserecorded in 1993/94, ranging from 18.0 to 20.4 �C.

Water and tissue nutrients

In the 1993/94 bioassays mean DIN concentrationsranged from 48 mg m�3 at site 6 to 3677 mg m�3 atsite 1 (Table 2). At sites 2, 4 and 5 a strong seasonalpattern was observed in the DIN concentrations withthe lowest concentrations being observed in Februaryand March. Site 6 also showed a seasonal pattern withthe lowest concentrations recorded in December, Jan-

uary and February. The other sites showed little or noevidence of a seasonal pattern in DIN concentrations.

The mean FRP concentration ranged from 3.7 mgm�3 at site 7 to 143 mg m�3 at site 3 (Table 2). Sites 2,4 and 7 showed a strong seasonal pattern in FRP con-centration, with sites 2 and 6 recording their lowestconcentrations in March and April; at sites 4 and 5the lowest concentrations were also recorded in May.The lowest concentrations at site 7 were recorded inFebruary, March and April. At the sites sampled inWhirinaki Arm of Lake Ohakuri in 1994/95 the meanDIN concentration ranged from 7.1 mg m�3 at site 3to 789 mg m�3 at site 9 at the head of the Arm. Thelower mean concentrations recorded in the 1994/95study were a result of the sampling being conductedduring the summer period when nutrient concentra-tions were frequently lower as observed in the 1993/94data at sites 4, 5 and 6. The mean FRP concentrationsincreased consistently moving up the Arm of the Lakefrom 12.4 mg m�3 at site 1 to a high of 147.2 mg m�3

at site 9.The mean C:N ratio in the 1993/94 bioassays

ranged from 7.1:1 to 16.6:1 with an overall range of5.3 to 22.3. At sites 2, 4, 5, 6 and 7 where the DIN con-centration was frequently below saturation, seasonalpatterns were observed in the C:N ratio with the ratioincreasing over the summer period when DIN concen-trations in the water were low. At sites where the DINconcentration was consistently above saturation therewas no seasonal variation in the C:N ratio. A similarresult was observed in 1994/95 with the mean C:N ratioranging from 16.9:1 to 7.2:1 moving up the Arm alongthe increasing DIN gradient. In the 1993/94 bioassaysthe mean N:P ratio ranged from 3.4:1 to 7.1:1 with anoverall range of 0.7:1 to 13.1:1. There was no seasonalpattern shown in the N:P ratio at any of the study sites.

The range of N:P ratios recorded in 1994/95 wasmuch narrower than the previous year ranging from0.85:1 to 4.3:1 with site means ranging from 1.6:1 atsites 3 and 4 to 4.5:1 at site 1.

Growth and nutrient relationships

To investigate the relationship between growth rate andnutrient concentrations, data from the primary grow-ing season from both years were used. This includedall data from the 1994/95 bioassays and bioassay 1–7in 1993/94 (Figure 4). The data from site 1 in 1993/94were removed from the analysis due to the extremelyhigh DIN concentrations (Tables 2, 3) and unusual sea-sonal growth patterns. Analysis of the site 1 data alone

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Figure 3. Growth rate B and temperature E at sites (a) 3, (b) 8, (c) 10, for experiments in 1993/94.

Table 2. Between site variation in growth rate and nutrient concentrations: Growthrates of H. reticulatum averaged over 11 sampling periods in 1993/94.

Site Growth rate DIN FRP

doublings d�1 mg m�3 mg m�3

Mean Range Mean Range Mean Range

1 0.13 0.05–0.20 3677 1798–4278 71.6 14.0–338

2 0.07 <0.01–0.15 73 4.8–163 8.9 4.7–15.5

3 0.16 <0.01–0.28 461 214–785 143 76.5–294

4 0.10 <0.01–0.21 198 4.5–715 75.5 47.1–109

5 0.09 <0.01–0.21 132 4.0–552 48.5 20.0–83

6 0.04 <0.01–0.12 48 3.0–156 27.2 9.3–71.7

7 0.10 <0.01–0.18 220 8.9–32.8 3.7 1.7–5.5

8 0.18 <0.01–0.30 915 358–2325 26.4 6.7–38.0

9 0.14 <0.01–0.29 587 488–678 41.0 0.0–51

10 0.20 <0.01–0.33 930 836–1029 75.5 59.5–96

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Table 3. Between site variation in growth rate and nutrient concentrations: Growthrates of H. reticulatum averaged over 5 sampling periods in 1994/95 in WhirinakiArrn of Lake Ohakuri.

Growth rate DIN FRP

doublings d�1 mg m�3 mg m�3

Site Mean Range Mean Range Mean Range

1 0.03 0.01–0.07 17.8 3.15–48.1 12.4 8.8–20.9

2 0.13 0.06–0.23 30 17.0–45.3 38.9 25.4–46.3

3 0.16 0.08–0.22 7.1 2.95–22.5 68.6 56.0–79.7

4 0.15 0.08–0.22 13.2 5.3–28.8 92.7 74.8–107.3

5 0.16 0.09–0.19 23.9 1.9–82.3 103.3 75.2–124.9

6 0.13 0.09–0.20 40.5 1.9–128.7 110.2 86.6–130.8

7 0.16 0.11–0.20 61.2 27.3–170 112.8 91.9–136.5

8 0.17 0.10–0.21 243.7 150–307 132.9 108.7–149.9

9 0.21 0.17–0.26 789.0 361–1425 147.2 115.7–177.3

showed no relationship between growth rate and DIN,FRP or temperature.

The relationship between growth rate and DIN con-centration can be described by a hyperbolic curve (Fig-ure 4a) with saturation of growth rate at approximately200 mg m�3. Tissue N content had a range of 0.5 to 4.8and also showed a hyperbolic relationship to DIN con-centration with a Ks of 29 mg m�3 DIN (Figure 4b).There was no relationship between growth rate andFRP concentration (Figure 4c).

Discussion

The seasonal growth pattern of an alga is a resultof a complex set of interactions involving factorssuch as temperature, light, nutrient concentrations andloss factors. The experiments in 1993/94 showed astrong seasonality in the growth of H. reticulatum withhigh growth rates recorded in early spring through toFebruary–March (Table 1), with no growth recordedduring June and July (except site 1). These patternsdo not appear to be related to seasonal changes inDIN concentrations as these patterns were observedat sites 3, 8 and 10 where DIN and FRP concentra-tions were above saturation throughout the year. Astrong correlation between growth rate and temperature(Figure 3) was not observed suggesting other factorswere responsible for the seasonal cycle observed. Thetemperatures recorded during these bioassays were allwithin the range reported by Hawes and Smith (1993)for H. reticulatum cells to be viable. They reported

an optimal temperature for growth of 25 �C with nogrowth occurring below 6 �C.

Irradiance levels may have contributed significant-ly to the seasonal pattern as no growth was recordedin June and July (except site 1), when lowest ambi-ent light conditions occur. Canale et al. (1982) showedboth temperature and light had a significant influenceon both photosynthesis and respiration on Cladopho-ra in Lake Huron where a strong seasonal cycle wasobserved. The measurement of loss factors in this typeof field bioassay experiment is very difficult and wasnot attempted in these experiments. However observa-tion showed no evidence of grazing in the cages duringthe experiments, hence it is unlikely to cause seasonalchanges.

There is no obvious explanation for the lack ofseasonality in growth rate recorded at site 1. Low tem-perature does not appear to be a likely explanation, assites 3, 8 and 10 all had similar temperature minima of8.0 �C. The DIN concentrations (Table 2) recorded atsite 1 were above saturation through out the year andare not a likely explanation as several other sites whichshowed no growth had DIN concentrations above thoserequired to saturate growth throughout the winter peri-od. There may however be other chemical factors thatwere not measured which may contribute to a complexset of factors which support H. reticulatum growth atthis site over the winter. For example Sorensen (1950)suggested low organic concentrations were a require-ment for good H. reticulatum growth, in contrast toCorillion and Geurlequin (1977) who recorded growthonly in organic rich waters.

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Figure 4. Relationship between (a) growth rate and DIN concentration, (b) Tissue N content (%) and DIN concentration, (c) growth rate andFRP concentration 1993/94 experiments B , 1994/95 experiments E .

The results of this field study have shown that theH. reticulatum present in New Zealand has a low nitro-gen requirement for growth. DIN concentrations above200 mg m�3 were shown to support saturated growth,(Figure 4a) and optimal tissue N content (Figure 4b).H. reticulatum was also shown to have a high affinityfor N with a Ks for tissue N content of 29 mg m�3

(Figure 4b). These results are consistent with laborato-ry studies conducted by Hawes and Smith (1993) whoshowed a Ks for tissue N content of 18 mg m�3 and thatoptimal tissue N content was maintained at NO3 con-centrations above 100 mg m�3. Observational studiesby Hall and Cox (1995) also suggested a low nitro-

gen requirement for growth with DIN concentrationsabove 30 mg m�3 shown to support healthy popula-tions of H. reticulatum. These concentrations are wellbelow those reported for other nuisance species such asCladophora with a Ks for tissue N content of 590 mgm�3 and a DIN concentration of 700 mg m�3 to satu-rate growth (Gordon et al., 1981).

The low nitrogen requirement to support growth ofH. reticulatum potentially explains why this specieshas been able to form substantial nuisance blooms inwater bodies that had not previously supported abun-dant algal growth. The region of New Zealand whereH. reticulatum has become established (Figure 1) is

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acknowledged as a region where nitrogen frequentlylimits algal growth (White, 1983) and hence nuisancegrowths of species with high nitrogen requirementshave not become established.

FRP does not appear to be a controlling factor ofH. reticulatum growth in the region with no relation-ship between FRP and growth rate (Figure 4c). FRPconcentrations were rarely (Tables 2, 3) below the 5 mgm�3 suggested by Hawes and Smith (1993) as neces-sary to support rapid growth of H. reticulatum. This isalso consistent with Hall and Cox (1995) who showedno relationship between H. reticulatum health and FRPconcentration in field populations.

The tissue content N:P ratio is often used as anindicator of the nutrient status of algae. An N:P ratioof 10–20 is considered to represent the optimum rangefor phytoplankton (Goldman et al., 1979). The meantissue N:P ratio was calculated for all tissue samplescollected during these experiments where both DINand FRP were above saturation for both growth andtissue content, 200 and 5 mg m�3 respectively. Underthese conditions the mean N:P ratio was 4.5� 1.1. Thisis below the N:P ratios of 6–13 which were reportedto support good growth of Cladophora (Gordon et al.,1981) and the 6–9 which Wallentinus (1981) showedsupported highest photosynthetic rates in Cladophora.These very low N:P ratios for nutrient sufficient cellssupports the suggestion by Hawes and Smith (1993)that H. reticulatum is unable to store nitrogen. Theinability to store nitrogen contrasts with both Ulva andCladophora which have been reported to store cellularnitrogen (Fujita et al., 1989; Gordon et al., 1981).This potential inability to store nitrogen may lead tothe very low N:P ratios recorded for nutrient sufficientH. reticulatum cells.

In conclusion this study has shown a strong season-al growth cycle for H. reticulatum in New Zealand withhigh growth rates occurring from August to February.We have also shown that the H. reticulatum present inNew Zealand has a low N requirement for growth anda high affinity for DIN, which are key factors in theestablishment of nuisance growths of H. reticulatum inthis country.

Acknowledgements

This research was supported by funding provided bythe Foundation of Research, Science and Technol-ogy, New Zealand. Aleki Taumoepeau and RohanWells provided invaluable assistance with field work.

Ian Hawes, John Clayton and Trevor Atkins provid-ed helpful discussions and suggestions for improvingthis manuscript. Megan Hickman, John Hawken, FayRichards and Stuart Pickmere assisted with analysis ofwater and tissue samples.

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