Dissolved oxygen and nutrient budgets in a phytotreatment pond colonised

by Ulva spp.

Marco Bartoli1,*, Daniele Nizzoli1, Luigi Vezzulli2, Mariachiara Naldi1, Giorgio Fanciulli2,Pierluigi Viaroli1 & Mauro Fabiano21Dipartimento di Scienze Ambientali (DSA), Universita di Parma, Parma, Italy2Dipartimento per lo Studio del Territorio e delle sue Risorse (DIP.TE.RIS), Universita di Genova, Genova, Italy(*Author for correspondence: Tel.: +39-0521-906000; Fax: +39-0521-905402; E-mail: bartoli@dsa.unipr.it)

Key words: phytotreatment ponds, Ulva harvesting, net budgets of oxygen, C, N, P

Abstract

Net daily budgets of dissolved oxygen (O2), dissolved inorganic carbon (DIC), dissolved inorganicnitrogen (DIN = NH4

++NO2)+NO3

)) and soluble reactive phosphorus (SRP) were determined in apond colonised by Ulva spp. This pond received wastewater from a land-based fish farm and was usedas a phytotreatment plant. Three consecutive 24-h cycles of measurements were performed with 8–14samplings per day. Water samples were collected at the inlet and outlet of the pond and budgets wereestimated from differences between inlet and outlet loadings. The first cycle was started when Ulvabiomass was 8 kg m)2, as wet weight. The second cycle was performed after the harvest of �20% ofthe macroalgal biomass and the third after the harvest of another �20% of the remaining biomass.Ulva removal was very fast (<1 h) and samplings for cycles 2 and 3 were started two hours afterharvesting, so that the whole experiment lasted �80 h. When Ulva biomass was at its maximum, theaquatic system was heterotrophic with an O2 demand of 519 mol d)1 and a net regeneration of DIC(2686 mol d)1), NH4

+ (49 mol d)1) and SRP (2.5 mol d)1). The DIC to O2 ratio was an indicator ofpersistent anaerobic metabolism. Following the first harvest intervention, this system displayed aprompt response and shifted toward a lower O2 demand (from )519 to )13 mol d)1), with a lesserregeneration degree of NH4

+ (11.4 mol d)1) and DIC (1066 mol d)1). After the second Ulva removalthe net budget of SRP became negative ()1.0 mol d)1). By integrating these results over the three dayscycle we estimated that in order to operate an efficient nutrient control and maintain macroalgal matsin a healthy status the optimal Ulva biomass should be well below �4 kg m)2 as wet weight. Abovethis threshold, self-limitation would render most of the algal mat unable to exploit light and nutrients.An efficient removal of nitrogen and phosphorus could be attained through the management of mac-roalgal biomass only with an optimisation of recipient surface to nutrient loading ratio.

Introduction

A number of carbon, nitrogen and phosphorusmass balances carried out in mariculture systemsrevealed that fish retain no more than 20–30% ofthe supplied food (Penczak et al., 1982; Phillipset al., 1985; Hall et al., 1990, 1992; Holby & Hall,1991). Wastewaters from land-based fish farms are

characterised by high organic matter, dissolvedand particulate nutrient concentrations, and lowoxygen content (Porter et al., 1987; Krom &Neori, 1989; Boyd, 1990; Porrello et al., 2003).Phytotreatment ponds have been constructed andtested in order to control nutrient loads from fishfarms and prevent eutrophication in the adjacentwater bodies (Ryther et al., 1975; Shpigel et al.,

Hydrobiologia (2005) 550:199–209 � Springer 2005P. Viaroli, M. Mistri, M. Troussellier, S. Guerzoni & A.C. Cardoso (eds),Structure, Functions and Ecosystem Alterations in Southern European Coastal LagoonsDOI 10.1007/s10750-005-4379-8

1993; Krom et al., 1995; Porrello et al., 2003). Thefunctioning of these systems is based on naturalprocesses such as sedimentation, microbial activityand assimilation by primary producers. Due totheir rapid growth rates, great storage capacityand resistance to environmental stress, macroalgaeare widely employed for this purpose (Kissil et al.,1992; Neori et al., 1996; Schuenhoff et al., 2003).There is certainly a complex relationship betweenmacroalgal biomass and nutrient removal effi-ciency: maximum nutrient control is a compromisebetween the amount of algae biomass and theenvironmental conditions in which the algae grow(Fig. 1). A high biomass has a great potential fornutrient removal via assimilation but is con-strained by environmental parameters such as lightavailability, water stagnation and the establish-ment of anoxic conditions in the deep layers(Viaroli et al., 1996a, b). Furthermore, one of themost common problems in semi-enclosed eutro-phic systems like lagoons, embayments and pondsis that macroalgae tend to accumulate in packedmasses and, due to self-limitation, to collapse(Viaroli et al., 2001). The collapse generally resultsin the sudden death of macroalgae, in the deposi-tion of huge amounts of labile organic matter andthe establishment of anaerobic respiration path-ways (Giordani et al., 1996, 1997). In the specificcase of phytotreatment ponds, the collapse of theproduction sets to zero the removal efficiency andin the more extreme situation may generate pulsesof nutrients and sulphides from decomposingbiomass, as generally occurs in coastal lagoons(Viaroli et al., 2001).

The assessment of macroalgal biomass thresh-olds for the optimisation of nutrient control via

assimilation is a primary target in order to definemanagement strategies for phytotreatment ponds,avoid dystrophic crises and prevent coastaleutrophication. Strategies for the control of mac-roalgal biomass have been elaborated in coastallagoons with a modelling approach (De Leo et al.,2001; Cellina et al., 2003). Overall, due to the largespatial scale, the evaluation of the effect of mac-roalgal control on nutrient cycling is difficult toobtain. In contrast, in small-scale studies, C, Nand P abatement was calculated very accuratelyand correlated with primary producers biomass.Neori et al. (1996) and Shuenhoff et al. (2003),investigating integrated fish-seaweeds culture sys-tems, estimated reduction of N export up to 30%when Ulva lactuca biomass was maintained at 1–2 kg m)2 as wet weight (ww). In these systems, dueto small scale, excess biomass was regularly re-moved and the residual biomass of Ulva wascontinuously suspended in the water column by airbubbling.

In this paper, a full-scale experiment was carriedout in order to identify optimal managementstrategies for an efficient assimilation of nutrientsby macroalgae in phytotreatment ponds. Net dailybudgets of dissolved oxygen (O2), dissolvedinorganic carbon (DIC), dissolved inorganicnitrogen (DIN = NH4

++NO2)+NO3

)) and solu-ble reactive phosphorus (SRP) were determined ina pond colonised by Ulva spp. Budgets were cal-culated when Ulva biomass was close to the col-lapse threshold (8–10 kg ww m)2) and after twoconsecutive harvestings of about 20% of thestanding biomass. Relationships between standingmacroalgal biomasses and budgets were thenanalysed with respect to nutrient removal.

Figure 1. Schematic representation of three phytotreatment ponds with increasing biomass of macroalgae. At pond (a) (low biomass)

potential growth of primary producers is probably maximum due to water circulation and light penetration within algal mats but

overall nutrient removal is low due to low biomass. At pond (c) (high biomass), self-limitation and scarce light penetration result in

little or no growth; stagnant anoxic zones are probably existing within the mat. Pond (b) represents a good compromise between

environmental conditions and standing biomass resulting in a more efficient nutrient control.

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Materials and methods

Study area

The Falesia fish farm, located at Piombino on theTyrrhenian coast of Tuscany (central Italy), pro-duces annually about 200 tonnes of seabream(Sparus aurata) and seabass (Dicentrarchus labrax)(Fig. 2). Fish are farmed in a series of paralleltanks having a total water volume of �6000 m3

and a mean depth of approximately 1.5 m. Thefish tanks are supplied with 250 l s)1 of waterpumped from the sea in the proximity of a powerplant. During winter time the water temperature iskept close to 18 �C. Outflowing waters are deliv-ered into a series of 3 phytotreatment ponds hav-ing a total volume of �2000 m3 with a mean depthof 1.2 m. To avoid contamination of undergroundwater the bottom of the ponds is covered by athick plastic sheet. Here, green macroalgae (Ulvasp.) grow spontaneously and are not harvested.During the study period (May 2003) the last basinof the phytotreatment plant was entirely colonisedby a floating mat of Ulva, whilst in ponds 1 and 2the macroalgal biomass was covering approxi-mately 30–50% of the pond surface.

Experimental design

Experiments started on the 5th of May and con-sisted of three consecutive 24-h cycles of investiga-tion focusing on the pond 3 were macroalgalbiomass was close to its maximum. During eachcycle, water sampleswere collected at the inflow andoutflow of the pond every 2–3 h. Water samples

were analysed for dissolved oxygen (O2), DIC, DINand SRP. At the end of each 24-h cycle approxi-mately 20% of the standing macroalgal biomasswas rapidly removed (<1 h); the harvested biomasswas drained and weighed.

Concentrations of O2, DIC, DIN and SRPwere multiplied by water flow, obtaining hourlyloads that were further integrated over 24 h. Thenet daily budgets were then estimated as the dif-ference between inlet and outlet loads. In addition,net daily balances of O2 were simultaneously cal-culated at ponds 1 and 2.

Analyses

During each sampling temperature, pH, salinityand O2 concentration were measured in situ withan YSI 556 multiple probe. Water samples(�200 ml) were filtered with GF/F Whatman fil-ters within 1 h from collection and immediatelyfrozen for later analyses. Ten ml of unfilteredwater were analysed for total DIC by titrationwith 0.1 M HCl (Anderson et al., 1986). Filteredwater was analysed for NH4

+ (Bower & Holm-Hansen, 1980), NO2

) (Golterman et al., 1978),NO3

) as NO2) after reduction with cadmium, and

SRP (Valderrama, 1977). Irradiance was measuredat 1 h intervals with a quantum sensor (DeltaOHM HD 9021).

Ulva biomass was estimated before the begin-ning of each investigation cycle. Eight replicateswere randomly collected in pond 3 with a cylinder(i.d. 70 cm, height 120 cm) trapping the algaealong the whole water column. The macroalgalbiomass within the cylinder was then collected,

Figure 2. Location of the fish farm with the phytotreatment ponds.

201

gently washed with pond water and weighed freshand after oven desiccation at 70 �C for 4–6 h.

Ulva was harvested from the pond with longrakes and packed in a series of perforated tanksgenerally used for fish harvesting. All the materialwas drained to remove most of the water and thenweighed.

Results

Water quality and Ulva biomass

During the experiment water column temperatureswere rather constant with diurnal variation <2 �C;values ranged between a minimum of 22.6 �C and amaximum of 24.4 �C. Salinity was 38 ppt and noappreciable increase was evidenced from inflow atpond 1 and outflow at pond 3 probably due to fastrenewal (mean residence time �2.5 h). pH of theinflowing wastewater was slightly below 7 and, de-spite macroalgal photosynthetic activity, peaksmeasured at midday in phytotreatment ponds werenever above 7.3. Dissolved oxygen in inflowingwater was constant during the experiment withconcentration of �110 lM, equivalent to a �50%saturation, whilst wide fluctuations were measuredwithin the ponds, as reported in the following par-agraph. Effluents from the fish farm were charac-terised by mean concentrations of 3.3 mM DIC,190 lM DIN and 9 lM SRP; NH4

+ (180 lM) wasthe dominant form of inorganic nitrogen.

On the 5th of May, at the starting of the 1stcycle of measurements, Ulva biomass was3 ± 1 kg ww m)2 in ponds 1 and 2 and 8 ± 3 kgww m)2 in pond 3. At the end of the first series ofsamplings, approximately 500 kg ww of Ulva wereremoved from pond 3 and at the end of the secondcycle approximately 400 kg ww were furtherremoved.

Irradiance was similar for cycles 1 and 3 withvalues of 61.6 and 62.4 E m)2 d)1, whilst it waslower during cycle 2 (34.2 E m)2 d)1) because ofcloudy weather.

Dissolved oxygen during the three 24-h cycles

The evolution of oxygen concentrations measuredat the inlets and outlets of the three ponds isshown in Figure 3. Pond 1 received wastewater

from the fish farm with O2 concentrations roughlyconstant during the 3-day period. The macroalgalphotosynthesis and the system respiration resultedin wide fluctuations (0–200 lM) of dissolved oxy-gen at the three outlets. On average, the waterdelivered by each pond had less oxygen than theinflowing waters, suggesting the dominance ofrespiration over photosynthetic processes. A fewexceptions were represented by values measuredduring the light period in pond 1 during the firstcycle and in pond 3 after the second harvesting,when water at the outlets was more oxygenated.

Instantaneous differences between inlet andoutlet loads are shown in the right side panels ofFigure 3. Widest fluctuations in hourly oxygenbalance were calculated during the first cycle atpond 1 where net system production and respira-tion reached 50 and )60 mol O2 h

)1, respectively.A similar trend was observed for pond 2 but day-night differences were less clear. In the pond 3,during the first day the hourly balance of O2 wasnegative most of the time and not correlated withirradiance. Here, on the 1st day just before dawn, awhole basin oxygen demand of )70 mol O2 h

)1

was estimated, which was equivalent to theimpressive value of )155 mmol O2 m

)2 h)1. Afterthe removal of part of the biomass, hourly bal-ances changed progressively and became morelight dependent. Average oxygen flux during day-light shifted from )40 to 29 mol O2 h

)1 whilstnight consumption slightly decreased (from )26to )18 mol O2 h

)1). In all ponds, the relationshipbetween light evolution and hourly oxygen balancewas clear even if peaks of oxygen production wereslightly delayed compared to the irradiance ones.This was probably due to the slow water circula-tion and solute diffusion within packed Ulva mats.Here, anoxic water masses originated during thenight were progressively saturated during lighttime. One interesting exception was represented bythe last cycle of measurements in the pond 3where, due to the lower biomass and morehomogeneous water circulation, light and oxygenflux curves were almost synchronous (Fig. 3).

Inorganic carbon, nitrogen and phosphorusbalances in pond 3

Inorganic nutrient concentrations were measuredonly at the inlet andoutlet of pond 3during the three

202

cycles. Hourly net balances for DIC, SRP andDIN,together with irradiance, are shown in Figure 4.DIC, SRP and NH4

+ concentrations were generallyhigher at the outlet during the whole monitoringperiod, giving positive net balances and indicatingthat pond 3 was regenerating nutrients. However,net budgets decreased after someUlva biomass washarvested. An opposite pattern was evidenced fornitrite and nitrate whose concentrations generallydecreased through the pond. In this latter case,Ulvaharvesting resulted in less negative NO2

) and NO3)

net balances during the last cycle (Fig. 4).The daily evolution of DIC concentrations was

inversely correlated with irradiance and thusopposite to dissolved oxygen: values measured

during night hours (up to 4.1 mM) were generallyhigher than those measured during daylight (downto 2.9 mM). On average, DIC concentration washigher at the outlet, in particular during the nighthours of the first cycle when Ulva biomass washighest. This resulted in net DIC production in thedark (up to 480 mol DIC h)1, equivalent to a fluxof �1.1 mol DIC m)2 h)1). A net consumption ofDIC was measured exclusively in the central partof the day with values ranging between )24and )290 mol DIC h)1.

Inlet and outlet NH4+ concentrations were

similar during the whole experiment with differ-ences ranging between –15 and 15 lM. Both val-ues were, however, widely fluctuating in the 24 h

Figure 3. Temporal evolution of dissolved oxygen concentration (lM) at the inlet and outlet of the three ponds (left) and hourly net

oxygen balance (right) in the ponds. Continuous line in the right panels represents irradiance.

203

with peaks (up to 252 lM) measured during thenight due probably to lower assimilation rates byUlva and bottom regeneration. Minimum values(down to 99 lM) were measured at midday. Nethourly balances were mostly positive, in particularduring night hours (when the pond regenerated upto 20.6 mol NH4

+ h)1) and negative in a fewoccasions during light hours (when the pond tookup a maximum of )16 mol NH4

+ h)1) (Fig. 4). Theremoval of the Ulva biomass resulted in morefrequent negative ammonium balances during thethird cycle of measurements.

The evolution of SRP concentrations over thethree cycles of measurements was overlapping thatof ammonium with values increasing at night (upto 10.5 lM) and decreasing during the day (downto 4.8 lM). During cycles 1 and 2, outlet concen-trations were generally higher than those measuredin the inlet, with differences in the order of 0.5 lM.During the third cycle differences decreased and insome cases SRP concentration was lower in theoutlet. Consequently net hourly balances, rangingbetween )1.8 and 1.5 mmol SRP h)1, resultedmostly positive on cycles 1 and 2 and close to zeroor slightly negative on cycle 3.

During the course of the experiment, nitrite andnitrate followed an opposite trend compared toDIC, NH4

+ and SRP consisting in an increaseduring the daylight hours and a drop during thenight. Inlet and outlet values changed synchro-nously and ranged between 1.8 and 10.6 lM,generally decreasing from the inlet to the outletwith maximum differences, measured in the dark,of 2.2 and 2.6 lM for NO2

) and NO3), respectively.

Hourly balances for these ions were mostly nega-tive with a maximum of )2.5 mol NOx

) h)1

meaning a net abatement of NOx) within pond 3.

The effect of Ulva harvesting on the balance ofNO2

) and NO3) was thus not clear (Fig. 4).

Discussion

In the pond 3, hourly budgets of O2, DIC, DINand SRP were integrated over 24 h in order tocalculate net daily balances (Figs. 5 and 6). Duringthe first cycle oxygen balance was )519 molO2 d

)1, equivalent to a mean daily fluxof )1.15 mol O2 m

)2 d)1. Such oxygen demand isabout 3 times higher than the respiration rate

Figure 4. Net hourly balances (outlet-inlet) of DIC, SRP, NH4+ and NOx

) measured at pond 3 during the three cycles. Continuous line

represents irradiance.

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()0.4 mol O2 m)2 d)1) measured in full summer in

a eutrophic coastal lagoon by dark mesocosmincubation of sediment with Ulva thalli (Bartoliet al., 2001). The negative balance indicates that inthe pond 3photosynthetic productionswas probablylimited by the excessive biomass (8±3 kg ww m)2)and was not capable to meet respiration rates andchemical oxygen consumption over the wholebasin. Harvesting of Ulva biomass had an imme-diate positive effect on the whole oxygen dynamicspromoting favourable conditions for macroalgalgrowth and primary production. Net daily balanceof oxygen remained negative but decreasedto )190 mol O2 d

)1 in the 2nd cycle andto )13 mol O2 d

)1 in the 3rd cycle (Fig. 5). Thistrend suggests that an additional harvestingintervention would have probably turned the netoxygen balance to become positive and the wholesystem to become net autotrophic. At the controlponds 1 and 2, where Ulva was not harvested, theoxygen daily balance was always negative, rangingfrom )290 to )400 mol O2 m

)2 d)1, and did notchange significantly during the three cycles (datanot shown). The imbalance between productionand respiration processes in pond 3 was confirmedby DIC daily balances that were largely positivefor all cycles. However, in pond 3, biomass re-moval resulted in a net reduction of the DICbudget that shifted from 2686 (1st cycle) to1066 mol DIC d)1 (3rd cycle).

Respiratory quotients calculated from hourlyDIC and O2 fluxes measured in the dark in the firstday ranged from 8 to 22. These values were farabove 2, which are assumed to be typical for

eutrophic sediments with low oxygen penetration(Hargrave & Phillips, 1981). This indicates thatmost of the microbial activity at the bottom of thepond and probably within the Ulva mat wasstrictly anaerobic. The pond bottom was made of aplastic sheet, to avoid water table contamination.Thus, the sediment was constituted by fragmentsof Ulva and by particulate matter such as notingested food or fish faecal pellets, which wererecognised as extremely labile substrate fuellingmicrobial activity. In this anaerobic system the endproducts of microbial metabolism, such assulphides, cannot be trapped by sedimentary iron(II) pools and, due to low night oxygen content inthe water column, cannot by oxidised (Giordaniet al., 1996). Presence of free sulphides results inwater toxicity and prevents the reutilisation ofoutflowing water.

The partial or total reuse of water delivered byponds into the fish tanks is a target of many fishfarmers and is a practice suggested by differentauthors in order to save water and energy (Krom& Neori, 1989).

Ammonium and SRP concentrations were onaverage higher in the outflowing water meaningthat during the experiment there was a net releaseof mineral nutrients from decomposing Ulva bio-mass, undigested food or fish faecal pellets(Fig. 6). Ammonium balance was 48.9 molNH4

+ d)1 on 1st cycle and became less positive on3rd cycle (11.4 mol NH4

+ d)1), after the excess ofUlva was removed. The balance of the 2nd cycle(89.5 mol NH4

+ d)1) could be explained by red-uced NH4

+ assimilation by Ulva due to the cloudy

Figure 5. Net daily balances of O2 and DIC calculated for pond 3 during three 24-h cycles of measurements. Between cycles about 20%

of Ulva standing stock was removed from the pond.

205

weather (the irradiance, 34.2 E m)2 d)1, was halfthe one measured during the 1st and 3rd cycles).To some extent, the difference between initial andfinal NH4

+ balance (37.5 mol NH4+ d)1, about

75% less than the initial value) was explained insmall part by the lower ammonium regenerationby harvested dead biomass (a fraction of the 40%removed) and in great part by ammonium nitrified

or assimilated by healthy Ulva thalli. Once againthis is probably the result of a better water circu-lation, improved light penetration within the Ulvabed and higher oxygen availability. Still, theremaining biomass (�4 kg ww m)2) was not ableto take up NH4

+ significantly.SRP daily balance was positive during 1st and

2nd cycle with values of 2.5 and 6.2 mol SRP d)1,respectively and was negative during 3rd cycle()1.0 mol SRP d)1). Despite being insignificantcompared to overall incoming SRP load, theabatement of this element is another demonstra-tion of the extreme reactivity of the Ulva mat andof the importance of biomass control. Higher SRPregeneration measured during 2nd cycle, despitethe harvesting of Ulva, is in agreement with whatobserved for ammonium and seems more relatedto the lower light intensity and little to no assim-ilation by the macroalgae.

Nitrite and nitrate represented a small fractionof the DIN (�5%) but their daily balances werealways negative (Fig. 6) meaning that outflowingwaters had a lower NOx

) content than the inflow-ing ones. Maximum NOx

) removal was measuredon 1st cycle ()32 mol NOx

) d)1) whilst valuesof )6.7 and )19.3 mol NOx

) d)1 were measuredrespectively on 2nd and 3rd cycle. In the pond 3,considering the unlimited availability of ammo-nium, the most important sink of NOx

) was notrepresented by macroalgae but by anaerobic pro-cesses like denitrification or nitrate ammonifica-tion occurring at the bottom of the pond or withinthe anoxic water mass (Naldi, 1994; Christensenet al., 2000). The harvesting of Ulva, stimulatingwater circulation and bottom oxygenation, wasunfavourable for NOx

) control via anaerobic pro-cesses in these sediment-devoid ponds and couldexplain the trend of NOx

) shown in Figure 6. Bycombining values of daily balances of NH4

+ andNOx

) it was calculated that after the second Ulvaharvesting pond 3 was abating 8.2 mol DIN d)1

whilst it was regenerating inorganic nitrogen on1st and 2nd cycle (16 and 83 mol DIN d)1).

We thus demonstrated how the removal ofexcess biomass was effective in the short term. Inthree days an almost dystrophic pond became asink for DIN and SRP. Unfortunately the experi-ment was limited to 2 harvesting and it wasnot possible, with our data, to calculate atwhich biomass nutrient uptake would be highest.

Figure 6. Net daily balances of NH4+, SRP and NOx

) calculated

for pond 3 during three 24-h cycles of measurements. Between

cycles about 20% of Ulva standing stock was removed from the

pond.

206

However, as found in the literature (Neori et al.,1996; Shuenhoff et al., 2003) we can argue that thisthreshold should be far below 4 kg ww m)2,which was the remaining biomass after the secondharvest. It is worth to consider that during theperiod of this investigation mean daily loads ofDIN and SRP reaching pond 3 were estimated in4000 mol NH4

+ d)1, 132 mol NO3) d)1, 83 mol

NO2) d)1 and 182 mol SRP d)1. During the 3rd

cycle DIN and SRP uptake were �8.2 and�1.0 mol d)1, respectively. Overall, they wereequivalent to 0.2 and 0.6% of the incoming loads.Even if nutrient uptake could be further stimulatedby another Ulva harvesting it seems extremelydifficult to reach even a 10% abatement of DINand SRP. One can assume that the pond efficiencydepends upon three main factors: (a) smalldimension of ponds (whose water volume is 1/3 ofthat used to grow fish), (b) too short retention time(less than 3 h) and (c) extremely high nutrientconcentrations in wastewater (190 lM DIN, 9 lMSRP). Further, the lack of sediment does notfavour SRP precipitation/adsorption and the highNH4

+/DIN ratio, together with the low O2 con-centration, do not favour the loss of N via deni-trification coupled to nitrification. If the oxidationof NH4

+ were improved at the beginning of phy-totreatment ponds and NO3

) concentrationincreased, denitrification could play a major role inDIN removal. Still, assuming denitrification ratesas high as those found in eutrophic coastal envi-ronments (i.e. �2 mmol N m)2 h)1) (Dong et al.,2000; Bartoli et al., 2001), nitrogen removal atpond 3 would be around 21 mol NO3

) d)1 and thusequivalent to 0.5 % of the incoming DIN load.

Finally, a theoretical maximum nitrogen re-moval for pond 3 as assimilated DIN was cal-culated. We assumed for Ulva a 20% dailyincrease of the standing stock and a thallus Ncontent of 5%. These values are on the high sideof the range of possible values for, respectively,daily growth rate and thallus N in Ulva (Lavery& McComb, 1991; Viaroli et al., 1996b), but arereasonable for this macroalga when nutrients areavailable and the standing biomass is notexceeding 2–3 kg ww m)2. With these assump-tions, when in pond biomass is 200 g dw m)2,daily Ulva growth can exert a N uptake equiva-lent to 0.14 mol DIN m)2 d)1. Integrating thisvalue for the pond surface (�450 m2) N uptake is

70 mol DIN d)1, which represents only 1.75% ofthe incoming DIN load. In order to keep mac-roalgal uptake around these values, daily removalof produced biomass and disposal of harvestedthalli are necessary and represent an additionalcost for fish-farmers. In the considered phyto-treatment pond, mainly due to its small size,beneficial effects for water quality are negligibleand in the specific case of nitrogen, removalefficiency cannot be higher than 5% of DIN load.

Conclusions

Presented results come from an in situ experimentcarried out on a real scale phytotreatment systemreceiving water from a land-based fish farm pro-ducing on average 200 ton year)1 of seabass andseabream. These kinds of fish farms, together withthose using floating cages in the coastal area, arevery common all over the European coasts andwill further increase in number to satisfy thegrowing demand for food fish supply (FAO, 2001;Tacon & Forster, 2001). Fish farming practicesenhance coastal eutrophication with demonstratednegative consequences for water, sediment andplant communities (Bell et al., 1989; Holmer &Kristensen, 1992; Naylor et al., 2000). The reduc-tion of their negative environmental impact is akey issue for ensuring long-term sustainability ofthe industry. With respect to nutrient control, theco-culture of seaweeds as organisms extractingpollutants from wastewater has been indicated bymany authors as a promising perspective (Troellet al., 2003 and references therein) but most of thestudies are preliminary and lack real spatial andtemporal scales. Results of the present study set‘‘extracting seaweeds’’ into a more realistic per-spective as they clearly demonstrate that: (a) fre-quent harvests of produced algal biomass isnecessary in order to keep algal mats healthy andprevent dystrophic events; (b) biomass control hasan immediate, positive effect on the functioning ofphytotreatment plant; (c) at the studied site there isan apparent discrepancy between the pollutantload and the capacity of the phytotreatment pondsto receive and metabolise it and thus most of theproduced nutrients (>95%, �4200 mol DIN d)1

and �180 mol SRP d)1) are discharged in thecoastal area.

207

Our opinion is that in western countries therealisation of low-impact, integrated fish andmacroalgae production is still very far, mainly dueto the need of large surfaces, active and costlymanagement of phytotreatment systems for bio-mass harvest and, above all, lack of technology forbiomass reutilisation.

Acknowledgements

We wish to thank the director and staff of Falesias.r.l. (Piombino, Italy) for their invaluable collab-oration during the sampling activity and for kindlyallowing us for the present study. A special thankis given to the ‘eel working group� for logisticsupport in removal operations. This work wasfunded by the Agenzia Regionale per lo Sviluppoe l�Innovazione nel settore Agricolo-forestale(ARSIA, Regione Toscana).

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