Marine Pollution Bulletin 68 (2013) 38–45

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Marine Pollution Bulletin

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Influence of size and density on filtration rate modeling and nutrient uptakeby green mussel (Perna viridis)

Chayarat Tantanasarit a, Sandhya Babel a,⇑, Andrew J. Englande b, Shettapong Meksumpun c

a Environmental Technology Program, School of Biochemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT), Thammasat University, P.O. Box 22,Pathum Thani 12121, Thailandb Department of Environmental Health Sciences, School of Public Health and Tropical Medicine, Tulane University, New Orleans, LA 70112-2704, USAc Department of Marine Science, Faculty of Fisheries, Kasetsart University, Bangkok 10900, Thailand

a r t i c l e i n f o

Keywords:Green musselMussel densityFiltration rateNutrient uptakeKinetics

0025-326X/$ - see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.marpolbul.2012.12.027

⇑ Corresponding author. Tel.: +66 2986 9009x2307E-mail address: sandhya@siit.tu.ac.th (S. Babel).

a b s t r a c t

This study investigates green mussel filtration rates based on variation of the mussel size and density,and attempts to correlate these with the amount of Chaetoceros calcitrans consumed by kinetic modeling.The filtration rates were found to be more effective in small mussels and with greater volumes of seawa-ter/mussel which represent low mussel densities in the mussel farms. Under field condition, the firstorder kinetic model is useful for evaluation of mussel filtration rate. However, the composite exponentialkinetic model was determined to better describe filtration rates in a close system. Higher ratios of sea-water volume L/g DW mussel tissue, resulted in an increasing filtration rate until a maximum plateauwas reached at 10.37 L/h/g DW tissue as determined by first order kinetics. Based on the filtration rate,carbon, nitrogen, and phosphorus uptake by green mussels were found to be 2128.72, 265.41, and66.67 mg/year/indv, respectively.

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1. Introduction

Enrichment of nutrients through runoff into estuarine areascauses eutrophication and red tide. This phenomenon results in ra-pid reduction of dissolved oxygen (DO). Green mussels (Perna vir-idis) act as a biofilter which may help in removing these nutrients.The mussels have been particularly well documented as greening/cleaning the coast through filtration of contaminants in the watercolumn (Rajagopal et al., 2006; Gao et al., 2008). This filtrationcauses a reduction in phytoplankton and turbidity which resultsin increased water clarity. Moreover, the green mussel is a subsis-tence mariculture in Thailand. Department of Fisheries, Ministry ofAgriculture and Cooperatives (2011) reported that in year 2009,green mussels were cultured in 34,696,000 m2 and the mass pro-duction of green mussel was reported to be 193,626 tons/year. Thisaccounted to 64.16% of total mass of shellfish production in Thai-land. By culturing and harvesting mussel as commercial food,nutrients are removed from aquatic system. There is also a sourceof income for farmers.

The green mussel has been of interest to researchers due to itshigh productivity and ability to withstand fluctuating environmen-tal conditions (Rajagopal et al., 2006). In turbid seawater (sus-pended solids up to 1000 mg/L), green mussels were found to be

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effective at filtration (Cheung and Shin, 2005) and depicted a highcapacity for phytoplankton uptake (Rajesh et al., 2001). Further-more, green mussels have been observed to successfully removeorganic wastes discharged from intensive farming activities(Yokoyama et al., 2002) and fish farms (Gao et al., 2006). Greenmussels have been used by shrimp farmers to filter luminous bac-teria from pond water to reduce disease in shrimp (Tendencia,2007). Thus, based on the research of several investigators (Haa-mer, 1996; Gao et al., 2008), it can be said that through the filtra-tion process, green mussels can effectively control phytoplanktonabundance and remove high nutrient contents from mariculturesites. Optimization of this process can lead to enhanced cultureof green mussels as well as a means of nutrient mitigation in mar-ine ecosystems.

Many researchers have documented that food availability is thekey factor influencing feeding behavior of bivalves (Hawkins et al.,1998; Madon et al., 1998; Wong and Cheung, 2001). At high plank-ton densities, green mussels achieved high filtration rates until athreshold level at 105 cells/mL at which pseudofeaces productionstarted (Rajesh et al., 2001). Although the filtration rate decreasesat very high food level (total particulate matter (TPM) higher than1000 mg/L), green mussels have ability to reject particles byincreasing pseudofaeces production. In this process the rejectedparticles, which do not go through the stomach, are wrapped inmucus and expelled without having passed through the digestivetract (Wong and Cheung, 2001). The study on feeding and growthrates of green mussel in Hong Kong by Wong and Cheung (2001)

C. Tantanasarit et al. / Marine Pollution Bulletin 68 (2013) 38–45 39

found that filtration rates correlated with green mussel dry weighttissue. Later, Gao et al. (2008) observed that green mussel filtrationrates also correlated with shell length. Although many studies havefocused on factors which influence green mussel filtration rates,there remains little information correlating filtration rate andgreen mussel density (seawater L/g DW tissue).

Hypothesis of this study is that green mussel size and densitycan affect the filtration rate. The correlation between filtration rateand green mussel density as affected by seawater quantity is themain focus of this research. Thus, this study aims to: (1) investigatefiltration behavior in small (shell length 2.8–3.0 cm) and large(shell length 6.8–7.0 cm) green mussels; (2) investigate kineticmodels that best describes filtration rates; and, (3) assess nutrientuptake potential by green mussel based on the models developed.Presence of inorganic material in food can increase ingestion offood as demonstrated on Mytilus galloprovincialis (Babarro and Rei-riz, 2006).

2. Methodology

2.1. Collection and preparation of green mussels and seawater

Small mussels (shell length 2.8–3.0 cm or 2.91 ± 0.06 cm) andmarket size mussels (shell length 6.8–7.0 cm or 6.94 ± 0.1 cm)were collected from a culture area at Sriracha Fisheries ResearchStation, Thailand and transported to the laboratory located at theFaculty of Fisheries, Kasetsart University, Bangkok, Thailand. Eachmussel was separated by gently cutting byssus threads. Musselswere cleaned and all attachments were removed. These musselswere then cultured in 4 fiberglass tanks (200 L) designed with aseawater filtration system and continuous aeration. Each tank con-tained approximately 50 mussels. Seawater after filtration was re-circulated and was totally changed (100%) daily. Then, the newseawater having same water quality as initial was replenished toavoid ammonia toxicity from the mussels excretion. During accli-mation, the salinity and temperature were maintained at 30 psu(practical salinity units) and 28 �C, respectively. Mussels were fedwith the marine diatom Chaetoceros calcitrans (C. calcitrans) andwere allowed to acclimatize one week prior to initiation ofexperiments.

Seawater was also collected from Sriracha Fisheries ResearchStation and analyzed for salinity, temperature, pH, and dissolvedoxygen (DO) using a handheld multi-probe (model: YSI 600QS).This seawater was stored in a storage tank and was filteredthrough ignited Whatman glass microfiber filters GF/C before em-ployed in filtration experiments.

2.2. Experiment design

All filtration experiments were conducted in the laboratory. C.calcitrans was chosen as representative of marine phytoplanktonin estuarine areas. Chlorophyll a concentration was analyzed as de-scribed in Parsons et al. (1984). Changes of chlorophyll a concen-tration in the seawater occurred due to green mussels consumingC. calcitrans via filtration.

Green mussel densities were presented in terms of surroundingseawater per individual mussel (L/indv). A green mussel densitynormally found in mussel farms was used to design the filtrationexperiment. Green mussel densities in the mussel farm were calcu-lated based on number of mussels divided by total volume of sea-water inside the farm.

The modeling of nutrient uptake by green mussels was devel-oped based on green mussel filtration rate. This study only consid-ers nutrient (carbon, nitrogen and phosphorus) uptake fromseawater. It does not consider nutrients released from green mus-

sels and interaction between atmosphere and seawater. Thus, thenutrient uptake by green mussels was computed by multiplyingthe mussel filtration rate with carbon, nitrogen and phosphoruscontents in C. calcitrans.

2.3. Calibration curves

Pure cultures of C. calcitrans from a laboratory stock were sub-cultered using Conway medium (Walne, 1974) and cells werecounted using a haemocytometer. A calibration curve correlatingchlorophyll a concentration and C. calcitrans cells density wasestablished by varying C. calcitrans of 0.5, 1, 2, 3, 4, 5, 7, 8, 9, 10,20, 45, 90 and 180 � 103 cells/mL in seawater and analyzingrespective chlorophyll a at each concentration. Based on this cali-bration curve, an initial C. calcitrans concentration of 5000 cells/mL was chosen. This was done to simulate the conditions in termsof amount of chlorophyll a as present in mussel farms.

Calibration curve between green mussel shell length and dryweight tissue was also developed for discussing results.

2.4. Filtration experiments

The filtration experiments were divided into three main exper-iments. Each experiment was conducted in triplicate. The firstexperiment aimed to evaluate green mussel filtration behaviorbased on size and density of green mussel by varying the volumeof seawater L/individual (indv). The experiment was done for smalland large size of green mussel. Each size of mussel was divided intofour treatments; the volume of seawater was varied from 0.5, 1.0,5.0 and 10 L/indv. Each mussel was placed in the middle of the sea-water and individually cultured in cylindrical water tanks filledwith filtered seawater (30 psu and 28 �C) containing pure culturesof the unicellular diatom C. calcitrans at 5000 cells/mL. Gentle aer-ation was done in the tank to avoid settlement of the diatom cells.Each treatment was divided into 6 durations; 1, 2, 3, 4, 5, and 6 h.For each duration, only one mussel was employed. Chlorophyll aconcentration was monitored before and after of each duration.

A second experiment on green mussel filtration was conductedto investigate filtration rate in short duration under batch systemconditions. A large mussel (6.8–7.0 cm) was individually culturedin 10 L of filtered seawater representing low density of green mus-sel. Initial concentration of C. calcitrans in seawater was 5000 cells/mL. The experiment was divided into 12 durations; 5, 10, 15, 20,25, 30, 35, 40, 45, 50, 55, 60 min. For each duration, only one mus-sel was employed and the initial and final chlorophyll a concentra-tions were analyzed.

In order to investigate if the green mussel continuously uptakechlorophyll a throughout the day, an experiment on filtration wascarried out for 24 h. For this, one large mussel (shell length 6.8–7.0 cm) was cultured in 10 L of filtered seawater. The same musselwas used throughout the experiment. Every 3 h, the entire seawa-ter was removed and replaced by seawater containing C. calcitrans5000 cells/mL. Reduction of C. calcitrans was observed by analyzingchlorophyll a concentration before and after the seawater was re-placed. The total duration lasted 24 h.

2.5. Determination of nutrient contents in C. calcitrans

C. calcitrans cells from pure culture were filtered through ignitedWhatman glass microfiber filter (GF/F). The samples were fumedwith concentrated hydrochloric acid for 24 h at room temperatureto remove extracellular calcium carbonate or carbohydrate beforeCHN analysis using a 2400 Series II, Perkin Elmer for carbon andnitrogen. The persulfate digestion method was used to determinephosphorus content as described in Standard Methods (APHA,2009).

Fig. 1. Calibration curve between chlorophyll a concentration and C. calcitrans cellsdensity.

Fig. 2. Relationship between shell length and dry weight (DW) tissue of greenmussel (P. viridis).

40 C. Tantanasarit et al. / Marine Pollution Bulletin 68 (2013) 38–45

2.6. Estimation of filtration rate

Green mussel filtration rates (FR) were calculated from chloro-phyll a concentration reduction according to first order chemicalkinetics (Coughlan, 1969) as follows:

Ct ¼ C0e�Kt

where Ct is chlorophyll a concentration at time t (lg/L); C0 is chlo-rophyll a concentration at time 0 (initial) (lg/L); K is first order rateconstant (h�1); and t is time (h).

A composite exponential kinetic model was also employed toinvestigate chlorophyll a removal by filtration. It was applied todetermine the FR by calculating the decrease in chlorophyll a con-centration according to the kinetic equation shown below.

St ¼ S1e�K1t þ S2e�K2t ; where S0 ¼ S1 þ S2

where St is chlorophyll a concentration at time t (lg/L); S0 ischlorophyll a concentration at time 0 (initial) (lg/L); S1 is chloro-phyll a initial concentration for removal described by K1 (lg/L);S2 is chlorophyll a initial concentration for removal described byK2 (lg/L); K1 is the rate constant (h�1) for initial chlorophyll a con-centration S1; and K2 is the rate constant (h�1) for initial chloro-phyll a concentration S2 (Brezonik, 1994).

The filtration rate (FR) was next computed from the determinedkinetic constants by the following equation.

FR ¼ KMn

where FR is filtration rate (L/h/g DW tissue); K is the rate con-stant (h�1) from first order kinetics or composite exponentialkinetics; n is g DW green mussel tissue; and, M is volume of seawa-ter (L) (applied from Coughlan, 1969).

3. Results

The mussels were collected from a mussel farm on December13, 2010. Basic seawater parameters are presented in Table 1.The values were found to be similar to that obtained by previousresearchers (Gao et al., 2008; Wong and Cheung, 2001). Under ac-tual field conditions as in mussel farm, green mussel densitieswere found to be around 5 L of seawater/indv.

Employing calibration curve as in Fig. 1, chlorophyll a concen-tration in seawater can be converted to C. calcitrans cells density.The relationship between green mussel tissue dry weight and shelllength is presented in Fig. 2.

Experimental results reflecting the filtration rates at differentdensities, sizes and time durations are shown in Fig. 3. It can beseen that at a lower density of green mussel (high volume of sur-rounding seawater (L/indv)), greater cumulative chlorophyll a up-take rates were observed at each hour. Maximum chlorophyll auptakes were observed during the first hour for both small andlarge mussels, and were followed by a slower uptake rate. An in-crease in seawater volume resulted in a higher filtration rate for

Table 1Characteristics of Seawater collected from Green Mussel Farm, Sriracha FisheriesResearch Station, Chonburi Province (December 13, 2010).

Parameters Minimum Maximum Average Standarddeviation (SD)

Salinity (psua) 30.92 30.96 30.94 0.02Temperature (�C) 28.24 28.34 28.29 0.03pH 7.78 8.01 7.90 0.07DO (mg/L) 5.43 7.76 6.50 0.88Chlorophyll a (lg/L) 3.12 4.45 3.92 0.58

a psu, practical salinity unit.

both sizes until a plateau was reached (Fig. 3). The plateau for max-imum chlorophyll uptake for the 2.8–3.0 cm mussels was approx-imately 30 lg/indv; whereas, for the 6.8–7.0 cm mussels it wasabout 25 lg/indv.

Fig. 4 shows the reduction of chlorophyll a concentration overtime due to green mussel filtration. When the experiment was con-ducted for small duration to observe the chlorophyll a uptake, itwas found that the uptake was higher initially. Highest chlorophylla uptake was observed in the first 5 min (16.7%). No further uptakewas observed after 30 min (total uptake 30.5% of initial chlorophylla concentration). From the initial chlorophyll a concentration of4 lg/L, it can be seen that after 60 min around 3 lg/L of chlorophylla was still remaining in the water column.

To investigate the green mussel filtration behavior throughoutthe day (24 h), chlorophyll a concentration was kept constant byreplacing C. calcitrans every 3 h. Before each replacement of C. cal-citrans, the percentage of chlorophyll a reduction from the initialamount was estimated. These reductions are presented as chloro-phyll a uptake by green mussel (%). As shown in Fig. 5, results indi-cate that green mussels have the ability to uptake chlorophyll aover 24 h at an almost constant rate with an average uptake of59.7 ± 8.8% of initial chlorophyll a concentration.

Filtration rate (FR) models were developed based on the first fil-tration experiment with varying green mussel size and density(seawater volumes L/indv) for 6 h duration. Other filtration exper-iments were designed to describe mussel filtration behavior. Thefiltration rates were computed from reduction of chlorophyll aconcentrations over time and data were fitted with first orderand composite exponential kinetics models. Corresponding K rateconstants and FR were determined as summarized in Table 2. Itcan be seen that the experimental data fit better for the composite

Fig. 3. Cumulative chlorophyll a uptake (lg/indv) by P. viridis in varying volumes of seawater (L/indv) and mussel size (cm).

Fig. 4. Reduction of chlorophyll a concentration (lg/L) in seawater during cultureof a P. viridis (6.8–7.0 cm) in 10 L seawater.

Fig. 5. Chlorophyll a uptake (%) by P. viridis (6.8–7.0 cm) in 24 h by replacing C.calcitrans every 3 h.

C. Tantanasarit et al. / Marine Pollution Bulletin 68 (2013) 38–45 41

exponential kinetics model than first order kinetic model as indi-cated by the higher coefficient of determination (R2). The compos-ite exponential kinetic model clearly demonstrate that in thelonger run (over 6 h), the green mussel had two phases of filtrationwhich can be termed ‘‘biphasic kinetics’’. Results from this kineticmodel also indicated that the initial filtration rates FR1, computedfrom K1, were much greater than secondary filtration rates FR2,computed from K2 (Table 2). Only results from the large musselwith 0.5 L/indv did not correlate well to composite exponentialkinetics. Results for this case, however, correlated well using firstorder kinetics.

Carbon (C), nitrogen (N) and phosphorus (P) contents of C. cal-citrans were found to be 34.86 ± 3.63, 4.35 ± 0.46, and1.09 ± 0.91 pg/cell, respectively, based on three replications of theanalysis. The C:N:P ratio was 82:9:1 (mol:mol). These results dem-onstrate that the carbon content in C. calcitrans is slightly higherthan for other marine diatoms which are characterized by C:N:Pratios of 75:10:1 (mol:mol) (Quigg et al., 2003).

4. Discussion

4.1. Green mussel filtration behavior

This study simulated conditions prevalent in a mussel farm interms of phytoplankton and mussel densities as a function of

surrounding seawater volume (L/indv or L/g DW tissue). Resultshelp in understanding mussel filtration behavior as discussedbelow.

This research shows the influence of green mussel density, rep-resented as amount of seawater per individual, on green mussel fil-tration efficiency. It was found that at each hour, the filtration ratesof small (2.8–3.0 cm) and large (6.8–7.0 cm) mussels increasedwith higher seawater volumes (L/indv) until a plateau was reached.As shown in Fig. 3, a greater chlorophyll a uptake demonstrated ahigher filtration rate. The increase in filtration was more pro-nounced for smaller size mussels and larger amounts of seawater(5 and 10 L/indv). At lower volumes of seawater (0.5 and 1 L/indv),the mussels filtration ability was found to be lower and this condi-tion represented a higher green mussel density. This suggests thatat lower green mussel densities, a higher filtration rate and an in-creased chlorophyll a uptake rate were observed.

There is not much research done on the influence of bivalvedensities on filtration rates. However, previous research reportedthat dense bivalve populations resulted in a reduction in theirgrowth rate. A greater growth in shell and tissue weight was foundat low densities for pectinids in suspended culture (Cote et al.,1993). Significantly higher maximum length and weight valueswere observed for the mussel, Mytilus galloprovincialis in sites hav-ing lower densities than those at higher densities in ‘‘mussel beds’’(Xavier et al., 2007) and in ‘‘suspended culture’’ (Cubillo et al.,2012). These authors suggested that high bivalve densities result

Table 2Green mussel filtration rate using different kinetic models.

Shell length (cm) Volume of seawater First order kinetics Composite exponential kinetics

L/indv L/g DW tissue K (h�1) FR R2 K1 (h�1) FR1 Time K2 (h�1) FR2 Time R2

2.8–3.0 cm 0.5 3.70 0.166 0.61 0.7582 0.96 3.57 0–2 h 0.046 0.17 2–6 h 0.97811 7.63 0.148 1.13 0.7123 3.73 28.49 0–1 h 0.092 0.70 1–6 h 0.99615 37.31 0.266 9.93 0.9861 1.04 38.73 0–5 h 0.154 5.75 5–6 h 0.975210 74.63 0.133 9.93 0.9192 2.02 150.97 0–5 h 0.100 7.46 5–6 h 0.9727

6.8–7.0 cm 0.5a 0.36 0.356 0.13 0.9992 – – – – – – –1 0.71 0.211 0.15 0.8827 0.68 0.48 0–2 h 0.059 0.04 2–4 h 0.92705 3.54 0.132 0.47 0.8939 1.82 6.43 0–2 h 0.054 0.19 2–3 h 0.999510 7.06 0.107 0.76 0.8105 2.10 14.78 0–2 h 0.059 0.42 2–6 h 0.9984

Note: FR, Filtration rate (L/h/g DW tissue); FR1, Filtration rate at K1 (L/h/g DW tissue); FR2, Filtration rate at K2 (L/h/g DW tissue).a This treatment follows first order kinetics.

42 C. Tantanasarit et al. / Marine Pollution Bulletin 68 (2013) 38–45

in greater competitive pressure and limited food sources. Whenstudying effect of density, results from this study showed a reduc-tion in filtration rate and lower uptake of chlorophyll a in densergreen mussel population. Results indicate that density affects thefiltration rate.

Greater chlorophyll a removal in small mussels demonstrateshigher metabolic and filtration rates as compared to large mussels,especially, at larger seawater volumes (Fig. 3). From the filtrationrates expressed as L/g DW tissue (Table 2), it can be clearly seenthat small mussels have filtration rate higher than large mussels.In addition, higher mortality and less activity were observed inlarge mussels when acclimated to experimental conditions. Thisfinding can be explained as increased sensitivity towards adverseenvironmental conditions with increasing size of the animals (Jor-gensen, 1990). Findings, contradict research evaluating the influ-ence of body size on green mussel filtration rates (Rajesh et al.,2001) which indicates that for large mussels, the filtration rate ishigher than for smaller mussels for all algal densities studied.However, this may be due to high algal concentrations (over30,000 cells/mL) used in the study. The gills of small mussel mayclog with dense algae, hence, the lower filtration rates obtained.A similar reduction of filtration rates due to a large amount ofavailable food has also been recorded by other researchers (Haw-kins et al., 1998; Wong and Cheung, 2001).

Research on the blue mussel, Mytilus edulis, also found an influ-ence of age on filtration rates (Sukhotin et al., 2003). In senile mus-sel (older than 6 years old), the pumping rate reduced. Theseresults support the physiological signs of senescence in blue mus-sel, such as rapid decrease of growth rate and net growth effi-ciency, lowering of ATP and phosphagens levels in tissue, declineof citrate synthase activity and intracellular pH (Sukhotin and Port-ner, 2001) as well as an exponential increase in the accumulationof lipid peroxidation products (Sukhotin et al., 2002). Based on re-sults from our study, large mussel filtration rates were lower thanthose of small mussels. This can be explained by the fact that senilemussels experience a drastic reduction in somatic growth due tolowering of the feeding rate, together with decreasing metabolicdemands.

It was noticed that both sizes of green mussel have a limitationof chlorophyll a removal. As shown in Fig. 3, at very low volumes ofseawater (0.5 and 1.0 L/indv), there still remained chlorophyll a inthe water column even though the water was completely circu-lated and exposure was for 6 h. A preliminary experiment on cul-turing large mussels (shell length 6–7 cm) for 3 days alsoproduced similar results with a final chlorophyll a residual remain-ing of 2.3 lg/L (on average). Clausen and Riisgard (1996) foundthat in the blue mussel (Mytilus edulis), filtration reached a thresh-old level at chlorophyll a concentration at around 0.5 lg/L. Similarfindings were reported by Riisgard et al. (2003).

In this study, rapid uptake of chlorophyll a was observed duringthe first 5 min (Fig. 4) and first hour (Fig. 3) in one and 6 h filtrationexperiments, respectively. This may be because of opening andclosing of valve based on the chlorophyll a concentration and thataffects the filtration rate. This is supported by other research inblue mussels that demonstrate the rhythm of opening/closing oftheir valves correlates to filtration rate. At low concentrations ofchlorophyll a, mussels reduce filtration by closing their valves(Dolmer, 2000). They quickly re-open when suddenly more phyto-planktons are added (Riisgard et al., 2003). Interestingly, it remainsuncertain that when the mussels are full whether they will closetheir valves or keep filtering but not digesting, so their excretionis released as pseudofaeces.

Chlorophyll a uptake by green mussels was found at a constantrate when steady algal concentrations were supplied over 24 h. Asdiscussed above, filtration is affected by closing/opening of valvesdepending on algal concentration (Riisgard et al., 2003). Moreover,the investigation also found that green mussels opened their valvesover the entire duration of the experiment. Results agree with themeasurement of filtration rates by the bay scallop (Argopecten irra-dians) (Palmer, 1980). Palmer (1980) found that the scallop contin-uously performed filtration, as their valves were opened in excessof 24 h, and a constant filtration rate was observed. Other evidencethat individual bivalves filter food particles at a steady rate over along time period has been shown for asidians (Fiala-Medioni,1978). Although the mussels continuously filter all particles, thisphenomenon does not guarantee that these particles are digestedat a constant rate since pseudofaeces production might vary overtime.

Green mussel filtration behavior as investigated in this study, issummarized below. At low density of green mussels, correspond-ing to a higher volume of seawater/indv, an increase in filtrationrate was observed. Previous studies report that the mussel filtra-tion rates increase with larger mussel body mass (Rajesh et al.,2001). However, in this study it was found that small musselsare more active and less sensitive than large mussels. Thus, higherfiltration rates were observed for small mussels. Green mussels ac-tively filter chlorophyll a initially and thereafter at slower rates asthe chlorophyll a concentration is reduced under batch conditions.When ambient conditions were simulated by providing a constantalgal concentration, the filtration of green mussel and chlorophyll aremoval were found to be steady over 24 h.

4.2. Evaluation of green mussel filtration rate by kinetic models

Various techniques can be used to estimate green mussel filtra-tion rates. In this study, the first order kinetic model as describedin Coughlan (1969) is employed to correlate data obtainedthroughout the total duration of the experiment. This method

ModelsFirst order 1 hr ) 0.96026 hr 0.8994Composite exponentialK1 ) 0.8222

Models Equation R2

1 hr ) 0.96026 hr 0.8994

K1 ) 0.8222

Fig. 6. Filtration rate (FR) determined from calculations of kinetic models as afunction of varying seawater volume.

C. Tantanasarit et al. / Marine Pollution Bulletin 68 (2013) 38–45 43

has been chosen to estimate bivalve filtration rates for decades(Rajesh et al., 2001; Leverone et al., 2007; Vijayavel et al., 2007;Wang et al., 2011).

Chlorophyll a concentration decreases over time due to greenmussel filtration. In the natural environment, input of chlorophylla is continuous. To simulate this condition, filtration rates werecalculated using the first hour removal rate in order to avoid thecondition of chlorophyll a reduction over time in batch experi-ments (Fig. 6). It should be noted, however, that the filtration ratesdetermined using first order kinetics only approximated the initialremoval since the log versus time plot was not linear over the en-tire sampling time.

Observed data reflected a two phase removal process which isbetter described kinetically by the composite exponential model.Table 2 clearly demonstrates that at longer durations (more thanone hour), chlorophyll a reduction in the batch system was a twophase process with a high initial uptake followed by a much lowerremoval rate. Therefore, it is suggested that the most accuratemethod to compute filtration rates in close systems are using K1

determined from the composite exponential kinetic model.Units of filtration rate are often expressed as volume of seawa-

ter L/h/indv. In this study, units of filtration rate were normalized

Table 3Comparison of filtration rate based on kinetic models for the green mu

Model Shell length(cm)

Seawat(L/indv

Under laboratory experimentsFirst order 2–3 10(Calculated from 1 h data) 6–7 10First order 2–3 10(Calculated from 6 h data) 6–7 10Composite exponential (K1) 2–3 10

6–7 10First order 2 0.5

2.5–3.0 0.16.4–6.7 1.7

Under field observationFR = 0.81 W0.85 7 –FR = 0.31SL0.7 7 –

to volume of seawater L/h/g DW tissue. Filtration rates were calcu-lated using first order kinetics and compared to those obtainedusing composite exponential kinetics which yielded different fil-tration rates (Table 2). All results correlated well with the volumeof seawater L/g DW tissue and all fit very well using either model(Fig. 6). The results indicate that green mussels have a higher filtra-tion rate at higher volumes of seawater (low green mussel density)but there exists an upper capacity.

As previous discussed, mussel density affects individual filtra-tion efficiency. The correlation between seawater (L/g DW tissue)and filtration rate (FR) as shown in Fig. 6 also supports this finding.At a lower density of green mussels, a higher filtration rate was ob-served. However, a maximum filtration capacity is observed athigher seawater volumes (exceed 70 L/g DW tissue). This is re-ferred to as the critical low green mussel density.

A comparison of results with previous researchers (Table 3)shows a variation in filtration rates due to different experimentaldesigns. Under laboratory conditions, evaluation of filtration usinga first order kinetic model for the first hour of exposure comparesclosest with previous research as shown in Table 3. However, thisstudy found that to describe filtration in a closed system, the com-posite exponential model is a better fit with a high coefficient ofdetermination (R2) (Table 2). Results also agree with previous re-search on filtration behavior (Palmer, 1980; Riisgard et al., 2003)indicating that bivalves quickly open their valves and rapidly reacha maximum filtration rate after algae were introduced into theaquarium. Hence, considering longer duration in a closed system,green mussel filtration rates can be evaluated using kinetic con-stants obtained from the composite exponential kinetic model,which better describes the filtration behavior.

Moreover, filtration rates which are estimated from 6 h by usingfirst order kinetic models are in agreement with the estimationfrom ambient conditions (Wong and Cheung, 2001; Gao et al.,2008). Our results also agree with the field study of mussel feedingactivity, which reported clearance rates ranging between 1 and10 L/h/indv (MacDonald and Ward, 2009).

4.3. Nutrient uptake by green mussels

Effective filtration by mussels can cause rapid depletion of sus-pended particles in a water column (Peterson and Heck, 1999; Dol-mer, 2000; Rajesh et al., 2001). Examples include organic matterfrom fish farming (Gao et al., 2006; Liutkus et al., 2012) and shrimpwaste (Van Khoi and Fotedar, 2012). Changes in mussel chemicalcomposition and biomass in the absence of phytoplankton con-firms that nutrient content in phytoplankton cells is assimilatedinto mussel meat as required for mussel growth (Strohmeier

ssel, Perna viridis (W = g DW tissue, SL = Shell length (cm)).

er)

Filtration rate(L/h/indv)

Reference

3.57 This study4.05 This study1.33 This study1.07 This study20.23 This study20.95 This study0.71 Leverone et al. (2007)1.10 Wang et al. (2011)3.50 Rajesh et al. (2001)

1.09 Wong and Cheung (2001)1.21 Gao et al. (2008)

44 C. Tantanasarit et al. / Marine Pollution Bulletin 68 (2013) 38–45

et al., 2008). In this study, nutrient uptake rate calculations arebased on green mussel filtration rates.

The marine diatom, C. calcitrans was chosen as a representativeof marine phytoplankton in estuarine and coastal areas because itis the dominant genus and is highly consumed by bivalves (Pernetet al., 2012). The estimation of nutrient uptake does not take intoaccount nutrient interaction between the atmosphere and seawa-ter. It only considers the aquatic system. Constant filtration/uptakerate with constant food supply (algal concentration) is assumed asin the previous discussion for 24 h mussel filtration behavior.

All filtration kinetics models were computed based on thereduction of chlorophyll a concentration, and that refer to chloro-phyll a uptake rates. Filtration rates have been defined as the vol-ume of water filtered completely free of particles per unit of time(Winter, 1978). In this study, chlorophyll a concentrations are con-verted to concentrations of C. calcitrans cells (Fig. 1). Nutrient up-take by green mussels can therefore be estimated by multiplyingthe carbon, nitrogen, and phosphorus contents in C. calcitrans cellsby the amount of plankton and mussel filtration rate. For instance,in this study for an initial C. calcitrans cell density of 5000 cells/mL,culturing green mussel shell length 7 cm, mussel density at a sea-water loading of 5 L/indv, and using first order kinetics at 6 h dura-tion, the filtration rate can be determined. Consequently thecarbon, nitrogen, and phosphorous quantities which can poten-tially be uptaken by green mussels, can be calculated as 2128.72,265.41 and 66.60 mg/year/indv, respectively.

Nutrient uptakes observed from this research were higher ascompared with the field observations of nutrient removals fromfish farms by green mussels in Hong Kong (Gao et al., 2008). Gaoet al. (2008) reported that the nutrient assimilation capacity of a7 cm mussel for carbon, nitrogen, and phosphorus was 1476.9,160.3 and 36.7 mg/year/indv, respectively. Differences in the re-sults from Gao et al. (2008) may be due to constant algal concen-tration used in this study and different environment conditions.Moreover, to understand nutrient removal, both nutrients uptakeand release must be evaluated. This study only considers nutrientuptake by green mussels. Nutrient release and turnover in aquaticsystems, known as the nutrient biogeochemical cycle, requires adeeper understanding to clarify ‘‘net nutrient removal’’ by greenmussel as noted by Stadmark and Conley (2011).

5. Conclusions

Green mussels effectively remove nutrients from the water col-umn by consuming phytoplankton. The rate of filtration is the keyin determining the capacity for phytoplankton uptake. Mussel fil-tration behavior in this study indicates that an increase in seawatervolume (L/indv) or lowering the mussel density, results in a higherfiltration rate for both small and large mussels. Under ambientconditions (estuarine and coastal areas), the filtration rates whichwere calculated using a first order kinetic model with 6 h dataare close to those obtained by other researchers (Wong and Che-ung, 2001; Gao et al., 2008). However, under a closed system andlong term durations, filtration rates are better described by thecomposite exponential kinetic model. The correlation between fil-tration rates and seawater volume (L/g DW tissue) can help in esti-mating the mussel filtration rate under ambient conditions. Themaximum filtration rate value was obtained at 10.37 L/h/g DW tis-sue. By multiplying the filtration rate with the amount of carbon(C), nitrogen (N) and phosphorus (P) contents in phytoplanktoncells, the CNP uptake by green mussels can be calculated as2,128.72, 265.41 and 66.67 mg/year/indv, respectively, by cultur-ing green mussel (shell length 7 cm, seawater 5 L/indv). Resultsdemonstrate that green mussel can act effectively as a biofilterand assist in removing nutrients from seawater. Although this

study does not include nutrients released from green mussel, thiscritical information in ‘‘nutrient uptake’’ can assist to estimatepurification of nutrients and chlorophyll a in marine ecosystemswith productive commercial mariculture. Thus, further study isrecommended to focus on green mussel excretion in order to morecompletely understand net nutrient removal by green mussels.

Acknowledgements

We are grateful to the Royal Golden Jubilee Ph.D. program (RGJ)under the Thailand Research Fund (TRF) for providing the funds forthis study. Thank are also due the Department of Marine Sciences,Faculty of Fisheries, Kasetsart University for providing laboratoryspace, and Sriracha Fisheries Station for cooperation in collectingsamples of green mussels and seawater.

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