Annual Variations of Biogenic Element Contents of Manila Clam (Ruditapes philippinarum) Bottom-Cultivated inJiaozhou Bay, China

ZAN Xiaoxiao, XU Binduo, ZHANG Chongliang, and REN Yiping

College of Fisheries, Ocean University of China, Qingdao 266003, P. R. China

Annual Variations of Biogenic Element Contents of Manila Clam (Ruditapes philippinarum) Bottom-Cultivated in
Jiaozhou Bay, China

ZAN Xiaoxiao, XU Binduo, ZHANG Chongliang, and REN Yiping*

College of Fisheries, Ocean University of China, Qingdao 266003, P. R. China

Manila clam (Ruditapes philippinarum) was monthly sampled from its benthic aquaculture area in Jiaozhou Bay from May 2009 to June 2010. The annual variations of major elemental composition, organic content, fatness and element ratio of Manila clam were examined. The element removal effect of clam farming in Jiaozhou Bay was analyzed based on natural mortality and clam harvest. The results indicated that the variation trend of carbon content in shell (Cshell) was similar to that in clam (Cclam). Such a variation was higher in summer and autumn than in other seasons, which ranged from 9.10 ± 0.13 to 10.38 ± 0.09 mmol g-1and from 11.28 ± 0.29 to 12.36 ± 0.06 mmol g-1, respectively. Carbon content of flesh (Cflesh) showed an opposite variation trend to that of shell in most months, varying from 29.42 ± 0.05 to 33.64 ± 0.62 mmol g-1. Nitrogen content of shell (Nshell) and flesh (Nflesh) changed seasonally, which was relatively low in spring and summer.NshellandNfleshvaried from 0.07 ± 0.009 to 0.14 ± 0.009 mmol g-1and from 5.46 ± 0.12 to 7.39 ± 0.43 mmol g-1, respectively. Total nitrogen content of clam ranged from 0.50 ± 0.003 to 0.76 ± 0.10 mmol g-1with a falling tend except for a high value in March 2010. Phosphorus content of clam (Nclam) fluctuated largely, while phosphorus content of shell (Pshell) was less varied than that of flesh (Pflesh).Pshellvaried from 0.006 ± 0.001 to 0.016 ± 0.001 mmol g-1; whilePfleshfluctuated between 0.058 ± 0.017 and 0.293 ± 0.029 mmol g-1.Pclamranged from 0.015 ± 0.002 to 0.041 ± 0.006 mmol g-1. Carbon and nitrogen content were slightly affected by shell length, width or height. Elemental contents were closely related to the reproduction cycle. The removal amounts of carbon, nitrogen and phosphorus from clam harvest and natural death in Jiaozhou Bay were 2.92×104t, 1420 t and 145 t, respectively. The nutrient removal may aid to reduce the concentrations of nitrogen and phosphorus, the main causes of eutrophication, and to maintain the ecosystem health of Jiaozhou Bay.

Jiaozhou Bay; Manila clam;Ruditapes philippinarum; biogenic element; ecological effect

1 Introduction

Coastal ecosystem has the highest productivity among marine ecosystems, but it is also strongly affected by human activities (Riebesellet al., 2007). As an important species in the coastal ecosystem, clam was cultured at high densities, which will strongly influence the distribution of biogenic elements and matter cycle (Zhanget al., 2005; Shenet al., 2010). Shellfish culture is a fast-growing industry with a global annual yield of 1.35×107tons, of which the yield of Manila clam (Ruditapes philippinarum) accounted for up to 24.1% (FAO, 2011). In China, most clam culture industries are located in Liaodong and Shandong peninsulas (Anonymous, 2011).

Previous studies indicate that shellfish including Manila clam play an important role in the matter cycle of coastal ecosystem. Shellfish filter the particle organic release matter, feed dissolved organic matter and plankton, and organic matter through excretion process, purifying waters to some extent (Nakamura, 2001; Riisgaardet al., 2004). Some researchers found that the purification ability of clam is affected by some indirect factors (e.g., plankton bloom, Navarro and Thompson, 1997; Navarrete-Mieret al., 2010). In addition, the physiological activity of clam facilitates releasing nutrient elements from marine sediments (Zhanget al., 2005; Dumbauldet al., 2009), further affecting the community structure of plankton and benthos as well as biogeochemical cycle (Nakamura and Kerciku, 2000; Bartoliet al., 2001; Viaroliet al., 2006).

Nutrient elements are stored in the shell of shellfish through bio-calcification (Goulletquer and Wolowicz, 1989; Carréet al., 2006; Takesueet al., 2008). Thus the nutrient composition of shell can indicate the historical environment condition under which clam live (Kleinet al., 1996; Hicksonet al., 1999). In addition, some metal elements are also gathered in shellfishviabio-concentration (Chong and Wang, 2001; Fukunaga and Anderson, 2011). After death, elements in shell are deposited in sediment and can only return to matter cycleafter a long geological time (Hicksonet al., 1999; Forrestet al., 2009). Since elements are stored in shells, they can be removed from marine ecosystem when clam are harvested (Kasparet al., 1985; Neoriet al., 1998; Nizzoliet al., 2006).

Although the biogenic element contents in shellfish and the element removal amountviashellfish harvest have been examined in some relevant studies, further researches on the elemental excretion, biodeposition and filtration activity of marine bivalves in farming cycle are appreciated. It is widely recognized that only a minor fraction of elements, such as nitrogen and phosphorus, are exported with harvested clam at the end of farming cycle (Navarro and Thompson, 1997; Bartoliet al., 2001; Lodeiroset al., 2001; Nizzoliet al., 2006; Forrestet al., 2009; Nizzoliet al., 2011). The element removal amountviashellfish death was not documented appropriately. To date, very few systematic studies have addressed the dynamics of biogenic elements of clam and the ecological effects of calm farming on ecosystem. Therefore, the dynamics of biogenic element contents, the relationship among element content and clam growth and reproduction cycle and the ecological effects of biogenic elements removal through clam harvest and shell deposition on ecosystems need be examined.

Jiaozhou Bay is the major habitat of Manila clam due to its suitable environmental conditions (Wu and Pan, 1992). The shellfish cultivation species in Jiaozhou Bay mainly include Manila clam (R. philippinarum),Sinonovacula constrifta,Potamocorbula laevisand among others.R. philippinarumis one of the most important commercial clam species.

In this study, the dynamics of content of biogenic elements of Manila clam was investigated. The aims were 1) to analyze the annual variation of biogenic element content of Manila clam and explore its relationship with growth and reproduction cycle; and 2) to disclose the biogenic element removal amountviaclam harvest and shell sediment and its ecological effect on Jiaozhou Bay ecosystem.

2 Materials and Methods

2.1 Study Area

This study was performed in Jiaozhou Bay, a semiclosed and fan-shaped natural bay. It situates in 35?38?N–36?18?N, 120?04?E–120?23?E, with an area of 362.4 km2and an average depth of 7 m. It connects with Yellow Sea through a channel, 3 km in width, with a maximum depth of 64 m. Several rivers enter into Jiaozhou Bay, which include Daguhe River, Licunhe River and Yanghe River. The bay receives an annual freshwater flux of about 8.61×108m3(Editorial Board of Annals of Bays in China, 1993). The investigation sites located in the cultivation area of Manila clam (36?6?30??N–36?11?10??N, 120?6?00??E–120?12?10??E), where the water depth ranged from 4 to 7 m, bottom water temperature varied between -0.97 and 25.98℃, and salinity fluctuated between 29.08 and 31.60 in our studying period. The sediment type in the investigation area is sand-clay.

2.2 Sampling and Processing

Three samples were randomly selected each survey from selected area (Fig.1). Samples were collected with a clam collector, 40 cm in width, and each sample plot was 1 m2. Clam individuals were collected from May 2009 to June 2010, one survey each month. February and April 2010 were excluded due to unfavourable weather conditions. All clam individuals were brought to laboratory, weighed (with the precision of 0.1 g) and temporarily cultivated in aquatic tank before further measurements.

Fig.1 Sampling stations of Manila clam survey in Jiaozhou Bay.

The annual catch data of Manila clam in Jiaozhou Bay was gathered by our research group based on the surveys of local fisherman and statistics bureau of the towns around Jiaozhou Bay. The data was employed to calculate the element removal amount through Manila clam harvest in Jiaozhou Bay.

At each sampling site, clam, 500 g in total weight, were randomly selected and counted with the density of individuals at each sampling site estimated. Fifty individuals were selected randomly at each site with their length, width and height measured with a micrometer. The measure was read to the nearest 0.01 mm. Individual fresh weight was measured with electronic scale (with the precision of 0.01 g). Shell and flesh were separated with dissecting knife. Fresh weights were measured with electronic balance to the nearest 0.01 g. The dry weights of shell and flesh were weighed with electronic balance (0.0001 g) after drying to constant at 60℃ (Goulletquer and Wolowicz, 1989; Lodeiroset al., 2001).

As the dry clam shell and flesh are very light, fifty clam individuals were sorted to two groups according to shell and flesh, and the element content was measured each month. The samples each group were smashed, sifted by 40 mesh sieve, and then weighed. The ashes of shell and flesh were measured after removing organic matter by heating in muffle furnace at 475℃ ± 5℃ for 36 h. Carbon and nitrogen contents were measured withCHNS-O Elemental Analyzer, and phosphorus content was measured with color comparison method of ammonium vanadate-molybdate (Lan, 2002). The unit of element content was converted to mmol g–1, mmol each gram of either shell or flesh.

2.3 Data Analysis

The fatness of Manila clam was described with condition index (CI) (Walne and Mann, 1975; Walne, 1976).

whereWdFrepresents dry flesh weight (g ind-1), whileWdSdenotes dry shell weight (g ind-1). The annual removal amounts of biogenic elements through clam harvest in Jiaozhou Bay ecosystem are calculated as

whereWdtrepresents the dry weights (t) of harvested clam shell or flesh in montht,Pidenotes the molar mass of elementiper unit dry shell or dry flesh (mmol g–1),Miis the molar mass of elementi(g mmol–1). The molar mass of carbon, nitrogen and phosphorus are 12.02, 14.01 and 30.97 g mol–1, respectively.

The element removal amount with shell deposit from dead individuals is calculated as

where monthly mortality was calculated as

and monthly shell deposit mass was calculated as

Dtis the mean clam density of investigation area in montht(ind m-2).

All data were processed by Predictive Analytics Software (PASW) Statistics 18.0 and EXCEL. Thet-test was used to study the difference in element content of flesh and shell. The monthly differences in element content of shell and flesh were analyzed by One Way ANOVA and multiple comparisons (L.S.D.), respectively. The correlation between element content and shell size (length, width and height), individual dry weight and condition index were investigated by Pearson correlation analysis (twotailed) (Du, 2009; Vittinghoffet al., 2010).

3 Results

3.1 Monthly Variations of Dry Weight and Condition Index of Manila Clam

The variations of dry weight of shell, flesh and clam individuals of bottom-cultivated Manila clam in Jiaozhou Bay were significantly different among months (One-way ANOVA,P<0.01, Fig.2a). The monthly variation of dry weight of shell (1.39–3.50 g ind-1) was similar to that of clam individuals (1.50–3.82 g ind-1), which increased and reached the highest in June 2010.

Condition index (7.16–13.01) and the dry weight of flesh (0.15–0.33 g ind-1) showed very similar patterns from May 2009 to June 2010, with peak values occurring in August 2009 and March 2010 (Fig.2b). One-way ANOVA indicated that the condition index were significantly different among months (P<0.01).

Fig.2 Monthly variation of dry weight and condition index of bottom cultivated Manila Clam in Jiaozhou Bay (Mean ± S.D.). a, dry weight of shell, flesh and clam individual clam individual; b, condition index.

3.2 Monthly Variations of Biogenic Element Contents of Manila Clam

As showed in Fig.3, the average element contents of dry shell were 9.82 ± 0.38 mmol g-1, 0.11 ± 0.02 mmol g-1and 0.01 ± 0.00 mmol g-1forCshell,Nshell, andPshell, respectively, which were lower than those of dry flesh (Cflesh: 32.17 ± 1.36 mmol g-1,Nflesh: 6.44 ± 0.61 mmol g-1,Pflesh:0.21 ± 0.07 mmol g-1). Thet-test indicated that the element contents (carbon, nitrogen and phosphorus) of dry flesh are significantly higher than those of dry shell (P<0.01). In addition, the organic contents of both dry shell and dry flesh were 3.85% ± 1.10% and 87.40% ± 2.33%, respectively.

Fig.3 Monthly variation of element contents of dry shell, dry flesh and dry clam individual of bottom cultivated Manila Clam in Jiaozhou Bay (Mean ± S.D.).

Carbon content in dry shell (Cshell) and gross carbon content in dry clam individuals (Cclam) showed similar variation trends (One-way ANOVA,P>0.05), coinciding with the variations ofCIand dry weight of flesh (Fig.2, Fig.3a).CshellandCclamremained stable among months in each season and changed seasonally, increasing in summer and autumn and falling in winter and spring. Carbon content in dry flesh (Cflesh) was significantly different among months (One-way ANOVA,P<0.01), and was opposite to that ofPCshellandPCclamfrom May 2009 to January 2010 (Fig.3a).

Nitrogen content in dry shell (Nshell) and dry flesh (Nflesh) had an overall ascending trend from May to December 2009, and declined in other months (One-way ANOVA,P<0.01, Fig.3b). The gross nitrogen content in dry clam individuals (Nclam) had an overall dwindling trend in the study period, with highest value in March 2010 (0.50–0.76 mmol g-1, Fig.3c).

Phosphorus content in dry flesh (Pflesh) and dry clam individuals (Pclam) accumulated over the summer to a maximum before spawning and declined to a minimum after spawning, showing significant monthly changes (One-way ANOVA,P<0.01, Fig.3c, d). Phosphorus content in dry shell (Pshell) lagged behind that of dry flesh and was a bit less thanPflesh(One-way ANOVA,P<0.01, Fig.3d).

Pearson correlation analysis indicated thatNfleshandNshell,NfleshandCflesh,CshellandPfleshwere positively correlated (r1=0.691,P=0.000;r2=0.485,P=0.004;r3=0.691,P=0.049, Table 1).CshellandCclam,PfleshandPclamwere also positively correlated (r1=0.816,P=0.000;r2=0.823,P=0.000). There was no significant correlation among other elements (P> 0.05) (Table 1).

The ratio ofCshelltoCclam(Cshell/clam, 72.11%-80.58%) andNshelltoNclam(Nshell/clam, 10.13%–20.73%) exhibited overall ascending trends from May 2009 to January 2010, and declined from January and June 2010(Fig.4a).Cshelloccupied a large proportion inCclam.Cshell/clamandNshell/clamwere higher in winter months while the dry weight of flesh was low. The ratio ofPshelltoPclam(Pshell/clam, 17.43%–63.00%) had two peak values in May and November 2009, which corresponded with the minimum dryweight of flesh in May and a lowPfleshin November 2009 (Fig.2a, Fig.3d, Fig.4a). The ratios of element contents of dry flesh to dry clam individuals were opposite to those of dry shell to dry clam individuals (Fig.4b).

Table 1 Pearson correlation coefficients (r) among biogenic element contents and dry weight and condition index of bottom cultivated Manila clam in Jiaozhou Bay (two-tailed)

Fig.4 Ratios of element contents of dry shell and flesh to dry clam of bottom cultivated Manila clam in Jiaozhou Bay (Mean ± S.D.).

3.3 Variations of Stoichiometry C:N and N:P Ratio

As showed in Fig.5, C:N ratio for both dry shell and dry flesh, with the range of 71.99–122.46 and 4.52–6.06, had a similar variation trend from May 2009 to June 2010. In general, the C:N ratio values were low during winter months when the nitrogen contents were high and carbon contents were low (Figs.3a, b), and then increased stead-ily to maximum values in May and June 2010 (Fig.5). The atomic ratios of C:N in dry flesh ranged from 3.87 to 5.19.

N:P ratio in dry flesh was higher in May and November 2009 whenPfleshwas lower than those in other months andNfleshwas relatively high (Figs.3b, d). N:P ratio of flesh ranged from 20.73 to 119.83 for all months, and varied from 20.73 to 37.73 excluding May and November 2009 (atomic ratio 17.77–32.34). The N:P ratio in dry shell (7.49–21.79) was higher in December 2009 due to the minimumPshelland maximumNshelland lower in May and November 2009 coinciding with lowerNshelland higherPshellthan those in adjacent months (Fig.3b, d; Fig.5).

Fig.5 Variation of C:N ratio and N:P ratio in dry shell and dry flesh of bottom cultivated Manila clam in Jiaozhou Bay (Mean ± S.D.).

The ratio of total carbon to total nitrogen (TC:TN ratio) showed a dramatic variation (15.80–23.42, Fig.6), and was positively correlated toCshelland C:N in dry flesh (r1=0.400,P=0.019;r2=0.406,P=0.017). The variation trend of the ratio of total nitrogen to total phosphorus (TN:TP ratio) was similar to N:P of dry flesh (18.28– 44.19,r=0.915,P=0.000), being higher in May and December 2009 than in other months with minimumP(Clam)(Fig.3c, Fig.6).

Fig.6 Variations of TC: TN ratio and TN: TP ratio of bottom cultivated Manila clam in Jiaozhou Bay (Mean ± S.D.).

3.4 Removal Amount of Biogenic Elements with the Harvest and Natural Death of Manila Clam

Based on the annual yield statistics of clam harvest in Jiaozhou Bay, the annual yield of fresh clam was 2.5×105t. The annual yield of dry clam was 1.37×105t, of which 1.24×105t was dry shell weight and 1.3×104t was dry flesh weight. As the average C, N and P contents of dry shell and dry flesh were 9.82, 0.11 and 0.01 mmol g-1, 32.17, 6.44 and 0.21 mmol g-1, respectively (Fig.3), annual removal mass of C, N and P in dry shell by harvest were 1.45×104t, 180 t and 35 t, with annual removal mass of C, N and P in dry flesh by harvest at 5000 t, 1120 t and 85 t, respectively (Table 2). In general, the annual removal mass of C, N and P elements by harvest were 1.95×104t, 1300 t and 120 t, respectively (Table 2).

Table 2 The removal mass of carbon, nitrogen and phosphorus from harvest and natural death of bottom cultivated Manila clam in Jiaozhou Bay

The total annual natural mortality of Manila clam individuals at in Jiaozhou Bay was 45.40% from May 2009 to June 2010. The dry weight of shell deposited onto sea floor was 1.03×105t, of which carbon, nitrogen and phosphorus were 1.21×104t, 149.6 t and 29.1 t, respectively. During the investigation period, the removal mass of carbon, nitrogen and phosphorus from harvest and natural death were 3.16×104t, 1449.6 t and 149.1 t in total, respectively (Table 2).

4 Discussion

4.1 Dynamic Variations of Biogenic Element Contents in Shell, Flesh and Clam Individuals

This study has shown the monthly variations of biogenic element contents ofR. philippinarumcultivated in Jiaozhou Bay. The biogeochemical process of element contents was closely associated with individual growthand reproduction cycle of bivalves (Robertet al., 1993; Elseret al., 1996; Kimet al., 2005). In this study, the biogenic element contents varied dramatically during the growth and reproduction process of Manila clam. Carbon and nitrogen contents of dry flesh were negatively correlated with reproduction (Table 1), since they played important roles in the reproductive activity. During the gonad development, a large amounts of carbon and nitrogen were consumed to generate energy (Lodeiroset al., 2001; Narváezet al., 2008), and converted into lipid as structural constituents in membranes of gonads (Birkelyet al., 2003; Ojeaet al., 2004; Martínez-Pitaet al., 2012). According to Patrick (2006), sexual maturation in bivalves was closely related to the breakdown of carbon content. Several studies showed that bivalves had the highest phospholipid content when it reached sexual maturation (Martinez, 1991; Delgadoet al., 2004; Serdar and L?k, 2009), and similar variations of carbon contents of Manila clam was found in this study.

The first decline in carbon contents and increase in phosphorous content of flesh, occurring from May to August 2009, were possibly directly associated with metabolizing carbohydrates to lipids. As the clam grew, the phosphorous content of flesh increased significantly. Phosphorous loss and nitrogen increase accompanied spawning activity. The drop of carbon in clam individuals was synchronous with phosphorous accumulation (Table 1), as the clam utilized large amounts of carbon and nitrogen to generate energy and accumulated phospholipids for the reproduction at the beginning of breeding season. After breeding season, the dry weight of clam fell; while the growth rate increased. Carbohydrate and protein accumulated gradually in the clam, as a result of nutrient assimilation. The result of this study were consistent with those previous conclusion that carbon contents of flesh significantly decreased in the ripeness stage, while it was enhanced before gametogenesis (Berthelinet al., 2000; Narváezet al., 2008; Martínez-Pitaet al., 2012).

Growth and reproduction of the clam had little effect on the carbon content of shell in the study period. It might be related to the structure of clam shell. It has been reported that CaCO3was the main composition of clam shell, accounting for as high as 98.8% of the shell of Manila clam with little variation among different ontogenic stages (Fanet al., 2005; Liuet al., 2008). In present study, it was found that carbon is the major component of clam shell, accounting for a large proportion of the total.

The variations of biogenic element contents were also closely related to the environmental factors. During the winter, biogenic element contents were quite low (Fig.3). Temperature is the main factor that affects the survival rate of Manila clam (Zarnoch, 2006; Zarnochet al., 2008). High mortality has been observed in this study as the temperature dropped below 3℃, occurring from Dec ember 2009 to January 2010. The assimilation rate dropped with the falling of water temperature in winter. The low assimilation rate and alimentation efficiency resulted in low growth rate of the clam (Zhang, 2002). Clam consumed large amounts of nutrients to meet its energy demand, and the element contents of clam decreased gradually in winter. Low temperature, together with starvation and winter mortality were probably the main causes of low element contents of Manila clam in winter. The biogenic element contents of shellfish in aquaculture waters of Sishili Bay and Sacca di Goro lagoon were close to our results (Zhouet al., 2002; Nizzoliet al., 2006; Nizzoliet al., 2011). The shell length of clam individuals in this study was from 25.12 to 32.96 mm. Further studies on the biogenic element contents at different sizes are required to evaluate if they have the similar variation trends to this study.

4.2 Stoichiometry Analysis of C:N and N:P Ratio

The change in the stoichiometric ratio of carbon, nitrogen, and phosphorus (C:N:P ratio) was mainly determined by the variation of phosphorus content (Vanniet al., 2002). Organisms must adjust their C:N:P ratio to the changing growth rate. Individuals with a high growth rate usually have low ratios values of C:N, C:P and N:P (Elseret al., 2003; Makinoet al., 2003). Related studies indicated that C:N and N:P ratio of cultured clam were maintained within a certain range, and their variations among different sea areas were mainly affected by nitrogen content (Goulletquer and Wolowicz, 1989; Zhouet al., 2002; Nizzoliet al., 2011). Zhouet al. (2002) found that the atom ratio of carbon to nitrogen in clam flesh was 4.64, and Nizzoliet al. (2011) showed that C:N ratio was 4.0 while N:P ratio was 26 ± 2. In this study, C:N and N:P atomic ratio of flesh were 4.52–6.06 and 17.77–32.34, respectively. C:N and N:P ratio of present study in Jiaozhou Bay was similar to those results reported.

In this study, as the nitrogen content of clam was lower in winter than that in summer, C:N ratio was higher in winter. After February 2010, the nitrogen content of clam dropped with the increase of condition index. N:P ratio was higher when phosphorus content was low, and was affected by phosphorus content greatly. The variation of phosphorus content of clam before and after breeding was larger than those of carbon and nitrogen contents.

4.3 The Ecological Effect of Removal of Biogenic Elements on Bay Ecosystem

As clam grows, a fraction particulate matter is converted into biomass (Lodeiroset al., 2001; Nizzoliet al., 2006; Forrestet al., 2009). Thus, the nutrients incorporated into the biomass can be removed by crop harvesting (Kasparet al., 1985; Nizzoliet al., 2006). The annual removal of carbon of shell in Jiaozhou Bayviaharvest and natural mortality of Manila clam was 2.66×104t. It was 84.2% of the annual removal of carbonviaharvest and natural mortality (Table 2). The annual removal of carbon by fleshviaharvest accounted for about 10.0% of the annual production of organic carbon in Jiaozhou Bay (Li, 2006; Wang, 2009; Tanet al, 2011). According to Zhang (2007) and Qi (2011), the annual nitrogen and phosphorous input into Jiaozhou Bay was 3.59×108mol and 1.06×107mol, respectively. During the investigationperiod, the annual removal of nitrogenviaharvest and natural mortality was 1449.6 t (1.03×108mol); and the annual removal of phosphorous was 149.1 t,i.e. 4.59×106mol. They amounted to 28.7% and 43.3% of the external nitrogen and phosphorous input to Jiaozhou Bay, respectively. These quantities acccounted for a considerable percentage of the external input of nutrients in Jiaozhou Bay in this study.

Relevant studies have disclosed the removal effect of carbon, nitrogen and phosphorus. Bartoliet al. (2001) found that the anthropogenic removal amounts of nitrogen and phosphorus were 46 t and 10 t in Sacca di Goro lagoon, which accounted for 5% and 25% of the external input amount, with the peak of annual yield 1.5×104t. Zhouet al. (2002) concluded that the removal amounts of nitrogen and phosphorus due to the clam and algae aquaculture in Sishili Bay were more than 850 t and 78 t. Nizzoliet al. (2006) showed that the removal amounts of nitrogen and phosphorus through clam harvest were 16 t and 0.9 t, when the clam annual yield was 6000 t. Compared with the previously studies, these quantities were larger in this study. The high yield of Manila clam in Jiaozhou Bay was the main cause of higher removal of element contents.

In the present study, there was no anthropogenic feeding in the clam enhancement and aquaculture in Jiaozhou Bay. The land-source nutrients provided Manila clam with rich feedings. The removal of carbon can also reduce the CO2concentration of water and carbon accumulation in the bay. The removal of biogenic element contents by harvest and natural death of Manila clam would help to control the biomass of phytoplankton, control eutrophication indirectly, and reduce the risk of environment deterioration. The harvest of clam can also transfer the hard-recycled elements to easily-used form for human. Therefore, the benthic aquaculture of Manila clam has important ecological significance for maintaining the ecological balance in Jiaozhou Bay.

Furthermore, the optimum enhanced seeding density and maximum economic removal of biogenic element contents of Manila clam in Jiaozhou Bay are needed to be studied. According to Ren (2006), the culture density of Manila clam in this study has exceeded its ecological carrying capacity in Jiaozhou Bay, and therefore the culture density of Manila clam must be reduced to capacity certain level for the sustainable development of Manila clam aquaculture and the maintenance the ecosystem health of Jiaozhou Bay. Many studies showed that integrated multi-trophic aquaculture (IMTA) might be a feasible method to reduce the pressure from shellfish culture on the ecological environment, and bring a healthy development of aquaculture (Chopinet al., 2001; Neoriet al., 2004; Troellet al., 2009; Zhanget al., 2009). However, water pollution is at a critical level in Jiaozhou Bay. Although the clam culture made a great contribution to the removal of biogenic element, it is still very important to control the pollution emission into the waters. Measures should be taken to prevent Jiaozhou Bay ecosystem from collapse, and to establish an environment-friendly ecological farming to keep the sustainable development of the clam industry.

Acknowledgements

The work was funded by the Public Science and Technology Research Funds Projects of Ocean (Grant No. 200805066), National Natural Science Foundation of China (Grant No. 41006083) and Shandong Provincial Natural Science Foundation, China (Grant No. ZR2010 DQ026). We are grateful to those students and crew for their assistances in data collection during the surveys.

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(Edited by Qiu Yantao)

(Received August 22, 2012; revised September 6, 2012; accepted April 1, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014

* Corresponding author. Tel: 0086-532-82032960

E-mail: renyip@ouc.edu.cn