| Large consumers play important roles in the circulation of materials of ecosystems. Herbivores and filter-feeding bivalves are one of the most important factors influencing primary produces and their feeding-defecation activities is a significant part of the circulation of materials of ecosystems. The ecology roles of bivalves in natural and aquaculture ecosystems were reviewed. The feeding-defecation of whooper swans(Cygnus cygnus) overwintering in Swan Lake of Rongcheng and ecophysiological parameters of suspension-cultured bay scallops were studied. The present study included the investigation of shoot height structure and shoot weight structure of eelgrass in different sites of Swan Lake, the diet composition of whooper swans and biogenic element(carbon, nitrogen and phosphorous) and heavy metals content of their major food sources and feces, seasonal variations of the effect of biodeposition and respiration by bay scallops(Argopecten irradians) cultured in Laizhou Bay. The main results are as followed,(1) The population structure of eelgrass in Swan Lake was investigated in May and June of 2014. Results showed that, in May, the shoot weight structure of eelgrass population in East bank and Central Sandbank were similar, both with a shape of “vase” The proportion of(10.0-14.9cm)group and(5.0-19.9cm)group were the highest and the proportion of eelgrass higher than 19.9cm declined with increasing in shoot height. The shoot height structure of eelgrass in South bank were similar with the other two sites in the range of under 45 cm, whereas the proportion of 45cm- group was also high with a value of 18.28%. By contrast, in June, the proportion of groups under 25 cm declined, suggesting that the recruited individuals are little. The groups in the range of over 25 cm increased. The decline in eelgrass higher than 45 cm probably indicated the death of high individuals. The shoot weight structure of eelgrass in three sites were all distributed in a pyramid shape in May and June. Generally, the proportion of eelgrass declined with increasing shoot weight.(2) Results of fecal microscope analysis and stable carbon isotope analysis both showed that eelgrass and wheat seedlings are the most significant food sources of whooper swans wintering in Swan Lake and there is a great diet switch in their diet composition during their overwintering period. In the early and middle wintering period, the whooper swans mainly feed on eelgrass, the proportion of eelgrass in their diet from November of 2011 to February of 2012 was 78.85%±7.91%, 58.68%±24.33%, 79.67%±21.10% and 49.65%±34.55%, respectively. In late wintering period, however, the whooper swans nearly feed exclusively on wheat seedlings. For the food sources, P is highest in eelgrass and C and N were highest in wheat seedlings. The total nitrogen and phosphorus content of feces averaged 2.62% and 0.34% of the dry weight, with a range of 1.06-3.85% and 0.13-0.59% respectively. The whooper swan population(~1500 swans) in Swan Lake could defecate 403.2 kg of dung including 10.56 kg N and 1.37 kg P, to the lakebed every day.(3) Zn, As and Cr in chaetomorpha(Chaetomorpha linum), Cd in eelgrass(Zostera marina L.) blades, Pb in eelgrass rhizomes, Cu in ulva(Ulva pertusa), and Hg in wheat seedlings(Triticum aestivum), had the highest concentrations amongst the selected food sources. The concentrations of Cd in swan feces of aquatic diet were significantly higher than those of terrestrial diet; and the concentrations of Cr, Pb and Hg in feces of terrestrial diet were significantly higher than those of aquatic diet. C. cygnus could accumulate Cd and Hg by ingestion of wheat seedlings.(4) The biodeposition rate of the suspension-cultured bay scallop(Argopecten irradians) in Laizhou Bay was in situ determined using sediment traps from September through November in 2013 and biogeochemical characters of the natural sediment and biodeposits were analyzed in laboratory. As well as these, the relationship between biodeposition rates of A. irradians was discussed for the first time. Results showed that the biodeposition rates of particulate matter by A. irradians were rather high and it fall within the upper range reported previously for other filter-feeding bivalves. The biodeposition rates of total particulate matter, carbon, nitrogen and phosphorus by bay scallops were 1.16×103, 8.88, 1.82 and 0.54 mg(g tissue dry wt)-1d-1. The allometric relationship between biodeposition rate(BDR; g/(ind·d)) and tissue dry weight(TDW; g/ind) of the scallop was modeled using the formula: BDR=a Wb, and the value of a ranged from 0.99~1.24, b ranged from 0.55~0.66. But the allometric relationship did not exist when the surface water temperature dropped below 10℃. The biodeposition of bay scallop contributed to the enrichment of organic matter, organic carbon, organic nitrogen and organic phosphorus in sediment. The heavy metals(As, Cr, Cd, Cu, Pb, Zn, Hg) content were also improved in the biodeposits from scallops, especially in October and November.(5) The respiration rate of the suspension-cultured bay scallops(Argopecten irradians) in Laizhou Bay were determined in field using bottle respirator from August to December. The oxygen consumption rates of A. irradians were high from August to October with the values ranging from 1.40 to 3.19 mg(ind?h)-1. When the water temperature dropped to 6.6℃ in December, the oxygen consumption rates declined to the lowest, even for the biggest scallops, the corresponding values was only 0.50 mg(ind?h)-1. The oxygen consumption rate of bay scallop also respond allometricly with tissue dry weight and its values of a ranged from 0.45~3.27 and values of b ranged from 0.61~1.08.(6) For the total scallops cultured in Laizhou bay, 1140 t total particulate matter was deposited to the seabed each day in the aquaculture zone, including 9.74 t C, 1.82 t N, 0.48 t TP, 0.05 t As,-4.28×10-5t Cd, 0.16 t Cr, 0.05 t Cu, 0.03 t Pb, 0.18 t Zn and 4.17×10-4t Hg. In addition, 2.87 t nitrogen, including 2.07 t ammonia nitrogen was excreted by the scallops. The large quantities of biodeposits produced by densely cultured bay scallops may compensate for the deficiency of organic matter in seabed and increase the secondary production rate of benthos. The aggregation of Musculus senhousia and Echinocardium cordatum rather than small opportunistic species indicated that the biodeposition by bay scallops increased the food supply to benthos and enhanced the energy flux in benthic food wed. The large-scale aquaculture of A. irradians might play an important role in benthic-pelagic coupling and phytoplankton control in coastal culture zone of Laizhou Bay. Meanwhile, large amounts of nitrogen and phosphorus were removed from the system with the harvesting, significantly relieving the eutrophication pressure on the costal water in Laizhou Bay. |
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