Monthly change of community structure of zoobenthos in Xiangxi Bay after impoundment of the Three Gorges Reservoir,China

Xiangxi Bay,a typical bay of the Three Gorges Reservoir(TGR)in China,was formed in June 2003 after the impoundment of the TGR.We investigated the community structure of zoobenthos in Xiangxi Bay from August 2003 to August 2004.Benthic abundance and biomass were calculated per unit area of sediment.A total of 26 taxa were identified.They included two families of Lamellibranchia(7.7%of the total taxa number),two families of Oligochaeta(50%),Chironomidae(38.5%),and one other animal(3.8%).The average density and biomass were 276 ind./m2 and 0.301 g/m2,respectively.Oligochaetes and chironomids constituted the major groups,and Limnodrilus hoffmeisteri,Procladius sp.,Branchiura sowerbyi,Tubificidae sp1.and Polypedilum scalaenum group sp.were the common taxa in this reservoir system.Among them,L.hoffmeisteri and Procladius sp.were the dominant species,and contributed 37.0%and 28.3%of the total density,respectively.The standing crop of zoobenthos in Xiangxi Bay was relative low,compared with other similar reservoirs.Our studies demonstrate that the standing crop increased gradually with time.The total density and biomass in August 2004 were 8.8 and 14.3 times those of the initial impoundment,separately.Oligochaetes did not become the sole dominant group,indicating that the sediment loading in Xiangxi Bay may be not serious.

In Situ Filtration Rates of Blue Mussels(Mytilus edulis)Measured by an Open-Top Chamber Method

Blue mussels, Mytilus edulis , form dense beds of both commercial and ecological importance, and many attempts have been made to determine their filtration rate. The total time in which mussels actually utilise their filtration capacity in nature varies greatly, making in situ methods for filtration rate measurements relevant. Further, it is being debated to what extend filtration rates measured in the laboratory using cultivated algal cells may apply for mussels in nature. In the present study, we have used an open-top chamber setup in order to allow repeated in situ filtration rate measurements of M. edulis using ambient natural phytoplankton and free-living bacteria. We found that the in situ measured filtration rates are comparable to filtration rates obtained in laboratory studies using different methods and controlled diets of cultivated algal cells. Further, we found that the retention efficiency of free-living bacteria was between 22.2% and 29.9%, in good agreement with values from laboratory studies. Our findings support the assumption that mussels in nature tend to use their filtration capacity when the phytoplankton concentration is above a certain lower trigger level.

Freshwater snail and shrimp differentially affect water turbidity and benthic primary producers

Benthic macroinvertebrates play key roles in shallow aquatic ecosystems and can contribute substantially to aquatic food webs.However,how macroinvertebrates with different behaviors(for example,pertaining to locomotion,foraging and burrowing)impact water quality and primary producers has not been fully explored.Here,we performed two consecutive microcosm experiments to test the effects of(1)macroinvertebrates with different behaviors(a low mobility scraper aquatic snail Bellamya aeruginosa and a high mobility shredder freshwater shrimp Macrobrachium nipponense)and(2)different shrimp biomasses on water clarity and the composition of benthic primary producers.The results showed that presence of snails significantly increased the biomass of filamentous green algae and decreased the biomass of periphyton.In contrast,presence of shrimp significantly decreased the biomass of filamentous green algae and increased the biomass of periphyton,and these effects were biomass dependent.Filamentous green algae disappeared when shrimp biomass reached 9.8 g m^(-2).No interactive effects of snail and shrimp presence were found.This could be attributed to different food preferences by the two consumers,with snails preferring periphyton(mainly diatoms)and shrimp preferring filamentous green algae.The presence of snails decreased water turbidity,while shrimp increased water turbidity,which showed a hump-shaped response to shrimp biomass with a peak at 24.2 g m^(-2).These results are likely because the snail is a low mobility grazer and can filter suspended particles,while the shrimp is a high mobility shredder with burrows,which can strongly disturb sediment.The decrease in water disturbance at high shrimp biomass might be due to food limitation,thus reducing burrowing and foraging activities.Neither snail nor shrimp affected the biomass of H.verticillata,while the biomass of V.spinulosa increased with shrimp biomass.The reason for this could be that shrimp increased nutrient availability and decreased the growth of filamentous green algae that compete with macrophytes.Our study demonstrated that different macroinvertebrates have complementary functions in benthic habitats;thus,maintaining macroinvertebrate diversity is important for shallow aquatic ecosystems.Furthermore,the freshwater shrimp M.nipponense could be a potential consumer to control filamentous green algal blooms in its native range,but their biomass should be taken into consideration.