| Amphipod is one of the main mesograzers in the estuary, intertidal zone and saline or brackish lagoon in temperate areas; it affects the community structure of macroalgae by 'selective feeding'. In eutrophic water, grazing pressure from amphipod could control the accumulation of macroalgal biomass, otherwise, excessively high density of these fast-growing seaweeds would damage their inner environment, and sinking of the rotten algae would deteriorate the sediment, which have negative impact on the ecosystem of the seaweed bed.In Yudang Lagoon, ultra-eutrophication of the water caused severe macroalgal bloom in recent years. Although seaweeds could absorb large quantities of nutrients and inhibit the bloom of microalgae, dense canopies of nuisance macroalgae may deteriorate the quality of the sediment and dead seaweeds will freely float in the surface of the water, which are disadvantageous to the landscape construction of Yundang Lagoon. By investigation of the temporal and spatial distribution of amphipods in the inner part of Yundang Lagoon and experiments on the feeding ecology and life cycle of A. valida, we discussed the impacts of amphipod grazing pressure, especially from A. valida, on the macroalgal community in this study, which would provide effective knowledge for the management of Yundang Lagoon. Main results are as follows:In Yundang Lagoon, seaweeds mainly consist of Ulva lactuca, Rhizoclonium sp., Enteromorpha clathrata and Gracilarla tenuistipitata in Stn. 13~#. Macroalgal canopies of U. lactuca can be seen every month during investigation, and reach its maximal biomass in August, 2008, while canopies of E. clathrata came out from late autumn (07.11) to early winter (08.01), and reach the maximal biomass in Novenber, up to 2995 g FW/m~2.G.tenuistipitata had remarkable patchiness from 07.12 to 08.03, and Rhizoclonium sp. came out in the beginning of March and the area tended to expand at the end of investigation. Distribution pattern of seaweeds changed gradually from random in December to zonal distribution in February. Macroalgal canopies were dominated first by E.clathrata then U.lactuca, and finally by Rhizoclonium sp. together with U.lactuca.Amphipods were absolutely the dominant group in Yundang Lagoon which accounts for 85.8-98.7 % of the total abundance of zoobenthos. Dominant amphipods dwelling on the seaweeds thalli consisted of Corophium uenoi, Ampithoe valida, Grandidierella japonica, Grammaropsis laevipalmata and Caprella panantis, the dominance of which were 0.34-0.82, 0.01-0.17,0.02-0.34, 0.02-0.05 and 0.05-0.29, respectively. G.japonica reached the highest abundance during the late of autumn (07.11) and early winter (08.01), and the abundance found in E. clathrata seemed to be much higher than that in U. lactuca. There was certain negative correlation between the abundance of G.japonica and that of A. valida (r=-0.64). And the correlation of the abundance of C. uenoi,A. valida and G. laevipalmata was significant (r>0.9) in macroalgal canopies dominated by U.lactuca. The change of amphipod abundance did not synchronize the change of macroalgal biomass. Amphipods lagged one month behind. In Yundang Lagoon, amphipods had high secondary production in macroalgal community with 6.16 g DW /m~2/6mo. for C. uenoi, 9.49 g DW/m~2/6mo. for G. japonica, and 29.17 g DW/m~2/5mo for A.valida, respectively.A.valida has compensatory feeding behavior, which is a strategy to adapt the changing environment. U. lactuca (40.6±11.2 %) and Rhizoclonium sp. (57.1±10.7 %) were preferably consumed by A. valida, and green algae were their agreeable habitats. The feeding selectivity of A.valida seems nothing to do with nutritional traits of seaweeds, but related to the living habit. Meanwhile, physio-chemical properties of macroalgae also contribute to the feeding choice.The optimal growth temperature for A. valida ranges from 20℃to 30℃.The growth rate could be approximately regarded as linear in the early stage of life cycle, and the growth rate was found to be much higher in male. Female individuals appeared to take up a higher proportion at temperature 15℃,up to 0.68±0.09, while in the rest treatments of A. valdia approached 0.5 in F/F+M. The proportion of female individuals increased with age. Higher temperature shortened the time of sexual maturity for A.valida. Similarly, female arrived at ovigerous status much earlier at 25 ℃and 30℃than at 15℃and 20℃.However, body length of ovigeous female A. valida for the first time (8.01±0.79 mm) was not related to temperature. Through observation, we found that A. valida would breed continuously and actively during life cycle. Reproductive rhythm of A.valida at temperature 15℃was 14.7±2.1 days, which was nearly 1.5 times of the other three control groups. A.valida had high fecundity, and the number of juveniles released per brood was not significantly relevant to temperature, but to the body length of A.valida.In summary, A.valida, a dominant mesograzer in Yundang Lagoon, whose grazing pressure reduced the accumulation of macroalgal biomass, plays great role in stability of pattern of macroalgal community because of the excessively high reproductive potential and high individual growth rate. Since the increment in abundance of A.valida fall behind that of Viva lactuca, Ulva lactuca will inevitably accumulate in a short period of time causing damage to the substrate when grazing pressure was low. Therefore reducing the biomass of macroalgae by removing redundant macroalgae artificially would be a feasible strategy for managing the macroalgal ecosystem in Yunudang Lagoon.
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