|Other Abstract||A total of 30 shallow lakes, located along the middle and lower reaches of the Yangtze River, were selected for this study. We aimed at presenting a snapshot of crustacean zooplankton communities and their relations to environmental factors, and then tested two hypotheses: (1) that there is a consistent relationship between crustacean biomass and trophic indicators among lake groups with different trophic conditions；(2) that larger size structure in aquaculture lakes is associated with higher grazing pressure. Assessment was made for the relative importance of nutrients and zooplankton biomass in determining the phytoplankton biomass in subtropical shallow lakes, and for the relative importance of physicochemical factors and crustacean zooplankton as determinants of rotifer density and species distribution. Finally, a life-table demographic study was performed to reveal the responses of Moina micrura in aquaculture lakes to high concentrations of chemical cues from planktivorous fish, and the underlying mechanism accounting for the dominance of M. micrura under planktivorous fish predation was also discussed.
The study lakes showed a wide range of trophic status, with total phosphorus (TP) ranging from 0.008 to 1.448 mg L-1, and chlorophyll a (Chl a) from 0.7 to 146.1 µg L-1, respectively. 38 species of Crustacea were found, of which Cladocera were represented by 25 taxa (20 genera), and Copepoda by 13 taxa (11 genera). The most common and dominant species were Bosmina coregoni, M. micrura, Diaphanosoma brachyurum, Cyclops vicinus, Thermocyclops taihokuensis, Mesocyclops notius and Sinocalanus dorrii. Daphnia was rare in abundance. There was a general trend from mesotrophic to hypertrophic lakes: the diminishing significance of large herbivorous cladocerans (Daphnia) was accompanied by the increasing predominance of cyclopoids (Cyclops, Thermocyclops and Mesocyclops) and small cladocerans (Moina, Diaphanosoma). Canonical correspondence analysis showed that except for four species (D. hyalina, S. dorrii, C. vicinus and M. micrura), almost all the dominant species had the same preference for environmental factors. Temperature, predatory cyclopoids and planktivorous fishes seem to be the key factors determining species distribution.
Biomass of cladocerans and copepods also varied greatly, with cladocerans being dominant in most lakes. TP was a better trophic indicator than Chl a to predict crustacean biomass. Within the three groups of lakes, however, there was no consistent relationship between crustacean biomass and trophic indicators. The possible reason might be that top-down and bottom-up control on crustaceans vary with lake trophic status. The lack of significant negative correlation between crustacean biomass and Chl a suggests that there was little control of phytoplankton biomass by macrozooplankton in these shallow subtropical lakes.
Mean body length (a proxy of size structure) of crustaceans, was positively correlated with increasing Chl a (r2 = 0.40, P = 0.000) and TP (r2 = 0.38, P = 0.000), contrary to the empirical studies. However, the ratio of zooplankton to phytoplankton biomass decreased significantly with increasing TP (r2 = 0.27, P = 0.005) and mean body length (r2 = 0.46, P = 0.000). Meanwhile, size structure showed no significant contributions in explaining residual variations of chlorophyll-phosphorus relationship (P = 0.231). These results indicate that larger size structure of crustaceans was not always associated with higher grazing pressure. It is likely that in aquaculture lakes, size structure of crustacean zooplankton was of minor importance in the control of phytoplankton biomass, and it was mainly regulated by fish predation. Our results and previous empirical studies might be a reflection of two different stages of lake eutrophication and fish predation intensity.
Factor analysis and multiple linear regressions showed that phytoplankton biomass was positively correlated with TN, NH4+, NO3- and TP, but did not show any negative relationship to zooplankton biomass. Meanwhile, contrary relationships were observed between phytoplankton biomass and the mass ratio of TN/TP in spring and summer, indicating that in nutrient-richer lakes the dominant phytoplankton species have different preferences for TN/TP ratio. The insignificant top-down control of phytoplankton biomass might be attributed to the dominance of small-sized crustaceans and low crustacean biomass resulting from cyanobacterial dominance and planktivorous fish predation as well as other factors. Thus, it is likely that nutrients were more important than zooplankton biomass in explaining the total variance of phytoplankton biomass in the subtropical lakes.
Multiple linear regressions showed that 38.5% and 34.1% of the variance in rotifer density were respectively explained by physicochemical factors (including Chl a, TN, temperature, depth, pH, transparency) and crustaceans (including S. dorrii, Daphnia, D. brachyurum, C. vicinus, Bosmina, Schmackeria forbesi, Diaptomidae), suggesting that these two sets of explanatory variables might have similar impacts on rotifer density. Redundancy analysis showed that the variance of rotifer species distribution explained by crustaceans and physicochemical factors was 26.9% and 28.9%, respectively. Further analysis demonstrated that the adjusted variance explained by pure crustaceans and pure physicochemical factors was 10.9% and 13.6%, respectively. Moreover, these two percentages were not statistically different (P = 0.261), indicating that crustaceans and physicochemical factors had equal contributions in determining rotifer species distribution. The distribution of rotifers was strongly associated with the gradient of trophic status, and their coexistence with crustaceans seemed to be determined by their preferences for trophic status and the morphological characteristics of particular rotifers.
During the life table demographic study, increasing concentration of planktivorous fish cues had no significant effect on M. micrura growth and size at first production as well as size of neonates in first brood. Total offspring, mean clutch size, life span and clutch number also did not show any significant difference between treatments (Post-Hoc, Tukey-test, P > 0.05). The generation time and intrinsic rate of population increase showed a narrow range between treatments. These results indicate that M. micrura might have accustomed themselves to the cues from their long-term coexistence with planktivorous fish. However, total offspring, mean clutch size and intrinsic rate of population increase were much larger than values reported from previous studies, suggesting that M. micrura might have already evolved a strong resistance to predation of planktivorous fish through changes of life history. In addition, decreased generation time, age and size at first reproduction might be also responsible for their resistance to predation of planktivorous fish.|