Reservoir trophic states and the response of plankton in Guangdong Province
广东省大中型供水水库营养现状及浮游生物的响应

The trophic states of 18 drinking water supply reservoirs and the response of plankton were surveyed in Guangdong Province in 2000. Total nitrogen (TN), total phosphorus (TP) and Secchi disk depth (SD) varied from 0.15 to 7 15mg/L, 0 003 to 0 387mg/L and 0 4 to 6 3m, respectively. They all showed high correlation with chlorophyll a that varied between 0 6 and 32 3mg/m 3. Trophic state indexes (TSI) based on TN, TP, SD and chlorophyll a were between 23 7 and 55 1, without apparent seasonal variation. Among the investigated reservoirs, two were oligotrophic, four oligo mesotrophic, three meso eutrophic, two eutrophic and the others mesotrophic. Reservoirs in the coastal area had higher trophic level, especially in Pearl delta area that is quite developed in Guangdong Province.In the oligotrophic and oligo mesotrophic reservoirs, Chlorophyta and Bacillariophyta were the most important taxa and the proportion of Chrysophyta was distinctly higher than that in the other trophic level reservoirs, while that of Cyanobacteria was low. In the mesotrophic reservoirs, Chlorophyta and Bacillariophyta were also the important taxa, but the proportion of Cyanobacteria increased greatly with the increasing of trophic level. In the meso eutrophic and eutrophic reservoirs, Cyanobacteria was the dominant group, but other groups such as Chlorophyta, Bacillariophyta and Euglenophyta were also well represented. The response of crustacean zooplankton to trophic status was not distinct, for fish played a key role in structuring zooplankton community. Heavy predation from the juvenile and adult of filter feeding fish might result in the relative scarcity of Cladocera and Calanoida, notably the efficient filter feeder Daphnia in most reservoirs. Cladocera were primarily dominated either by Bosmina longirostris, Bosminopsis deitersi, Ceriodaphnia spp. or Diaphanosoma brachyurum, and Copepoda by Cyclopoida. Compared with crustacean zooplankton, rotifer responded to trophic status more distinctly. In the meso eutrophic and eutrophic reservoirs, more rotifer species was found and eutrophic indicators, Brachionus spp., Trichocerca spp., Diurella stylata or Asplanchna priodonta were the dominant groups. In the other trophic level reservoirs, the dominant groups were species with a wide ecological spectrum and oligotrophic indicators.