Long-term zooplankton body size and species changes in a subtropical lake: implications for lake management

Analysis of long-term (1970-2002) zooplankton body sizes in Lake Kinneret, as recorded in an histori- cal zooplankton sample archive, combined with re-analysis of published long-term zooplankton taxonomic abun- dance data revealed previously undetected patterns in zooplankton biomass and demonstrates a profound long-term impact of planktivorous fi sh predation intensity driven by fiharvest practices. During the 1970s, crustacean zooplankton biomass remained relatively constant at a mean (±SD) of 1.50 ± 0.26 g m -3 , dropped precipitously in the 1980s reaching 0.57 g m -3 by 1993, and rebounded to 1.33 ± 0.19 g m -3 in the early 2000s. The 1980s decline refl ects decreased cladoceran biomass due to a decline in mean body mass in the three dominant cladoceran genera (Diaphanosoma, Ceriodaphnia, Bosmina) and decreased copepod biomass that resulted from a shift in dominance by large Mesocyclops ogunnus to small Thermocyclops dybowskii. Although crustacean body sizes remained small through the early 2000s, cladoceran and copepod densities increased by ~34 and > 300 %, respectively, after 1994 leading to a dramatic increase in crustacean zooplankton biomass. These patterns indicate an intensifi cation in fi predation throughout the 1980s and 1990s, and although zooplankton biomass rebounded in 1996-2002 to ~80 % of the 1970s levels due to sharp increases in crustacean densities, smaller cladoceran and copepod body sizes sug- gest that Lake Kinneret zooplankton continue to be strongly infl uenced by intense fi sh predation.