USING DAPHNIA EPHIPPIA PIGMENTATION AND SIZE TO DETERMINE THE TROPHIC IMPACTS OF FISH PREDATION DURING THE LAST 8000 YEARS

Past trophic levels were determined by analyzing the resting egg; or ephippia of the zooplankton, Daphnia, that are preserved in a sediment core from Twin Ponds, VT. Daphnia occupy an important position on the pelagic food chain as consumers of phytoplankton and the prey of planktivorous and juvenile fish. The presence of Daphnia in a waterbody is an important control on algal growth and water turbidity. Daphnia and their ephippia respond to fish predation by changing their size and pigmentation. Ephippia have a small area and low pigmentation when fish predation is high and a large area and high pigmentation in the presence of low fish predation. The ephippia from 5ml samples were counted, then 20 random ephippia from each sample were analyzed for size (area and length) and pigmentation by using pixel color threshold values of 75 and 125. Observed morphological differences between species were normalized using z-cores.

Before 3.0 cal ka BP, ephippia were generally larger in size and changes in size and pigmentation did not always occur simultaneously. However, increased ephippia size did occur during times of greater ephippia abundance. These results suggest varying but low levels of fish predation. After 3.0 cal ka BP, ephippia were smaller and pigmentation was more variable. Additionally, changes in ephippia abundance and average size and percent pigmented area are positively correlated and indicate increased and variable levels of fish predation. Changes in fish predation resulted in centennial-scale changes in Daphnia abundance and primary productivity. Comparison of these results with geochemical data, indicate that longer lake stratification and lack of dissolved oxygen reduced fish predation and led to more abundant, larger, and darker ephippia. This research will be further compared with regional paleoclimate records and historic land use to determine the impact of environmental change on aquatic trophic conditions.