Grantee Research Project Results

Two exotic Daphnia species were found in the sediments of Onondaga Lake exclusively during the period of peak pollution (Hairston et al. 1999a, Duffy et al. 2000). These were Daphnia exilis, which has only previously been described from shallow, fishless, saline lakes west of the Mississippi River, and D. curvirostris, which is native to saline coastal lakes in Europe. It is likely that the increase in salinity caused by industrial pollution made Onondaga Lake habitable for these species.

Native Daphnia species from Onondaga Lake, D. pulicaria, D. ambigua, and D. galeata mendotae were absent during the period of peak pollution by heavy metals, and were replaced by exotics. They are present in lake sediments deposited prior to 1900, but are absent from the early part of this century until about 1990, when they reappear. At the sediment depths that these species are absent, D. exilis and D. curvirostris diapausing eggs become abundant. The native species currently cause a marked "clear-water phase" during May of each year when they cause a marked decline in primary producer biomass. During the 1960s and 1970s, when the exotic species were present, a clear-water phase was also present, but occurred much later in the summer due to a different seasonal phenology with maximum abundance in August for D. exilis and D. curvirostris.

The data from the cores from the reference lake (Oneida Lake) show no invasions by exotics, nor do they show the extended period of absence of the native Daphnia. There are interesting patterns of species replacements over periods on the order of decades that have important implications for the coexistence of species in changing environments, however (Hairston 1996, 1998; Hairston & Caceres 1996).

Three traps at the shallow site and three at the deep site had a mechanism installed that permitted the sediment below the trap to be stirred to investigate the effect of sediment disturbance on hatching timing and rate. We found that hatching rates of Daphnia did not differ between treatments, but that hatching extended later into the summer in stirred traps. This shows that human impacts on sediment dynamics may have a significant impact on the role that the egg bank plays in zooplankton communities.

Sediment emergence traps were deployed at two sites in Onondaga Lake. Only D. galeata mendotae was found in the traps, even though eggs of five species of Daphnia occur in the lake sediments, and even though a few D. pulicaria were found in the water column of the lake in late summer. As in Oneida Lake, hatching occurred principally in May and June, although there was a return to some hatching after fall turnover. Sediment stirring increased hatching from the egg bank in Oneida Lake, but had no measurable effect in Onondaga Lake.

Several other collaborations have come out of this project including a paper on (1) the role of diapausing egg banks in the coexistence of competing copepod species in German lakes (Santer et al. 2000), which is an idea presented previously for copepods by Hairston and Bohonak (1998), (2) the record of fish introductions in high mountain lakes in Austria (Perry et al., manuscript), and (3) the record of phosphorus inputs to Cayuga Lake (Hairston et al., submitted).