LAUSR

The Great Plains of North America are projected to become more arid as the climate changes over the next century. Salinity levels for lakes in this region are linked to climate, suggesting that lakes will become more saline as the climate becomes drier. One group of organisms that might be affected by increased salinity levels are the zooplankton, aquatic invertebrates that transfer energy from primary producers to macroinvertebrates and fish. Little is known about the ability of zooplankton to adapt to long-term increases in lake salinity levels. For my thesis, I used resurrection ecology to examine how a common zooplankter, Ceriodaphnia dubia, responded to long-term salinity changes in Moon Lake, North Dakota; a lake that has experienced fluctuating salinity levels and for which a history of salinity levels has been reconstructed from diatoms. My thesis had three objectives: 1) To determine how salinity influences the rate of hatching of C. dubia resting eggs in sediment core slices of different depths; 2) To determine if the salinity tolerance of C. dubia has changed through time; and 3) To determine if C. dubia, has existed in Moon Lake throughout the last 170 years, or if the species was absent from the lake during periods of high salinity. I collected seven sediment cores from Moon Lake and counted the number of Ceriodaphnia eggs present in 1 cm intervals of each core over the last 170 years. I then performed experiments to determine the salinity levels that were most likely to induce hatching in C. dubia eggs through time, and I performed toxicity tests to determine if salinity tolerance of the species has changed through time in concert with the salinity history of the lake. My egg counts showed that C. dubia was present in Moon Lake throughout all core layers during the past 170 years, even through periods of intense drought and high lake salinity levels. My experiments also showed that C. dubia eggs preferred to hatch in saltier water following periods of drought and in fresher water following wet periods. Similarly, my toxicity experiments