Exploring the Salinity Tolerance of Zooplankton Communities on the Great Plains

Palaeoecological data shows fluctuating salinity levels in lakes across the Great Plains due to alternating wet and dry periods over the last two millennia. Droughts caused by this natural variation may be exacerbated by anthropogenic climate change, as the Great Plains are expected to become drier over the coming decades. As conditions become drier, the evaporation of water will increase the salinity of lakes across the region. Increases in salinity may significantly impact biodiversity, including important members of lake food webs such as zooplankton. Due to the region's history of alternating wet and dry periods, zooplankton may have adapted to withstand changing salinity levels. However, to understand how expected salinity increases may affect zooplankton communities, it is necessary to understand the limits of their salinity tolerance. While several studies have mapped the distribution of zooplankton species in lakes on the Great Plains, none have examined the salinity tolerance of common species. To better understand how salinity changes will affect zooplankton, my study has two objectives: 1) Determine the salinity tolerance of common zooplankton species; and 2) Test for intraspecific differences in salinity tolerance of zooplankton originating from lakes with differing salinity levels. Zooplankton communities were collected from several lakes across the Great Plains and were cultured in environmental chambers and used for acute salinity tolerance tests to examine the limits of their salinity tolerance. Three species were used in experiments, including Ceriodaphnia laticaudata, Daphnia magna and Daphnia schodleri. The half-effective concentration (EC50) results showed that zooplankton from higher salinity lakes tended to have a higher tolerance compared to those originating from lower salinity lakes. This suggests that the osmoregulatory capacity of zooplankton may have evolved in response to higher salinity levels. Our results also showed that the upper salinity tolerance for common species was >8 g/L, ii suggesting that our study species may weather moderate increases in salinity. However, during the Dust Bowl years in the 1930s, salinity in some lakes in the region, such as Moon Lake, North Dakota, reached 25 g/L. Climate change forecasts predict droughts greater than those experienced during the Dust Bowl years by 2060. Therefore, despite the high tolerance observed in this study, some species may not persist in affected lakes. Further studies on a diversity of species should be done to see if other zooplankton might tolerate expected salinity increases.