Lack of dormancy to protect diversity: Decrease in diversity of active zooplankton community observed in lake with depauperate egg bank

Abstract

The study of zooplankton communities in freshwater resources under anthropogenic pressures rarely includes the simultaneous assessment of dormant embryos in bottom sediments and active life-stages in the water column. A coastal lake with a history of coal-ash contamination and disruption by hurricanes provided an ideal opportunity to demonstrate the power of examining both dormant and active zooplankton. The primary objective of this study was to evaluate changes in structure of a multicellular zooplankton community that is under simultaneous pressure from anthropogenic pollution and hurricane-induced flooding. To evaluate change in community structure, the active zooplankton community in 2015 was compared to that observed in 1985. Shannon-Wiener and Simpson indices demonstrate that diversity of the active zooplankton community decreased during this 30-year span. In total, 31% of zooplankton species were lost, and new colonization accounts for 27% of species richness. Dominant species of all major taxonomic groupings changed. Because most zooplankton in freshwater lakes depend on dormant embryos to reestablish active populations after major disruptions, dormant embryos in the sediment “egg bank” were also quantified. Dormant cladoceran ephippia are present in bottom sediments, but dormant copepods and rotifers are missing. The existence of a dormant egg bank that is less diverse than the active community in a freshwater lake is unprecedented, and a depauperate “egg bank” would certainly impair community recovery after severe flooding from hurricanes. It is argued that a paradigm shift is needed in the ecological assessment of inland lakes in order to account for the critical role that dormant embryos (egg banks) play in freshwater zooplankton communities. Two challenges to achieving this are that 1. long-term monitoring is expensive and 2. data on dormant zooplankton are rarely available. This study provides an example of how to conduct such studies by leveraging historic data when long-term monitoring is not possible.