Rotenone Decreases Hatching Success in Brine Shrimp Embryos by Blocking Development: Implications for Zooplankton Egg Banks

Sediment from lake with missing egg bank is toxic to hatchlings of model zooplankton: A reason to consider obligate dormancy in toxicological assessment

Over the last 60 years, valuable progress was made in the standardization of environmental monitoring with model zooplankton. However, obligate dormancy in zooplankton life cycles is not yet considered in standardized toxicology methods. Most zooplankton from coastal and inland waters use dormancy as a critical ecological strategy, and exposure to toxicants during dormancy or resurrection from dormancy alters developmental patterning and hatching success. The present study accounts for this by using both standardized and novel toxicology assays to assess the impacts of coal ash contaminated sediments and water on development, hatching, and survivorship of model zooplankton. The results demonstrate that standardized assays with rotifer and cladoceran models detect no toxicity in surface water and sediment pore water from Lake Sutton, North Carolina, USA. By contrast, novel toxicity assays with cladoceran and anostracan models demonstrate that development and larval survivorship are negatively impacted by Lake Sutton water and sediment. Embryos of Artemia franciscana display developmental patterning and hatching aberrations that match those observed in previous studies with metals when hatched in filtered surface water or pore water after a period of anoxia-induced dormancy. Larval survivorship in Daphnia magna and A. franciscana also decreases when post-diapause embryos are hatched in the presence of sediment. The effects of whole sediment on larval survivorship are not explained by coal ash impacts on water pH. These data provide an explanation for the missing egg bank and historic community restructure in Lake Sutton. The data also demonstrate a need for standardized assays that include dormant life stages.

The ultrastructure of resurrection: Post-diapause development in an Antarctic freshwater copepod

The copepod, Boeckella poppei, is broadly distributed in Antarctic and subantarctic maritime lakes threatened by climate change and anthropogenic chemicals. Unfortunately, comparatively little is known about freshwater zooplankton in lakes influenced by the Southern Ocean. In order to predict the impact of climate change and chemicals on freshwater species like B. poppei, it is necessary to understand the nature of their most resilient life stages. Embryos of B. poppei survive up to two centuries in a resilient dormant state, but no published studies evaluate the encapsulating wall that protects theses embryos or their development after dormancy. This study fills that knowledge gap by using microscopy to examine development and the encapsulating wall in B. poppei embryos from Antarctica. The encapsulating wall of B. poppei is comprised of three layers that appear to be conserved among crustacean zooplankton, but emergence and hatching are uniquely delayed until the nauplius is fully formed in this species. Diapause embryos in Antarctic sediments appear to be in a partially syncytial mid-gastrula stage. The number of nuclei quadruples between the end of diapause and hatching. Approximately 75% of yolk platelets are completely consumed during the same time period. However, some yolk platelets are left completely intact at the time of hatching. Preservation of complete yolk platelets suggests an all-or-none biochemical process for activating yolk consumption that is inactivated during dormancy to preserve yolk for post-dormancy development. The implications of these and additional ultrastructural features are discussed in the context of anthropogenic influence and the natural environment.

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

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.

Embryos of an Antarctic zooplankton require anoxia for dormancy, are permeable to lipophilic chemicals, and reside in sediments containing PCBs

Zooplankton in Antarctic maritime lakes face challenges imposed by anthropogenic chemicals. Studies on temperate species suggest that lipophilic chemicals will accumulate in dormant embryos of Antarctic zooplankton and decrease hatching success, thereby threatening centuries of accumulated genetic diversity that would increase population resilience in the face of climate change. We evaluated the potential for lakes to act as sinks for legacy pollutants in the maritime Antarctic by testing sediments for polychlorinated biphenyls (PCBs) previously identified in soil, flora and fauna of lake catchments. Direct tests of embryo permeability to chemicals are confounded by potential adhesion of chemicals to the embryo surface and limited biomass available. Therefore, in order to assess the potential for lipophilic chemicals to penetrate and passively accumulate in dormant embryos of Antarctic lacustrine zooplankton, we evaluated the effect of anoxia on post-diapause development in the calanoid copepod, Boeckella poppei, and then used chemical anoxia induced by rotenone as a reporter for permeability of these embryos to moderately lipophilic chemicals. The data presented demonstrate that embryos of B. poppei from Antarctic lake sediments will passively accumulate moderately lipophilic chemicals while lying dormant in anoxic sediments. Implications for legacy POPs in sediments of Antarctic maritime lakes are discussed.