THE INFLUENCE OF DROUGHT AND RE-ACIDIFICATION ON ZOOPLANKTON EMERGENCE FROM RESTING STAGES

Abstract

The recovery of biota in lakes damaged by cultural acidification may be influenced by a number of factors, including El Nino-related droughts. Long-term zooplankton records from Swan Lake, Sudbury, Canada, indicated that crustacean species richness temporarily increased from an annual mean of 10 species to 18 species in the year following a two-year drought and subsequent re-acidification of the lake. We hypothesized that this unexpected increase in richness was the result of an emergence of zooplankton from resting stages that were historically deposited in the lake sediments. The drought and re-acidification event resulted in several changes in the lake that may have triggered the emergence of zooplankton, including desiccation of littoral sediments (and the zooplankton resting stages residing there), increased water clarity, increased profundal temperature, and increased oxygen concentration in the bottom waters. Using in situ emergence traps, we investigated the influence of desiccation, light, temperature, and oxygen concentration on the emergence of crustacean zooplankton from resting stages. Responses were species-specific; four taxa had higher emergence when sediments were dried over the winter, the emergence of six taxa was influenced by temperature, and the emergence of three taxa was influenced by light. This suggests that climatic events, such as droughts, that alter the physical and chemical properties of lakes may alter zooplankton communities by triggering the emergence of resting stages residing in the lake sediments. This is particularly significant if, as in Swan Lake, emerging zooplankton are faced with inhospitable water quality. Under this scenario, emergence may act to deplete the egg bank, reducing the number of potential colonists available to repopulate the lake when environmental conditions improve.