Recent ecological trends in lower trophic levels of the international section of the St. Lawrence River: a comparison of the 1970s to the 2000s

Abstract

Changes from the 1970s to 2000s in phosphorus and chlorophyll levels, water transparency, zooplankton, and benthic communities in the upper, International Section of the St. Lawrence River were evaluated using trend data from limnological surveys. The influence of Lake Ontario as a source for riverine production was evident in the upper river. Total phosphorus levels from 1976 to 1978 (average ~20 μg/l) sampled during a period of nutrient pollution declined (to 6–7 μg/l) following abatement. As expected, water transparency indicated by summer Secchi depths showed an opposite response increasing from ~3.5 m in the 1970s to >10 m by 2003 but declined to 6–7 m in more recent years. Zooplankton communities have experienced declines in overall densities, and the community has changed but its primary components, Bosmina and Diacyclops remain. Ceriodaphnia lacustris abundance declined substantially from the 1970s to the recent time period while Chydorus sphaericus increased and Eurytemora affinis and Cercopagis pengoi had first appearances. In contrast, benthic invertebrate biomass increased substantially between time periods. Increases in families and occurrence of gastropods were observed, but the primary components–chironomids, amphipods, and oligochaetes–were consistent between the periods with the exception of dreissenid mussels. Dreissena was dominated by D. bugensis, and both D. bugensis and D. polymorpha show evidence of recent declines. Conversely, high abundance of round goby (Apollonia melanostomus) has promoted new trophic pathways. Downstream attenuation in nutrients and chlorophyll, and increased transparency suggest continued strong effects of Lake Ontario on the downstream river environment, but increased energy in the benthos has likely promoted greater in situ production.