PS 1-15
Zooplankton community and water quality responses to the cessation of rainbow trout stocking in a Minnesota lake
Lake eutrophication can occur from the ‘bottom-up’, as a result of increased nutrient loading, or from the ‘top-down’, if changes in food web structure at higher trophic levels cascade downward and cause increases in algal biomass. Square Lake, historically the clearest of the more than 950 lakes in the Minneapolis-St. Paul (Minnesota, USA) metropolitan area has experienced a trend of declining water clarity due to increased algal biomass over the past 30 years, but the cause(s) for the eutrophication trend have yet to be conclusively determined. Research over the past decade has investigated potential bottom-up and top-down causes the decline in water clarity, and has identified predation by rainbow trout (annually stocked since the early 1980s) on large-bodied zooplankton grazers (Daphnia pulicaria) to be the likely cause for eutrophication trend. To enable an evaluation of the hypothesis that rainbow trout predation is responsible for the decline in the lake’s water clarity, the Minnesota Department of Natural Resources initiated a three-year moratorium on trout stocking in the fall of 2012. Results presented here compare zooplankton and water quality data from the first year of the moratorium (2013) to two recent years (2010 and 2012) when the lake was stocked with trout.
Results/Conclusions
Results from the first year of the trout-stocking moratorium (2013) show significant changes in Square Lake’s zooplankton community compared to 2012 and 2010. Water column concentrations of the large-bodied Daphnia species (D. pulicaria) were significantly higher throughout the summer and D. pulicaria was also found at higher levels in shallow water samples than in recent years. Conversely, densities of a smaller-bodied Daphnia species (D. mendotae) were lower in 2013 compared to 2012 and 2010. In addition, measures of the lake’s trophic state improved in the summer of 2013 compared to 2012 and 2010: 1) there was a prominent ‘clear-water phase’ from mid-May into June when D. pulicaria reached its highest densities, 2) average water clarity (secchi depth) was greater, 3) surface water algal biomass (Chl a) was lower, and 4) dissolved oxygen in deep water persisted at higher concentrations in 2013 compared to recent years. In summary, results from the first year of trout-stocking moratorium preliminarily support the hypothesis that predation by trout on large-bodied herbivorous zooplankton over the past 30 years is the primary cause for the pattern of eutrophication in Square Lake. Additional data from the next two years will enable a more conclusive evaluation of this hypothesis.