Interactions within a lake’s food web have an important influence on the system as a whole; however, less is understood about the radiating effects from trophic level loss, especially intermediate levels. Historically fishless, Lake Minnewaska, a mesotrophic lake in NY has been in ecological flux recently with the loss of an intermediate trophic level. In 2008, a minnow species, Golden Shiners (GS), were unintentionally introduced, followed by the Largemouth Bass (LMB) in 2011. The introduction of fish caused a trophic cascade, resulting in algal blooms and decreasing water clarity. GS were extirpated between fall 2013 and spring 2014 and as a result, the LMB have been experiencing a gradual decline since 2014. We predicted that other ecosystem variables would be affected following the disappearance of the intermediate trophic level. With the loss of their predator, we expected zooplankton to increase in both density and individual size, increasing water transparency, and decreasing hypolimnetic anoxia. From 2014 - 2016, we collected zooplankton samples, Secchi depth as a proxy for water transparency, and dissolved oxygen profiles from the surface of the water to 20 m throughout each ice free season.
Results/Conclusions
Average density of zooplankton has decreased from 23.4 individuals/L in 2013, to 4.7 individuals/L in 2016. The mean individual size of Daphnia has increased from 0.35 mm in 2013 to 1.02 mm in 2016, suggesting that the larger zooplankton are more abundant once free from predation. After the loss of GS, algal biomass decreased resulting in recovering water clarity. The mean summer Secchi depth has increased from 3.5 m in 2012 to 6.3 m in 2016. Conversely, hypolimnetic anoxia has significantly increased throughout the lake as a likely result of accumulating organic matter in the lake sediments during 2010 - 2012. Though algal biomass has been decreasing since 2012, the anoxia levels from the bloom have yet to show the signs of recovery to pre-GS conditions. These results help elucidate the importance and effects of the recent changes to the ecosystem. We intend to use this data to determine if recovery to conditions prior to fish introduction can be achieved naturally or if conservation measures need to be taken.