AN OLIGO-MESOTROPHIC LAKE PARTIALLY RETURNS TO PAST TROPHIC STATUS AND ECOSYSTEM STRUCTURE UPON LOSS OF AN INTERMEDIATE TROPHIC LEVEL

The overall water quality of a freshwater lake ecosystem can be influenced by interactions in its food web; however less is understood about the radiating effects from trophic level loss, especially intermediate levels. Historically fishless, Lake Minnewaska, an oligo-mesotrophic lake in NY, has been in ecological flux following the loss of an intermediate trophic level. In 2008 Golden Shiners (Notemigonus crysoleucas, hereafter GS), a minnow species, was unintentionally introduced, followed by Largemouth Bass (Micropterus salmoides, hereafter LMB) in 2011. The introduction of fish caused a trophic cascade, leading to algal blooms and decreased water clarity. GS were extirpated between fall 2013 and spring 2014, and as a result the LMB population has 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, water transparency to increase, and hypolimnetic anoxia to decrease. From 2014 - 2016, we measured Secchi depth as a proxy for water transparency, dissolved oxygen profiles from the water surface to 20 m, and collected zooplankton samples throughout each ice free season.

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 larger zooplankton are more abundant since predation pressure was lifted. 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 hypoxia 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 hypoxia 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 management intervention is needed.