The zooplankton community during the introduction and loss of a zooplanktivorous fish species in a temperate lake
Lake Minnewaska, located within Minnewaska State Park, southeastern New York, is a historically clear, acidic, and until recently, fishless lake. In 2008, a minnow species, Notemigonus cryosoleucas, was introduced into the lake and has caused a trophic cascade in Lake Minnewaska as the lake transitions from acidic to neutral and oligotrophic to mesotrophic. N. cryosoleucas have been feeding on the zooplankton, who in turn feed on the phytoplankton found in the lake. With decreased predation, phytoplankton numbers have increased, shifting a once clear, blue lake to opaque and green with an increase in phytoplankton biomass and decrease in water clarity. In 2012, a new species of piscivorous fish, Micropterus salmoides (Largemouth Bass) was unintentionally introduced. Over the past two summers (2013 and 2014), we measured fish population size via electrofishing and mark and recapture techniques. We examined the change in fish populations, zooplankton size and density, and chlorophyll a as a proxy for algal biomass. Both zooplankton densities and chlorophyll concentrations were also recorded in neighboring lakes, Awosting and Mohonk, to serve as controls. Awosting is acidic (pH ~5), fishless and oligotrophic. Mohonk is slightly basic (pH ~7.5), stocked with fish and mesotrophic.
Following the introduction of N. cryosoleucas, phytoplankton biomass in Minnewaska has been more than three times that of the fishless Awosting, likely due to the suppression of zooplankton populations. Over the two years sampled, N. cryosoleucas populations went from an estimate of 15,000 in 2013 to zero in 2014 as N. cryosoleucas was neither captured nor observed in the lake. The extirpation of N. cryosoleucas was likely due to predation by M. salmoides as evidenced by the increase in M. salmoides average body length and 60% increase in population size. Although not observed, another factor could have been a major winter fish kill. Across all lakes, the density per liter of zooplankton has decreased from 2013 to 2014. However, in Minnewaska only, the average size of zooplankton has increased, due to the loss of the N. cryosoleucas population, who predate upon the larger zooplankton. The introduction of the N. cryosoleucas had an immediate effect on the Minnewaska ecosystem. Since their disappearance, the lake has increasing water clarity with some reverting back to pre N. cryosoleucas state; however, the lake remains neutral with mesotrophic nutrient concentrations.