Site Description:
Our three study lakes are located within 10 km of each other on the Shawangunk Ridge, New York, USA (Table 1). Lake Awosting has been fishless for 100 years due to geographic isolation and acidic conditions. Fish introductions to Lake Minnewaska over the past decade have altered its food web interactions and ecosystem function (Richardson et al., 2016; Richardson et al., 2019). Currently, Micropterus salmoides (largemouth bass) is the only fish species present in Minnewaska following the extirpation of a previous minnow species in 2013 (Charifson et al., 2015). Both Minnewaska and Awosting have recovered from acidic conditions (Richardson et al., 2018) and have pH > 6. Mohonk Lake has been regularly stocked with popular freshwater sport fish since 1920 (Smiley and Huth, 1983) and currently has a diverse fish community including planktivores such as Notemigonus crysoleucas (golden shiner) and piscivores (e.g., M. salmoides and Salvelinus namaycush, lake trout).
Zooplankton community composition sampling:
To assess zooplankton community composition, we quantified density and biomass from 10-m vertical tows that were collected with an 80-um Wisconsin plankton net. Vertical tows were collected mid-day (between 1000 h and 1400 h) during the summers of 2013 to 2016 (n=3 to 5 times per year in Minnewaska only), 2017 (n=4 for each lake) and 2018 (n=3 for Minnewaska and Mohonk; n=2 for Awosting). Zooplankton were narcotized with Alka-Seltzer and preserved with 70% ethanol following collection in the field. Once preserved, we counted and measured both crustaceans and large rotifers with a compound microscope (40x to 100x magnification), counting 100 total individuals per sample. Cladocerans were identified to family, copepods to order, and rotifers to genus. Because the depth at sampling locations for Awosting, Minnewaska, and Mohonk were 25, 22, and 12 m, respectively, 10-m tows excluded the zooplankton community residing deep in each lake. Therefore, these data were only used to examine how zooplankton community composition from 0-10 m differs across lakes, with a focus on diurnal epilimnion and metalimnion populations (see Richardson et al. 2019).
Zooplankton vertical distribution sampling:
To assess zooplankton vertical distribution, we collected zooplankton in Minnewaska, Mohonk, and Awosting during three separate sampling campaigns (19-20 Jul, 24 Aug, and 19-20 Sep 2017) from four discrete depths to estimate their vertical distribution in the water column. The four depths were: 1 m, 5 m, 8 m, and 15 m (12 m for Mohonk given shallower maximum depth; Table 1). These depths were chosen based on light, temperature, and oxygen conditions in each lake: 1 m was where light and temperature were highest, 5 m (just above the thermocline) was where DO was generally high, 8 m (below the thermocline) was where light and temperature were lower, and 15 m (or 12 m for Mohonk) was where light, DO, and temperature were lowest. The deepest sampling depths were chosen based on long-term sampling protocols at each lake (Richardson et al., 2019).
In all three lakes, we used a 1-L Van Dorn bottle to collect zooplankton from ~10 L of lake water at each depth. Zooplankton were then filtered through a 125-mm mesh sieve, narcotized with Alka-Seltzer, and preserved in 70% ethanol. Following field preservation, we counted and measured both crustaceans and large rotifers with a compound microscope (40x to 100x magnification) to estimate density and biomass. For most samples (n = 25), we counted ~100 total individuals, crustaceans and rotifers combined; for the remaining samples with lower densities (n = 11), we counted [?] 70% of the concentrated sample (stopping after 26 aliquots of 1.2 mL). Cladocerans were identified to family, copepods to order, and rotifers to genus.
References:
Charifson, D. M., Hith, P. C., Thompson, J. E., Angyal, R. K., Flaherty, M. J. and Richardson, D. C. (2015) History of Fish Presence and Absence Following Lake Acidification and Recovery in Lake Minnewaska, Shawangunk Ridge, NY. Northeastern naturalist 22, 762-81.
Richardson, D. C. et al. (2019) Serial Introductions Modify a Trophic Cascade and Partially Mitigate Changes in Lake Ecosystem Structure. Freshw. Sci. 38, 642-653.
Richardson, D. C. et al. (2018) Watershed Management and Underlying Geology in Three Lakes Control Divergent Responses to Decreasing Acid Precipitation. Inland Waters 8, 70-81.
Richardson, D. C., Charifson, D. M., Stanson, V. J., Stern, E. M., Thompson, J. E. and Townley, L. A. (2016) Reconstructing a Trophic Cascade Following Unintentional Introduction of Golden Shiner to Lake Minnewaska, New York, USA. Inland Waters 6, 29-33.
Smiley, D. and Huth, P. C. (1983) Fish Disappearance in the Northern Shawangunks between 1862 and 1982. Research Report. Mohonk Preserve, New York.
