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:
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.
