North Temperate Lakes LTER: Zooplankton - Madison Lakes Area 1997 - current

Zooplankton samples for the 4 southern Wisconsin LTER lakes (Mendota, Monona, Wingra,
Fish) have been collected for analysis by LTER since 1995 (1996 Wingra, Fish) when the
southern Wisconsin lakes were added to the North Temperate Lakes LTER project. Samples are
collected as a vertical tow using an 80-micron mesh conical net with a 30-cm diameter
opening (net mouth: net length ratio = 1:3) consistent with sampling conducted by the
Wisconsin Dept. Natural Resources in prior years. Zooplankton tows are taken in the deep
hole region of each lake at the same time and location as other limnological sampling;
zooplankton samples are preserved in 70% ethanol for later processing. Samples are usually
collected with standard tow depths on most dates (e.g., 20 meters for Lake Mendota) but not
always, so tow depth is recorded as a variate in the database. Crustacean species are
identified and counted for Mendota and Monona and body lengths are recorded for a portion of
each species identified (see data protocol for counting procedure); samples for Wingra and
Fish lakes are archived but not routinely counted. Numerical densities for Mendota and
Monona zooplankton samples are reported in the database as number or organisms per square
meter without correcting for net efficiency. [Net efficiency varies from a maximum of about
70% under clear water conditions; net efficiency declines when algal blooms are dense
(Lathrop, R.C. 1998. Water clarity responses to phosphorus and Daphnia in Lake Mendota.
Ph.D. Thesis, University of Wisconsin-Madison.)] Organism densities in number per cubic
meter can be obtained by dividing the reported square-meter density by the tow depth,
although adjustments for the oxygenated depth zone during the summer and early fall
stratified season is required to obtain realistic zooplankton volumetric densities in the
lake's surface waters. Biomass densities can be calculated using literature formulas for
converting organism body lengths reported in the database to body masses. Sampling
Frequency: bi-weekly during ice-free season from late March or early April through early
September, then every 4 weeks through late November; sampling is conducted usually once
during the winter (depending on ice conditions). Number of sites: 4 Note: for a period
between approximately 2011 and 2015, a calculation error caused density values to be
significantly greater than they should have been for the entire dataset. That issue has been
corrected.