DOI: 10.6073/pasta/ec3d0186753985147d4f283252388e05
title: Madison Wisonsin Lakes Zooplankton 1976 - 1994

description: Zooplankton of the four Yahara River chain of lakes (Mendota, Monona, Waubesa, and Kegonsa) were sampled and analyzed by the Wisconsin Department of Natural Resources (WDNR) during 1976 to 1994 as part of a long-term limnological sampling program directed by Richard C. Lathrop. Methods for zooplankton sampling and laboratory analyses were previously summarized for Lake Mendota (Lathrop 1998); methods were similar for the other three Yahara lakes although Waubesa and Kegonsa were only sampled for zooplankton during 1976-1985. In general, zooplankton were sampled at the deepest location of each lake by pulling a conical net vertically through the water column (i.e., vertical tow sample) during 1976-1994. Sampling was usually done on a biweekly schedule during the open water period and at least once through the ice at the deepest region of each lake, although in some years lakes Waubesa and Kegonsa were only sampled monthly during the open-water period. On a few dates during high winds or during winter, sampling was conducted in slightly shallower water than at the deepest lake location. Tow depths for Mendota and Monona are included in the data set. Tow depths for Waubesa and Kegonsa have not been transcribed yet, but generally ranged between 9.5-10.0 meters and 7.5-8.0 meters, respectively. The conical zooplankton net had a 15-cm diameter opening ("small net") and a net filtering area to opening area ratio of about 11. The net was made of Nitex screening with a mesh size of 75-80 um (#20 net) for all years except for 1976 when the mesh size was about 153 um (#10 net). The smaller-meshed net was initially chosen so that rotifers would be quantitatively collected along with crustacean zooplankton. For the small net, a vertical tow sample was taken from approximately 0.5 m off the lake bottom. The net was pulled to the lake surface at approximately 0.3 m per sec. After the net was raised and the sides rinsed, the sample was transferred to a bottle and preserved with formalin in early years, sugared formalin for a few years, and then formalin plus seltzer water was the preservative used after 1986. For each zooplankton sample obtained by the small net, organisms were identified to species and enumerated in three separate 1-mL subsamples obtained by a Hensen-Stemple pipette. Subsamples were taken directly from the sample bottle (sample volume ca. 150-250 mL), transferred to a 1-mL Sedgewick-Rafter cell and counted using a compound microscope. For each subsample, individual body lengths for all Daphnia encountered were recorded using an eyepiece micrometer; 5-10 organisms of other cladocerans and various life stages of copepods (i.e., nauplii, copepidites, and adults) were also measured. Based on specified size criteria, juvenile and adult Daphnia of each species were recorded separately. These size criteria were given in Luecke et al. (1990) for samples collected in 1976-1989. The criteria varied in subsequent years but were generally 1.3 mm for D. pulicaria, 1.2 mm for D. mendotae, the most common Daphnia species encountered. If smaller Daphnia were encountered with eggs in their brood chamber, then the adult size was adjusted. Thus, the distinction of adult versus juvenile Daphnia densities recorded in each sample must be viewed as only an index of the two age groups. Calanoid and cyclopoid copepodites were counted separately as two distinct groups without regard to species or life stages. All nauplii were counted as a single group with no distinction made between calanoid and cyclopoid species. All densities and length measurements in the three subsamples were then averaged for each species' life stage. Numerical density estimates (N per meter squared) for each species or zooplankton group as reported in the data set were then computed by multiplying the average subsample count by the subsampling dilution factor (i.e., volume of sample in bottle) and then dividing by the net opening area (0.01767 meter squared). Biomass estimates for each species or zooplankton group (gram per meter squared) can then be computed using the average length (mm) recorded in the data set using length-weight relationships published in the literature. One caveat for interpreting the zooplankton data for 1976-1994 is that the net efficiency of the vertical tow net used is less than 100%, and also variable depending on the amount of algae clogging the net (Lathrop 1998). Under clear water conditions (Secchi disc greater than 8 m), the net efficiency of the small net was determined to be approximately 58%. However, net efficiency was only about 42% during a period with moderate blue-green algal densities (Secchi disc 1.8 m). The relatively small mesh size (75-80 um) of the net was initially chosen in order to capture rotifers, which are recorded in the database. Beginning in 1991 a larger 30-cm diameter ("large") net with a 75-80 um mesh size was also used, which became the standard sampling net used in 1995 coincident with the lake sampling program being conducted by the North Temperate Lakes Long-Term Ecological Research (NTL-LTER) Project. After 1994, the "small" net was no longer used for sampling zooplankton on the Yahara lakes. This data set also contains large net data for years 1991 through 1994 for Mendota and for 1994 for Monona. Leptodora counts from the small net are available for Mendota (1976-1989), Monona (1976-1987), Kegonsa and Waubesa (1976-1985). Leptodora counts from the large net are available for Mendota (1991-1993). All Leptodora counts were performed on the entire sample by dumping the sample bottle into a container. Individual length measurements for some zooplankton sampled with the small net are available for Mendota (1988-1989, 1992-1994) and Monona (1994). Individual length measurements for zooplankton sampled with the large net are available for Mendota (1991-1993). Daphnia eggs counts (number of eggs per adult daphnia in the sample) from samples taken with the small net are included for Mendota (1976-1991), Monona (1976-1993 except 1989), and Kegonsa and Waubesa (1976-1985). Egg counts represent the combined number of free eggs found in each subsample plus eggs still remaining in adult Daphnia as many times adult Daphnia had eggs expelled from their brood chambers once the preservative was added (especially in years when the preservative was just formalin). Once the total number of eggs for either D. pulicaria or D. mendotae was counted, then the estimate of the number of eggs per adult Daphnia was calculated while recognizing that the criteria for separating adult versus juvenile Daphnia is not without error. Thus, the number of eggs per adult number is best used as an index to determine if the Daphnia population was increasing or not growing due to food limitation. Number of sites: 4 Sampling Frequency: varies