Zooplankton Methods, LOLA 2003 and 2008 - Background

Extensive surveys for each Great Lake occur generally on a rotating five-year schedule. The binational lakewide sampling effort of Lake Ontario for 2003 and 2008 is known as LOLA (Lake Ontario Lower foodweb Assessment). Standard sites on several north-south transects were sampled once during isothermal (April) and twice during stratified (July-September) conditions. Ecosystem indicators included water chemistry, microbial food web, phytoplankton, zooplankton, and benthos. Two ships were used- the EPA’s R/V Lake Guardian and Canadian Coast Guard ship CCGS Limnos. The project was similar in scope and design to the LOTT (Lake Ontario Trophic Transfer) program of 1990 and 1996. This database documents zooplankton indicators including the density, biomass, and average length of each zooplankton species.

Zooplankton sample collection

Two nets were used at each site. An epilimnion tow (referred to as E) sampled the upper 20 m in isothermal conditions or the water above the thermocline during stratified conditions. If water depth was less than 20 m the tow sampled from the bottom minus 2 m to the surface. The net had a 64-micron mesh nylon net and a 0.4 m diameter opening. A total water column tow (referred to as T) sampled from 2 m above the bottom to the surface (for all sites in 2003, for sites < 100 m in 2008) or to a maximum depth of 100 m (for sites > 100 m) in 2008. A 153-micron mesh nylon net with a 0.5 m diameter mouth was used for total water column tows. Both nets were equipped with Rigosha flowmeters (ID # recorded) to estimate the filtering efficiency of the net. Each flowmeter had been calibrated at the beginning of the field season using five vertical tows of the net ring without a net attached from 40 m depth to the surface and found to be consistent with the specifications of 11 turns per meter towed. Volume filtered (in m3) was then calculated by multiplying water depth filtered (from flow meter), and net mouth area (pi(3.14) times net radius (0.2 or 0.25) squared). Time of tow (Eastern Daylight Time) was recorded and identified as DAY, NIGHT, DAWN (within an hour of sunrise), or DUSK (within an hour of sunset). Zooplankton samples were preserved in 70% ethanol after being anesthesized using an effervescent antacid tablet. Duplicate tows of both nets were collected at 10% of the sites. Data from both tows are presented and identified as replicate 1 or 2.

Zooplankton identification, enumeration, and biomass estimation

Zooplankton samples were counted by Dee Geiling (LimnoServices of Burlington, Ontario) in 2003 and Dr. Claudio Tudorancea (Aquatic Bio-services of Kitchener Ontario) in 2008. When present, predatory cladocerans (Bythotrephes and Cercopagis) were separated from the main sample and later counted by Rachel Nagtegaal (Department of Fisheries and Oceans, Canada) in 2003 and Chris Hotaling (Cornell Biological Field Station) in 2008. Subsampling was done in terms of volume (ml) for the main sample or mass (mg) for the predatory cladoceran sample.

The goal was to identify and measure at least 400 animals in each sample. Each species and life stage of cladocera or copepoda were subsampled differently to ensure that average length was based on a sufficient number of measurements (at least 30 but not more than 100). Therefore, rather than a single subsample proportion per sample, the size of the subsamples were species-specific. A proportion (% of total) was calculated for each component before calculations of density and biomass for each taxonomic group. Three different sets of length (mm): dry weight (ug) regression equations (CBFS, EPA, and DFO)were used to estimate the dry weight of individuals (equations in Watkins et al. 2011). Biomass is presented as mg/m3 for the three sets of L-W coefficients. Lengths and calculated weights of individual zooplankton can be requested from the data owner.

Planktonic veliger larvae of exotic dreissenid mussels were counted in LOLA zooplankton samples but not included in the overall crustacean zooplankton biomass calculations. The veliger data are reported within a separate spreadsheet in this data package. Note that E nets have a fine mesh (64 um) that likely catch most veligers. However, the mesh of T nets (153 um) are larger and many veligers likely pass through. Only tow samples that contained veligers are listed in this spreadsheet, other stations included within the main zooplankton file had no veligers. Average size (um) and volumetric density (#/m3) of veligers are reported.

Measurement errors were screened by comparing lengths to documented size ranges for all species. Egg counts (attached and loose) included in the original data were not incorporated in this package. Life stage of Cercopagis (sex and instar number) are included in the 2003 data but were lumped together under Cercopagis pengoi in 2008.

References

Watkins, J.M., Rudstam, L.G. and Holeck, K. 2011. Length-weight regressions for zooplankton biomass calculations – A review and a suggestion for standard equations. Online: http://hdl.handle.net/1813/24566.

Published research that used previous versions of this dataset

Holeck, K., Watkins, J.M., Mills, E.L., Johannsson, O. Millard, S., Richardson, V., Bowen K. 2008. Spatial and long-term temporal assessment of Lake Ontario water clarity, nutrients, chlorophyll a, and zooplankton. Aquatic Ecosystem Health and Management 11:377-391.

Stewart, T. J., O. E. Johannsson, K. Holeck, W. G. Sprules, and R. O'Gorman. 2010. The Lake Ontario zooplankton community before (1987-1991) and after (2001-2005) invasion-induced ecosystem change. Journal of Great Lakes Research 36:596-605.