Water Resources Research Center, University of Minnesota
Newsletter or Bulletin
Although data were collected for two years, 1970 and 1971, the major portion of this research was carried out the second year. This research took place in western Lake Superior and most of the data were collected at two stations, Larsmont and Stony Point, which were twenty miles northeast of Duluth. Each of these stations included two sites, one a half mile and the second two miles from shore. The other area where samples were collected was at the Little Marais and Sugar Loaf Cove stations, some 70 miles north of Duluth.
The major purposes of this research were to study the productivity and the vertical, seasonal and horizontal distribution of the crustacean zooplankton population in western Lake Superior. A limited study of the biology of the copepod, Limnocalanus macrurus, was also conducted.
Productivity at the Larsmont and Stonv Point area averaged 323 crustaceans per 100 liters of water, and 60 grams per square meter (based on a fifty meter water column). Productivity at the Little Marais and Sugar Loaf Cove area averaged 95 crustaceans per 100 liters and 37 grams per square meter. In general, productivity decreased as the depth increased from zero to 50 meters. If a thermocline was present, then both the toted number of crustaceans and the biomass became relatively scarce below twenty meters. Cladocerans were most frequent1y found in the upper ten meters of the water column whereas copepods were present at every level. Adult copepods were usually heavier than adult cladocerans and
it was not unusual to find the mean weight of an organism at 50 meters
ten or more times that of one at five meters.
Productivity at the Larsmont and Stony Point area was bimodal during the sampling season; the first peak occurred in July and contained primarily copepods and the second, which was the seasonal maximum, occurred in September and contained both copepocls and cladocerans. Surface water temperatures were also bimodal during the sampling season; the peak recorded in July was thirteen degrces centigrade and sixteen degrees was reached in September. The cladoceran, Bosmina, became abundant after the water temperature reached five degrees in July, Another cladoceran, Dapnia,
Replaced Bosmina in September when the water temperature was about eleven degrees. Ephippia, the overwintering stage of Daphnia first appeared in late August. Three copepods, Diaptomus, Limnocalanus, and Cyclops were present during most of the sampling season. Limnocalanus was present at all depths from June to early August, but was most numerous at ten meters. When the water temperature warmed above twelve degrees, the population shifted downward and was usually below the thermocline during the davlight hours. At this time, they were most abundant at 40 meters, The copepod, Epischura, was numerous in the upper lavers after the water warmed above eleven degrees.
Productivity differences were found between the various sites and
stations. These differences point to the lack of homogeneity in the
horizontal distribution of the crustacean zooplankton population and
support the phenomenon of “zooplankton patchiness". Productivity levels
at the Little Marais and Sugar Loaf Cove area were from one-third to two-
thirds of those at Larsmont and Stony Point. The Larsmont station was
slightly more productive than Stony Point. The Stony Point inshore site
was slightly more productive than the offshore site. The period of maximum productivity occurred at the Larsmont inshore site amd at both Stony Point sites in September. Maximum productivity was recorded at the Larsmont offshore site in July. A phytoplankton bloom was observed at the Stony Point station on July 20, 1971, but was not seen on the same day at the Larsmont station.
Limnocalanus macrurus contrihuted to the greatest percentage of the crustacean biomass (often more than 90 percent) at depths 30, 40 and 50 meters in western Lake Superior. The male to female ratio established was 1:2. The mean lengths of mature males and females were, 2.09 and 2.16 millimeters, respectively. The length-weight correlation was: Dry weight (mg/100) = 3.31 length (mm) - 2.95. Two cladocerans, new to Lake Superior, were identified. They were: Alona guttata Sars and Holopedium gibberum Zaddach.
Conway, J.B. Odlaug, T.O. Olson, T.A. Ruschmeyer, O.R. 1973. Distribution, Composition and Biomass of the Crustacean Zooplankton Population in Western Lake Superior. Water Resources Research Center.
Water Resources Research Center
Conway, J.B.; Odlaug, T.O.; Olson, T.A.; Ruschmeyer, O.R..
Distribution, Composition and Biomass of the Crustacean Zooplankton Population in Western Lake Superior.
Water Resources Research Center, University of Minnesota.
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