The seasonal distribution and abundance of Cladocera and Copepoda in a Minnesota Dolomite Quarry

Ecological information on the annual cycle of distribution and abundance in the cladoceran and copepod species in a south central Minnesota dolomite quarry was collected during 1972-1973. Collections were made biweekly except for the period of winter ice cover when they were taken at approximately monthly intervals. Biological collections were supplemented with physicochemical determinations of water temperature, dissolved oxygen, and pH. Three sampling stations were selected to physically compartmentalize the quarry waters on the basis of vertical and horizontal differences in habitat types. The quarry presented a unique and little studied water body type characterized by seasonally highly stratified, relatively cold, and transparent waters. Aquatic macrophytes, represented only by a small Chara bed, were restricted by steeply sloping basin sides, an extensive algal mat which severely reduced light penetration, and a bottom substrate consisting largely of exposed, dolomite slabs. Data gathered on the microcrustacean species were subjected to analyses inspecting dispersion, vertical distribution, community composition, and species diversity. All species exhibited clumped microdistributions. Most populations were unevenly distributed in both horizontal and vertical components with temporal changes evident in their centers of distribution. Community comparisons showed temporal differences existed in the degree of overlap in species assemblages among the sampling stations. Stability in absolute species composition and in the relative abundances of species in common also showed different seasonal patterns among the stations. Species diversity values were positively related to habitat heterogeneity and, in part, to times of potentially high primary productivity. The component of evenness of relative abundances was more closely correlated to observed diversity values than was that of species richness. Treating Eucyclops agilis and E. speratus as an indistinguishable complex, fourteen species were collected. Of these, five were considered limnetic. Nine species were apparently perennial in occurrence, and of these six attained dominance at some time during the year. Momentary limnetic species dominants usually consisted of one cladoceran and one copepod, while one cladoceran and two copepods generally dominated the littoral assemblages. The quarry microcrustacean community is viewed as a dynamic unit exhibiting seasonal changes in species composition, relative abundances, and spatial distributions. Species presence and distribution seemed closely related to habitat suitability as controlled mainly by seasonal changes in environmental conditions. Changes in abundance, while obviously related to changing environmental and habitat conditions, were also effected by biological interactions, notably predation by other invertebrates.