LAKE SUPERIOR FOOD WEB – APOSTLE ISLANDS ONRW

Five open lake sites (Outer, Sand, Rocky, and Long Islands, Stockton Island at Presque Isle), and three lagoon sites (Outer, Stockton, and Michigan Island Lagoons) were sampled monthly over the summer of 1996. The water quality parameters of temperature, pH, conductivity, dissolved oxygen concentration, turbidity, total dissolved solids, Secchi disc transparency and light transmission were measured on-site. Triplicate water samples for the determination of phosphate, ammonia, silica, nitrate/nitrite, and chlorophyll a were collected at each site for analysis in the lab. Triplicate zooplankton and benthos samples were obtained and immediately preserved with formalin for later identification and enumeration. The open water sites, with the possible exception of Long Island, could be classified as pelagic-type environments and all can be considered to be oligotrophic with respect to total nitrogen, total phosphorus, and chlorophyll a concentration as well as Secchi disc transparencies. Dissolved oxygen concentrations were at saturation levels at all depths at all three sampling periods. Concentrations of nutrients, particularly silica, varied little among the open water sites during a given sampling period. Zooplankton samples obtained from the open water sites indicate distinct warm nearshore and cool offshore species assemblages as well as mixtures in the surge areas that move through the island channels. The exotic cladoceran, Bythotrephes cederstroemi was found at four of the sites during the August sampling. A variety of parthenogenic instar stages were found suggesting that Bythotrephes is either hatching in the warmer, shallower waters off of Long Island and is being moved to other islands or is being transported to the Apostle Islands from elsewhere. Benthic substrate types and low densities of benthic organisms suggest the presence of very strong scour zones between islands. All of the lagoon sites had low or nonexistent benthic organism populations and warm, highly colored acidic waters. All sites had very low levels of soluble nutrients. The variation in water quality parameters and zooplankton species composition among these sites was much greater than was observed between the open lake sites.