THE STRUCTURE OF THE PLANKTONIC FOOD-WEB IN THE ST.-LAWRENCE GREAT-LAKES

The structure of the planktonic food-web was studied during the spring (April/May) and summer (August) periods in 1993 to 1995 at twelve sta tions located in-the offshore region of all five Great Lakes. All comp onents of the planktonic food-web were collected from the same water s ample (with the exception of crustaceans), counted microscopically, co nverted to carbon units, and averaged over the euphotic zone. Due to p hosphorus load reductions and the impact of non-indigenous mussels in the lower lakes, physical/chemical characteristics of the lower lakes are becoming similar to those in the upper lakes. Spring total phospho rus and euphotic zone depth were relatively similar among all the stat ions (except western Lake Erie), ranging from 3 to 7 mu g/L and 21 to 26 m, respectively. During the summer total phosphorus concentrations were more variable, but ranged between 4 to 10 mu g/L at all stations except western Lake Erie. Planktonic biomass was correlated with total phosphorus concentration. Within a season, the structure of the plank tonic food-web was remarkably similar among all stations across all th e lakes. Of the seventeen food-web structure parameters examined, only two exhibited significant differences among stations during the sprin g isothermal period; only four parameters exhibited significant differ ences among stations during the summer. Small plankton were very abund ant in all the lakes. Picoplankton (0.2 to 2.0 mu m) biomass was appro ximately equal to the combined biomass of nannoand microplankton (2 to 200 mu m). For microorganisms (all organisms except crustaceans) auto trophic: heterotrophic ratios averaged 1.3 (spring = 1.1, summer = 1.5 ). The heterotrophic microorganism community was comprised of bacteria (mean = 65%), protozoans (mean = 32%), and rotifers (3%). Even though zebra mussel veligers were found in all the lakes except Lake Superio r, their contribution to microorganism biomass never exceeded 1%. Due to seasonal variation in crustacean abundance, the mean contribution o f major functional groups varied by season; producers (autotrophs), de composers (bacteria), micrograzers (protozoans and rotifers), and meso grazers (crustaceans) constituted 40%, 30%, 11%, and 19% of total plan ktonic carbon, respectively, during the spring, and 32%, 15%, 9%, and 43%, respectively, during the summer. The overall similarity in the st ructure of the planktonic food-web across all stations in the Great La kes was attributed to the strong influence of abiotic factors.