Seasonal Daphnia biomass in winterkill and nonwinterkill glacial lakes of South Dakota

We compared the seasonal biomass (??g dry weight/L) and body size of Daphnia pulex populations between South Dakota lakes that experienced nearly complete winterkill (n = 2) and those that did not experience fish winterkill the previous winter (n = 2). In spring (March-May), D. pulex biomass was substantially lower in winterkill lakes (0.4-1.9 ??g/L) than in nonwinterkill lakes (13.8-129.4 ??g/L). In summer months (June-July), D. pulex biomass increased in all lakes but was generally higher in winterkill lakes (332.1-469. 3 ??g/L) than in nonwinterkill lakes (88.6-204.2 ??g/L). By September, D. pulex biomass was substantially higher in winterkill lakes (243.8 ??g/L) than in nonwinterkill lakes (1.0 ??g/L). The mean size of D. pulex increased significantly from March to August in winterkill lakes but not in nonwinterkill lakes. The seasonal differences in D. pulex biomass and size may have been related to size-selective predation by planktivorous fishes because the ratio of spine length to body length in D. pulex was significantly lower in winterkill lakes (mean, 0.34) than in nonwinterkill lakes (mean, 0.43). Moreover, seasonal patterns in chlorophyll-a biomass were inversely related to D. pulex biomass. The production and subsequent hatching of resting eggs may be important for recolonization of Daphnia populations after severe winter conditions. The survival and growth of zooplanktivorous fish that are stocked to repopulate winterkill lakes may be increased by delaying stocking until Daphnia populations recover from severe winterkill.