Spatio-temporal trends in the density and condition of a secondary consumer, Bythotrephes, in southern Lake Michigan

Abstract

Natural gradients in temperature, nutrient loading, and primary productivity contribute to broad scale regional differences in the food web structure of large lakes, such as Lake Michigan. These factors influence spatial patterns of primary production and resource reliance of high trophic level consumers. Secondary consumers, such as larval fish and predatory zooplankton represent an important intermediate link within large lake food webs, but spatial patterns in their density and physiological condition have not been as thoroughly assessed. To analyze the spatial and temporal condition of secondary consumers in Lake Michigan, we sampled the ubiquitous spiny water flea, Bythotrephes cederströmii during the 2015 Cooperative Science and Monitoring field year in Lake Michigan. Monthly estimates of density, instar frequency, length-at-age, and RNA content of Bythotrephes were compared between the eastern and western shores of southern Lake Michigan. Condition indices differed seasonally between the eastern and western shores. During June-August, Bythotrephes were more abundant and in better condition along the eastern shore of Lake Michigan, but in September, all indices were comparable between transects. While this study focused on a single year, other studies across multiple years have demonstrated consistent upwellings and cooler temperatures along the western shore of southern Lake Michigan relative to the eastern shore. A temporal lag in preferable environmental conditions along the western shore may be common and lead to delayed growth and reduced physiological condition of secondary consumers, such as predatory zooplankton and late spring emerging larval fish.