Comparative Ecology of Pontoporeia Hoyi Populations in Southern Lake Michigan: The Profundal Region Versus the Slope and Shelf Regions

Abstract

This study investigates Pontoporeia hoyi ecology in the profundal region (approximated by a 97-m station) of southern Lake Michigan and compares these results with previous investigations conducted in the slope and shelf regions of the lake. Pontoporeia typically attains its maximum abundance in the slope region, suggesting that this region of the lake is the most favorable for amphipod growth and survival. Profundal and slope P. hoyi exhibited little seasonal variation in mean size while shelf-region populations exhibited strong seasonal variation in mean size. Deepwater sculpins, a major predator on profundal populations of P. hoyi, selectively consumed the largest amphipods: mean size consumed was 6–7 mm. The relative sparsity of larger (> 5 mm) P. hoyi in the profundal, slope, and shelf-regions of the lake may arise from intense size-selective fish predation on this size class of amphipods. Gut content studies revealed that profundal P. hoyi populations feed intermittently. Such feeding behavior was subtly different from that previously observed for slope P. hoyi populations: profundal populations feed more continuously but less intensively than slope populations. Regional differences in feeding behavior may be related to differences in food regime and to predation avoidance strategies. Pontoporeia hoyi apparently is capable of inhabiting a broad range of depth regimes by modifying its physiology (reproductive cycles, generation time) and behavior (feeding, motility) to adjust to spatial variations in temperature, food level, and predation.