ENVIRONMENTAL GRADIENTS AND THE HORIZONTAL DISTRIBUTION OF MICROCRUSTACEANS IN LAKES

1. The assemblage of suspension-feeding microcrustaceans in lakes chan ges along a habitat gradient from nearshore to offshore. This gradient of microcrustaceans was explored in relation to differences in macrop hytes and the associated changes in water chemistry, food resources an d types of predators. 2. Some microcrustacean species were littoral or limnetic specialists, while others changed their distribution along t his horizontal habitat gradient on a diel or seasonal basis. Distribut ion patterns were similar in a lake and a pond which differed in exten t of macrophyte habitat. 3. There was a large shift in the composition of sestonic food, indicating heterotrophic seston nearshore and more autotrophic seston offshore. Sit-and-wait predators of microcrustacean s (e.g. Enallagma spp.) dominated nearshore and cruising predators (e. g. Leptodora kindtii Focke) were more common offshore. 4. A reciprocal transplant experiment revealed that littoral specialists could surviv e equally well when fed littoral or limnetic seston, while limnetic sp ecialists performed poorly when fed littoral seston. Food resources ma y be important in determining where some microcrustacean species live along this horizontal habitat gradient.