LAKE OUTLETS AND THE DISTRIBUTION OF FILTER FEEDERS - AN ASSESSMENT OF HYPOTHESES

Dense assemblages of filter-feeding invertebrates at lake outlets, and their abrupt decline downstream, have been attributed to gradients in food (seston: quantity, quality, particle size). temperature, flow regime (discharge, turbulence, depth), substrate, competition, predation, and to the colonization cycle. In this paper we critically review the evidence for the several hypotheses. Empirical support is strongest for the three food hypotheses and for those addressing depth, competition, and the colonization cycle. The hypotheses are not independent because the capture of seston is dependent on certain environmental factors and the use of filtering sites. Certain species appear to be outlet specialists. More experimental tests of the hypotheses would not only clarify the population ecology of take-outlet filter feeders but also allow predictions to be made about food limitation and disturbance effects on the species assemblage. Lake-outlet habitats provide opportunities to test models of stream ecosystem function.