RELATIVE IMPORTANCE OF ALGAE AND EMERGENT PLANT LITTER TO FRESH-WATER MARSH INVERTEBRATES

We test the hypotheses that (1) algae are a major food source for marsh invertebrates and (2) plant litter serves mainly as cover for invertebrates and a substrate for algal growth rather than as a direct source of nutrition. Twelve enclosures (5-m diameter) were deployed in May in a nutrient-poor marsh of the Interlake region of Manitoba, six of which were fertilized to increase algal levels. The hardstem bulrush (Scirpus lacustris glaucus) litter present was replaced with a nonnutritive artificial substrate in half of the fertilized and unfertilized enclosures. Dominant taxa of nektonic herbivores-detritivores increased in abundance and biomass following algae manipulations but were not affected by the replacement of plant litter with a nonnutritive substrate. Epiphytic herbivores-detritivores responded to changes in both detritus and algal food sources, the importance of each factor varying among taxa. Fertilization resulted in early peaks of emergence for chironomids. In the first weeks of the study, chironomid emergence was not affected by the replacement of plant litter with a non-nutritive substrate. In August, however, more chironomids emerged in the treatments where plant litter was provided when compared with the artificial litter treatments.