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

We test the hypotheses that (1) algae are a major food source for mars h invertebrates and (2) plant litter serves mainly as cover for invert ebrates and a substrate for algal growth rather than as a direct sourc e of nutrition. Twelve enclosures (5-m diameter) were deployed in May in a nutrient-poor marsh of the Interlake region of Manitoba, six of w hich were fertilized to increase algal levels. The hardstem bulrush (S cirpus lacustris glaucus) litter present was replaced with a nonnutrit ive artificial substrate in half of the fertilized and unfertilized en closures. Dominant taxa of nektonic herbivores-detritivores increased in abundance and biomass following algae manipulations but were not af fected by the replacement of plant litter with a nonnutritive substrat e. Epiphytic herbivores-detritivores responded to changes in both detr itus and algal food sources, the importance of each factor varying amo ng taxa. Fertilization resulted in early peaks of emergence for chiron omids. In the first weeks of the study, chironomid emergence was not a ffected by the replacement of plant litter with a non-nutritive substr ate. In August, however, more chironomids emerged in the treatments wh ere plant litter was provided when compared with the artificial litter treatments.