EFFECTS OF ALGIVOROUS MINNOWS ON PRODUCTION OF GRAZING STREAM INVERTEBRATES

The algivorous minnow, Campostoma anomalum, strongly influences the distribution and standing crop of periphyton in north temperate streams. We conducted a 5 wk experiment in large, recirculating artificial streams to examine the effects of Campostoma on secondary production of two invertebrate grazers, the crayfish Orconectes virilis and the pulmonate snail Physella virgata. Treatments included experimental streams with snails alone, crayfish alone, snails and fish together, crayfish and fish together, and a non-grazed control. In this study, algivorous fish had significant impacts on production of co-occurring invertebrates. Crayfish were negatively impacted, apparently through resource monopolization by fish. Snail production was indirectly enhanced through fish grazing activities that reduced the algal overstory and may have made nonfilamentous algae more available to snails. Additionally, in the presence of Campostoma, snails exhibited a phenotypically-plastic shift in life-history traits. Our results show that these abundant, widespread freshwater fish can influence not only the secondary production of co-occurring invertebrate grazers, but life histories as well, and confirm the conclusions of other recent studies in assigning periphyton-grazing fish an important role in temperate stream ecosystems.