INVADING CRAYFISH IN A MICHIGAN STREAM - DIRECT AND INDIRECT EFFECTS ON PERIPHYTON AND MACROINVERTEBRATES

Crayfish are common inhabitants of North American streams and many spe cies are undergoing human-assisted range expansions. We studied the ef fects of an introduced crayfish (Orconectes rusticus) on benthic macro invertebrates and periphyton in a northern Michigan (USA) stream by 1) conducting a 46-d enclosure-exclosure experiment and 2) sampling bent hic communities along a longitudinal gradient in crayfish density. In stream enclosures, crayfish reduced total macroinvertebrate densities by 47-58% and herbivore densities by 55-72% relative to exclosures. Ov er the course of the experiment, periphyton chlorophyll a increased by 48-70% in enclosures compared to an increase of only 8% in exclosures . Periphyton biomass, however, did not vary across treatments. Periphy ton primary productivity increased 4-7 times in the presence of crayfi sh, probably because crayfish reduced grazer densities (indirect effec t) and removed non autotrophic components of the periphyton matrix (di rect effect). The longitudinal survey supported experimental results. At sites along a crayfish density gradient occurring over 3 km of stre am, periphyton chlorophyll a on rocks increased and macroinvertebrate density decreased with increasing crayfish density. These studies show that crayfish directly and indirectly affected the stream benthos, th ereby producing responses at more than 1 trophic level. Some responses were consistent with a trophic cascade, but crayfish increased food w eb connectance by consuming periphyton. Therefore, crayfish can have c omplex, multi-trophic-level effects on the food webs of invaded stream s.