EFFECTS OF AN OMNIVOROUS CRAYFISH (ORCONECTES RUSTICUS) ON A FRESH-WATER LITTORAL FOOD-WEB

Cascading trophic interactions are important in many freshwater pelagi c food webs, but their importance in more complex, omnivore-rich litto ral-zone food webs is less well known. We tested the existence of a tr ophic cascade involving omnivorous crayfish (Orconectes rusticus), mac roinvertebrates, periphyton, and macrophytes using 9-m(2) cages in the littoral zone of Plum Lake, Wisconsin, USA. Treatments in the replica ted (N = 4) experiment were crayfish enclosures, crayfish exclosures, and cageless references. During June-September, we measured macrophyte shoot numbers, macroinvertebrate numbers, and periphyton (on plastic strips) chlorophyll a, and dry mass (DM). We expected that crayfish fo raging would directly reduce abundance and change species composition of macrophytes and macroinvertebrates and would indirectly enhance per iphyton abundance by reducing the abundance of grazing snails. In encl osures, macrophyte and snail (but not nonsnail macroinvertebrate) dens ities declined significantly throughout the experiment, whereas densit ies of macrophytes, snails, and nonsnail macroinvertebrates increased in exclosures and cageless references. Some of the reduction in macrop hytes resulted from nonconsumptive fragmentation of macrophytes by cra yfish. Consistent with the cascading trophic interactions model, perip hyton chlorophyll a per unit surface area increased in enclosures, but declined in exclosures. Periphyton quality (as indexed by chlorophyll a/DM) also increased in enclosures relative to exclosures and cageles s references. However, because of large reductions in macrophyte surfa ce area (which periphyton colonizes) in enclosures, total amount of pe riphyton chlorophyll a in enclosures (relative to exclosures) probably declined while periphyton quantity per unit surface area and periphyt on quality increased. Thus, the impacts of crayfish omnivory on periph yton, expressed in two conflicting indirect effects, confirm the possi bility that omnivory can complicate cascading trophic predictions. Ove rall, results support the existence of strong trophic interactions in the littoral zone, in which omnivorous crayfish control abundance of m acrophytes, snails, and periphyton.