INTERACTION OF A BIOTIC FACTOR (PREDATOR PRESENCE) AND AN ABIOTIC FACTOR (LOW-OXYGEN) AS AN INFLUENCE ON BENTHIC INVERTEBRATE COMMUNITIES

We examined the response of benthic invertebrates to hypoxia and preda tion risk in bioassay and behavioral experiments. In the bioassay, fou r invertebrate species differed widely in their tolerance of hypoxia. The mayfly, Callibaetis montanus, and the beetle larva, Hydaticus mode stus, exhibited a low tolerance of hypoxia, the amphipod, Gammarus lac ustris, was intermediate in its response and the caddisfly, Hesperophy lax occidentalis, showed high tolerance of hypoxia. In the behavioral experiments, we observed the response of these benthic invertebrates, which differ in locomotor abilities, to vertical oxygen and temperatur e gradients similar to those in an ice-covered pond. With adequate oxy gen, invertebrates typically remained on the bottom substrate. As bent hic oxygen declined in the absence of fish, all taxa moved above the b enthic refuge to areas with higher oxygen concentrations. In the prese nce of fish mayflies increased activity whereas all other taxa decreas ed activity in response to hypoxia. Mayflies and amphipods remained in the benthic refuge longer and endured lower oxygen concentrations whe reas the vertical distribution of caddisflies and beetle larvae was no t influenced by the presence of fish. As benthic oxygen declined in th e presence of fish, all but the beetle larva reduced activity over all oxygen concentrations compared to when fish were absent. As benthic o xygen continued to decline, mayflies and amphipods moved above the ben thic refuge and were preyed upon by fish. Thus, highly mobile taxa una ble to tolerate hypoxia (mayflies and amphipods) responded behaviorall y to declining oxygen concentrations by migrating upward in the water column. Taxa that were less mobile (beetle larvae) or hypoxia-tolerant (caddisflies) showed less of a response. Taxa most vulnerable to fish predation (mayflies and amphipods) showed a stronger behavioral respo nse to predator presence than those less vulnerable (caddisflies and b eetle larvae). Because invertebrates differ in their ability to withst and hypoxia, episodes of winter hypoxia could have long-lasting effect s on benthic invertebrate communities either by direct mortality or se lective predation on less tolerant taxa.