RESPONSE OF STREAM INVERTEBRATES TO A GLOBAL-WARMING THERMAL REGIME -AN ECOSYSTEM-LEVEL MANIPULATION

We manipulated, in accord with global-warming predictions, the thermal regime of a permanent first-order stream near Toronto, Ontario, Canad a. We examined the effects of a 2-3.5 degrees C water-temperature incr ease on densities, biomass, species composition, and life histories of resident stream invertebrates. The stream was divided longitudinally at the source into two channels, one control and one experimental, and a before and after (BACI) design was employed such that one pre-manip ulation year was followed by 2 yr of the temperature manipulation. Cha nges in the experimental channel following commencement of the manipul ation included: (1) decreased total animal densities, particularly Chi ronomidae (Diptera); (2) earlier onset of adult insect emergence; (3) increased growth rates and precocious breeding in Hyalella azteca (Amp hipoda); (4) smaller size at maturity for Nemoura trispinosa (Plecopte ra) and H. azteca; and (5) altered sex ratios for Lepidostoma vernale (Trichoptera). These results partially corroborate previous laboratory and field studies. However, variation in the responses of individual target species to the manipulation was unexpected and may have been in fluenced by the genetic structure of local populations. We conclude th at levels of gene flow among habitats may be critical to the degree of impact seen as a result of large-scale thermal perturbation (e.g., gl obal warming).