SELECTION OF AN AQUATIC INDICATOR SPECIES TO MONITOR ORGANIC CONTAMINANTS IN TROPHICALLY SIMPLE LOTIC FOOD WEBS

A four-step procedure was used to identify an aquatic macroinvertebrat e with which to monitor organic contaminant burdens in trophically sim ple lake food webs of Alberta, Canada. We identified leeches (Oligocha eta: Hirudinea) as the potential indicator assemblage (Step 1), and th en documented their abundance and distribution in 16 lakes to identify a species-level bioindicator (Step 2). The latter two steps involved identifying the number and level of organic contaminants within the in dicator species (Step 3) and a comparison with other taxa within the f ood web (Step 4). The majority of lakes within the watershed contained seven to nine leech species with individuals of Nephelopsis obscura b eing one of the most abundant and widely distributed species that is s ufficiently large to allow for rapid collection of sufficient biomass for organic contaminant analyses. Concentrations of organic contaminan ts in leeches from 22 sites and in sediments (two lakes) within the wa tershed were low, and in the majority of cases less than analytical de tection levels (i.e., 0.05 mu g/kg wet weight). Nevertheless, leech ti ssues contained trace amounts of a fungicide (Le., hexachlorobenzene) and four pesticides including: pp'-DDE, pp'-DDD, representing intermed iate and final breakdown products of pp'-DDT, a-HCH (i.e., hexachloroc yclohexane) and g-HCH (Lindane). Concentrations of these contaminants did not differ significantly between leeches collected from lakes with in a national park compared to areas outside the park that may have be en more vulnerable to loadings of contaminants from industrial and agr icultural sources. Lastly, we compared organic contaminant concentrati ons in N. obscura with Gammarus lacustris and Brook's stickleback, Cul aea inconstans (Gasterosteidae). Organic contaminant concentrations in N. obscura were similar or higher than those present in G. lacustris and C. inconstans. Eight organic contaminants were found in N. obscura , whereas, C. inconstans and G. lacustris contained only three and fou r, respectively. The fact that N. obscura typically contained higher c oncentrations and greater numbers of detectable compounds suggest that it is a suitable indicator of organic contamination. While current le vels of organic contaminants in the Beaver Hills watershed are low, de spite the intense agriculture and industrial activities, they should b e monitored to act as an early warning indicator of ecosystem degradat ion.