Evaluating Direct Toxicity and Food-Chain Effects in Aquatic Systems Using Natural Periphyton Communities
Authors: Boston HL , Hill WR , Stewart AJ 
Published: March 2009

Periphytic algae are a ubiquitous and ecologically important component of many rivers and streams and are useful for assessing toxicity. Biotic communities in streams near Oak Ridge National Laboratory were apparently impacted by industrial activities; however, no one toxicant had been clearly implicated in these streams and consequently no exposure data were available. We used natural periphyton communities in several streams to (1) identify stream reaches with ambient toxicity, (2) characterize conditions for the growth of periphyton and other aquatic biota, and (3) evaluate contaminant movement and effects through foodchain interactions.

Algal biomass (as chlorophyll a) and photosynthetic (PS) rates (based on H14CO3 - incorporation) were measured monthly for two years at 15 sites. Data for periphyton PS were analyzed using analysis of covariance (ANCOVA) with biomass as the covariate. The ANCOVA generated a chlorophyll-adjusted photosynthetic (CAPS) rate for the periphyton at each site that provided a means to track and compare functional aspects (e.g., PS) of periphyton communities among sites where biomass differed. The CAPS rate appears to provide an index of the physiological condition of the periphyton that was (1) sensitive to stress, and (2) robust for different communities and for different light and nutrient levels based on experimental evidence and comparison with data for other biotic communities.

Commonly used periphyton evaluation techniques (e.g., species composition, percent organic content, and accrual rates) were used, determined, and compared with CAPS rates. A 23-day periphyton bioassay suggested that toxicity from chlorine in once-through cooling water could result in low taxonomic diversity and a patchy algal distribution at sites near cooling water discharges. These experiments showed that PS was more sensitive to chlorine stress than was either biomass or species composition.

In systems where toxic stress is intermittent, cumulative, or the result of biotic interactions, integrative measures of natural communities, such as CAPS and biomass accrual, are relatively simple and useful for assessing toxicity. Data from these integrative measures were better correlated with data for other biotic communities than were the results of single species toxicity tests.

Periphyton at sites directly downstream of industrial discharges was enriched with various toxic metals [e.g., cadmium (Cd), chromium (Cr), and mercury (Hg)]. Because several taxa of periphyton-grazing invertebrates were coincidentally absent from these sites, we conducted laboratory experiments to assess the effects of contaminants on invertebrates. Experiments (five-week duration) showed that the growth of snails and grazing caddisflies on periphyton from three sites was positively correlated with algal biomass and was apparently not affected by the metals in the periphyton consumed. Thus, although periphyton may be good indicators of metal loading, these metals may not be rapidly transferred to consumers.