Responses of chlorophyll-a, organic matter, and macroinvertebrates to nutrient additions in rivers flowing through agricultural and forested land

I conducted field studies to determine if chlorophyll-a, organic matter, and macroinvertebrates would respond to nutrient changes in rivers in agricultural and forested areas of the Eastern Deciduous Forest in southwestern Ontario, Canada. Nutrient manipulations were examined using nutrient diffusing substrates (NDS) modified to permit independent sampling units. Results of a 5-6 week field experiment conducted in three forested and three agricultural rivers during mid-summer showed that concentrations of chlorophyll-a colonizing NDS varied among rivers and between land use types. As expected, concentrations of chlorophyll-a were higher in rivers flowing through agricultural than forested rivers. Variation among rivers within land use type was greater than between land use types in terms of organic matter and invertebrate responses to nutrient amendments. The NDS that were spiked with phosphate contained consistently higher quantities of phosphate than control or nitrate treatments after incubation. There were no significant differences in nitrate concentrations in NDS among treatments after incubation. Chironomids were the most abundant invertebrates that colonized NDS. Of the organisms examined, only one taxon (Tipulidae) appeared to be phosphate limited in forested rivers. Changes in the macroinvertebrate community would be expected after sustained additions of nutrients. The lack of significant differences between macroinvertebrates inhabiting natural rocks and those colonizing control NDS suggests that similarly designed substrata could be used to monitor macroinvertebrates in microhabitats.