Effects of Agrochemicals on Riparian and Aquatic Primary Producers in an Agricultural Watershed

Abstract

In agricultural watersheds, streams are intimately connected with croplands and may be inadvertently exposed to agrochemicals such as fertilizers and herbicides. Riparian plants and aquatic primary producers (aquatic plants, phytoplankton and periphyton) may be particularly affected by agrochemicals due to their taxonomic similarity to the intended targets (crop and weed species). The overall objective of this thesis was to assess the effects of fertilizers and the herbicide atrazine on riparian plants and aquatic primary producers. Effects were assessed across varying scales of observation ranging from empirical field studies at the watershed scale to in-situ experimental manipulations in two temperate streams to a laboratory concentration-response experiment. Twenty-four stream/river sites located across the South Nation River watershed, Canada ranged in surrounding agricultural land use (6.7-97.4 % annual crops) and in-stream concentrations of reactive phosphate (4-102 μg/L) and nitrate (3-5404 μg/L). A gradient of atrazine contamination spanning two orders of magnitude (56 d time-weighted-average concentrations of 4-412 ng/L) was observed using polar organic chemical integrative samplers (POCIS). A total of 285 riparian and aquatic plant species were identified with species richness ranging from 43-107 species per site. Atrazine and the percentage of surrounding annual crops had no statistically significant effects on community structure. In contrast, an increase in the percentage of non-native species, a decrease in submerged macrophytes and a decrease in overall floristic quality was observed along a gradient of increasing nitrate. Similarly, periphyton biomass increased with increasing nitrate across the watershed and was associated with the Chlorophyta. In contrast, no clear response was observed in periphyton exposed to nutrient enrichment and atrazine contamination in in-situ periphytometer experiments in two streams. Greenhouse concentration-response experiments provided evidence that the sensitivity of duckweed (Lemna minor) to atrazine was lower in populations previously exposed to the herbicide. However, the overall range in biomass 25% inhibition concentrations was small (19-40 μg/L atrazine). A clear gradient in agrochemical contamination was observed at the watershed scale and this research provided evidence of negative effects on riparian and aquatic primary producers. Effects of nutrients, specifically nitrate, superseded observable effects of the herbicide atrazine.