Testing the robustness of pelagic zooplankton as indicators of land use impacts on small lakes




Rieberger, Kevin

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I investigated the utility of nitrogen stable isotope signatures in zooplankton as water quality indicators in small lakes. The d15N of organisms such as fish, mussels, insects, and aquatic plants have been linked to land use in several studies, however, I believe pelagic zooplankton in lakes provide comparable information to these with the benefit of being commonly available and easier to collect. To determine the potential use of this tool in water quality assessments, I collected samples from a total of 61 lakes throughout British Columbia. To investigate the seasonal patterns and inter-annual consistency of calanoid copepods and Daphnia d15N, I analyzed data collected from eight coastal lakes over several years. Seasonal variability in zooplankton d15N was observed for several small, temperate lakes with peak values generally occurring in the winter and spring. Sampling schedules should therefore include this critical period of maximum d15N. Calanoid d15N was consistently higher and less variable than Daphnia d15N, and therefore selected as the preferred taxonomic group. A strong relationship between mean d15N and density of septic systems was demonstrated and spring calanoid d15N was consistent over time in the absence of changes in land use. Consistent seasonal patterns in zooplankton d15N on a year-to-year basis support the application of this parameter in water quality trend analysis. To determine the physical factors that influence spring calanoid copepod d15N, the role of several watershed and limnological characteristics in 22 British Columbia lakes was investigated. The density of residential lots, as a proxy for septic density, within the riparian zones of lakes and their tributary streams was the only consistent significant predictor of d15N. While this suggests residential land use and septic density influenced calanoid d15N in these lakes, there are likely additional factors that contribute to the final signature measured. These factors include: the contribution of different N sources and subsequent isotopic mixing, the physical characteristics of individual lakes and differences in lake-specific chemical or biological processing of N. Some weaknesses in the study were identified. Most of the lakes studied had residential development as the dominant land use and a better representation of other land use types, such as agriculture, would have been beneficial. The lakes studied were exposed to relatively low or high levels of septic densities and more lakes exposed to moderate levels of development may have explained more variation. Finally, the majority of lakes studied were located in coastal areas and greater regional representation of lakes may have illustrated regional differences in seasonal trends. In my research, I have demonstrated an application for calanoid d15N in water quality assessments and resource management. When used in conjunction with other information, such as land use and water chemistry, d15N provides insight to nutrient sources for a particular lake, tracks changes in water quality over time, and can help guide management decisions.



Lake ecology, Land use, British Columbia