How moderate agriculture affects the food web of macroinvertebrates in fresh water streams in Kings County,Nova Scotia

Abstract

Moderate agriculture changes physical and chemical factors in streams resulting in a fertilizing effect by increasing available nutrients to macroinvertebrates through increased detritus breakdown and algal growth (Karr & Schlosser, 1978; Guliset al. 2006; Young et al., 2008; Hurynet al., 2002; Paul et al., 2006; Gulis&Suberkropp, 2003; Magbanuaet al,.2010). Increases in species tolerant to agricultural inputs have been observed as well as a basal carbon shift towards algal sources which is detectible through carbon isotope analysis (Wetzel et al., 1997; Guliset al., 2006; Bunn et al., 1999; England et al. 2004). The goal of this study was to detect changes in the food web within 4 study streams and relate it to agriculture in the stream watershed. Three streams with agriculture and one without were visited in April 2010 for samples of invertebrates, water, detritus, stream width, temperature and flow over a 2km length of the stream. Then again in June 2010 for canopy cover and water over the same 2 km area as well as 500 metres upstream from the sample sites. Ephemeroptera, Tricoptera and Plecoptera(EPT) were sorted and counted from invertebrate samples and 5 predator species (R. minor, R. vibox, R. fuscula, I. montana, and S. naica) were also counted and measured to determine instar. Physical, chemical and invertebrate results were compared within and between streams using the general linear model and a t-test was used to determine a significant change in carbon isotopes. Wheaton, the stream with the most agriculture, had the highest conductivity, densities of EPT, Ephemeroptera, Plecoptera, R. vibox, S. naicaand I. montana. The reference stream, with the least agriculture, had the lowest densities of EPT, Ephemeroptera, Tricoptera, and I. montana. The higher densities show a fertilizing effect in Wheaton. The other two streams had very different levels of agriculture in the entire watershed while having a closer level in a 50 metre buffer of the stream. The two streams also had some very similar densities which could mean the agriculture in the buffer is what is influencing the invertebrates the most. The carbon isotope results showed that the most downstream site in Turner Brook was more depleted in C13 than the furthest upstream site. This shows a basal carbon diet shift downstream.