In a 16000 km2 area of southwestern Ontario, almost 15% of all streams have been enclosed (buried) largely for agricultural purposes. ArcGIS was used to characterize the natural features of catchments and to calculate enclosedness (proportion of stream network enclosed; x̅ = 16.5%, n = 10106). Catchments with the highest enclosedness received \u3e990 mm of precipitation annually and had high drainage density (\u3e1.9 km/km2), while catchments with the lowest enclosedness receivedannually, were characterized by clay and undrumlinized till plains and had relatively shallow water tables (m). These natural features influence where enclosure is likely to occur, however, the final decision to enclose a stream may also be based on social, economic and political factors.
Effects of enclosedness on fish and benthic macroinvertebrate (BMI) assemblages were determined using multiple visit sampling (MVS) at 10 sites in the Ausable River Basin (ARB), and single visit sampling (SVS) at 157 sites across southwestern Ontario (SWO). Among SVS-SWO sites, enclosedness was positively correlated to BMI density (r = 0.195) and estimated abundance (r = 0.266) and the proportion of herbivorous/insectivorous fish species (r = 0.187), and negatively correlated to the proportion of insectivorous/piscivorous fish species (r = -0.167). There were no correlations with enclosedness among the MVS-ARB sites. However, fish species richness and abundance, and BMI richness, density and diversity were most variable with sampling date at these sites, suggesting that multiple visit sampling may provide a more complete description of biotic assemblages in these agricultural streams. For species present at 25 – 75% of SVS-SWO sites, logistic regression showed increased likelihood of finding two fish species (Blacknose Dace, Central Stoneroller) and two BMI taxa (Asellidae, Lebertiidae) and decreased likelihood of finding two BMI taxa (Tabanidae, Physidae) with increasing enclosedness.
Enclosedness has modest effects in streams already impacted by agricultural practices and should be considered a potential threat to aquatic ecosystems. These findings provide a first step toward understanding headwater loss through enclosure and should be taken into consideration in future decisions to enclose streams
