Cottonwood Creek Ecological Assessment

This report describes the third year of study conducted on Cottonwood Creek. The water quality, physical habitat and biotic community of Cottonwood Creek were investigated during the summer of 2005. Water temperature loggers were placed within seven different locations extending from the headwaters to near the mouth. Physical measures of channel shape, slope, substrate and large woody debris were conducted at four locations representing different channel conditions. Macroinvertebrates were sampled using the ASCI methods, and relative juvenile fish abundance was determined using baited minnow traps. Water samples were collected four times at eight locations and analyzed for pH, specific conductance, turbidity, dissolved oxygen, nitrate and nitrite-N, ammonia-N, total phosphorus, and total dissolved phosphorus; and four times at three locations and analyzed for total fecal coliform bacteria. Nutrient limitation was tested using nutrient diffusing substrata. Stream water temperatures were consistently low above Cornelius Lake and within Dry Creek. Water temperatures exceeded State Standards on multiple occasions from the outlet of Nekleson Lake to Surrey Road. Water temperatures were highest at lake outlets and decreased when flowing through well-vegetated channels. Juvenile fish distribution appeared to be affected by water temperatures. Stream water pH values were above neutral, specific conductance was relatively high, near 200 μS/cm. Turbidity was consistently low, below 1 NTU. Low dissolved oxygen concentrations were recorded within the slow-moving macrophyte-filled reach from Wasilla Lake to Edlund Road. Nitrate concentrations were below detection limits, but ammonia nitrogen concentrations were high relative to total or total dissolved phosphorus on most dates. Molar ratios of nitrogen to phosphorus suggested nitrogen limitation during base-flow, but following storm events, ratios decreased supporting nitrogen limitation. Results of the nutrient diffusing experiment supported nitrogen limitation but were not conclusive, likely reflecting the temporal variability in available elements. Water quality was assessed as “good” to “excellent” using samples and metric scores for low-gradient fine-substrate streams, but “poor” to “fair” using metric scores for low-gradient coarse-substrate streams. Macroinvertebrate metric scores decreased slightly from measures in 1998 and 2000. Cottonwood Creek Ecological Assessment 7/15/2006 ARRI