Impaired Biota: Assessing Landscape Linkages to In-Stream Biota

Impaired Biota 303(d) listings in Minnesota are estimated to increase several fold based on the increased biological assessment effort of the Minnesota Pollution Control Agency (MPCA). The use of biological metrics in an index of biological integrity (IBI) does not yield specific information about watershed system stress. Investigative work is necessary to link temporal and spatial clues that may offer inference about potential stressors.Little Rock Creek, a designated trout stream and coldwater tributary to the Mississippi River north of St. Cloud, Minnesota provided an opportunity to test varying watershed assessment tools. Cultivated row-crop irrigation on sandy soils has increased over the last two decades. Additionally, numerous reports of decreasing trout numbers coincided with the increase in irrigation. Based on a 1999 in-stream biological assessment for fish, a low fish IBI score suggested impaired aquatic life triggering the need for a Total Maximum Daily Load (TMDL) study.Historic records of irrigation withdrawal permits and ground water observation wells were examined for trends. Biological sampling was conducted in the summers of 2007 and 2008, along with geochemical sampling and field water quality measurements for snowmelt, stormflow and base-flow conditions. Stream channel aggradation was measured via mechanical shear using probing rod; these tests were conducted as part of the TMDL investigation at various locations along the creek.Preliminary results suggest ground water withdrawal has increased over the last decade; this in-turn, may have influenced the main channel bed which appeared to fill with loose sand reducing pool habitat quality. Dissolved oxygen was below the water quality standard on numerous occasions. NO3-N was elevated in main-channel reaches associated with intensive irrigation agriculture. Biological data showed warmwater fish species dominating the stream. Based on the current data, intensive row-crop irrigation has emerged as the dominant aquatic life stressor.