Post-flood and Post-restoration Biotic Sampling of Fond du Lac and Martin Branch Streams

Abstract

Fond du Lac Environmental Program requested sampling of two stream reaches in late summer of 2012. One reach (Station 2) on Fond du Lac Creek was damaged by the Solstice Flood of 2012, while a second reach (Marshall Rd) on the Martin Branch had received restoration work. During the early summer of 2012 the St. Louis River River watershed endured a high flood stage which caused easily-observable changes in water quality, bathymetry, and presence of submerged aquatic vegetation within the estuary. Many streams draining into the St. Louis River and estuary were also damaged by the flooding. Damage to Fond du Lac Creek included a road and culvert washout with associated scouring of the stream bed at Station 2. Sampling of Martin Branch was to document any changes due to a new and improved culvert just downstream of the sampled reach at Marshall Rd. The Natural Resources Research Institute's macroinvertebrate sampling crew sampled both reaches for aquatic macroinvertebrates and in-stream and riparian zone habitat at the end of August, 2012. These results were compared with samples collected by NRRI in mid-September of 2010 from these same reaches using the same methods and sampling gear. both streams showed evidence of physical changes to their channels between 2010 and 2012 that were reflected in noticeable but not substantial changes in their macroinvertebrate assemblages. Fond du Lac Creek's flood damage seems to have created or enlarged a pool and enlarged the channel, particularly bankfull width. It is possible that these changes will reduce the ability of baseflow to adequately fill the channel during low flow periods. The flood also appears to have coarsened the channel substrate and removed all of the large woody debris from the wetted area of the channel. These changes appear to have provided more niches for increased Ephemeroptera taxa richness (although there has been some splitting of genera that adds to this perception), but EPT proportional composition of the assemblage dropped slightly. Combined with increased proportions of chironomids, amphipods and isopods, and oligochaetes, the changes resulted in a significantly lower EPA tolerance score for the stream in 2012 than in 2010. Martin Branch faired somewhat better between 2010 and 2012, gaining a riffle that was not seen in 2010 sampling and experiencing a coarsening of the substrate in the pool and run areas. These physical changes were accompanied by a more than doubling of the taxa richness of Ephemeroptera, Plecoptera, and Trichoptera, including gaining a greater proportion of net-spinning Trichoptera. Chironomid taxa richness also nearly doubled between 2010 and 2012, and they increased in proportional composition of the reach assemblage. With the increase in chironomid taxa came an associated decrease in several non-insect groups, which dropped the non-insect proportion of the assemblage by 50% from 2010. However, this did not result in a change in either tolerance score calculated for the reach.