Assessing the cumulative impacts to near-shore, in-water habitat

Principal Investigator: Bruce Vondracek.

Students: Jennifer Keville and Jessie Lepore.

Status: Project completed 30 June 2013. 

Human structures related to shoreline development, such as docks, boatlifts, and other structures, and disturbance from recreational activity may have a cumulative impact on aquatic ecosystems. Near-shore areas (< 4.5 m deep) often contain most of the vegetation and are generally the spawning area for fish. Several studies have addressed the effects of incremental changes on lake ecosystems despite ongoing concerns about the rate and extent of near-shore, in-water habitat alterations, and expansion of in-lake structures. However, there is limited knowledge about the cumulative effects of human activities on aquatic habitat, water quality, and fish populations, which has hindered regulatory authorities and lake managers to guide landowners toward lower impact practices. We assessed the extent of near-shore vegetation, fish, and coarse woody structure along a gradient of shoreline development and develop a framework to assess cumulative impacts on whole lake systems. We used aerial photos and existing DNR data to estimate whole lake disturbances of ~100 lakes in the Northern Lakes and Forests Ecoregion. We also conducted assessments of a subset of lakes (~30) at the individual lot scale, to quantify impacts to vegetation, fish, and coarse woody structure along a gradient of shoreline development and shoreline types. Finally, we assessed the biovolume, species, and areal coverage of aquatic macrophytes in 12 lakes (included in the 30 lake subset). We used our research to develop a model to predict the cumulative impact of development on aquatic ecosystems, providing a tool to guide lake managers toward sustainable near-shore, in-water development.

We found that the littoral zone contains all of the vegetation within a lake and is critical to the physical and biological integrity of lakes. Aquatic macrophytes and coarse woody structure (CWS) provide refuge, foraging area, and spawning substrate for many fish species. The goal of this study was to evaluate shoreline development by measuring a number of variables that reflect human activity including, terrestrial vegetation, physical alterations and in-lake structures. Previous studies have found reductions in abundance of aquatic vegetation and CWS; however, few studies have quantified the specific influence of docks on aquatic habitat structure. CWS and three measures of macrophyte abundance increased with distance to the nearest dock structure. Presence of CWS and emergent species were significantly and negatively related to lake-wide dock density. We intensively investigated effects of lakeshore development on nearshore habitat across 11 northern Minnesota lakes using the Minnesota Department of Natural Resources’ Score Your Shore (SYS) survey to assess development intensity. Developed sites (a residence and dock present) had lower macrophyte species richness, emergent, and floating-leaf macrophytes and CWS than undeveloped sites (no residence, no dock). SYS score was a significant factor in models of most macrophyte community variables, supporting the hypothesis that site-scale development intensity is related to littoral vegetation. A fish Index of Biological Integrity decreased as the density of docks increased for the 11 intensively studied lakes. Development density across 29 lakes and 114 lakes were also examined, but less intensively. Effects of development in these less intensively studied lakes were less apparent for most lake macrophyte and fish community variables than for the intensively studied lakes. These findings suggest that riparian management on residential lots and reduced removal of aquatic macrophytes and CWS could improve fish habitat at both local and lake-wide scales of development.