Freshwater macrophyte communities in lakes of variable landscape position and development in northern Wisconsin, U.S.A.
Introduction
Freshwater macrophytes are the pivot point of interaction between fish, invertebrates, waterfowl, plankton, bacteria, and water and sediment chemistry in lakes. Alterations to aquatic plant community structure will have cumulative effects on the entire lake ecosystem (Carpenter and Lodge, 1986, Wilcox and Meeker, 1992, Jeppesen et al., 1998).
Freshwater macrophyte abundance relies on many limnological factors. These include the temperature and chemistry of the water, the amount of light available to the plants, the substrate in which the plants are growing, the exposure to wave action, groundwater inflow, the extent of herbivory within the lake, and the presence of human development in the watershed and on the lakeshores (Pip, 1979, Barko et al., 1982, Duarte and Kalff, 1986, Lodge et al., 1986, Chambers and Kalff, 1987, Lodge et al., 1989, Rørslett, 1991, Nichols and Lathrop, 1994, Vestergaard and Sand-Jensen, 2000b, Radomski and Goeman, 2001, Egertson et al., 2004, Rasmussen and Anderson, 2005).
Substantial shoreline vegetation alteration by human activity has been documented. In Minnesota, developed lake shorelines have 66% less floating-leaf and emergent vegetative cover than undeveloped shorelines (Radomski and Goeman, 2001). Many lakeshore homeowners remove aquatic vegetation adjacent to their property to create beach conditions. The construction of docks and alteration of shoreline with riprap, concrete, and seawalls can also destroy aquatic macrophytes (Engel and Pederson, 1998). The addition of docks can mean an increase in boating traffic as well. Increased boating activity, including larger boats and personal watercraft, directly damages macrophytes and increases wave disturbance (Liddle and Scorgie, 1980, Murphy and Eaton, 1983, Ostendorp et al., 1995). Native freshwater macrophyte densities can also be reduced significantly by exotic species introduced by humans (e.g., Orconectes rusticus, Myriophyllum spicatum; Lodge and Lorman, 1987, Boylen et al., 1999, Wilson, 2002).
The topographic position of a lake in a hydrological network affects a number of factors that may influence macrophyte communities and their response to human disturbance. Lakes that are low in the drainage network tend to receive a larger proportion of their water budgets from groundwater than lakes higher in the landscape because groundwater flow follows surface water drainage (Brown, 1995). Conductivity, alkalinity, algal abundance, fish species richness, pH, [Ca2+], [Cl−], and dissolved silica all may increase lower in the landscape (Kratz et al., 1997, Riera et al., 2000). These environmental variables may then affect aspects of habitat, water quality, and biota within lake ecosystems. Together and separately, human development and landscape position influence many of the variables that constrain macrophyte community composition and structure.
We evaluated the presence and composition of freshwater macrophyte communities on gradients of landscape position and human development. This study examines the changes in this important littoral structure along these two gradients simultaneously. The individual relationships between aquatic macrophytes and environmental variables have been well documented elsewhere (see above); here we assess the relative importance of each variable across a landscape and how those variables differ in importance for distinct macrophyte communities.
Section snippets
Site description
Vilas County is located in north central Wisconsin, sharing a boundary with the Upper Peninsula of Michigan. It ranges from 469 to 573 m above sea level and has a total area of 263,490 ha of which 17% is water (Natzke and Hvizdak, 1986). Because of its lakes, streams, wetlands, and forests, Vilas County is a popular tourist destination. Lakeshore plots of land have become very popular sites for vacation homes and the human population has more than doubled in Vilas County since the 1970s.
Chemical and physical limnology
Over the course of four summers (2001–2004), we assessed the chemical and physical limnology of each of the 60 study lakes (15 lakes each summer). Measurements were taken at 1 m depth (with the exception of Secchi depth, maximum depth, and the shoreline development factor) at the deepest part of each lake. The variables include conductivity, alkalinity, chlorophyll a concentration, pH, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total nitrogen unfiltered (N), total
Results
The 60 study lakes had low to moderate alkalinity and conductivity, but differed greatly in light availability, intensity of human land use and crayfish abundance (Table 1). Manicured lawns and docks were the most common landcover types associated with human presence in our 480 study sites (Table 2).
Lake-level environmental factors
The overwhelmingly prominent environmental parameter influencing macrophyte community composition in these lakes is alkalinity. Because lakes that are low in the drainage network tend to receive a larger proportion of their water budgets from groundwater than lakes higher in the landscape (Brown, 1995), alkalinity often increases in lakes lower in the landscape (Kratz et al., 1997, Riera et al., 2000). Macrophyte species groups 2 and 3 were found predominantly in low alkalinity, high landscape
Acknowledgements
We are indebted to S. Van Egeren, L. Zeglin, K. Werner, A. Marburg, A. Fowler, J. Hansen, K. Wagner, A. Newman, J. Jakubowski, T. Aschkenase, A. Rea, G. Pledger, and T. Bland who helped with data collection. We thank B. Benson for helpful discussions on statistical analyses. This work was funded by the NSF under Cooperative Agreement #DEB-0083545 (Biocomplexity).
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Present address: Wisconsin Department of Natural Resources, Rhinelander, WI 54501, USA.