Brian R. Herwig

Impacts of Lakeshore Residential Development on Coarse Woody Debris in North Temperate Lakes

Coarse woody debris (CWD) is a critical input from forested watersheds into aquatic ecosystems. Human activities often reduce the abundance of CWD in fluvial systems, but little is known about human impacts on CWD in lakes. We surveyed 16 north temperate lakes to assess relationships among CWD, riparian vegetation, and shoreline residential development. We found strong positive correlation between CWD density and riparian tree density (r2 = 0.78), and strong negative correlation between CWD density and shoreline cabin density (r2 = 0.71) at the whole-lake scale. At finer spatial scales (e.g., between sampling plots), correlations between CWD and riparian vegetation were weaker. The strength of relationships between CWD and riparian vegetation was also negatively influenced by the extent of cabin development. Overall, there was significantly more CWD in undeveloped lakes (mean of 555 logs/km of shoreline) than in developed lakes. Within developed lakes, CWD density differed between forested sites (mean of 379 logs/km of shoreline) and cabin-occupied sites (mean of 57 logs/km of shoreline). These losses of CWD will affect littoral communities in developed north temperate lakes for about two centuries. Because CWD is important littoral habitat for many aquatic organisms, zoning and lake management should aim to minimize further reductions of aquatic CWD and woody vegetation from lakeshore residences.

Managing Macrophytes to Improve Fish Growth: A Multi-lake Experiment

Abstract Macrophyte harvesting often has been suggested as a way to improve fish growth and size structure in lakes with high densities of submergent macrophytes and stunted fish populations. However, previous experimental tests have provided no clear consensus on whether the technique works for management. We conducted a series of whole-lake manipulations to test the effects of macrophyte removal on growth of bluegill and largemouth bass. We selected four lakes in southern and central Wisconsin for experimental manipulation and nine others for controls. In August 1994, we removed macrophytes from approximately 20% of the littoral zone by cutting a series of evenly spaced, deep channels throughout each treatment lake. In the first year after manipulation, we observed substantially increased growth rates of some age classes of both bluegill and largemouth bass in treatment lakes relative to controls. Growth rates of other age classes were less responsive to manipulation. We observed increased bluegill and ...

BIOTIC INTERACTIONS AS DETERMINANTS OF ECOSYSTEM STRUCTURE IN PRAIRIE WETLANDS: AN EXAMPLE USING FISH

Wetlands are abundant throughout the prairie pothole region (PPR), an area comprising over 700,000 km2 in central North America. Prairie wetland communities are strongly influenced by regional physiography and climate, resulting in extreme spatial and temporal variability relative to other aquatic ecosystems. Given the strong influence of abiotic factors, PPR wetland communities have been viewed traditionally in the context of their responses to chemical and physical features of landscape and climate. Although useful, this physical-chemical paradigm may fail to account for ecosystem variability due to biotic influences, particularly those associated with presence of fish. Spatial and temporal variability in fish populations, in turn, may reflect anthropogenic activities, landscape characteristics, and climate-mediated effects on water levels, surface connectivity, and hydroperiods. We reviewed studies assessing influences of fish on prairie wetlands and examined precipitation patterns and biological data from PPR wetlands in east-central North Dakota and western Minnesota, USA. Our review and analysis indicated that native fish influence many characteristics of permanently flooded prairie wetlands, including water clarity and abaundance of phytoplankton, submerged macrophytes, and aquatic invertebrates. We suggest that ecologists and managers will benefit from conceptual paradigms that better meld biotic interactions associated with fish, and perhaps other organisms, with chemical and physical influences on prairie wetland communities.

Effects of aquatic insect predators on zooplankton in fishless ponds

We removed the surface-orienting aquatic insects from a fishless pond to determine their predation effects on zooplankton behavior and size structure. A second fishless pond served as the unmanipulated reference system in this two year study. In the reference pond and the treatment pond prior to manipulation, daphnids exhibited pronounced diel vertical migrations. Following the removal of surface-orienting aquatic insects from the treatment pond, daphnid migration changed to a reverse migration strategy that was significantly different from that observed in the reference system. Average daphnid body size increased significantly following predator removal in the treatment system. Our data indicate that predation by aquatic insect predators, such as notonectids and dytiscids, may affect daphnid migration behavior in fishless systems. Vertical migration by daphnids may allow coexistence with surface-orienting insects in ponds that are deep enough to provide a spatial refuge from these predators.

Short- and Long-Term Evaluation of Passive Integrated Transponder and Visible Implant Elastomer Tag Performance in Muskellunge

Abstract Fisheries professionals charged with managing muskellunge Esox masquinongy frequently seek population information that requires the ability to identify cohorts as well as individuals; hence, reliable tagging methods are needed. Our approach was to simultaneously assess multiple marking techniques on different life stages of muskellunge over short and long time scales. We evaluated the short-term detection of visible implant elastomer (VIE) and passive integrated transponder (PIT) tags in muskellunge fingerlings in experimental ponds. We compared survival (relative to that of control fish) and tag retention for two PIT tagging locations (the cheek and dorsal musculature) and one VIE location (the jaw). Overwinter survival did not differ between tagged and untagged fish (84–98%), but overwinter PIT tag retention was lower for the cheek (92%) than the dorsal musculature (100%). We also fin-clipped and VIE-tagged (jaw) 1,651 muskellunge fingerlings from 1997 to 2002 to evaluate long-term tag retentio...

Factors Influencing Fish Distributions in Shallow Lakes in Prairie and Prairie-Parkland Regions of Minnesota, USA

Fish exert strong influences on shallow lakes, but managers lack empirical models useful for predicting fish distributions at landscape scales. We used classification and regression tree analysis (CART), and regression to predict fish presence/absence (P/A), richness, and community composition in 82 shallow lakes distributed among two regions (prairie and prairie-parkland) along the eastern margin of the Prairie Pothole Region in western Minnesota, U.S.A. A CART model for fish P/A using downstream connections to fish sources and maximum depth correctly classified ≥92% of our study sites, indicating the rare fishless sites observed in our study were either isolated or shallow. Fish richness was positively related to both lake and watershed size. Given that many fish species have strong negative influences on shallow lake ecological characteristics, we conclude that future conservation efforts should focus on protecting shallow, isolated basins, or reducing surface connectivity among basins as these factors were decisive in promoting fish populations. Such management strategies should help to maintain current levels of fish richness and enhance richness of aquatic birds, amphibians, plants, and invertebrates.

Relative importance of phosphorus, fish biomass, and watershed land use as drivers of phytoplankton abundance in shallow lakes

Phytoplankton abundance in shallow lakes is potentially influenced by ambient phosphorus concentrations, nutrient loading accentuated by human activities in lake watersheds, and abundance of planktivorous and benthivorous fish. However, few studies have simultaneously assessed the relative importance of these factors influencing phytoplankton abundance over large spatial scales. We assessed relative influences of watershed characteristics, total phosphorus concentrations, and fish biomass on phytoplankton abundance in 70 shallow lakes in western Minnesota (USA) during summer 2005 and 2006. Our independent variables included total phosphorus (TP), benthivore biomass, planktivore biomass, summed planktivore and benthivore biomass (summed fish), areal extent of agriculture in the watershed, region (prairie versus parkland lakes), and year. Predictive models containing from one to three independent variables were compared using an information theoretic approach. The most parsimonious model consisted of TP and summed fish, and had over 10,000-fold greater support compared to models using just TP or summed fish, or models comprised of other variables. We also found no evidence that relative importance of predictor variables differed between regions or years, and parameter estimates of TP and summed fish were temporally and spatially consistent. TP and summed fish were only weakly correlated, and the model using both variables was a large improvement over using either variable alone. This indicates these two variables can independently increase phytoplankton abundance, which emphasizes the importance of managing both nutrients and fish when trying to control phytoplankton abundance in shallow lakes.

Comparing Effects of Lake- and Watershed-Scale Influences on Communities of Aquatic Invertebrates in Shallow Lakes

Constraints on lake communities are complex and are usually studied by using limited combinations of variables derived from measurements within or adjacent to study waters. While informative, results often provide limited insight about magnitude of simultaneous influences operating at multiple scales, such as lake- vs. watershed-scale. To formulate comparisons of such contrasting influences, we explored factors controlling the abundance of predominant aquatic invertebrates in 75 shallow lakes in western Minnesota, USA. Using robust regression techniques, we modeled relative abundance of Amphipoda, small and large cladocera, Corixidae, aquatic Diptera, and an aggregate taxon that combined Ephemeroptera-Trichoptera-Odonata (ETO) in response to lake- and watershed-scale characteristics. Predictor variables included fish and submerged plant abundance, linear distance to the nearest wetland or lake, watershed size, and proportion of the watershed in agricultural production. Among-lake variability in invertebrate abundance was more often explained by lake-scale predictors than by variables based on watershed characteristics. For example, we identified significant associations between fish presence and community type and abundance of small and large cladocera, Amphipoda, Diptera, and ETO. Abundance of Amphipoda, Diptera, and Corixidae were also positively correlated with submerged plant abundance. We observed no associations between lake-watershed variables and abundance of our invertebrate taxa. Broadly, our results seem to indicate preeminence of lake-level influences on aquatic invertebrates in shallow lakes, but historical land-use legacies may mask important relationships.

Fish Influences on Amphibian Presence and Abundance in Prairie and Parkland Landscapes of Minnesota, USA

Abstract Many amphibian populations are declining, and increased understanding of the drivers of amphibian presence and abundance will help in their conservation. In 2005 and 2006 we estimated relative abundance of larvae of two common amphibian taxa, Tiger Salamanders (Ambystoma tigrinum) and ranid tadpoles (Northern Leopard Frog Lithobates pipiens and Wood Frog Lithobates sylvaticus), in 75 shallow lakes in prairie and parkland areas in Minnesota. We used a two-step procedure in which we first modeled presence-absence with data from all lakes and then modeled abundance only in lakes where the amphibian taxa were present. For the two amphibian taxa, a generalized linear mixed effects model was used to examine the effects of dynamic factors like fish abundance and static (timeframe of study) variables like fish community type, depth, and adjacent land cover. Fish variables had the greatest influence but differed for the presence and abundance models. Salamander and tadpole presence was inversely correlated to the abundance of benthivorous fish, while salamander abundance was best explained by total fish abundance and tadpole abundance by fish community type. We did not detect influences of land cover types on the amphibian taxa we surveyed. Our findings are important because they complement previous studies documenting that negative correlations with fish extend beyond piscivores to include both planktivores and benthivores. Hydrological changes in our study landscape (e.g., installation of drainage networks, wetland consolidation) associated with agriculture and other land uses have likely increased the distribution and abundance of fish populations, thus dampening amphibian breeding success.

Biotic Manipulations of Aquatic Ecosystems

Large-scale biotic manipulations have been performed in aquatic habitats to evaluate how ecosystem structure alters processes and dynamics. These experiments have involved either direct manipulation of species or the habitat structure that mediates species interactions. Manipulations at smaller experimental scales can not encompass the natural biology of wide-ranging species or the system-level heterogeneity that has critical effects on the interactions between community and ecosystem dynamics (e.g., Schindler et al. 1997). Manipulations of aquatic ecosystems are often used to explore the responses of possible management actions or, in fact, are large management experiments (e.g., Kitchell 1992; Hansson et al. 1998; Olson et al. 1998). In this chapter we review briefly some of the techniques used in controlled experimental settings and in natural resource management that have involved biotic manipulations of aquatic ecosystems. We also discuss some considerations for design of ecosystem experiments, and refer to important directions for future ecosystem research. We have limited our review to experiments in freshwater systems, and cite only selected examples of biotic manipulations. For more detailed descriptions of some of the methods we discuss, readers should consult Nielsen and Johnson (1983), Downing and Rigler (1984), and Cooke et al. (1993).