Anett S. Trebitz

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 ...

An Introduction to the Practice of Ecological Modeling

odeling has become an important tool in the study of ecological systems,as a scan of the table of con tents of a ny major eco l ogical journ a l makes abundantly clear. A number of books have recently been published that provide excellent advice on mo del construction, building, and use (e.g.,Gotelli 1995, Gurney and Nisbet 1998, Roughgarden 1998) and add to the classic literature on mo deling ecological systems and their dynamics (e.g., Maynard Smith 1974, Nisbet and Gurney 1 9 8 2 ) . Un fortu n a tely, h owever, l i t t l e — i f a ny — of t h i s growing literature on e cological modeling addresses the motivation to model and the initial stages of the modeling process, information that beginning students would find useful. Fast computers and graphical software packages have removed much of the drudgery of creating models with a programming language and opened new avenues of model construction,use,and even misuse. There are many reasons why a student might want to consider modeling as a component of his or her education. Models p rovide an opportunity to explo re ideas regarding ecological systems that it may not be possible to field-test for logistical, political, or financial reasons. Often, learning occurs from apparently st range results and unexpected sur prises. The process of formulating an e cological model is ext remely helpful for organizing one’s thinking , bringing hidden assumptions to light, and identifying data needs. More and more,students want to “do something” with modeling but are not sure how to get started. The goals of this article are to outline issues concerning the value of ecological models and some possible motivations for mo deling, and to provide an entry point to the established modeling literature so that those who are beginning to think about using models in their research can integrate modeling usefully. We therefore envision the typical reader to be an advanced undergraduate, a beginning graduate student, or a new modeler. We first consider some of the values of models and the motivation for modeling. We then discuss the steps involved in developing a mo del from an initial idea to something that is implemented on a computer, outlining some of the decisions that must be ma de along the way. Many excellent texts and journal articles deal with the technical details of models and model construction; we do not attempt to replace this literature, but rather try to make the reader aware of the issues that must be considered and point to some of the sources we have found particularly useful. We b egin with the assumption that the reader has decided that he or she would like to “do something” with modeling as part of his or her research (Figure 1). It is important to recognize the difference between models and the modeling process. A model is a representation of a particular thing, idea, or condition. Models can be as simple as a verbal statement about a subje ct or two boxes c onnected by an arrow to represent some r elationship. Alternatively, models can be ext remely c omplex and detailed, such as a mathematical description of the pathways o f nitrogen t ransformations within ecosystems. The model ing process is the series of steps taken to convert an idea first into a conceptual model and then into a quantitative m odel . Because part of what eco l ogists do is revi s e hypotheses and collect new data, the model and the view of nature that it r epresents often und ergo many changes from the initial conception to what is d eemed the final product. The discussion that follows is organized to consider issues in a sequence similar to what a new modeler would encounter. Because individuals’ backgrounds differ, the sequence is not fixed. We map one possible route through the sorts of decisions that will most lik ely need to be considered; this course is derived from our individual experiences plus the collective knowledge o f our reviewers. We begin with conceptual models because many people, even self-labeled nonmodelers, formulate conceptual models.

Water Quality in Great Lakes Coastal Wetlands: Basin-wide Patterns and Responses to an Anthropogenic Disturbance Gradient

ABSTRACT We present water quality data from 58 coastal wetlands, sampled as part of a larger effort investigating effects of nutrient enrichment and habitat disruption in the Laurentian Great Lakes. Our sampling design selected sites from across a gradient of agricultural intensity within combinations of biogeographic ecoprovince and wetland hydromorphic type and captured a large range in water quality. Levels of total nutrients (N and P), and various measures of particulate concentration, water clarity, and ionic strength were strongly associated with agricultural intensity in the watershed, and could be effectively aggregated into an overall principal component-based water quality descriptor. Lake Erie wetlands had the highest nutrient levels and lowest water clarity, while wetlands in Lakes Superior and Huron had the lowest nutrient levels and clearest water. Lake Ontario wetlands had clearer water than would be expected from their nutrient levels and position on the agricultural intensity gradient. Dissolved oxygen, silica, pH, and dissolved organic carbon (DOC) were independent of agricultural intensity but DOC was responsible for low water clarity in some Lake Superior wetlands. Simple classification by hydromorphic type (riverine or protected) did not explain water quality differences among wetlands exposed to similar agricultural intensity levels, so finer hydrologic classification may be desirable. Results are used as a basis for discussing research and information needs underlying development of water quality criteria and monitoring programs for coastal wetlands of the Great Lakes.

Responsiveness of Great Lakes Wetland Indicators to Human Disturbances at Multiple Spatial Scales: A Multi-Assemblage Assessment

ABSTRACT Developing indicators of ecosystem condition is a priority in the Great Lakes, but little is known about appropriate spatial scales to characterize disturbance or response for most indicators. We surveyed birds, fish, amphibians, aquatic macroinvertebrates, wetland vegetation, and diatoms at 276 coastal wetland locations throughout the U.S. Great Lakes coastal region during 2002–2004. We assessed the responsiveness of 66 candidate indicators to human disturbance (agriculture, urban development, and point source contaminants) characterized at multiple spatial scales (100, 500, 1,000, and 5,000 m buffers and whole watersheds) using classification and regression tree analysis (CART). Non-stressor covariables (lake, ecosection, watershed, and wetland area) accounted for a greater proportion of variance than disturbance variables. Row-crop agriculture and urban development, especially at larger spatial scales, were about equally influential and were more explanatory than a contaminant stress index (CSI). The CSI was an important predictor for diatom indicators only. Stephanodiscoid diatoms and nest-guarding fish were identified as two of the most promising indicators of row-crop agriculture, while Ambloplites rupestris (fish) and Aeshna (dragonflies) were two of the strongest indicators of urban development. Across all groups of taxa and spatial scales, fish indicators were most responsive to the combined influence of row-crop and urban development. Our results suggest it will be critical to account for the influence of potentially important non-stressor covariables before assessing the strength of indicator responses to disturbance. Moreover, identifying the appropriate scale to characterize disturbance will be necessary for many indicators, especially when urban development is the primary disturbance.

Characterizing Seiche and Tide-driven Daily Water Level Fluctuations Affecting Coastal Ecosystems of the Great Lakes

ABSTRACT Seiches in the Great Lakes probably play a role similar to that of tides in estuaries in organizing the structure and function of coastal wetlands and embayments, but information needed to test this idea is lacking. Past Great Lakes work has focused on enumerating frequencies of oscillation but without addressing their combined influence. Information on seiche magnitude is sparse and focused on extremes rather than typical levels, and tools that integrate magnitude and frequency components to derive net day-scale effects are lacking. This study uses water level time series to characterize daily fluctuation regimes for 51 stations around the Great Lakes. Distributions of fluctuation magnitude typically had long upper tails, with some level of activity always present. Logarithmic mean daily water level range varied from ∼4 cm in Lake Ontario to > 20 cm in Lake Erie, with largest values at the ends of lakes and in large bays. Oscillation frequency patterns were spatially variable and had both seiche and tide components. One-half the daily sum of water level increments is a computationally tractable metric of fluctuation intensity that integrates magnitude and frequency. This metric is directly interpretable as the column of water moved by all seiche and tide modes combined, which when multiplied by an area of interest yields the volume of water involved. Logarithmic mean values for this metric ranged from ∼10 cm in Lake Ontario to > 50 cm in Lake Erie. Data and tools provided will support future efforts to establish seiche and tide influences on Great Lakes wetlands and embayments.

Relative Role of Lake and Tributary in Hydrology of Lake Superior Coastal Wetlands

Abstract Despite the documented importance of hydrodynamics in influencing the structure and function of Great Lakes coastal wetlands, systematic assessments of coastal wetland hydrology are lacking. This paper addresses this gap by describing patterns in lake and tributary inputs, water residence times, and mixing regimes for a suite of western Lake Superior wetlands that differ in the amount of tributary and seiche flow they receive. We show that variability in tributary flows among wetlands and over time is far greater than variability in seiche-driven water movements, and that the amount of tributary flow strongly influences wetland hydrology via effects on water mixing and residence times, seiche size, mouth closures, and relative amounts of main and off-channel areas. Wetland seiche amplitudes were reduced in systems with small mouth openings and wetland mouth size was correlated with tributary flow. All wetlands experienced seiche-driven water level oscillations, but there was lake water intrusion only into those wetlands where tributary outflow was small relative to the seiche-driven inflow. Wetlands in settings exposed to long-shore sediment transport exhibited periodic mouth closures when stream flows were low. The absolute and relative size of lake and tributary inputs must be explicitly considered in addition to wetland morphology and landscape setting in studies seeking to understand determinants of coastal wetland structure, function, and response to anthropogenic stressors.

Exotic and Invasive Aquatic Plants in Great Lakes Coastal Wetlands: Distribution and Relation to Watershed Land Use and Plant Richness and Cover

ABSTRACT We use data from inundated-area surveys of 58 coastal wetlands spanning a gradient of anthropogenic impacts across all five Laurentian Great Lakes to describe the distribution of nine exotic and invasive taxa of aquatic plants. We found plants that were exotic or have invasive strains to be substantially more prevalent in wetlands in Lakes Erie and Ontario than in Lakes Superior and Huron, with Lake Michigan wetlands intermediate. Najas minor (slender naiad), Butomus umbellatus (flowering rush), and Hydrocharis morsus-ranae (European frogbit) were restricted to the lower lakes and rarely dominant. Myriophyllum spicatum (Eurasian milfoil), Potamogeton crispus (curly pondweed), Lythrum salicaria (purple loosestrife), Phalaris arundinacea (reed canary grass), Phragmites australis (common reed), and Typha sp. (cattail) were more widespread and except for P. crispus, often among the dominant taxa. None of the submerged or floating-leaf exotic taxa were associated with altered total plant cover or richness, although M. spicatum, P. crispus, and native Stuckenia pectinatus (sago pondweed) were positively associated with agricultural intensity in the watershed (a surrogate for nutrient loading). Emergent P. australis, L. salicaria, and Typha were more likely to be present and dominant as agricultural intensity increased, and were associated with elevated emergent cover and decreased emergent genera richness. Effects of dominant taxa on plant cover and richness were readily detected using ordinal data from 100 m inundated segments but were harder to discern with data aggregated to the wetland scale. The sum of shoreline-wide abundance scores for four easily identified taxa (S. pectinata, P. australis, Typha, and L. salicaria) is proposed as a rapidly-measured indicator of anthropogenic disturbance across the Great Lakes.

Hydroacoustic estimates of abundance and spatial distribution of pelagic prey fishes in Western Lake Superior

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) are a valuable prey resource for the recovering lake trout (Salvelinus namaycush) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha−1 with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha−1), followed by the Duluth Minnesota region (20.22 kg ha−1), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha−1). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.

A review of selected ecosystem services provided by coastal wetlands of the Laurentian Great Lakes

Significant ecosystem services derive from the coastal wetlands of the Laurentian Great Lakes even though two-thirds of the original coastal wetlands have been lost since European settlement, and the remaining 126,000 ha of U.S. coastal wetlands and ≥70,000 ha of Canadian wetlands are affected by anthropogenic stressors. Published information indicates that wildlife habitat, fisheries support, and water quality improvement are significant ecosystem services provided by Great Lakes coastal wetlands that should be strongly considered during management decision making. 30 species of waterfowl, 155 breeding bird species, and 55 species of reptiles and amphibians are supported by coastal wetland habitats across the Basin. Nearly all sport and commercial Great Lakes fish species use coastal wetlands for life-cycle functions, and Great Lakes food webs are supported by wetland export of young sport and forage fish. Biological responses indicate declines in the wildlife and fishery services with increasing levels of anthropogenic disturbance. Extrapolation from a single well-studied system suggests that, Basin-wide, coastal wetlands may retain nearly 4000 tonnes P and 53,000 tonnes N per year, but additional studies are needed to support these estimates and determine stressor effects. Coastal wetlands appear to retain sediments over long time scales, but may either retain or release sediments during storm events. Extrapolation of carbon sequestration from other wetland types suggests that less than 90 g C yr−1 might be retained across the Basin. Wild rice production provides a culturally important ecosystem service, and coastal protection may be locally significant where fringing wetland remain. To support management decisions, quantitative relationships between specific stressors or land use practices and the delivery of ecosystem services are needed, as are ecosystem service indicators to measure those responses.

Turbidity Tolerances of Great Lakes Coastal Wetland Fishes

Abstract Despite recent interest in assessing the condition of fish assemblages in Great Lakes coastal wetlands and a concern for increasing turbidity as a major stressor pathway influencing these ecosystems, there is little information on fish tolerance or intolerance to turbidity on which to base wetland assessment metrics. Existing studies have borrowed tolerance designations from the stream literature, but they have not confirmed that the designations apply to Great Lakes wetlands or that designations based on tolerance to degradation in general apply to turbidity in particular. We used a published graphical method to determine turbidity tolerances of Great Lakes fishes based on their pattern of occurrence and relative abundance across coastal wetlands spanning a turbidity gradient. Fish composition data were obtained from fyke-net and electrofishing surveys of 75 wetlands along the U.S. shoreline of the Laurentian Great Lakes, representing a turbidity range of approximately 0–110 nephelometric turbid...