Mary T. Bremigan

Linking Landscapes and Food Webs: Effects of Omnivorous Fish and Watersheds on Reservoir Ecosystems

Abstract Ecologists increasingly recognize the need to understand how landscapes and food webs interact. Reservoir ecosystems are heavily subsidized by nutrients and detritus from surrounding watersheds, and often contain abundant populations of gizzard shad, an omnivorous fish that consumes plankton and detritus. Gizzard shad link terrestrial landscapes and pelagic reservoir food webs by consuming detritus, translocating nutrients from sediment detritus to the water column, and consuming zooplankton. The abundance of gizzard shad increases with watershed agriculturalization, most likely through a variety of mechanisms operating on larval and adult life stages. Gizzard shad have myriad effects on reservoirs, including impacts on nutrients, phytoplankton, zooplankton, and fish, and many of their effects vary with ecosystem productivity (i.e., watershed land use). Interactive feedbacks among watersheds, gizzard shad populations, and reservoir food webs operate to maintain dominance of gizzard shad in highly productive systems. Thus, effective stewardship of reservoir ecosystems must incorporate both watershed and food-web perspectives.

Cross‐scale interactions: quantifying multi‐scaled cause–effect relationships in macrosystems

Ecologists are increasingly discovering that ecological processes are made up of components that are multi-scaled in space and time. Some of the most complex of these processes are cross-scale interactions (CSIs), which occur when components interact across scales. When undetected, such interactions may cause errors in extrapolation from one region to another. CSIs, particularly those that include a regional scaled component, have not been systematically investigated or even reported because of the challenges of acquiring data at sufficiently broad spatial extents. We present an approach for quantifying CSIs and apply it to a case study investigating one such interaction, between local and regional scaled land-use drivers of lake phosphorus. Ultimately, our approach for investigating CSIs can serve as a basis for efforts to understand a wide variety of multi-scaled problems such as climate change, land-use/land-cover change, and invasive species.

Accounting for Multilevel Data Structures in Fisheries Data using Mixed Models

Multilevel data structures are those that have a hierarchical structure, in which response variables are measured at the lowest level of the hierarchy and modeled as a function of predictor variables measured at that level and higher levels of the hierarchy. For example, a multilevel data structure may consist of measurements taken on individual fish (lower level) that are nested within lakes or streams (higher level). Multilevel data structures are a common feature in fisheries research. We provide simulated fisheries data examples, similar in structure to other published studies, to illustrate the application of multilevel models and discuss how hypothesis testing and inferences can be incorrect if multilevel data structures are ignored. Ignoring multilevel data structures has implications for the use of commonly-used ordinary least squares (OLS) approaches to test hypotheses and to make inferences. Multilevel models are an alternate approach that circumvents problems associated with traditional OLS methods and allows valid inferences to be made.

Effects of Macrophyte Bed Architecture on Largemouth Bass Foraging: Implications of Exotic Macrophyte Invasions

Abstract Invasive, canopy-forming macrophyte species such as Eurasian watermilfoil Myriophyllum spicatum and hydrilla Hydrilla verticillata are now widespread throughout North American lakes and reservoirs. However, their effects on economically and ecologically important species such as largemouth bass Micropterus salmoides are insufficiently understood. Canopy-forming exotic macrophytes typically displace structurally diverse native macrophytes. In aquaria, we tested whether exotic macrophyte canopies negatively affect age-0 largemouth bass foraging success compared with diverse macrophyte assemblages. In addition, we explored the relative effects of macrophyte density, and prey density of bluegills Lepomis macrochirus on largemouth bass foraging success. Bluegill prey density did not significantly affect largemouth bass foraging success. However, largemouth bass experienced shorter search times and greater attack and consumption rates in moderate plant density treatments (compared with dense) and diver...

Prey Selection by Larval Fishes as Influenced by Available Zooplankton and Gape Limitation

Abstract Feeding success during the first weeks of life is critical to determining survival and ultimate year-class strength of fishes. To compare the relative influence of gape limitation and available zooplankton on prey size selection among the larvae of three species of freshwater fishes, we gathered data on fish gape size, prey size, and size-specific prey selection in lakes and reservoirs. These variables were compared among black crappies Pomoxis nigromaculatus from a lake that contained large zooplankton as prey and white crappies P. annularis and gizzard shad Dorosoma cepedianum (a potential competitor of white crappie) from reservoirs that contained small zooplankton. In three Ohio reservoirs (i.e., small-zooplankton systems), available zooplankton and larval stages of white crappies and gizzard shad were collected once per week during April through September 1987 and 1988. Although mean prey size of white crappies continued to increase with fish size, mean prey size of smaller-gaped gizzard sha...

Using Landscape Limnology to Classify Freshwater Ecosystems for Multi-ecosystem Management and Conservation

Governmental entities are responsible for managing and conserving large numbers of lake, river, and wetland ecosystems that can be addressed only rarely on a case-by-case basis. We present a system for predictive classification modeling, grounded in the theoretical foundation of landscape limnology, that creates a tractable number of ecosystem classes to which management actions may be tailored. We demonstrate our system by applying two types of predictive classification modeling approaches to develop nutrient criteria for eutrophication management in 1998 north temperate lakes. Our predictive classification system promotes the effective management of multiple ecosystems across broad geographic scales by explicitly connecting management and conservation goals to the classification modeling approach, considering multiple spatial scales as drivers of ecosystem dynamics, and acknowledging the hierarchical structure of freshwater ecosystems. Such a system is critical for adaptive management of complex mosaics of freshwater ecosystems and for balancing competing needs for ecosystem services in a changing world.

VARIABLE GIZZARD SHAD RECRUITMENT WITH RESERVOIR PRODUCTIVITY: CAUSES AND IMPLICATIONS FOR CLASSIFYING SYSTEMS

Achieving sustainable prey fish assemblages that support sport fish predator populations is a fundamental challenge to fisheries managers. Among Midwestern and Southeastern (USA) reservoirs, gizzard shad, Dorosoma cepedianum, have been widely stocked to improve predator growth. However, these stockings have yielded highly variable effects on sport fish, due in part to highly variable recruitment of gizzard shad. To determine whether reservoir productivity can be used to classify reservoirs according to recruitment of gizzard shad, we quantified gizzard shad recruitment along a mesotrophic to hypereutrophic productivity gradient. We sampled 12 reservoirs during May through June 1993, to evaluate the hypothesis that larval gizzard shad foraging success and survival increase with reservoir productivity. Both hatch abundance and survival of larval gizzard shad correlated positively with total phosphorus concentrations (TP), an indicator of reservoir productivity. Abundance of 15-mm (total length) larval gizzard shad survivors, an indicator of age-0 year class strength, increased by two orders of magnitude across TP concentrations. Larval gizzard shad foraging success increased with availability of preferred, small zooplankton prey. However, abundance of small zooplankton did not increase with reservoir TP concentrations, and larval survival did not increase with foraging success. These results provide mechanistic understanding for the relative lack of gizzard shad in mesotrophic reservoirs, and the dominance of gizzard shad in hypereutrophic reservoirs. In hypereutrophic reservoirs, negative effects of gizzard shad on sport fish may be alleviated by reducing phosphorus loading from the watershed, suggesting a watershed approach to this fishery and water quality problem.

Zooplankton Selectivity by Larval Yellow Perch in Green Bay, Lake Michigan

Recruitment of yellow perch (Perca flavescens) in southern Lake Michigan has been poor since 1989, whereas in Green Bay, Lake Michigan, larval yellow perch have been more abundant. Therefore, investigation of factors influencing yellow perch early life stages in Green Bay, and comparison with southern Lake Michigan conditions, can lend insight into factors limiting recruitment of this species in southern waters of the lake. We investigated diets and selectivity of larval yellow perch in Green Bay, to identify conditions of zooplankton availability that limit larval yellow perch. Larval yellow perch and zooplankton were collected in Green Bay during May and June 1998 and 1999. We identified, counted, and measured available zooplankton and larval yellow perch stomach contents, estimated larval gape size, and compared larval diets to zooplankton availability. Similarities between 1998 and 1999 in larval yellow perch diet composition and selectivity indicate that zooplankton are most likely to limit small (< 8 mm TL), gape-limited larvae, whose diets are comprised almost exclusively of copepod nauplii and small (< 0.5 mm) copepods. Differences between 1998 and 1999 further implicate the importance of small copepods. In 1999, when small copepods were less abundant in Green Bay than in 1998, a higher proportion of larvae had empty guts, and growth of larvae was slower. In southern Lake Michigan, densities of small copepods have declined precipitously since the late 1980s. Taken together, our findings indicate that availability of suitable prey for larval yellow perch in Lake Michigan warrants further investigation as a potential factor limiting recruitment during the last decade.

Experimental Assessment of the Influence of Zooplankton Size and Density on Gizzard Shad Recruitment

Abstract Available prey influence fish recruitment, especially when size-dependent processes, such as starvation or predation, dictate larval survival. Yet, for zooplanktivorous larval fishes. quantitative assessments of how zooplankton density and quality (i.e., size or taxonomic composition) influence foraging success, growth, and survival are rare. With small-scale experiments, we tested whether high densities of small zooplankton promote recruitment of age-0 gizzard shad Dorosonia cepedianum from the larval (<25 mm total length, TL) to the juvenile (≥25 mm TL) stage. In our first experiment, in which larval and early juvenile gizzard shad (10–29 mm TL) were exposed to a range of densities of small zooplankton, foraging success varied substantially but generally increased at zooplankton densities over 200/L for larvae less than 20 mm TL and at densities over 100/L for individuals 20 mm TL or longer. In a second experiment. larval gizzard shad (15 mm TL), after 2 weeks in 1-m3 enclosures, survived more ...

Effects of Land Use on Lake Nutrients: The Importance of Scale, Hydrologic Connectivity, and Region

Catchment land uses, particularly agriculture and urban uses, have long been recognized as major drivers of nutrient concentrations in surface waters. However, few simple models have been developed that relate the amount of catchment land use to downstream freshwater nutrients. Nor are existing models applicable to large numbers of freshwaters across broad spatial extents such as regions or continents. This research aims to increase model performance by exploring three factors that affect the relationship between land use and downstream nutrients in freshwater: the spatial extent for measuring land use, hydrologic connectivity, and the regional differences in both the amount of nutrients and effects of land use on them. We quantified the effects of these three factors that relate land use to lake total phosphorus (TP) and total nitrogen (TN) in 346 north temperate lakes in 7 regions in Michigan, USA. We used a linear mixed modeling framework to examine the importance of spatial extent, lake hydrologic class, and region on models with individual lake nutrients as the response variable, and individual land use types as the predictor variables. Our modeling approach was chosen to avoid problems of multi-collinearity among predictor variables and a lack of independence of lakes within regions, both of which are common problems in broad-scale analyses of freshwaters. We found that all three factors influence land use-lake nutrient relationships. The strongest evidence was for the effect of lake hydrologic connectivity, followed by region, and finally, the spatial extent of land use measurements. Incorporating these three factors into relatively simple models of land use effects on lake nutrients should help to improve predictions and understanding of land use-lake nutrient interactions at broad scales.