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Cisco (Coregonus artedi) are threatened by climate change and lake eutrophication, and their oxythermal habitat can be assessed with TDO3, the water temperature at which dissolved oxygen equals 3 mg·L−1. We assessed the influence of TDO3 on cisco habitat use, genetic diversity, diets, and isotopic niche in 32 lakes ranging from oligotrophic to eutrophic. Results showed that as TDO3 increased, cisco were captured higher in the water column, in a narrower band, with higher minimum temperatures and lower minimum dissolved oxygen. TDO3 was also negatively related to cisco allelic richness and expected heterozygosity, likely driven by summer kill events. Moreover, TDO3 influenced the isotopic niche of cisco, as fish captured deeper were more depleted in δ13C and more enriched in δ15N compared to epilimnetic baselines. Lastly, cisco in high TDO3 lakes consumed more Daphnia, had fewer empty stomachs, and achieved larger body size. Our work identifies specific characteristics of cisco populations that respond to climate change and eutrophication effects and provides a framework for understanding responses of other cold-water species at the global scale.

Zebra mussels (Dreissena polymorpha) filter feed phytoplankton and reduce available pelagic energy, potentially driving fish to use littoral energy sources in lakes. However, changes in food webs and energy flow in complex fish communities after zebra mussel establishment are poorly known. We assessed impacts of zebra mussels on fish littoral carbon use, trophic position, isotopic niche size, and isotopic niche overlap among individual fish species using δ13C and δ15N data collected before (2014) and after (2019) zebra mussel establishment in Lake Ida, MN. Isotope data were collected from 11 fish species, and from zooplankton and littoral invertebrates to estimate baseline isotope values. Mixing models were used to convert fish δ13C and δ15N into estimates of littoral carbon and trophic position, respectively. We tested whether trophic position, littoral carbon use, isotopic niche size, and isotopic niche overlap changed from 2014 to 2019 for each fish species. We found few effects on fish trophic position, but 10 out of 11 fish species increased littoral carbon use after zebra mussel establishment, with mean littoral carbon increasing from 43% before to 67% after establishment. Average isotopic niche size of individual species increased significantly (2.1-fold) post zebra mussels, and pairwise-niche overlap between species increased significantly (1.2-fold). These results indicate zebra mussels increase littoral energy dependence in the fish community, resulting in larger individual isotopic niches and increased isotopic niche overlap. These effects may increase interspecific competition among fish species and could ultimately result in reduced abundance of species less able to utilize littoral energy sources.

Groundwater exchanges with most lakes are rarely quantified because there are many technical challenges to quantification. We investigated a lakebed mapping approach to infer the relative areas of groundwater exchange in 12 prairie shallow lakes and five Laurentian mixed forest shallow lakes in Minnesota, USA in 2011. We used a relatively common approach (seepage meters) to provide baseline information on the magnitude and direction of flow at four locations in each lake. To expand from point measurements to the whole-lake scale, we explored use of specific conductivity as a cheaper and more time efficient proxy for groundwater discharge to lakes. We validated the approach at near shore stations in each lake where seepage meter measurements and specific conductivity surveys overlapped. Specific conductivity surveys provided a similar assessment of groundwater discharge compared to seepage meters for 50% of the lake-sampling period combinations. The lakebed mapping approach, when validated for a lake with a limited number of seepage meter (or alternative methods) measurements, offers the advantages of being more time and labor efficient over the use of a similar number of seepage meter monitoring locations; seepage meters (or piezometers, for example) are costlier in terms of equipment and labor, even for single-lake studies. We show the combined approach could provide useful baselines for understanding and mapping groundwater exchange in shallow lakes.

Total phosphorus (TP) is commonly used to assess water quality in shallow lakes and other surface waters. Shallow lakes require special consideration because they can transition between two alternative stable states: (1) a clear-water state that typically supports abundant submerged vegetation and provides high quality wildlife habitat, and (2) a turbid-water state with frequent algal blooms and poor habitat quality. A shallow lake’s TP level in relation to critical TP tipping points determines whether the lake is in a highly resilient clear state, a highly resilient turbid state, or whether the lake is in a dynamic region where either state is possible. Further, management options differ for highly resilient clear lakes versus lakes that may be in the turbid state. For example, resilient clear lakes may be assigned special watershed and shoreline protection to preserve their healthy conditions. On the other hand, managers may plan to allocate resources for assessment and possible in-lake management for lakes that have TP levels where the turbid state is possible. Managers would benefit from models that can predict whether a lake is in a resilient clear state or may be in the turbid state without physically visiting a lake to collect water samples or other in-lake data. We used TP data from 118 shallow lakes in Minnesota and previously estimated TP tipping points to classify lakes as either highly resilient clear lakes (“stable-clear”) or possibly turbid (“bistable-turbid”). We used random forest methodology to build models for predicting these two TP classes using both remotely sensed watershed-scale predictors and in-lake variables, and we performed recursive feature elimination to find the most parsimonious model. We demonstrate that TP class can be predicted using only watershed-scale remotely sensed variables and that land cover and use, soil texture, and ecoregion are key predictor variables for TP class. We highlight how our model predictions can be used as indicators to help make management decisions for a set of shallow lakes.

Biogeochemistry patterns in shallow lakes are influenced by both in-lake factors such as ecosystem state as well as watershed-level factors such as land use, but the relative importance of in-lake versus watershed factors is poorly known. This knowledge gap makes it difficult for lake mangers to prioritize efforts on watershed versus in-lake strategies for stabilizing the clear-water state. We studied 48 shallow lakes in Minnesota, USA to assess the relative influence of lake size, land use in watersheds, and ecosystem state (turbid versus clear) on water column total nitrogen (TN) and total phosphorus (TP), as well as δ

The influence of zebra mussels (Dreissena polymorpha) on phytoplankton abundance is well known, but their community‐level impact on energy flow is less clear. Reduced phytoplankton abundance could increase reliance of fish and aquatic invertebrates on alternative energy sources such as epiphyton and benthic algae. We assessed impacts of zebra mussels on energy flow by comparing food webs in two Minnesota, USA, lakes during summers of 2013 and 2014. Lake Carlos had a dense population of zebra mussels, while upstream Lake Ida was free of zebra mussels until this study began and maintained low densities during our study. We used baseline‐corrected (BC) δ¹³C to test whether fish and littoral invertebrate primary and secondary consumers were more reliant on littoral carbon in Carlos compared to Ida. We also used BC δ¹⁵N to determine if trophic position of fish species differed between lakes. Lastly, we compared isotopic niche space by estimating standard ellipse areas for fish species in Carlos and Ida lakes, and tested whether the community‐level range of trophic levels, reliance on littoral carbon and standard ellipse area differed between lakes. Results showed invertebrate secondary consumers had more enriched BC δ¹³C in Carlos than in Ida, indicating greater reliance on littoral energy. Mixing models indicated that 10 of 11 fish species were more reliant on littoral carbon in Carlos, with littoral carbon use in the 10 species 1.5‐fold higher in Carlos. Isotopic niche analysis also showed increased littoral reliance in Carlos fish, as the same 10 fish species in Carlos had statistically distinct ellipses that were enriched in δ¹³C. Mixing models also indicated that seven of 11 fish species analysed had significantly higher trophic positions in Lake Carlos. In contrast, community‐scale metrics for fish showed no difference between lakes in the range of trophic levels, range of reliance on littoral energy, or size of standardised ellipse area of isotopic niche space. This indicates that, despite most individual fish species increasing their reliance on littoral energy and shifting upwards in trophic position, the overall size of the community isotopic niche area remained similar between lakes. Our results indicate that zebra mussels have community‐wide impacts on energy flow in lakes, with invertebrate predators and many species of fish increasing their reliance on littoral energy sources, and most species of fish shifting to higher trophic positions. A key question is whether increased water clarity associated with zebra mussels can increase littoral production sufficiently to compensate for higher demand. If not, it is plausible that invertebrate and fish production will decline due to increased intra‐ and inter‐specific competition.

Stable isotopes 13C and 15N are often used in lake ecosystems to assess energy sources and trophic positions, respectively. However, δ13C and δ15N are also influenced by internal biogeochemical processes in epilimnetic and hypolimnetic habitats in lakes, but the extent to which biogeochemical processing mediates isotope values between these two habitats, and whether these patterns are influenced by lake productivity is not known. We sampled δ13C and δ15N in epilimnetic mussels, Chaoborus, cisco (Coregonus artedi), and seston and zooplankton in the epilimnia and hypolimnia of 22 Minnesota (USA) lakes ranging from oligotrophic to eutrophic. We also measured lake temperature–oxygen profiles and light levels to assess factors influencing isotope patterns. Isotope samples were baseline‐corrected using epilimnetic mussels in each lake (sample—mussel) to control for watershed‐level differences in isotope values. Results showed δ13C in epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco became more depleted in δ13C relative to epilimnetic mussels in low‐productivity lakes where light penetrated into the hypolimnion, while epilimnetic seston δ13C stayed similar to mussel δ13C in all lakes. This pattern was likely due to hypolimnetic phytoplankton in clearwater lakes incorporating more respired CO2, which is depleted in δ13C, and subsequently passing depleted δ13C values up the food chain. Results also showed habitat differences in δ15N with epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco becoming more enriched relative to epilimnetic mussels in low‐productivity lakes with higher O2 levels in the hypolimnion, while epilimnetic seston δ15N remained similar to mussel values. The δ15N pattern is consistent with the idea that denitrification and microbial degradation enriched hypolimnetic seston relative to epilimnetic seston in low nutrient lakes, while enhanced epilimnetic primary production enriched epilimnetic δ15N seston relative to hypolimnetic seston in high nutrient lakes. Our results indicate isotopic differences between epilimnetic and hypolimnetic organisms that change along productivity gradients and suggest that microbial processes and the light regime are important drivers.

Research on lake eutrophication often identifies variables affecting amounts of phosphorus (P) and nitrogen (N) in lakes, but understanding factors influencing N:P ratios is important given its influence on species composition and toxin production by cyanobacteria. We sampled 80 shallow lakes in Minnesota (USA) for three years to assess effects of watershed size, proportion of watershed as both row crop and natural area, fish biomass, and lake alternative state (turbid vs. clear) on total N : total P (TN : TP), ammonium, total dissolved phosphorus (TDP), and seston stoichiometry. We also examined N:P stoichiometry in 20 additional lakes that shifted states during the study. Last, we assessed the importance of denitrification by measuring denitrification rates in sediment cores from a subset of 34 lakes, and by measuring seston δ15 N in four additional experimental lakes before and after they were experimentally manipulated from turbid to clear states. Results showed alternative state had the largest influence on overall N:P stoichiometry in these systems, as it had the strongest relationship with TN : TP, seston C:N:P, ammonium, and TDP. Turbid lakes had higher N at given levels of P than clear lakes, with TN and ammonium 2-fold and 1.4-fold higher in turbid lakes, respectively. In lakes that shifted states, TN was 3-fold higher in turbid lakes, while TP was only 2-fold higher, supporting the notion N is more responsive to state shifts than is P. Seston δ15 N increased after lakes shifted to clear states, suggesting higher denitrification rates may be important for reducing N levels in clear states, and potential denitrification rates in sediment cores were among the highest recorded in the literature. Overall, our results indicate lake state was a primary driver of N:P dynamics in shallow lakes, and lakes in clear states had much lower N at a given level of P relative to turbid lakes, likely due to higher denitrification rates. Shallow lakes are often managed for the clear-water state due to increased value as wildlife habitat. However, our results indicate lake state also influences N biogeochemistry, such that managing shallow lakes for the clear-water state may also mitigate excess N levels at a landscape scale.

Ecological shifts in shallow lakes from clear-water macrophyte-dominated to turbid-water phytoplankton-dominated are generally thought of as rapid short-term transitions. Diatom remains in sediment records from shallow lakes in the Prairie Pothole Region of North America provide new evidence that the long-term ecological stability of these lakes is defined by the legacy of large regime shifts. We examine the modern and historical stability of 11 shallow lakes. Currently, four of the lakes are in a clear-water state, three are consistently turbid-water, and four have been observed to change state from year to year (transitional). Lake sediment records spanning the past 150-200 yr suggest that (1) the diatom assemblage is characteristic of either clear or turbid lakes, (2) prior to significant landscape alteration, all of the lakes existed in a regime of a stable clear-water state, (3) lakes that are currently classified as turbid or transitional have experienced one strong regime shift over the past 150-200 yr and have since remained in a regime where turbid-water predominates, and (4) top-down impacts to the lake food-web from fish introductions appear to be the dominant driver of strong regime shifts and not increased nutrient availability. Based on our findings we demonstrate a method that could be used by lake managers to identify lakes that have an ecological history close to the clear-turbid regime threshold; such lakes might more easily be returned to a clear-water state through biomanipulation. The unfortunate reality is that many of these lakes are now part of a managed landscape and will likely require continued intervention.

Shallow lake managers seek strategies to improve water quality and ecological features of these habitats, but lake responses are unpredictable and factors responsible for changes are often unclear. We summarized results of eight whole-lake rehabilitation projects in Minnesota, USA, an area with many shallow waters highly impacted by anthropogenic activities. To assess lake responses, we compared characteristics of managed sites to those of other regional shallow lakes manifesting clear- or turbid-state conditions. Managed lakes showed modest similarity to clear-water reference lakes in terms of phytoplankton (as chlorophyll a), nutrients, and submerged aquatic plants. Responses of aquatic invertebrate communities were more equivocal, with relatively little similarity to clear-water sites following management. These patterns indicate that these lakes either failed to undergo transitions to clear-water states, or that clear-water conditions did not persist throughout the 2–3 year period following treatment and prior to our evaluation. We believe these results show responses of shallow lakes that have been pushed beyond boundaries where they maintain sufficient natural resilience to resist local stressors. This means that shallow lake rehabilitation efforts will not always succeed and that, when improvements occur, management may need to be repeated to maintain favorable ecological conditions in highly modified landscapes.

Shallow lakes process large amounts of carbon (C) via gross primary production (GPP) and respiration (R), but C fluxes are highly variable among lakes. We used a two-prong approach to determine whether C fluxes differed between two alternative stable states observed in shallow lakes. First, we used a replicated whole-lake experiment where we manipulated fish densities in four experimental lakes to induce shifts from the phytoplankton-dominated state (turbid state) to a submersed macrophyte-dominated state (clear state), and determined whether whole-lake GPP, R, and net aquatic production (NAP) changed in response to the manipulation. We also compared lake metabolism in the four experimental lakes to four lakes in a turbid state and four lakes in a clear state. Second, we used sediment cores from 68 shallow lakes to test whether modern burial rates of organic C differed between lakes in clear and turbid states. Biomanipulation in the experimental lakes reduced abundance of fish and phytoplankton and increased abundance of aquatic invertebrates and submerged macrophytes. However, there was no significant change in GPP, R, or NAP. Similarly, GPP, R, and NAP did not differ among experimental lakes, turbid-state lakes, or clear-state lakes. Lastly, organic C burial in sediments did not differ between lakes in clear vs. turbid states, though variability among sites was high. High light and nutrient availability facilitate rapid transitions between two alternative groups of competing, rapidly growing primary producers in shallow lakes. These characteristics facilitate relatively uniform C fluxes at the ecosystem scale despite substantial differences in community structure.

Fish and aquatic plants influence aquatic invertebrate assemblages in shallow temperate lakes, but estimating their relative influence is difficult because invertebrates respond to multiple factors operating simultaneously at various temporal and spatial scales. Co-correspondence analysis (CoCA) may prove useful for clarifying community relationships. We sampled assemblages in 104 shallow lakes in 5 study areas across 4 ecological regions of Minnesota, USA. We used CoCA to test for patterns among aquatic invertebrates, submerged plants, and fish. After controlling for influences of fish with CoCA, we used direct gradient analysis to assess influences of study area, lake state (turbid or clear), and watershed size on the abundance of zooplankton and macroinvertebrates. CoCA indicated high co-correspondence between fish and macrophytes across study areas, with weaker associations among zooplankton, macroinvertebrates, and fish. After removing the effects of fish, zooplankton and macroinvertebrates s...

Ecosystems sometimes undergo dramatic shifts between contrasting regimes. Shallow lakes, for instance, can transition between two alternative stable states: a clear state dominated by submerged aquatic vegetation and a turbid state dominated by phytoplankton. Theoretical models suggest that critical nutrient thresholds differentiate three lake types: highly resilient clear lakes, lakes that may switch between clear and turbid states following perturbations, and highly resilient turbid lakes. For effective and efficient management of shallow lakes and other systems, managers need tools to identify critical thresholds and state-dependent relationships between driving variables and key system features. Using shallow lakes as a model system for which alternative stable states have been demonstrated, we developed an integrated framework using Bayesian latent variable regression (BLR) to classify lake states, identify critical total phosphorus (TP) thresholds, and estimate steady state relationships between TP and chlorophyll a (chl a) using cross-sectional data. We evaluated the method using data simulated from a stochastic differential equation model and compared its performance to k-means clustering with regression (KMR). We also applied the framework to data comprising 130 shallow lakes. For simulated data sets, BLR had high state classification rates (median/mean accuracy >97%) and accurately estimated TP thresholds and state-dependent TP-chl a relationships. Classification and estimation improved with increasing sample size and decreasing noise levels. Compared to KMR, BLR had higher classification rates and better approximated the TP-chl a steady state relationships and TP thresholds. We fit the BLR model to three different years of empirical shallow lake data, and managers can use the estimated bifurcation diagrams to prioritize lakes for management according to their proximity to thresholds and chance of successful rehabilitation. Our model improves upon previous methods for shallow lakes because it allows classification and regression to occur simultaneously and inform one another, directly estimates TP thresholds and the uncertainty associated with thresholds and state classifications, and enables meaningful constraints to be built into models. The BLR framework is broadly applicable to other ecosystems known to exhibit alternative stable states in which regression can be used to establish relationships between driving variables and state variables.

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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.

We examined macrophyte-environment relationships in shallow lakes located within the Prairie Parkland and Laurentian Mixed Forest provinces of Minnesota. Environmental variables included land cover within lake watersheds, and within-lake, water and sediment characteristics. CCA indicated that sediment fraction smaller than 63 μm (f

We assessed the relative infl uence of total phosphorus and piscivore biomass on the abundance of benthivores, soft-rayed planktivores, spiny-rayed planktivores, zooplankton and phytoplankton in 69 shallow lakes in the prairie and parkland areas of Minnesota, USA. Piscivore biomass was the best predictor for three of these response variables, exhibiting a negative relationship with soft-rayed planktivores, a positive relationship with benthivores, and a weaker positive relationship with large-bodied cladocerans. Total phosphorus and piscivores comprised the best model for predicting spiny-rayed planktivores, while neither variable showed any strong relationship to small-bodied cladocerans. Total phosphorus was positively related to phytoplankton abundance, and was the best predictor among all candidate models. Moreover, contrary to predictions of trophic cascade theory, the relationship between chlorophyll a and total phosphorus did not diff er between lakes with and without piscivores. Our results indicated top-down infl uences of piscivores extended through parts of two trophic levels, but failed to infl uence zooplankton – phytoplankton interactions, leaving phytoplankton abundance constrained largely by total phosphorus. Lack of a relationship between piscivores and phytoplankton was likely due to high densities of larval planktivores less susceptible to piscivory, as well as positive infl uences of spiny-rayed planktivores and benthivores on algal abundance. Th ese results support the idea that top – down infl uences of piscivores on phytoplankton abundance may be reduced in more diverse fi sh communities where some prey species are less susceptible to piscivory.

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.

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...

Numerous studies have demonstrated alternative regimes in shallow lake ecosystems around the world, with one state dominated by submerged macrophytes and the other by phytoplankton. However, the stability of each regime, and thresholds at which lakes shift to the alternative regime, are poorly known. We used a cross-sectional analysis of 72 shallow lakes located in prairie and parkland areas of Minnesota, USA, during 2005 and 2006 to assess the occurrence of alternative regimes and shifts between them. Cluster analysis revealed two distinct groups of lakes characterized not only by different macrophyte abundance and chlorophyll a levels but also by different total phosphorus–chlorophyll a relationships. Thirtynine lakes were macrophyte- and 23 lakes phytoplankton-dominated in both years, whereas 10 sites shifted sharply between those regimes. We failed to detect a universal shifting threshold in terms of chlorophyll a or total phosphorus. However, 95% of the lakes with chlorophyll a concentrations less than 22 l gl -1 were in a clearwater regime, whereas 95% of the lakes with chlorophyll a higher than 31 l gl -1 were in a turbid regime. Total phosphorus less than 62 l gl -1 was an accurate predictor of lakes in a stable clearwater regime, whereas a large change in biomass of planktivores and benthivores between years was the only variable weakly related to regime shifts. Our results support the theoretical prediction that regime thresholds vary among lakes. We recommend that lake managers focus on improving resilience of clear regimes in shallow lakes by reducing nutrient loading, rather than attempting to identify and manage complex triggers of regime shifts.

Dense aggregations of waterfowl, often caused by loss of native wetlands and increased waterfowl numbers, can result in the destruction of wetland vegetation and agricultural crops, increase the risk of infec- tious disease outbreaks, and decrease water quality. Problems related to water quality may be particularly se- vere in arid regions of the southwestern United States, where water quality and quantity are contentious is- sues. Over 40,000 Lesser Snow Geese (Chen caerulescens caerulescens) and Ross' Geese (Chen rossii) winter annually at Bosque del Apache National Wildlife Refuge, New Mexico. Daily feeding bouts by geese move large quantities of nutrients from farm fields where they feed to managed wetlands where they roost. Using energy and mass balance models, population estimates, daily and seasonal migration patterns, and feeding behaviors, we estimated the mass and ratio of nitrogen to phosphorus of nutrients loaded by geese into the wetlands of the Bosque del Apache National Wildlife Refuge. Loading rates peaked in late November 1995 at more than 300 kg nitrogen per day and over 30 kg phosphorus per day. As feeding behaviors changed through the winter and bird densities declined, loading rates fell. Our estimates suggest that in the winter of 1995-1996, bird-borne nutrients supplied nearly 40% of the nitrogen and 75% of the phosphorus entering the primary wetland used for geese roosting. High loading rates by geese are a consequence of their colonial roosting behavior; over 90% of the geese roost on 10% of the wetland area. The effects of nutrient loading could be reduced by increasing flushing rates or dispersal of roosting waterfowl. The loss of natural wetlands, however, limits options for expanding wetland area available to waterfowl, and it may be difficult to in- crease flushing rates in this arid region. Thus, management of waterfowl and the nutrients they move will continue to be an important issue for wetlands of wildlife refuges.

The fathead minnow Pimephales promelas occurs in high densities in wetlands of the prairie pothole region (PPR) of North America, but food resources sustaining these populations are poorly known. We assessed population dynamics and prey consumption of fathead minnow populations in three PPR wetlands for 2 years. Fish density peaked at 107 fish per m 2 for all age classes combined. Larval and juvenile fish dominated these populations in terms of abundance and accounted for 83% of total prey consumption. Detritus dominated fish diets, representing 53%, 40% and 79% of diet mass for larval, juvenile and adult fish respectively. Detritus consumption was positively related to minnow density and negatively related to invertebrate abundance, but only for adult fish. Seasonal production:biomass ratios were unrelated to proportions of detritus in the diet for all ages of fish, indicating that detritus is an important food resource capable of meeting metabolic demands and sustaining fish growth in high-density populations. Detritus consumption may also weaken links between abundance of invertebrate prey and minnows, promoting dense fish populations with strong, consistent influences on wetland ecosystems.

We assessed naturally occurring stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) for available food resources and consumers in the mainstream channel of the Mississippi River. Isotopic ratios were assessed for organic sources and organisms at two different sites during a fall, spring, and two summer seasons. Terrestrial C4 plants did not appear to be an important carbon source for consumers in the mainstream channel. A mixing model, IsoSource, indicated that terrestrial C3 vegetation, suspended algae, and epixylon were at times important food resources for large river consumers. Many consumer signatures fell outside the mixing polygon defined by these sources, indicating that there was a 13C-depleted food resource for which we did not account. We could not distinguish precisely whether downstream allochthonous and autochthonous carbon, or in situ production, was the dominant food resource supporting consumers in these systems. However, our data suggest that in situ organic matter sources can be important. Consumer δ13C and δ15N signatures intermediate between several sources indicated widespread omnivory in the river reaches that we studied. To fully understand food web structure and energy sources in complex large river ecosystems, an integrative approach that combines related empirical data sets is needed.

We measured concentrations of multiple elements, including rare earth elements, in waters and sediments of 38 shallow lakes of varying turbidity and macrophyte cover in the Prairie Parkland (PP) and Laurentian Mixed Forest (LMF) provinces of Minnesota. PP shallow lakes had higher element concentrations in waters and sediments compared to LMF sites. Redundancy analysis indicated that a combination of site- and watershed-scale features explained a large proportion of among-lake variability in element concentrations in lake water and sediments. Percent woodland cover in watersheds, turbidity, open water area, and macrophyte cover collectively explained 65.2 % of variation in element concentrations in lake waters. Sediment fraction smaller than 63 μm, percent woodland in watersheds, open water area, and sediment organic matter collectively explained 64.2 % of variation in element concentrations in lake sediments. In contrast to earlier work on shallow lakes, our results showed the extent to which multiple elements in shallow lake waters and sediments were influenced by a combination of variables including sediment characteristics, lake morphology, and percent land cover in watersheds. These results are informative because they help illustrate the extent of functional connectivity between shallow lakes and adjacent lands within these lake watersheds.

We used both direct measurement and bioenergetic methods to examine nitrogen and phosphorus (P) excretion by fathead minnows in two wetlands over 2 yr. We also assessed the potential importance of nutrient flux from fish by comparing excretion estimates to empirically derived estimates of algal P demand. Population-level estimates of P excretion across the four lake-years were high relative to other published studies, peaking at 11.7 mg m 23 d 21

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 biologi...

Carbon and nitrogen stable isotopes (δ13C and δ15N, respectively) were measured for locally abundant primary producers, detritus, aquatic invertebrates, and fish in backwater lakes of two large floodplain rivers in an effort to understand patterns of energy dependence and trophic structure within these habitats. We observed trophic enrichment values for 15N that were within the ranges observed for other aquatic systems but were often considerably lower than the 3.4‰ typically assumed in stable isotope studies. Production based on benthic and terrestrial material, combined with planktonic production for larval fish, appears to be an important energy source supporting consumers in backwater lakes of large floodplain rivers. Our isotopic data challenge the conventional wisdom that macrophytes are centrally involved in supplying carbon to aquatic consumers in shallow lakes and that food webs in most lakes are sustained almost exclusively by carbon fixed by phytoplankton.

Wildlife refuges are sites of dense aggregations of wintering waterfowl. Refuge managers are concerned about local water-quality effects and the amount of birdborne nutrient load that might cause eutrophication in roosting ponds and(or) be exported to downstream systems. We initiated this research effort to test the hypothesis that daily feeding migrations by geese represented a significant source of nutrients to wetland systems at the Bosque del Apache National Wildlife Refuge in the middle Rio Grande River valley of New Mexico. We documented the role of geese in translocating nutrients through a combination of time budget and mass balance models for birds, bioassays of phytoplankton growth, and stable isotope methods that trace the source of nitrogen to food webs. Geese increased the nutrient loading rates in some wetland ponds by up to 40% for total nitrogen and 75% for total phosphorus. Bioassays revealed that nitrogen was consistently limiting to primary production by algae in the ponds. Chlorophyll levels increased in proportion to bird densities. Fish and crayfish from the ponds intensively used by birds had very low stable nitrogen signatures relative to those from the Rio Grande River. This result derived from the low 15 N in alfalfa and corn, which were the primary forage for birds, and accumulated through food webs in proportion to bird use of individual ponds as roosting areas. In general, the wetland ponds functioned efficiently in retaining the birdborne, allochthonous nutrient load and only modest amounts of the total nitrogen or phosphorus were exported downstream.

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.

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.

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.

Summary 1. Anthropogenic landscape modifications have resulted in increased water depths and greater connectivity among the remaining shallow lakes throughout eastern portions of the Prairie Pothole Region of North America. This has created conditions favourable for the establishment of planktivorous fish populations, notably fathead minnows Pimephales pr?rnelas. Such fish populations may reach high densities and reduce Zooplankton and macroinvertebrate abundance and diversity. Resulting reductions in algal grazing by Zooplankton favour transition to a turbid-water state, characterized by increased phytoplankton, decreased water clarity, and reduced macrophytes. Ultimately, such lakes are characterized by low habitat complexity and biodiversity, and conditions less favourable for waterbirds and other wetland-dependent species. 2. We conducted a 3-year study to assess the efficacy of walleye Sander vitreus stocking as a means of initiating trophic cascades, thus testing the ecological theory that piscivore addition can promote shifts from the turbid to the clear-water state by suppressing planktivores (fathead minnows), increasing Zooplankton and macroinvertebrates, and reducing phytoplankton. Treatments consisted of six sites stocked with walleye fry, six lakes stocked with walleye age-1 and older, and six control lakes containing antecedent fathead minnows only. 3. Stocking walleye fry resulted in much lower densities of fathead minnows and higher densities of cladocerans and some macroinvertebrates. Chlorophyll a also decreased in the fry treatment sites. Stocking age-1 and older walleye induced few changes at the stocking density we tested. 4. The success of walleye fry appears to be partly attributable to their rapid diet shift to fish prey and their ability to consume and suppress all life stages of planktivores (fry, juveniles, and adults). However, most responses to walleye fry additions were short-lived, indicating that achieving long-term improvements may require repeated restocking. 5. Synthesis and applications. The available evidence indicates that catchment-level restoration alone is often of limited utility in restoring clear-water conditions in shallow lakes. Our results indicated that stocking piscivorous fish has the potential to improve ecological characteristics of shallow lakes, overcoming tendencies towards phytoplankton dominance. We suggest that managers consider using piscivorous fish stocks, ideally in conjunction with catchment-level restoration measures, to stimulate improvement of ecological characteristics in shallow lakes.

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).