Habitat coupling in lake ecosystems

Lakes are complex ecosystems composed of distinct habitats coupled by biological, physical and chemical processes. While the ecological and evolutionary characteristics of aquatic organisms reflect habitat coupling in lakes, aquatic ecology has largely studied pelagic, benthic and riparian habitats in isolation from each other. Here, we summarize several ecological and evolutionary patterns that highlight the importance of habitat coupling and discuss their implications for understanding ecosystem processes in lakes. We pay special attention to fishes because they play particularly important roles as habitat couplers as a result of their high mobility and flexible foraging tactics that lead to inter-habitat omnivory. Habitat coupling has important consequences for nutrient cycling, predator-prey interactions, and food web structure and stability. For example, nutrient excretion by benthivorous consumers can account for a substantial fraction of inputs to pelagic nutrient cycles. Benthic resources also subsidize carnivore populations that have important predatory effects on plankton communities. These benthic subsidies stabilize population dynamics of pelagic carnivores and intensify the strength of their interactions with planktonic food webs. Furthermore, anthropogenic disturbances such as eutrophication, habitat modification, and exotic species introductions may severely alter habitat connections and, therefore, the fundamental flows of nutrients and energy in lake ecosystems.     

Nutrient dynamics and ecosystem metabolism in the Bay of Blanes (NW Mediterranean)

The dynamics of the nutrient pools and their stoichiometry as well as their control by ecosystem metabolism (benthic and planktonic) and benthic–pelagic exchanges (sedimentation rates and sediment waterfluxes) were examined in the Mediterranean littoral (Blanes Bay, NE Spain). Dissolved organic nitrogen comprised about half of the nitrogen present in the water column and the carbon pool was dominated by the inorganic pool (95% of the carbon present in the water column). The dissolved and particulate organic pools were deficient in P relative to C and N, indicating a rapid recycling of P from organic matter. The pelagic compartment was heterotrophic, supported by significant allochthonous inputs of land material, which also contributed greatly to the sedimentary inputs (37% of total sedimenting carbon). In contrast, the benthic compartment was autotrophic, with the excess net benthic community production balancing the deficit in pelagic community production, leading to metabolic equilibrium at the station studied. Sedimentary inputs of nitrogen, phosphorus and silicon exceeded the benthic release, indicating that the benthic compartment acted as a sink for nutrients, consistent with its autotrophic nature. Carbon inputs to the benthic compartment also exceeded requirements, due to the allochthonous subsidies to the system, so that the benthic compartment stored or exported organic carbon. An erratum to this article can be found at .     

Effects of Terrestrial Organic Matter on Aquatic Primary Production as Mediated by Pelagic–Benthic Resource Fluxes

Flows of energy and matter across habitat boundaries can be major determinants of the functioning of recipient ecosystems. It is currently debated whether terrestrial dissolved organic matter (tDOM) is a resource subsidy or a resource subtraction in recipient lakes. We present data from a long-term field experiment in which pelagic phosphorus concentration and whole-ecosystem primary production increased with increasing tDOM input, suggesting that tDOM acted primarily as a direct nutrient subsidy. Piecewise structural equation modeling supports, however, a substantial contribution of a second mechanism: colored tDOM acted also as a resource subtraction by shading benthic algae, preventing them from intercepting nutrients released across the sediment–water interface. Inhibition of benthic algae by colored tDOM thus indirectly promoted pelagic algae and whole-ecosystem primary production. We conclude that cross-ecosystem terrestrial DOM inputs can modify light and nutrient flows between aquatic habitats and alter the relative contributions of benthic and pelagic habitats to total primary production. These results are particularly relevant for shallow northern lakes, which are projected to receive increased tDOM runoff.     

Marine eutrophication and benthos: the need for new approaches and concepts

In this review, using examples drawn from field observations or experimental studies, our goals are (i) to briefly summarize the major changes, in terms of species composition and functional structure, occurring in phyto and zoobenthic communities in relation to nutrient enrichment of the ecosystems; particular interest is given to the major abiotic and biotic factors occurring during the eutrophication process, (ii) to discuss the direct and indirect influences of benthic organisms on eutrophication and whether the latter can be controlled or favoured by benthos; most benthic species play a major role in the process of benthic nutrient regeneration, affecting primary production by supplying nutrients directly and enhancing rates of pelagic recycling; experimental studies have shown that the impact of benthic fauna on benthic–pelagic coupling and nutrient release is considerable. Thus, once the eutrophication process is engaged—that is, high organic matter sedimentation—it may be indirectly favoured by benthic organisms; benthos should always be considered in eutrophication studies, (iii) to evaluate the limits of our observations and data, highlighting the strong need for integrated studies leading to new concepts. Coastal ecosystems and benthic communities are potentially impacted by numerous human activities (demersal fishing, toxic contaminants, aquaculture…); in order to design strategies of ecosystem restoration or rehabilitation, we have to better understand coastal eutrophication and develop tools for quantifying the impacts; in order to achieve this goal, some possible directions proposed are: integrated studies leading to new concepts, model development based on these concepts and finally comparison of various ecosystems on a global scale.     

Shoreline bank construction modifies benthic–pelagic coupling of food webs

Management of the food web is important in maintaining ecosystem functions; however, the studies to assess anthropogenic impact on food web structures are very limited. Recently, shoreline development is considered to impact the littoral zone of lakes through loss of littoral habitats. Shoreline engineering would consequently modify the food web structure and coupling between benthic and pelagic habitats. Thus, we investigated whether lakeshore modification would change benthic–pelagic coupling by largemouth bass in 15 ponds, which differed in the proportion of concrete bank modification. We performed isotope mixing models to estimate benthic contribution to the fish species and tested the relationships with environmental factors including proportion of bank modification by general linear models. These results showed that the benthic contribution to largemouth bass was strongly related to the proportion of anthropogenic shoreline modification, while for bluegill sunfish, the benthic contribution was not changed with anthropogenic shoreline modification. Therefore, anthropogenic modification of shoreline strongly modified the food webs in ponds through the littoral habitat modification. The results suggested that we should pay attention to the anthropogenic impact on food web structures to maintain ecosystems.     

Contamination on sandflats and the decoupling of linked ecological functions

Benthic macrofauna can influence inputs and transformations of energy and matter in estuaries, affecting both the stocks of vital materials (e.g. carbon, oxygen) and the rates of key processes (e.g. organic matter decomposition, nutrient uptake). Although a number of studies have identified shifts in functional groups or biological traits in relation to anthropogenic stressors, there have been few field‐based assessments of changes in functioning associated with stress gradients. We used a comparative experimental approach to investigate functioning on two sandflats with differing exposures to urban contaminants. Apart from significant differences in sediment contaminant concentrations (43.2 ± 1.8 mg kg^?1 Zn and 15.6 ± 0.9 mg kg^?1 Pb at the Pollen site; 17.7 ± 0.7 mg kg^?1 Zn and 7.9 ± 0.9 mg kg^?1 Pb at the Waiheke site), the two sandflats were readily comparable: both had similar sediment grain size distributions and were dominated by the same macrofaunal species; and both were in non‐eutrophic New Zealand marine reserves with low ambient sediment organic matter content. To better understand the effects of contaminants on biologically mediated transformations of organic matter into inorganic nutrients, we manipulated sediment organic matter content and macrofaunal abundance in standardized treatments at each site. Fluxes of oxygen and ammonium, which are linked to key sandflat processes such as organic matter decomposition and benthic photosynthesis, were measured as response variables 1 week after the experimental manipulations. We predicted more efficient organic matter processing on the uncontaminated flat and thus expected to see elevated ammonium efflux in response to organic enrichment treatments at this site. Higher rates of benthic photosynthesis were predicted for plots with higher ammonium efflux, as ammonium is a readily utilizable form of limiting inorganic nitrogen. We documented significant positive relationships between ammonium uptake and benthic primary production on the uncontaminated flat, but weaker/insignificant relationships at the contaminated site. Our data were consistent with theories of increased variability and a decoupling of system processes with increasing amounts of stress.     

POSITIVE FEEDBACK AND THE DEVELOPMENT AND PERSISTENCE OF ECOSYSTEM DISRUPTIVE ALGAL BLOOMS1

Harmful algal blooms (HABs) have occurred with increasing frequency in recent years with eutrophication and other anthropogenic alterations of coastal ecosystems. Many of these blooms severely alter or degrade ecosystem function, and are referred to here as ecosystem disruptive algal blooms (EDABs). These blooms are often caused by toxic or unpalatable species that decrease grazing rates by planktonic and benthic herbivores, and thereby disrupt the transfer of nutrients and energy to higher trophic levels, and decrease nutrient recycling. Many factors, such as nutrient availability and herbivore grazing have been proposed to separately influence EDAB dynamics, but interactions among these factors have rarely been considered. Here we discuss positive feedback interactions among nutrient availability, herbivore grazing, and nutrient regeneration, which have the potential to substantially influence the dynamics of EDAB events. The positive feedbacks result from a reduction of grazing rates on EDAB species caused by toxicity or unpalatability of these algae, which promotes the proliferation of the EDAB species. The decreased rates also lower grazer‐mediated recycling of nutrients and thereby decrease nutrient availability. Since many EDAB species are well‐adapted to nutrient‐stressed environments and many exhibit increased toxin production and toxicity under nutrient limitation, positive feedbacks are established which can greatly increase the rate of bloom development and the adverse effects on the ecosystem. An understanding of how these feedbacks interact with other regulating factors, such as benthic/pelagic nutrient coupling, physical forcing, and life cycles of EDAB species provides a substantial future challenge.     

Sensitivity of Heterogeneous Marine Benthic Habitats to Subtle Stressors

It is important to understand the consequences of low level disturbances on the functioning of ecological communities because of the pervasiveness and frequency of this type of environmental change. In this study we investigated the response of a heterogeneous, subtidal, soft-sediment habitat to small experimental additions of organic matter and calcium carbonate to examine the sensitivity of benthic ecosystem functioning to changes in sediment characteristics that relate to the environmental threats of coastal eutrophication and ocean acidification. Our results documented significant changes between key biogeochemical and sedimentary variables such as gross primary production, ammonium uptake and dissolved reactive phosphorus flux following treatment additions. Moreover, the application of treatments affected relationships between macrofauna communities, sediment characteristics (e.g., chlorophyll a content) and biogeochemical processes (oxygen and nutrient fluxes). In this experiment organic matter and calcium carbonate showed persistent opposing effects on sedimentary processes, and we demonstrated that highly heterogeneous sediment habitats can be surprisingly sensitive to subtle perturbations. Our results have important biological implications in a world with relentless anthropogenic inputs of atmospheric CO₂ and nutrients in coastal waters.     

Benthic-pelagic interactions in shallow water columns: an experimentalist's perspective

Shallow water column benthic and pelagic communities are thought to be linked by trophic relationships, through life history or ontogenetic links, and by biologically or physically-mediated resuspension or sedimentation processes. It is often confusing and sometimes misleading to focus only on benthic or only on pelagic components of aquatic food webs, even though the literature on shallow water column experiments contains few experiments that give a balanced view of these components, or interactions between components in different habitats. The rarity of balanced experiments is especially troublesome because the most common types of manipulations in shallow water column experiments (fish and nutrients) often have rapid, direct effects on both kinds of habitats, or easily recognized indirect links between the two habitats that go unevaluated. Despite a large experimental literature on pelagic and benthic foodwebs (with less on both in the same systems), there appears to be continuing uncertainty about the importance to pelagic productivity of nutrients released from resuspended sediments, the role of macrobenthos in controlling plankton, and the efficacy and interaction of trophic cascades between pelagic and benthic communities.     

Importance of sediments in understanding nutrient cyclings in lakes

Inorganic and organic nutrients are continuously transported to lake bottoms by sedimentation. By various biological, physical, chemical and mechanical processes quantities of certain nutrients can be brought back to the free water again. This cycling between the sediments and water may occur according to various schemes dependent on lake type and bottom conditions. Lake morphology, temperature regimes, trophic level and sediment type can all strongly influence the size of nutrient pools and rates of turnover.The various activities of bacteria, benthic algae, macrophytes, benthic invertebrates and fish, in conjunction with influences of temperature, pH-values, Eh-values, water content, organic matter and elemental sediment composition, lead to the extremely complex nature of nutrient cycling. Three essential components of aquatic ecosystems are discussed, namely carbon, nitrogen and phosphorus.The objective of this paper is to illustrate in condensed form the heterogeneous nature of nutrient cycling processes. In addition, the importance of sediments in understanding nutrient cycling is discussed from a water management perspective.     

Lake morphometry predicts the degree of habitat coupling by a mobile predator

Habitat coupling is an ecosystem process whereby semi-discontinuous habitats are connected through the movement of energy and nutrients by chemical, physical or biological processes. One oft-cited example is that of littoral–pelagic coupling in lakes. Theory has argued that such habitat coupling may be critical to food web dynamics, yet there have been few empirical studies that have quantified ecological factors that affect the degree of habitat coupling in ecosystems. Specifically, the degree to which habitat coupling occurs across important physical gradients has largely been ignored. To address this, we investigate the degree of littoral habitat coupling (i.e. the degree to which a top predator lake trout, Salvelinus namaycush, derives energy from the littoral zone) along a gradient of lake shape, where lake shape modifies the relative quantity of coupled epilimnetic benthic and pelagic habitats within each lake. Herein we demonstrate that littoral habitat coupling is intensified in simple circular lakes compared to their reticulate counterparts in seven Canadian Shield lakes. Although the more reticulate lakes had larger areas of epilimnetic benthic habitat, littoral food sources comprised 11% compared to 24% of lake trout diet in reticulate and circular lakes, respectively. This heightened interaction in circular lakes also appears to translate into increased omnivory in more circular lakes compared to reticulate lakes such that lake trout of circular lakes have a significantly lower trophic position than lake trout of reticulate lakes (F_1,5=6.71 p=0.05). These results suggest that it is the accessibility of littoral production via thermal refugia, and not the amount of littoral production, that determines the degree to which lake trout couple littoral and pelagic habitats in lakes.     

Ecological responses to simulated benthic-derived nutrient subsidies mediated by omnivorous fish

1. Fish can play an important role in coupling benthic and pelagic habitats by consuming benthic prey and providing essential nutrients to algae in dissolved form. However, little is known about the factors affecting the magnitude of this nutrient subsidy. 2. Using laboratory and mesocosm experiments we evaluated how varying ingestion rates of bluegill sunfish (Lepomis macrochirus) affects fish excretion rates of both nitrogen (N) and phosphorus (P). During the 10‐week mesocosm experiment, we also evaluated how varying ingestion rates may affect plankton community dynamics, and nutrient flux between pelagic and benthic habitats. Lastly, bioenergetic/mass balance models were used to examine the nutrient stoichiometry of fish body composition and excretion products. 3. Under laboratory conditions, both N and P excretion rates increased with increased ingestion of benthic prey surrogates (earthworms). This effect was more pronounced for N than P. Furthermore, under the more realistic conditions of the mesocosm experiment ingestion rate had no significant effect on P excretion rate. 4. Increased fish ingestion rate in the mesocosm experiment increased total algal biomass and the flux of nutrients from the water column to sediments. Effects of variable ingestion were much stronger on periphyton biomass and algal sedimentation rates than on phytoplankton or zooplankton biomass or composition. 5. Fish body nutrient composition was greatly affected by ingestion rate. N content increased and P content decreased with ingestion rate. As a result, the N : P ratio of fish bodies also increased with ingestion rate. The N : P ratio of nutrients excreted by fish also increased with ingestion rate, counter to predictions of stoichiometric theory, which predicts that excreted N : P ratio is negatively correlated to body N : P. However, this finding can be explained by relaxing the assumption of constant nutrient assimilation rates, and our mass balance data suggest that assimilation rates vary indeed with ingestion rate. 6. Our study provides experimental evidence that translocation of benthic‐derived nutrients by fish can affect the flux of nutrients among habitats, while also suggesting that stoichiometry models need to better incorporate how variable ingestion rates affect nutrient assimilation and excretion rates.     

The Function of Marine Critical Transition Zones and the Importance of Sediment Biodiversity

Estuaries and coastal wetlands are critical transition zones (CTZs) that link land, freshwater habitats, and the sea. CTZs provide essential ecological functions, including decomposition, nutrient cycling, and nutrient production, as well as regulation of fluxes of nutrients, water, particles, and organisms to and from land, rivers, and the ocean. Sediment-associated biota are integral to these functions. Functional groups considered essential to CTZ processes include heterotrophic bacteria and fungi, as well as many benthic invertebrates. Key invertebrate functions include shredding, which breaks down and recycles organic matter; suspension feeding, which collects and transports sediments across the sediment–water interface; and bioturbating, which moves sediment into or out of the seabed. In addition, macrophytes regulate many aspects of nutrient, particle, and organism dynamics above- and belowground. Animals moving within or through CTZs are vectors that transport nutrients and organic matter across terrestrial, freshwater, and marine interfaces. Significant threats to biodiversity within CTZs are posed by anthropogenic influences; eutrophication, nonnutrient pollutants, species invasions, overfishing, habitat alteration, and climate change affect species richness or composition in many coastal environments. Because biotic diversity in marine CTZ sediments is inherently low whereas their functional significance is great, shifts in diversity are likely to be particularly important. Species introductions (from invasion) or loss (from overfishing or habitat alteration) provide evidence that single-species changes can have overt, sweeping effects on CTZ structure and function. Certain species may be critically important to the maintenance of ecosystem functions in CTZs even though at present there is limited empirical evidence that the number of species in CTZ sediments is critical. We hypothesized that diversity is indeed important to ecosystem function in marine CTZs because high diversity maintains positive interactions among species (facilitation and mutualism), promoting stability and resistance to invasion or other forms of disturbance. The complexity of interactions among species and feedbacks with ecosystem functions suggests that comparative (mensurative) and manipulative approaches will be required to elucidate the role of diversity in sustaining CTZ functions. Received 25 February 2000; accepted 31 January 2001.     

Interactions between temperature and nutrients across levels of ecological organization

Temperature and nutrient availability play key roles in controlling the pathways and rates at which energy and materials move through ecosystems. These factors have also changed dramatically on Earth over the past century as human activities have intensified. Although significant effort has been devoted to understanding the role of temperature and nutrients in isolation, less is known about how these two factors interact to influence ecological processes. Recent advances in ecological stoichiometry and metabolic ecology provide a useful framework for making progress in this area, but conceptual synthesis and review are needed to help catalyze additional research. Here, we examine known and potential interactions between temperature and nutrients from a variety of physiological, community, and ecosystem perspectives. We first review patterns at the level of the individual, focusing on four traits – growth, respiration, body size, and elemental content – that should theoretically govern how temperature and nutrients interact to influence higher levels of biological organization. We next explore the interactive effects of temperature and nutrients on populations, communities, and food webs by synthesizing information related to community size spectra, biomass distributions, and elemental composition. We use metabolic theory to make predictions about how population‐level secondary production should respond to interactions between temperature and resource supply, setting up qualitative predictions about the flows of energy and materials through metazoan food webs. Last, we examine how temperature–nutrient interactions influence processes at the whole‐ecosystem level, focusing on apparent vs. intrinsic activation energies of ecosystem processes, how to represent temperature–nutrient interactions in ecosystem models, and patterns with respect to nutrient uptake and organic matter decomposition. We conclude that a better understanding of interactions between temperature and nutrients will be critical for developing realistic predictions about ecological responses to multiple, simultaneous drivers of global change, including climate warming and elevated nutrient supply.     

Environmental Drivers of Benthic Flux Variation and Ecosystem Functioning in Salish Sea and Northeast Pacific Sediments

The upwelling of deep waters from the oxygen minimum zone in the Northeast Pacific from the continental slope to the shelf and into the Salish Sea during spring and summer offers a unique opportunity to study ecosystem functioning in the form of benthic fluxes along natural gradients. Using the ROV ROPOS we collected sediment cores from 10 sites in May and July 2011, and September 2013 to perform shipboard incubations and flux measurements. Specifically, we measured benthic fluxes of oxygen and nutrients to evaluate potential environmental drivers of benthic flux variation and ecosystem functioning along natural gradients of temperature and bottom water dissolved oxygen concentrations. The range of temperature and dissolved oxygen encountered across our study sites allowed us to apply a suite of multivariate analyses rarely used in flux studies to identify bottom water temperature as the primary environmental driver of benthic flux variation and organic matter remineralization. Redundancy analysis revealed that bottom water characteristics (temperature and dissolved oxygen), quality of organic matter (chl a:phaeo and C:N ratios) and sediment characteristics (mean grain size and porosity) explained 51.5% of benthic flux variation. Multivariate analyses identified significant spatial and temporal variation in benthic fluxes, demonstrating key differences between the Northeast Pacific and Salish Sea. Moreover, Northeast Pacific slope fluxes were generally lower than shelf fluxes. Spatial and temporal variation in benthic fluxes in the Salish Sea were driven primarily by differences in temperature and quality of organic matter on the seafloor following phytoplankton blooms. These results demonstrate the utility of multivariate approaches in differentiating among potential drivers of seafloor ecosystem functioning, and indicate that current and future predictive models of organic matter remineralization and ecosystem functioning of soft-muddy shelf and slope seafloor habitats should consider bottom water temperature variation. Bottom temperature has important implications for estimates of seasonal and spatial benthic flux variation, benthic–pelagic coupling, and impacts of predicted ocean warming at high latitudes.     

Water abstraction impacts stream ecosystem functioning via wetted‐channel contraction

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Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

Productivity and trophic structure of aquatic ecosystems result from a complex interplay of bottom‐up and top‐down forces that operate across benthic and pelagic food web compartments. Projected global changes urge the question how this interplay will be affected by browning (increasing input of terrestrial dissolved organic matter), nutrient enrichment and warming. We explored this with a process‐based model of a shallow lake food web consisting of benthic and pelagic components (abiotic resources, primary producers, grazers, carnivores), and compared model expectations with the results of a browning and warming experiment in nutrient‐poor ponds harboring a boreal lake community. Under low nutrient conditions, the model makes three major predictions. (a) Browning reduces light and increases nutrient supply; this decreases benthic and increases pelagic production, gradually shifting productivity from the benthic to the pelagic habitat. (b) Because of active habitat choice, fish exert top‐down control on grazers and benefit primary producers primarily in the more productive of the two habitats. (c) Warming relaxes top‐down control of grazers by fish and decreases primary producer biomass, but effects of warming are generally small compared to effects of browning and nutrient supply. Experimental results were consistent with most model predictions for browning: light penetration, benthic algal production, and zoobenthos biomass decreased, and pelagic nutrients and pelagic algal production increased with browning. Also consistent with expectations, warming had negative effects on benthic and pelagic algal biomass and weak effects on algal production and zoobenthos and zooplankton biomass. Inconsistent with expectations, browning had no effect on zooplankton and warming effects on fish depended on browning. The model is applicable also to nutrient‐rich systems, and we propose that it is a useful tool for the exploration of the consequences of different climate change scenarios for productivity and food web dynamics in shallow lakes, the worldwide most common lake type.     

Nutrient reduction and climate change cause a potential shift from pelagic to benthic pathways in a eutrophic marine ecosystem

The degree to which marine ecosystems may support the pelagic or benthic food chain has been shown to vary across natural and anthropogenic gradients for e.g., in temperature and nutrient availability. Moreover, such external forcing may not only affect the flux of organic matter but could trigger large and abrupt changes, i.e., trophic cascades and ecological regime shifts, which once having occurred may prove potentially irreversible. In this study, we investigate the state and regulatory pathways of the Kattegat; a eutrophied and heavily exploited marine ecosystem, specifically testing for the occurrence of regime shifts and the relative importance of multiple drivers, e.g., climate change, eutrophication and commercial fishing on ecosystem dynamics and trophic pathways. Using multivariate statistics and nonlinear regression on a comprehensive data set, covering abiotic factors and biotic variables across all trophic levels, we here propose a potential regime shift from pelagic to benthic regulatory pathways; a possible first sign of recovery from eutrophication likely triggered by drastic nutrient reductions (involving both nitrogen and phosphorus), in combination with climate‐driven changes in local environmental conditions (e.g., temperature and oxygen concentrations).     

Disentangling effects of multiple stressors on matter flow in a lake food web

Understanding the relative importance of multiple stressors is valuable to prioritize conservation and restoration measures. Yet, the effects of multiple stressors on ecosystem functioning remain largely unknown in many fresh waters. Here, we provided a methodology combining ecosystem modeling with linear regression to disentangle the effects of multiple stressors on matter flow, an important ecosystem function. Treating a shallow lake as the model ecosystem, we simulated matter flow dynamics during 1950s–2010s with different combinations of stressors by Ecopath with Ecosim (EwE) modeling and determined the relative importance of each stressor by generalized linear mixed models. We found that matter flow of the lake food web was highly dynamic, attributing to effects of multiple anthropogenic stressors. Biological invasion played the strongest role in driving the matter flow dynamics, followed by eutrophication, while biomanipulation (i.e., phytoplankton control by planktivorous fish stocking) was of little importance. Eutrophication had a stronger role on primary producers, pelagic food chain, and top predators, while biological invasion on consumers in the middle of food chains. The former was more important in driving the quantity of matter flow, while the latter on trophic transfer efficiencies. Scenario forecasting showed that reducing nutrients contents could largely shape the matter flow pattern, while biomanipulation had little effect. Our findings provided new insights into understanding the mechanistic links between anthropogenic stressors and ecosystem functioning by combining ecosystem modeling with linear regression.     

Ecosystem effects of water column minnows in experimental streams

We used red shiner (Cyprinella lutrensis) as a model to examine ecosystem effects of water column stream minnows (Cyprinidae) in experimental streams. Benthic primary productivity, benthic invertebrate abundance, water column nutrient concentrations, size distribution of benthic particulate organic matter (BPOM), and sedimentation rates were measured across a range of fish densities (0–26.6 fish m^–2) over a 35-day period. In addition, effects of fish density on algal standing crop and benthic invertebrates in experimental streams were examined over a longer time span (156 and 203 days). After 35 days, benthic primary productivity was positively associated with fish density, with an approximate three-fold increase in productivity between experimental streams stocked with no fish and those with 26.6 fish m^–2. No effects on other ecosystem properties were detected after 35 days. Additionally, there was no effect on algal standing crop after 156 or 203 days and no effect on benthic invertebrates after 203 days. Because red shiners fed primarily on terrestrial insects, this experiment suggests that water column minnows can affect primary productivity in streams by transporting nutrients from terrestrial sources to the benthic compartment of the ecosystem. However, this effect may only be important in streams or during periods when nutrients are limiting.