Soil microbes weaken the positive effect of an aquatic-terrestrial subsidy on plant performance

Aims Linkages formed through aquatic-terrestrial subsidies can play an important role in structuring communities and mediating ecosystem functions. Aquatic-terrestrial subsidies may be especially important in nutrient-poor ecosystems, such as the freshwater sand dunes surrounding Lake Michigan. Adult midges emerge from Lake Michigan in the spring, swarm to mate, and die. Their carcasses form mounds at the base of plants, where they may increase plant productivity through their nutrient inputs. However, the effect of aquatic-terrestrial subsidies on plant productivity could depend on other biotic interactions. In particular, soil microbes might play a key role in facilitating the conversion of nutrients to plant-available forms or competing for the nutrients by plants. Methods In a greenhouse experiment, we tested how carcasses from lake-emergent midges (Chironomidae) and soil microbes independently and interactively influenced the performance of a common dune grass, Calamovilfa longifolia. To determine whether midges influenced abiotic soil properties, we measured how midge additions influenced soil nutrients and soil moisture. Important findings Midges greatly increased plant biomass, while soil microbes influenced the magnitude of this effect. In the absence of soil microbes plant biomass was 7 times greater with midges than without midges. However, in the presence of soil microbes, plant biomass was only 3 times greater. The effect of midges might be driven by their nutrient inputs into the soil, as midges contained 100 times more N, 10 times more P, and 150 times more K than dune soils did. Our results suggest that soil microbes may be competing with plants for these nutrients. In sum, we found that midges can be an important aquatic-terrestrial subsidy that produces strong, positive effects on plant productivity along the shorelines of Lake Michigan, but that the impact of aquatic-terrestrial subsidies must be considered within the context of the complex interactions that take place within ecological communities.

Local and landscape drivers of aquatic-to-terrestrial subsidies in riparian ecosystems: a worldwide meta-analysis

Cross-boundary fluxes of organisms and matter, termed “subsidies”, are now recognized to be reciprocal and of roughly equal importance for both aquatic and terrestrial systems, even if terrestrial input to aquatic ecosystems has received most attention. The magnitude of aquatic to terrestrial subsidies is well documented, but the drivers behind these subsidies and their utilization by terrestrial consumers are characteristically local scale studies, limiting the inferences that can be drawn for broader geographic scales. We therefore built and analyzed a database of stable isotope data extracted from 21 studies worldwide, to identify both landscape and local scale variables that may affect the diet of terrestrial predators in riparian ecosystems. Our meta-analysis revealed a greater magnitude of aquatic-to-terrestrial subsidies (> 50%) than previously reported, albeit with large geographic and inter-annual variations. We demonstrated a large effect of landscape-scale factors on aquatic-to-terrestrial subsidies, particularly anthropogenic land use and tree cover. Local human population was the only relevant factor at the local scale. We also found that studies on landscape-scale and anthropogenic land use effects on aquatic-to-terrestrial subsidies are currently strongly under-represented in the ecological literature. Such studies are needed to improve our understanding of how land use and environmental change might influence future patterns of biodiversity and ecosystem function.

Predation by introduced fish can magnify the terrestrial arthropod subsidies in mountain lakes

A portion of the terrestrial subsidies to lentic habitats consists of arthropods. In high mountain, originally fishless lakes, terrestrial arthropods are an important seasonal food resource for introduced fish. Here we investigate how brook trout (Salvelinus fontinalis) can alter the input of terrestrial arthropods in 10 high mountain lakes contrasting for their stocking history (with and without fish or manipulated for fish eradication). We used a food consumption model to calculate the minimum and maximum number or biomass of arthropods consumed by fish, and we found that they can exceed, by several folds, the number or biomass of arthropods sinking into the lakes, at least under the metabolic rates expected for fish for most of the summer. We interpret this result as an indirect indication that arthropods usually cannot overcome the surface tension at the lake surface and that fish can work as a vector across the water–air interface. We infer that pathways for dead and live arthropods to leave the lakes do exist and fish can transfer into the water column many arthropods whose fate was leaving the lakes, which may have overlooked ecological and conservation implications.

Restoring dissolved organic carbon subsidies from floodplains to lowland river food webs: a role for environmental flows?

Environmental flows are managed events in river systems designed to enhance the ecological condition of aquatic ecosystems. Although not traditionally seen as important in lowland rivers, there is mounting evidence that terrestrial subsidies can be an important energy source in aquatic metazoan food webs. We argue that the apparent lack of importance of terrestrial subsidies to many lowland river food webs may reflect an artefact resulting from historical anthropogenic changes to lowland river–floodplain ecosystems, including the loss of lateral connectivity between rivers and their floodplains, changes in floodplain land use and carbon stores, and loss of sites of transformation within the main channel. The loss of floodplain subsidies to the main river channel can be partially redressed using environmental flows; however, this will require mimicking important aspects of natural high-flow events that have hitherto been overlooked when targeting environmental flows to a limited suite of biota. We suggest that key biotic targets for environmental flow releases may not be achievable unless river–floodplain subsidies are sufficiently restored. Environmental flows can go some way to addressing this shortfall, but only if floodplain subsidies to river channels are explicitly included in the design and management of environmental flows.