Energy, nutrients, and contaminants derived from aquatic ecosystems and transferred to terrestrial ecosystems are termed aquatic-terrestrial subsidies or, more simply, aquatic subsidies. Common examples of aquatic subsidies include organisms that move across habitat boundaries and deposit their nutrients as they decompose in terrestrial habitats or are consumed by terrestrial predators, such as spiders, lizards, birds, and bats. Aquatic insects that develop within streams and lakes before emerging as winged adults and moving to terrestrial habitats contribute to aquatic subsidies. Fish removed from aquatic ecosystems by terrestrial predators are another important example. Conversely, the flow of energy and nutrients from terrestrial ecosystems to aquatic ecosystems are considered terrestrial subsidies, like leaf litter entering a stream. Both aquatic subsidies and terrestrial subsidies are types of cross-boundary subsidies. Energy and nutrients are derived from outside the ecosystem where they are ultimately consumed.
Allochthonous describes resources and energy derived from another ecosystem; aquatic-terrestrial subsidies are examples of allochthonous resources. Autochthonous resources are produced by plants or algae within the local ecosystem Allochthonous resources, including aquatic-terrestrial subsidies, can subsidize predator populations and increase predator impacts on prey populations, sometimes initiating trophic cascades. Nutritional traits of autochthonous and allochthonous resources influences their use by microorganisms, animals and other consumers, even when they are readily available.
Resource subsidies Resource subsidies, in forms of nutrients, matter, or organisms, describe movements of essential resources across habitat boundaries to animals or other consumers. These inputs of resources can influence individual growth, species abundance and diversity, community structure, secondary productivity and food web dynamics.
Resource subsidies supplement the productivity of the recipient consumer, but the consumer has little impact on productivity of the resource. As a result, resource subsidies are described as "donor-controlled". The flux rate of the subsidy is independent of productivity in the recipient habitat. Aquatic-terrestrial resource subsidies are often strongly seasonal. Aquatic insect emergence is typically highest during the warm season, while terrestrial leaf fall into aquatic habitats is associated with autumn in temperate biomes. The timing of these resource-subsidy pulses is important to how they are used by predators and other consumers, and the impacts on predator-prey dynamics in recipient habitats. In some cases, subsidies can destabilize predator-prey dynamics in recipient habitats. For example, blooms of algae can increase insect productivity and emergence, resulting in growth of terrestrial predator populations.
The rate of resource subsidy fluxes is mediated by the permeability of ecotones and modified by physical and biological factors. Species interactions within donor habitats and variability in climate can both alter rates of cross-habitat resource fluxes. The response of recipient consumers to an influx of resources depends on conditions within the recipient habitat; effects are largest when other resources are scarce within the recipient habitat. Flows between terrestrial and stream environments are among the best studied cross-boundary subsidies.
Aquatic subsidies Aquatic subsidies are energy or nutrients that are transferred from the aquatic environment to the terrestrial environment. These aquatic subsidies vary spatially and seasonally. Subsidies support ecosystem functions and link interactions between species.
Marine anadromous fishes such as salmon provide a subsidy to freshwater and then terrestrial ecosystems through spawning and carcasses. These marine-derived nutrients provide resources to a range of species both in the stream and on land. Terrestrial species that feed on salmon include river otters, mink, bald eagles and bears. Stream invertebrates such as stoneflies, caddisflies and midges also derive energy and nutrients from salmon and, in turn, provide food to terrestrial species such as birds and bats. Animals are not the only benefactors of these aquatic subsidies, riparian plants can receive up to 26% of their nitrogen from salmon.
Lateral movement of nutrients and energy from the stream to the surrounding riparian zone and terrestrial environment beyond serve an important role in food webs. Flooding of a stream and the movement of organisms both act to transfer nutrients and energy sources to the terrestrial environment. Algae and fine organic matter washed up from high flows provide resources to herbivorous species and promote plant germination. These lateral movements are limited in how far they make it away from the stream without help, but terrestrial species can increase the distance that these subsidies travel. For example, the emergence of adult aquatic insects from streams is one of the most distinct and well studied forms of aquatic subsidies. They supply 25–100% of the energy or carbon to riparian species such as spiders, bats, birds, and lizards. Emergence of aquatic insects typically peaks in the summer of temperate zones, prompting predators to aggregate and forage along riparian and stream boundaries. These species typically feed near the water's edge but then when they leave to travel elsewhere, their feces will add nutrients to other environments. Another example of a terrestrial species that moves aquatic subsidies further inland is that of the brown bear. Brown bears consume a massive amount of salmon from streams, so much so, they are considered a keystone species. Brown bears have been shown to deliver as much as 84% of the nitrogen found in white spruce trees that are up to 500 meters from the stream on the Kenai Peninsula (Alaska, USA) through their interactions with aquatic subsidies.
Ecological importance of aquatic subsidies
Although inputs from the terrestrial environment to an aquatic one (terrestrial subsidies) have been studied extensively, aquatic inputs to the terrestrial environment (aquatic subsidies) haven't been as widely studied. Aquatic subsidies, however, can be extremely important in the terrestrial landscape and are generally of higher nutritional quality because they come from animal, rather than plant-based or detrital, sources. These aquatic subsidies may be more important than terrestrial prey for riparian predators in some ecosystems.
However, aquatic subsidies are also increasingly recognized as important sources of environmental contaminants to terrestrial food webs. Aquatic animals can accumulate pollutants in their tissues and exoskeletons (such as metals and polychlorinated biphenyls) and move them to riparian and terrestrial systems as they emerge or when they are consumed by terrestrial predators.
While aquatic subsidies provide a pathway for anthropogenic stressors to propagate from aquatic to terrestrial ecosystems, they are themselves being impacted by global change. Global warming and habitat modification change both the physiology and phenology of emerging aquatic insects as well as the physical boundary between water and land, which in turn affects their dispersal. In temperate regions, increasing temperature increases the growth and emergence rate of aquatic insects, while in tropical regions aquatic insect emergence rates decline.
Terrestrial subsidies Terrestrial subsidies are primary production on land that is transferred to aquatic ecosystems as litter fall or dissolved organic matter.
Terrestrial subsidies or allochthonous inputs into aquatic environments are a major component of organic carbon budgets for aquatic systems. In many ecosystems autochthonous production of carbon is not enough to support the food web and organisms rely on allochthonous to maintain secondary production. Aquatic ecosystems are generally heterotrophic; respiration exceeds production, suggesting the food web is supported externally. The carbon that enters the aquatic ecosystem from terrestrial inputs is taken up by micro-organisms like bacteria and fungi which are then consumed by higher trophic levels This microbial transfer of organic carbon has shown to support food webs in lakes and streams.
Organic carbon inputs into aquatic ecosystems come in multiple forms. The two main forms are dissolved organic carbon (DOC) or particulate organic carbon (POC). Particulate organic carbon includes living organisms like bacteria, phytoplankton, zooplankton, as well as detrital components. Dissolved organic carbon is organic carbon that has been broken down, is suspended, and considered soluble in water. Dissolved organic carbon has been shown to stimulate heterotrophic production in aquatic settings and heterotrophic bacteria can use dissolved organic carbon to support their growth. Particulate organic carbon also stimulates heterotrophic production which becomes available to bacteria or other micro-organisms through decomposition and other consumers by direct consumption.
Terrestrial invertebrates such as spiders, caterpillars, and ants are also an important form of terrestrial subsidy to aquatic ecosystems.
Drift-feeding fish can rely on falling terrestrial invertebrates for up to half of their annual energy budget. Variation in the flux of terrestrial invertebrates is dependent on the weather, time – annual and daily – and the riparian architecture. Warmer and more humid temperatures, generally associated with summer and early fall, facilitate greater invertebrate activity and thus larger subsidies, whereas wet seasons reduce the flux of terrestrial invertebrates. Daily, the input of terrestrial invertebrates is greatest during afternoons and evenings. Finally, riparian zones composed of closed canopy deciduous vegetation can support higher density and diversity of fishes compared to other vegetation types, due to the greater supply of terrestrial invertebrates.
Terrestrial leaf litter, wood inputs and deposition of pollen are important organic matter sources that augment benthic invertebrate productivity. In particular, these terrestrial subsidies are vital for detritivores and shredders and control their population sizes. Benthic invertebrate communities respond swiftly to changes in the supply of organic matter; the absence of litter stocks led to a drastic decline in productivity and predators in one experimental temperate stream system. Furthermore, provision of organic matter may increase productivity and create hypoxic conditions in streams; however, this is typically uncommon given the high turnover and low residence time of water. In the Mara River basin, though, substantial rates of organic matter and nutrient loading by hippopotami create subsidy overloads in hippo pools, stimulate anoxic conditions approximately three times a year, and cause multiple fish kill events.
Contaminants as aquatic-terrestrial subsidies Aquatic-terrestrial contaminant subsidies originating in the aquatic environment can be transported across ecosystem boundaries, primarily mediated by organisms. The transmission of contaminants can have negative ecological consequences that amplify up the food chain, including reduced nesting success of birds, disruptions to riparian food webs, and contamination of otherwise pristine environments. The mechanism of aquatic-terrestrial contaminant transfer can be especially influential when there are no additional sources of those contaminants to the terrestrial system.
Types of contaminant subsidies