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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
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s 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 Terrestrial ecosystems are ecosystems that are found on land. Examples include tundra, taiga, temperate deciduous forest, tropical rain forest, grassland, deserts. Terrestrial ecosystems differ from aquatic ecosystems by the predominant presence o ...
to
aquatic ecosystems An aquatic ecosystem is an ecosystem found in and around a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms—aquatic life—that are dependent on each other and on their environm ...
are considered terrestrial subsidies; 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 River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts.Angelier ...
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 cascade Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to redu ...
s. Nutritional quality of autochthonous and allochthonous resources influences their use by 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. Allochthonous resources are defined as originating outside of the ecosystem while autochthonous resources are derived within the ecosystem. For example, leaf fall into a stream would be an allochthonous resource. 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
ecotone An ecotone is a transitional area between two plant communities, where these meet and integrate. Examples include areas between grassland and forest, estuaries and lagoon, freshwater and sea water etc. An ecotone may be narrow or wide, and it ma ...
s 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 Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousa ...
fishes such as
salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...
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
invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...
s 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 Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
from salmon. Lateral movement of nutrients and energy from the stream to the surrounding
riparian zone A riparian zone or riparian area is the interface between land and a river or stream. In some regions, the terms riparian woodland, riparian forest, riparian buffer zone, riparian corridor, and riparian strip are used to characterize a ripari ...
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 Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
and fine organic matter washed up from high flows provide resources to
herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...
species and promote plant
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, ...
. 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 The brown bear (''Ursus arctos'') is a large bear native to Eurasia and North America. Of the land carnivorans, it is rivaled in size only by its closest relative, the polar bear, which is much less variable in size and slightly bigger on av ...
. Brown bears consume a massive amount of salmon from streams, so much so, they are considered a
keystone species A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in main ...
. 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 The Kenai Peninsula ( Dena'ina: ''Yaghenen'') is a large peninsula jutting from the coast of Southcentral Alaska. The name Kenai (, ) is derived from the word "Kenaitze" or "Kenaitze Indian Tribe", the name of the Native Athabascan Alaskan tribe ...
(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 Polychlorinated biphenyls (PCBs) are organochlorine compounds with the formula C12 H10−''x'' Cl''x''; they were once widely used in the manufacture of carbonless copy paper, as heat transfer fluids, and as dielectric and coolant fluids f ...
) 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 Phenology is the study of periodic events in biological life cycles and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation). Examples include the date of emergence of leav ...
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
ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
s 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 Autochthon, autochthons or autochthonous may refer to: Nature * Autochthon (geology), a sediment or rock that can be found at its site of formation or deposition * Autochthon (nature), or landrace, an indigenous animal or plant * Autochthonou ...
production of carbon is not enough to support the food web and organisms rely on allochthonous to maintain
secondary production In ecology, the term productivity refers to the rate of generation of biomass in an ecosystem, usually expressed in units of mass per volume (unit surface) per unit of time, such as grams per square metre per day (g m−2 d−1). The unit of mas ...
. Aquatic ecosystems are generally
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
ic;
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...
exceeds production, suggesting the
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...
is supported externally. The carbon that enters the aquatic ecosystem from terrestrial inputs is taken up by
micro-organisms A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in Ja ...
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 Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of wa ...
inputs into aquatic ecosystems come in multiple forms. The two main forms are
dissolved organic carbon Dissolved organic carbon (DOC) is the fraction of organic carbon Operational definition, operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometre, micrometers. The fraction remain ...
(DOC) or
particulate organic carbon Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 millimeters (53 μm) to 2 millimeters. Particulate org ...
(POC). Particulate organic carbon includes living organisms like
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
,
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
,
zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...
, as well as
detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephen ...
components. Dissolved organic carbon is organic carbon that has been broken down, is suspended, and considered
soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...
in water. Dissolved organic carbon has been shown to stimulate
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
ic production in aquatic settings and
heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
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 Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is ess ...
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 A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...
, including reduced nesting success of
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, disruptions to riparian
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...
s, 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

Various organic compounds, trace elements, metals, algal toxins, pesticides, and pharmaceutical waste products resulting from intentional or incidental releases via human activities can act as contaminant subsidies. After being loaded into waterways, contaminants that accumulate in the aquatic food web can return to terrestrial environments through consumption by organisms.


Movement pathways through animals

Organisms serve as the vector for transportation of contaminant subsidies across
trophic level The trophic level of an organism is the position it occupies in a food web. Within a food web, a food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the ...
s and aquatic-terrestrial ecosystem boundaries. Understanding the fate of aquatic-terrestrial subsidies is key to predicting their impact on terrestrial consumers.


Invertebrates

Aquatic invertebrate Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates, including the ...
s take up contaminants introduced to the environment via the
water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...
, by grazing on surfaces, and from contaminated sediment. These contaminants can have several fates depending on their biochemical properties. One, that contaminants like metals and
polycyclic aromatic hydrocarbon A Polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incine ...
s (PAHs) are preferentially shed into the
exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...
during
metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...
, and then recycled into the aquatic environment. Two, macroinvertebrates eaten during aquatic or larval stages transfer their contaminant burdens to higher aquatic trophic levels such as fish and those contaminants are retained by the aquatic environment. Contaminants that would otherwise be shed during metamorphosis are therefore most likely to be taken up by aquatic predators of larval stage insects. Three, larval aquatic macroinvertebrates can transfer contaminant subsidies directly to terrestrial environments following successful metamorphosis to their adult form. In particular, man-made organic contaminants like
polychlorinated biphenyl Polychlorinated biphenyls (PCBs) are organochlorine compounds with the formula Carbon, C12Hydrogen, H10−''x''Chloride, Cl''x''; they were once widely used in the manufacture of carbonless copy paper, as heat transfer fluids, and as dielectri ...
s (PCBs) can become concentrated in adults. Predator risk for the uptake of organic contaminants is higher when preying upon adult life stages of aquatic insects, and adult aquatic insects are more likely to be consumed by terrestrial predators such as birds. Terrestrial predatory invertebrates have also been identified as vectors of contaminant transport. In particular, riparian
spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...
s have been shown to move contaminants, such as
methylmercury Methylmercury is an organometallic cation with the formula . It is the simplest organomercury compound. Methylmercury is extremely toxic, and its derivatives are the major source of organic mercury for humans. It is a bioaccumulative environment ...
, originating in aquatic prey to the terrestrial environment.


Fish

Because many
fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
species
prey Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not ki ...
upon macroinvertebrates that may have taken up contaminants, fish are an important middle trophic level for contaminant transport. Subsequent consumption of fish from aquatic environments by terrestrial predators is a significant movement pathway for aquatic-terrestrial subsidies.
Anadromous Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousa ...
migratory fish, such as
salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...
, transport contaminants far distances and across aquatic ecosystem boundaries. The consumption of salmon by terrestrial predators, such as bears, when salmon return to freshwater ecosystems to
spawn Spawn or spawning may refer to: * Spawning, the eggs and sperm of aquatic animals Arts, entertainment and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** ''Spawn: Armageddon' ...
transfers marine-derived contaminant subsidies to terrestrial systems far removed from areas of contaminant uptake by the aquatic food web. Salmon can be the largest dietary source of marine-derived contaminants consumed by bears. Salmon-derived contaminants are also transported to recipient aquatic ecosystems where salmon spawn and/or die. Contaminants may be maternally transferred to eggs or recycled to the base of aquatic food for subsequent trophic transfer to higher trophic levels. Consumption of animals containing these contaminants by terrestrial predators is another pathway of aquatic-terrestrial subsidy transfer across large spatial scales.


Birds

Fish-eating birds are at the topmost trophic level of many aquatic food webs. As a result, birds are often the recipients of aquatic contaminant subsidies and transporters of aquatic contaminants to the terrestrial environment. An area of much research in birds is the tendency for contaminants present in the aquatic environment to
biomagnify Biomagnification, also known as bioamplification or biological magnification, is the increase in concentration of a substance, e.g a pesticide, in the tissues of organisms at successively higher levels in a food chain. This increase can occur a ...
to significant levels in predatory birds. This phenomenon was exemplified by
DDT Dichlorodiphenyltrichloroethane, commonly known as DDT, is a colorless, tasteless, and almost odorless crystalline chemical compound, an organochloride. Originally developed as an insecticide, it became infamous for its environmental impacts. ...
biomagnification in predatory birds during the 1960s in the US, which resulted in the collapse of many bird populations.
Migratory birds Bird migration is a seasonal movement of birds between breeding and wintering grounds that occurs twice a year. It is typically from north to south or from south to north. Migration is inherently risky, due to predation and mortality. The ...
share the same capacity for contaminant transport across vast distances as fish. This may be of particular concern with
Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...
migratory birds, as they have the ability to transport contaminants to environments with otherwise limited contaminant input. Birds can also recycle contaminants back to aquatic environments via
guano Guano (Spanish from ) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a le ...
.


Ecological consequences of contaminant subsidies


Impacts of contaminant subsidies on terrestrial predators

For
flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwin ...
and other metamorphosing insects, high burdens of Se, PCB, metals, synthetic nanoparticles, and other contaminants can decrease body and reproductive fitness, leading to reduced amounts of larvae metamorphosing and emerging from the water column as terrestrial adults. When contaminant exposure does not impact metamorphosis or emergence, emerging insects may carry high concentrations of contaminants that are readily
bioavailable In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. By definition, when a medication is administered intravenously, its bioavailability is 100%. H ...
to the terrestrial food web. Consuming these contaminated prey items can result in severe histological, circulatory, digestive, and reproductive issues in terrestrial predators like spiders, amphibians, reptiles, mammals, and birds. The large number of insects that some predators need to consume in proportion to body mass for survival raises the risk of contaminant
bioaccumulation Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance faster than it can be lost or eliminated by catabolism and excretion. T ...
, increasing the likelihood of developmental deformities and mortalities. This also can result in the
biomagnification Biomagnification, also known as bioamplification or biological magnification, is the increase in concentration of a substance, e.g a pesticide, in the tissue (biology), tissues of organisms at successively higher levels in a food chain. This inc ...
of organic and element subsidies like PCBs, selenium, and mercury by higher trophic levels that consume contaminated aquatic insects and their primary consumers like arthropods and fish. Contaminant levels in prey can be so highly concentrated that, for example, small-bodied
songbird A songbird is a bird belonging to the suborder Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as the scientific or vernacular name is Oscines, from Latin ''oscen'', "songbird". The Passeriformes contains 5,00 ...
chicks can experience adverse physiological effects from feeding on a single spider containing high levels of PCB (at less than 6,000
parts per billion In science and engineering, the parts-per notation is a set of pseudo-units to describe the small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measur ...
).


Ecosystem-wide impacts

Concentrated contamination of aquatic insect populations can facilitate a decline in the ecological health of aquatic and terrestrial ecosystems. Consumption of contaminated insects either continues the contaminant pathway up trophic levels or excretion returns the subsidies back into the sediment, a major sink of contaminants in aquatic environments. Due to the movement of subsidies through
lotic River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts.Angelier ...
systems and emergence patterns of flying insects, the source of contamination can be some distance away from the source of contamination and affected habitats. Furthermore, the massive biomass of insects compared to other animals, and the sequestration of organic contaminants in one water body, can lead to large amounts of contaminants being exported across many different terrestrial ecosystems. From a single creek, it was estimated that emerging insects exported around 6 grams of PCBs per year to land, which is equivalent to the amount exported by 50,000 migrating salmon in an entire watershed. The subsequent reduction in
recruitment Recruitment is #Process, the overall process of identifying, sourcing, screening, shortlisting, and interviewing candidates for Job (role), jobs (either permanent or temporary) within an organization. Recruitment also is the process involved in ...
from a lack of prey or consumption of contaminant subsidies can lead to local extirpations of fish, and aquatic and arachnivorous birds. The loss of biomass and reduced subsidy pathways deteriorate the complexity of aquatic and terrestrial food webs. As the biodiversity of a habitat decreases, its
ecological resilience In ecology, resilience is the capacity of an ecosystem to respond to a perturbation or Disturbance (ecology), disturbance by resisting damage and subsequently recovering. Such perturbations and disturbances can include stochastic events such as ...
to further contamination and food web restructuring also declines.


Measuring aquatic-terrestrial connections

Researchers use several tools to assess how terrestrial and aquatic
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...
s are connected. Stable
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s, particularly of carbon, nitrogen,
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
, and oxygen, can be used to determine what
resource ''Resource'' refers to all the materials available in our environment which are Technology, technologically accessible, Economics, economically feasible and Culture, culturally Sustainability, sustainable and help us to satisfy our needs and want ...
s
consumer A consumer is a person or a group who intends to order, or use purchased goods, products, or services primarily for personal, social, family, household and similar needs, who is not directly related to entrepreneurial or business activities. ...
s are eating. Other compounds, such as
fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...
, can also be used to trace food web connections between aquatic and terrestrial ecosystems. Stable carbon isotope ratios (ratio of
carbon 13 Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth. Detection by mass spectrometry A mas ...
(13C) to carbon 12 (12C)), are one of the most common methods used to measure the energy inputs and sources for aquatic ecosystems, and can be used to track flux of aquatic resources into riparian zones. Naturally-occurring variation in carbon stable isotope ratios can often distinguish organic matter produced by photosynthesis of terrestrial plants or aquatic algae. A more precise but also more expensive method requires adding a form of carbon labelled with an extreme ratio of carbon 13 (13C) to carbon 12 (12C) that does not naturally occur and which can be used to trace the movement of the added carbon through the ecosystem and food web. Once the tracer carbon has had time to go through the system, samples of water, algae, bacteria, and other organisms are collected and the ratios of carbon 13 (13C) to carbon 12 (12C) in their tissues are determined. A
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...
can then be drawn by tracing what organisms have taken up the tracer carbon and how much. Stable isotope ratios are measured using an isotope ratio mass spectrometer from dried organic samples. There is sometimes overlap between terrestrial plants and algae in naturally-occurring stable carbon isotope ratios, complicating their use in identifying aquatic-terrestrial subsidies. Stable isotope ratios of hydrogen (ratio of
deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
to hydrogen) can be used to distinguish terrestrial and aquatic primary production when carbon isotope ratios overlap. However, stable hydrogen isotope ratios of aquatic organisms can also be influenced by variation in the isotope ratios present in the water molecules of the aquatic environment. Stable isotope ratios of nitrogen are particularly useful in tracing fluxes of marine-derived resources such as anadromous fish into riparian and terrestrial environments.


Measuring contaminant subsidies and impacts

The movement of aquatic-terrestrial contaminant subsidies can first be measured by testing the water quality of sites with known contamination or near urban centers or factories that discharge chemical waste. This enables scientists to determine where contaminants are highly concentrated in aquatic habitats. Next, aquatic insects are often collected and analyzed for contaminant loads and to model any population changes. Aquatic insects are commonly studied to estimate water quality because many species are highly sensitive to pollution, resulting in community composition changes in contaminated waterbodies. Finally, researchers study histological, blood, gut, feather, and egg samples from predators to determine if contaminants are traveling up trophic levels via the consumption of contaminated prey and what negative effects this may have on predators.


See also

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References

{{Reflist Ecological connectivity Trophic ecology