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Aquatic insect, Caddisfly in its pupae stage

Energy and nutrients derived from aquatic ecosystems and transferred to terrestrial ecosystems are termed aquatic-terrestrial subsidies or, more simply, aquatic subsidies. The most common examples of aquatic subsidies involve 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[1][2]. This phenomenon is exemplified by aquatic insects that develop within streams and lakes before emerging as winged adults and moving to terrestrial habitats[3]. 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; both aquatic subsidies and terrestrial subsidies are types of cross-boundary subsidies. Energy and nutrients are derived from different sources and are available to the aquatic and terrestrial ecosystems.

Allochthonous and autochthonous are the main drivers of energy and nutrient inputs into the different systems and can be seen as a trophic system[4]. The strength of both top-down and bottom-up trophic cascades are increased when allochthonous resources are more abundant[5][6]. However, these resources could be readily available, but not nutritious[7].

Resource Subsidy

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Resource subsidy, manifested as nutrients, matter, or organisms, is the flux of energy across ecosystem boundaries[8]. The allochthonous inputs of resources can influence individual growth, species abundance and diversity, community structure, secondary productivity and food web dynamics[8]. Allochthonous resources are defined as originating outside of the ecosystem while autochthonous resources are derived within the ecosystem. For example, leaf fall into the stream would be an allochthonous resource.

Separate from cross-ecosystem predator-prey interactions, resource subsidies supplement the productivity of the recipient species with little to no effect on the source[9]. As a result, resource subsidies are donor-controlled and independent of the recipient consumer or productivity of the recipient habitat.[10][11]. Although donor productivity can influence predator-prey interaction due to pulse timing of biotic interactions.[12]. Subsidies can be stable until predator-prey interaction increases due to timing, emergence of insects, and timing of resource pulses that affect the community that prey on it[11]. An example is increased algal bloom which can advance the emergence of insects that lead to more terrestrial consumers[12].

This flooding stream is delivering aquatic subsidies to the surrounding terrestrial area.

The abundance and rate of resource subsidy fluxes are mediated by retention and permeability of ecotones and modified by both physical and biotic factors[13]. Fluxes are further altered by the amount of resources available, which are determined by factors of species interactions and climate[14]. Recipient habitats differentially respond to the influx of resource subsidies, experiencing the largest effect when comparable resources are low[15]. Similarly, the effect of subsidies fluctuate across and within taxonomic and functional feeding groups.[15] However, the effect of subsidy fluxes are volatile due to episodic changes, seasonal cycles, and pulses in recipient habitats[11][15]. Cross-ecosystem resource subsidy flows between terrestrial and stream environments are one of the most well studied forms of subsidies.[10]

Aquatic Subsidies

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Aquatic subsidies are energy or nutrient sources that are transferred from the aquatic environment to the terrestrial environment. [16] These aquatic subsidies vary spatially and seasonally, and can include resources that move laterally, downstream, and upstream. [17][18] Subsidies from all directions provide vital nutrients and energy to ecosystem functions and link interactions between species. [18]

Subsides that travel downstream are more common in freshwater systems and include organic matter that has been broken down through processes within the stream such as animals feeding on food resources and, of course, through feces generated by species upstream. [17] These downstream resources can be incredibly important within a watershed by providing nutrients to downstream habitats that may be lacking in these inputs, so this addition helps to bolster primary productivity and food webs. [17]

Upstream subsidies are generally from marine fishes such as salmon that contribute nutrients from carcasses and spawning events that help to support terrestrial food webs.[18] 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. [19] Stream invertebrates such as stoneflies, caddisflies and midges also derive energy and nutrients from salmon and, in turn, provide food to terrestrial species. [19] Animals are not the only benefactors of upstream aquatic subsidies, riparian plants can receive up to 26% of their nitrogen from salmon, alone. [19]

Lateral movement of nutrients and energy from the stream to the surrounding riparian zone and terrestrial environment beyond serve an important role in the food webs. [16] Flooding of a stream and the movement of organisms, both play a role in transferring nutrients and energy sources to the terrestrial environment. [20] Algae and fine organic matter washed up from high flows provide resources to herbivorous species and help to generate plant germination.[17] 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. [20] 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. [21] Emergence of aquatic insects typically peaks in the summer of temperate zones, prompting predators to aggregate and forage along riparian and stream boundaries. [21] 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. [20] Another example of a terrestrial species that moves aquatic subsidies further inland is that of the brown bear. [19] Brown bears consume a massive amount of salmon from streams, so much so, they are considered a keystone species. [19] 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 through their interactions with aquatic subsidies. [19]

Brown bear feeding on marine-derived nutrients in the form of salmon.

Ecological Importance of Aquatic Subsidies

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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. [22] 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. [16][23] Sometimes, these aquatic subsidies can even be relied on more heavily for nutrients than terrestrial subsidies in certain ecosystems. [24]

In addition to their nutritional value, however, aquatic subsidies are increasingly recognized as important sources of environmental contaminants to terrestrial food webs. [25] 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. [25]

Terrestrial Subsidies

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Terrestrial subsidies is primary production on land (allochthonous) that is transferred to aquatic ecosystems as litter fall or dissolved organic matter.[26]

Terrestrial subsidies or allochthonous inputs into aquatic environments are a major component of organic carbon budgets for aquatic systems.[27] In many ecosystems autochthonous production of carbon is not enough to support the food web and rely on production being enhanced or subsidized to maintain secondary production.[28] Aquatic ecosystems a generally heterotrophic; respiration exceeds production, suggesting the food web is supported externally.[29] The carbon that enters the aquatic ecosystem gets taken up by micro-organisms like bacteria and algae where the carbon may go up the trophic levels by being consumed.[29] This microbial-mediated transfer of organic carbon has shown to support food webs in lakes and streams.[29]

Organic carbon inputs into aquatic ecosystems come in multiple forms to be utilized. The two main forms of organic carbon inputs are dissolved organic carbon (DOC) or particulate organic carbon (POC).[30] Particulate organic carbon is living organisms like bacteria, phytoplankton, zooplankton, detrital components derived from living organisms, and litter fall.[31] Dissolved organic carbon is organic carbon that has been broken down, is suspended, and considered soluble in water.[32] Dissolved organic carbon has been shown to stimulate heterotrophic production in aquatic settings and that heterotrophic bacteria can use the allochthonous dissolved organic carbon as a carbon source.[30] Particulate organic carbon also stimulates heterotrophic production which becomes available to bacteria or other micro-organisms through decomposition and other consumers by direct consumption.[30]

Measuring Aquatic-Terrestrial Subsidies

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Using stable isotopes of hydrogen is a potential way of measuring the allochthonous and autochthonous energy inputss from aquatic or terrestrial subsidies.[33] This process is is done by first gathering organic matter samples such ass litter fall, algae, bacteria, and tissue samples from larger organisms.[33] The organic samples are then usually dried, ground down to a fine powder, and pyrolyzed at high temperatures to produce H2 and CO gases.[33] The H2 gas is analyzed for stable isotope composition using an isotope-ratio mass spectrometer to determine the amount of energy each sample has. [33] The energy input is determined by the amount of stable hydrogen isotopes present with more equaling higher energy input.

Carbon isotopes are another method used In measuring the energy inputs and sources for aquatic ecosystems. To measure the input from terrestrial to aquatic ecosystems a form of organic carbon with an easily traceable carbon isotope, usually carbon 13 (13C), is added to an aquatic system imitating terrestrial input.[34] The tracer carbon is then allowed to go through the system to be absorbed or taken up by organism.[34] Once the tracer carbon has had time to go through the system samples of water, algae, bacteria, and other organism are taken, measured for how much of the carbon isotope had mad it into them.[34] A food web can then be drawn by tracing what organisms have taken up the tracer carbon and how much.[34] Measuring of the isotopes is done by using an isotope ratio mass spectrometer from dried organic samples.[34]

Measuring stable nitrogen isotopes (15N) in aquatic animals and micro-organisms is used to measure the terrestrial energy inputs to the aquatic ecosystem. To do this samples of riparian arthropods and their potential aquatic and terrestrial food sources are collected, then frozen besides the arthropods.[35] Sampling of large aquatic animals like fish, small tissue sample are taken from the individual and frozen.[36] The arthropods are usually kept in water for at least a day to clear any gut contents and then frozen.[35] Once samples are collected they are all ground down to a fine powder to obtain a homogenized sample.[35] The nitrogen isotopic composition is then measured by putting ground down samples in a isotope ratio mass spectrometer to get the nitrogen composition.[35] The amount of 15N in the animal being tested helps determine the diet and how much terrestrial plan input there is into the system.[35] A high ratio of 15N detected in an aquatic animal or arthropod indicated that the individual gains most its energy from plant inputs and a low ratio indicates a diet based off predation rather than consumer.[35]

  1. ^ Schindler, Daniel E.; Smits, Adrianne P. (2016-10-13). "Subsidies of Aquatic Resources in Terrestrial Ecosystems". Ecosystems. 20 (1): 78–93. doi:10.1007/s10021-016-0050-7. ISSN 1432-9840.
  2. ^ BAXTER, COLDEN V.; FAUSCH, KURT D.; CARL SAUNDERS, W. (2005-01-18). "Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones". Freshwater Biology. 50 (2): 201–220. doi:10.1111/j.1365-2427.2004.01328.x. ISSN 0046-5070.
  3. ^ Nakano, S.; Murakami, M. (2001-01-02). "Reciprocal subsidies: Dynamic interdependence between terrestrial and aquatic food webs". Proceedings of the National Academy of Sciences. 98 (1): 166–170. doi:10.1073/pnas.98.1.166. ISSN 0027-8424.
  4. ^ Gregory, Stanley V.; Swanson, Frederick J.; McKee, W. Arthur; Cummins, Kenneth W. (1991-09). "An Ecosystem Perspective of Riparian Zones". BioScience. 41 (8): 540–551. doi:10.2307/1311607. ISSN 0006-3568. {{cite journal}}: Check date values in: |date= (help)
  5. ^ Leroux, Shawn J.; Loreau, Michel (2008-08-17). "Subsidy hypothesis and strength of trophic cascades across ecosystems". Ecology Letters. 11 (11): 1147–1156. doi:10.1111/j.1461-0248.2008.01235.x. ISSN 1461-023X.
  6. ^ Allen, Daniel C.; Vaughn, Caryn C.; Kelly, Jeffrey F.; Cooper, Joshua T.; Engel, Michael H. (2012-10). "Bottom-up biodiversity effects increase resource subsidy flux between ecosystems". Ecology. 93 (10): 2165–2174. doi:10.1890/11-1541.1. ISSN 0012-9658. {{cite journal}}: Check date values in: |date= (help)
  7. ^ Marcarelli, Amy M.; Baxter, Colden V.; Mineau, Madeleine M.; Hall, Robert O. (2011-06). "Quantity and quality: unifying food web and ecosystem perspectives on the role of resource subsidies in freshwaters". Ecology. 92 (6): 1215–1225. doi:10.1890/10-2240.1. ISSN 0012-9658. {{cite journal}}: Check date values in: |date= (help)
  8. ^ a b Richardson, John S.; Zhang, Yixin; Marczak, Laurie B. (2010-01). "Resource subsidies across the land-freshwater interface and responses in recipient communities: RESOURCE SUBSIDIES ACROSS THE LAND-FRESHWATER INTERFACE". River Research and Applications. 26 (1): 55–66. doi:10.1002/rra.1283. {{cite journal}}: Check date values in: |date= (help)
  9. ^ Richardson, John S.; Sato, Takuya (2015-04). "Resource subsidy flows across freshwater-terrestrial boundaries and influence on processes linking adjacent ecosystems: CROSS-ECOSYSTEM RESOURCE SUBSIDIES ACROSS THE WATER-LAND BOUNDARY". Ecohydrology. 8 (3): 406–415. doi:10.1002/eco.1488. {{cite journal}}: Check date values in: |date= (help)
  10. ^ a b Larsen, Stefano; Muehlbauer, Jeffrey D.; Marti, Eugenia (2016-04-25). "Resource subsidies between stream and terrestrial ecosystems under global change". Global Change Biology. 22 (7): 2489–2504. doi:10.1111/gcb.13182. ISSN 1354-1013.
  11. ^ a b c Sato, Takuya; El-Sabaawi, Rana W.; Campbell, Kirsten; Ohta, Tamihisa; Richardson, John S. (2016-04-21). "A test of the effects of timing of a pulsed resource subsidy on stream ecosystems". Journal of Animal Ecology. 85 (5): 1136–1146. doi:10.1111/1365-2656.12516. ISSN 0021-8790.
  12. ^ a b Terui, Akira; Negishi, Junjiro N.; Watanabe, Nozomi; Nakamura, Futoshi (2017-09-11). "Stream Resource Gradients Drive Consumption Rates of Supplemental Prey in the Adjacent Riparian Zone". Ecosystems. 21 (4): 772–781. doi:10.1007/s10021-017-0183-3. ISSN 1432-9840.
  13. ^ Ballinger, Andrea; Lake, P. S. (2006). "Energy and nutrient fluxes from rivers and streams into terrestrial food webs". Marine and Freshwater Research. 57 (1): 15. doi:10.1071/mf05154. ISSN 1323-1650.
  14. ^ Greig, Hamish S.; Kratina, Pavel; Thompson, Patrick L.; Palen, Wendy J.; Richardson, John S.; Shurin, Jonathan B. (2011-10-27). "Warming, eutrophication, and predator loss amplify subsidies between aquatic and terrestrial ecosystems". Global Change Biology. 18 (2): 504–514. doi:10.1111/j.1365-2486.2011.02540.x. ISSN 1354-1013.
  15. ^ a b c Marczak, Laurie B.; Thompson, Ross M.; Richardson, John S. (2007-01). "META-ANALYSIS: TROPHIC LEVEL, HABITAT, AND PRODUCTIVITY SHAPE THE FOOD WEB EFFECTS OF RESOURCE SUBSIDIES". Ecology. 88 (1): 140–148. doi:10.1890/0012-9658(2007)88[140:mtlhap]2.0.co;2. ISSN 0012-9658. {{cite journal}}: Check date values in: |date= (help)
  16. ^ a b c Ballinger, Andrea (2006). "Energy and nutrient fluxes from rivers and streams into terrestrial food webs". Marine and Freshwater Research.
  17. ^ a b c d Richardson, John (2009). "Resource Subsidies Across the Land-Freshwater Interface and Responses in Recipient Communities". River Research and Applications.
  18. ^ a b c Richardson, John (2014). "Resource Subsidy Flows Across Freshwater-Terrestrial Boundaries and Influence on Processes Linking Adjacent Ecosystems". Echohydrology.
  19. ^ a b c d e f Helfield, James M. (2006). "Keystone Interactions: Salmon and Bear in Riparian Forests of Alaska". Ecosystems. {{cite journal}}: line feed character in |title= at position 34 (help)
  20. ^ a b c Schindler, Daniel (2016). "Subsidies of Aquatic Resources in Terrestrial Ecosystems". Ecosystems.
  21. ^ a b Baxter, Colden (2005). "Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones: Prey subsidies link stream and riparian food webs". Freshwater Biology.
  22. ^ Collier, Kevin (2002). "A stable isotope study of linkages between stream and terrestrial food webs through spider predation". Freshwater Biology.
  23. ^ Bartels, Pia (2012). "Reciprocal subsidies between freshwater and terrestrial ecosystems structure consumer resource dynamics". Ecology.
  24. ^ Gray, L (1989). "Emergence, production and export of aquatic insects from a tallgrass prairie stream". The Southwestern Naturalist.
  25. ^ a b Walters, D.M. (2008). "The dark side of subsidies: adult stream insects export organic contaminants to riparian predators". Ecological Applications.
  26. ^ Doucett, Richard (2007). "Measuring terrestrial subsidies to aquatic food webs using stable isotopes of hydrogen". Ecology. 88: 1587–1592.
  27. ^ Carpenter, Stephen (2005). "Ecosystem subsidies: terrestrial support of aquatic food webs from 13C addition to contrasting lakes". Ecology. 86: 2737–2750.
  28. ^ Rubbo, Michael (2006). "Terrestrial Subsidies of Organic Carbon Support Net Ecosystem Production in Temporary Forest Ponds: Evidence from an Ecosystem Experiment". Ecosystems. 9: 1170–1176.
  29. ^ a b c Rubbo, Michael (2006). "Terrestrial Subsidies of Organic Carbon Support Net Ecosystem Production in Temporary Forest Ponds: Evidence from an Ecosystem Experiment". Ecosystems. 9: 1170–1176.
  30. ^ a b c Bartels, Pia (2012). "Terrestrial subsidies to lake food webs: an experimental approach". Oecologia. 168: 807–818.
  31. ^ Fisher, Thomas (1998). "Dissolved and Particulate Organic Carbon in Chesapeake Bay". Estuaries. 21: 215–229 – via JSTOR.
  32. ^ Sharp, Jonathan (1973). "Size Classes of Organic Carbon in Seawater". https://www.jstor.org/stable/2834468. 18: 441–447 – via JSTOR. {{cite journal}}: External link in |journal= (help)
  33. ^ a b c d Doucett, Richard (2007). "Measuring terrestrial subsidies to aquatic food webs using stable isotopes of hydrogen". Ecology. 88: 1587–1592.
  34. ^ a b c d e Bartels, Pia (2012). "Terrestrial subsidies to lake food webs: an experimental approach". Oecologia. 168: 807–818.
  35. ^ a b c d e f Paetzold, Achim (2005). "Aquatic Terrestrial Linkages Along a Braided-River: Riparian Arthropods Feeding on Aquatic Insects". Ecosystems. 8: 748–759.
  36. ^ Masese, Frank (2015). "Are Large Herbivores Vectors of Terrestrial Subsidies for Riverine Food Webs?". ecosystems.
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