Marine Subsidies

by

Debora Obrist

• LAST REVIEWED: 20 February 2024
• LAST MODIFIED: 20 February 2024
• DOI: 10.1093/obo/9780199830060-0251

Introduction

Ecosystems are closely linked through the exchange of resources. Marine subsidies refer to the cross-ecosystem flows of ocean-derived resources, including organisms, nutrients, and detritus. These subsidies occur in two forms: passive, resulting from deposition by abiotic forces such as gravity, wind, and waves, and active, through animal-mediated resource transport. These transfers often enrich recipient habitats with otherwise limiting nutrients, leading to enhanced productivity and causing a wide range of ecological impacts. The influence of marine subsidies extends across terrestrial, freshwater, and marine environments. On arid desert islands in Baja California, Mexico, the influence of marine subsidies to terrestrial ecosystems is particularly evident; here, the energy gained from passively donated marine detritus outweighs islands’ terrestrial productivity. Seabird guano, on the other hand, is an example of an actively deposited marine input in terrestrial ecosystems. This resource has been studied for decades, likely due to its high commercial value in the fertilizer industry. Meanwhile, Pacific salmon provide a classical example of marine subsidies to both freshwater and terrestrial systems. These fish feed and gain mass in the ocean before they spawn and die in their natal freshwater streams, facilitating the fertilization of both nutrient-limited streams and riparian terrestrial habitats. Finally, marine subsidies can also refer to cross-ecosystem movements between different types of marine habitats. For example, zooplankton feed on phytoplankton, which remove vast amounts of carbon dioxide from the atmosphere. Fecal pellets from upper-pelagic zooplankton slowly drift downward in the water column through gravity and passively subsidize deep ocean ecological communities. Marine animals, including whales, can move nutrients back upwards in the water column against the pull of gravity as they feed in the deep ocean and return to the sea surface to breathe, excreting feces and urine on the way. Through a process called upwelling, this upward movement of nutrients can also happen passively. Upwelling is a wind-driven phenomenon by which cold, nutrient-rich waters are forced up toward the sea surface, resulting in increased productivity in surface waters. Overall, marine subsidies link various ecosystems through the movement of marine-derived resources, and greatly impact the ecologies of recipient habitat flora and fauna. From fertilizing nutrient-limited streams to subsidizing deep ocean benthic communities, marine subsidies have significant ecological impacts across the globe.

General Overviews

Although not all explicitly focused on marine subsidies, several reviews and meta-analyses exist that provide good summaries of the state of research on spatial subsidies more generally. The comprehensive synthesis provided by Polis, et al. 1997 describes literature about how nutrient, detrital, and prey subsidies influence population, food web, and community dynamics. Polis, et al. 2004 follows up on this synthesis with a more specific review of the movements of marine inputs on islands and coastal ecosystems across the globe. Gounand, et al. 2018 describes how ecosystems worldwide are connected through flows of carbon. Subalusky and Post 2019 provides a useful, in-depth overview of the effects of animal resource subsidies, including those that originate in the marine environment. In terms of connections between specific ecosystems, several overviews exist which focus explicitly on marine subsidies to terrestrial ecosystems. These include a review by Colombini and Chelazzi 2003, which summarizes the influence of marine subsidies on sandy beach communities, and Cox, et al. 2020, which provides a review of the importance of shellfish subsidies in terrestrial habitats. Several overviews about Pacific salmon subsidies exist, including Naiman, et al. 2002, which provides an overview of the early literature on salmon-mediated ecological impacts, and Walsh, et al. 2020, which analyzes the relationships between spawning salmon densities and both terrestrial and freshwater aquatic ecosystems. For an overview of marine subsidies to marine ecosystems, see Turner 2015.

• Colombini, I., and L. Chelazzi. 2003. The influence of marine allochthonous input on sandy beach communities. In Oceanography and marine biology. Edited by R. N. Gibson and R. J. A. Atkinson, 115–159. Boca Raton, FL: CRC Press.

  DOI: 10.1201/9780203180570-21

  Synthesizes work on the importance of the accumulation of macrophytes and other organic materials on beaches. Explains the composition of such beach-cast subsidies and describes how they can vary in abundance with relation to various trends (e.g., seasonal, lunar, tidal), as well as due to the decomposition and use by different terrestrial and semi-terrestrial communities.

• Cox, K. D., H. L. Davies, K. H. Davidson, T. G. Gerwing, S. E. Dudas, and J. Francis. 2020. Shellfish subsidies along the Pacific coast of North America. Ecography 43.5: 668–681.

  DOI: 10.1111/ecog.04476

  First review to formally describe the importance of shellfish subsidies, with an emphasis on mollusks. Shellfish subsidies have been transported into terrestrial habitats by birds, mammals, and humans for thousands of years. Discusses shellfish influences on soil chemistry, forest productivity, and the diversity of producers across spatial and temporal scales.

• Gounand, I., C. J. Little, E. Harvey, and F. Altermatt. 2018. Cross-ecosystem carbon flows connecting ecosystems worldwide. Nature Communications 9.4825 (2018): 1–8.

  DOI: 10.1038/s41467-018-07238-2

  Provides a quantitative synthesis of the spatial flows of carbon connecting global ecosystems and makes inferences on how these flows impact ecosystem functioning, including those ecosystems connected to marine realms.

• Naiman, R. J., R. E. Bilby, D. E. Schindler, and J. M. Helfield. 2002. Pacific salmon, nutrients, and the dynamics of freshwater and riparian ecosystems. Ecosystems 5:399–417.

  DOI: 10.1007/s10021-001-0083-3

  Summarizes the effects of salmon-derived nutrients on freshwater and terrestrial ecosystems, as well as the atmospheric and marine processes governing salmon populations.

• Polis, G. A., W. B. Anderson, and R. D. Holt. 1997. Toward an integration of landscape and food web ecology: The dynamics of spatially subsidized food webs. Annual Review of Ecology and Systematics 28.1: 289–316.

  DOI: 10.1146/annurev.ecolsys.28.1.289

  Describes the importance of spatial flows among habitats for food web ecology through a large literature review, which includes landscape considerations and the various directions that spatial subsidies can move (e.g., from land to land or land to water, among many others). It distinguishes between published works on the movements of nutrients and detritus, of consumers, and of prey.

• Polis, G. A., F. Sánchez-Piñero, P. T. Stapp, W. A. Anderson, and M. D. Rose. 2004. Trophic flows from water to land: Marine input affects food webs of islands and coastal ecosystems worldwide. In Foodwebs at the landscape level. Edited by G. A. Polis, M. E. Power, and G. R. Huxel, 200–216. Chicago: Univ. of Chicago Press.

  Although this entire book is a great resource for information about cross-boundary ecosystem transfers, this chapter is a particularly good overview of the effects of marine subsidies on islands and coastal habitats around the world. It describes the factors determining the amounts of marine inputs, the relative contributions of marine inputs to terrestrial productivity, and the resulting impacts on terrestrial consumers.

• Subalusky, A. L., and D. M. Post. 2019. Context dependency of animal resource subsidies. Biological Reviews 94.2: 517–538.

  DOI: 10.1111/brv.12465

  A comprehensive review which provides a framework for understanding the context dependency of subsidies vectored by animals. Describes the variability in contributions of abiotic factors along with characteristics of the animal vectors, which determine the quantity, quality, timing, and duration of the input, and the resulting impacts on recipient ecosystems.

• Turner, J. T. 2015. Zooplankton fecal pellets, marine snow, phytodetritus and the ocean’s biological pump. Progress in Oceanography 130:205–248.

  DOI: 10.1016/j.pocean.2014.08.005

  Synthesis which contains many references relating to the ocean’s biological pump and how these resources move through the water column. Covers literature on zooplankton fecal pellets, marine snow, phytodetritus, and other components including sinking carcasses and feces from marine mammals.

• Walsh, J. C., J. E. Pendray, S. C. Godwin, et al. 2020. Relationships between Pacific salmon and aquatic and terrestrial ecosystems: Implications for ecosystem-based management. Ecology 101.9: e03060.

  DOI: 10.1002/ecy.3060

  An extensive synthesis of the various relationships observed between spawning salmon densities and ecological responses in terrestrial and aquatic ecosystems. Assesses general trends in responses through species abundance, species diversity, food provisioning, individual growth, concentrations of marine-derived isotopes, nutrient enhancement, and phenology. Finds a notable lack of studies evaluating ecosystem-level responses.