Estuaries and regulation of global climate
Estuaries are defined as semi-enclosed coastal bodies of water where seawater mixes with freshwater, creating unique environments that can vary widely in physical and chemical characteristics. These areas are categorized into different geomorphic types, such as fjords and coastal plain estuaries, and are stratified based on salinity. Estuaries play a critical role in regulating global climate due to their influence on biogeochemical cycles. As climate change progresses, factors like sea-level rise and changes in freshwater flow will significantly impact estuarine dynamics. For instance, alterations in freshwater input can shift salinity stratification and potentially lead to the loss of estuarine environments if they fill with sediment or are unable to adapt to rising sea levels.
Moreover, human activities—such as shoreline development, dam construction, and changes in land use—further complicate these dynamics by affecting nutrient delivery and sediment supply. The ecological ramifications of these changes are profound, with increased stratification and temperatures potentially leading to problems like hypoxia and shifts in marine species distributions. Understanding the complexities of estuaries is essential for predicting their future resilience amid ongoing climate challenges and human interventions.
Subject Terms
Estuaries and regulation of global climate
Definition
Oceanographer Gerardo Perillo has crafted perhaps one of the most well-considered definitions of an estuary from the dozens that had been put forward previously:
An estuary is a semi-enclosed coastal body of water that extends to the effective limit of tidal influence within which seawater entering from one or more free connections to the open sea, or other saline body of water, is significantly diluted with freshwater.
This definition encompasses seven geomorphic types of estuary in three categories of stratification. The geomorphic types are: high relief estuaries, such as fjords; moderate-relief, winding valleys, such as rias; low-relief, drowned river valleys, or coastal plain estuaries; deltaic estuaries; tectonic estuaries; bar-built estuaries; and seasonally blocked bar-built estuaries, or blind estuaries. In terms of salinity stratification, estuaries can be salt-wedge, partially mixed, or homogeneous.
Significance for Climate Change
Coastal systems in general and estuaries in particular exert a control over biogeochemical cycles and, therefore, play an important role in regulating global climate. In the face of a changing climate, sea-level rise, sediment supply, the persistence of barrier beaches and salt marshes, water temperature changes, and freshwater supply will all alter estuaries. Changes in the magnitude of freshwater flow or in its seasonal distribution will immediately shift the stratification of existing estuaries. For instance, greater spates will drive existing estuaries toward more stratified end members. Under extreme conditions, salinity gradients could be driven out onto the shelf. At high latitudes, the reduction or elimination of and sea-ice cover will redefine the freshwater budget of local estuaries.
A transgressive shoreline creates space for estuaries, but it may be inimical to them in some situations. Bar-built estuaries, in particular, rely on the maintenance of a barrier beach. A too rapid rise in sea level could make it impossible for the barriers to maintain themselves, drowning the former estuaries. In other situations, stronger seasonality of freshwater discharge may open formerly blind estuaries or turn ephemeral estuaries into permanent, bar-built estuaries. The condition will depend on the rate of sea-level rise and on the supply of littoral sand by processes that allow barrier beaches to maintain their integrity and to migrate shoreward in the face of rising seas.
In the geological perspective, estuaries are ephemeral features, prevalent at times of high sea-level stands, especially along coasts of low relief. Rising sea levels will shift isohalines inland, possibly turning formerly true-estuarine environments into marginal seas or merely arms of the ocean. At the same time, high-energy environments will shift toward the outer part of the coast. However, because estuaries are sediment traps, their continued existence is a balance between flooding associated with rising sea level and infilling.
Estuaries disappear when they eventually fill with sediment. Climate-induced changes in the amount and seasonality of rainfall, temperature, humidity, and vegetation over wide areas will all influence the rate of infilling of estuaries newly fighting for existence. Any increased sedimentation will compete with increased sea-level rise to shorten the life of future estuaries. For example, increased rainfall and more intense spates may enhance soil erosion, and changes in both temperature and humidity may accelerate chemical weathering. Northern estuaries might be expected to be exposed to higher sediment supplies more typically associated with semitropical regions.
Human responses to climate change will exert another influence on estuaries, especially because so much human development is concentrated around estuarine shorelines. Shoreline hardening can squeeze out salt marsh development, as well as arresting shoreline transgression. Human control of flooding—such as that provided by the Thames storm-surge barriers, those at Rotterdam and St. Petersburg, and that under construction in Venice—would limit the expansion of estuaries in some regions. The construction of dams to control flooding and water supply could force some estuaries to cope with decreases in both freshwater flow and nutrient fluxes. Changes in forestation and agricultural land-use must further alter the delivery of both nutrients and sediments to estuaries.
Although the physical shifts in estuarine conditions may be as yet poorly resolved, the potential and realized impacts on marine ecosystems are well recognized. Increasing stratification, coupled with elevated temperature, exacerbates problems of hypoxia that already plague many temperate estuaries. Freshening and warming will also change nutrient delivery to the estuary, perhaps increasing total nutrient delivery while simultaneously shifting higher concentrations seaward. Pelagic species will be forced into new patterns of larval transport and recruitment and changes in foraying. Changes in one or two key, leverage species may cause broad, community-level changes. Harmful algal blooms, and the spread of diseases will alter estuarine ecology further. In Long Island Sound, for example, a crash of the American lobster population has been suggested to result from immunodepression caused by the stress of warming water temperatures.
Bibliography
"Climate Adaptation and Estuaries." EPA, 28 May 2024, https://www.epa.gov/arc-x/climate-adaptation-and-estuaries#. Accessed 20 Dec. 2024.
Crossland, Christopher J., et al., eds. Coastal Fluxes in theAnthropocene. Berlin: Springer-Verlag, 2005.
Fairbridge, Rhodes W. “The Estuary: Its Definitive and Geodynamic Cycle.” In Chemistry and Biogeochemistry of Estuaries, edited by E. Olausson and I. Cato. New York: Wiley, 1980.
Harley, Christopher D. G., et al. “The Impact of Climate Change in Coastal Marine Systems.” Ecology Letters 9 (2006): 228-241.
Perillo, Geraldo M. E. “Definition and Geomorphic Classifications of Estuaries.” In Geomorphology and Sedimentology of Estuaries, edited by G. M. E. Perillo. Amsterdam: Elsevier, 1997.