What does an estuarine ecosystem look like?
Estuaries are critically important habitats for a vast array of wildlife. Not every estuarine ecosystem is structured in the same way; they take the form of mudflats, tidal channels, salt marshes, swamp forests, marsh grasses, etc.1 Dependent on light, nutrient inputs, salinity levels2, as well as its structural formation, each type of estuary supports a unique ecosystem. In all estuaries, however, the interface between salt and freshwater, compounded with the sheltered nature of the estuary, create a productive and dynamic environment primed to support a wide range of life1,3.
What role do estuaries play as ecosystems?
Estuaries support a large amount of life. In the summer months, the provide a home for bottlenose dolphins4. Snails and oysters take up year-round residence1 along with shrimp, crabs, and many sessile estuarine species5. Migratory birds use the estuaries as stopover locations during their migration6 and rely on the abundant fish life to serve as a food source7. They also support numerous forms of vegetative life.
This vegetative life, along with cyanobacteria and other microorganisms, combine to give estuaries considerably high rates of primary production. Estuaries also generally have relatively shallow depth, as well as low currents and weak tide amplitude when compared to coastal habitats. These features combined with consistent salinity and long residence time of the water make estuaries the ideal nurseries for finfish and shellfish, one of the most important roles that they play5.
Why are estuarine ecosystems uniquely vulnerable to humans?
Every species supported by an estuarine ecosystem plays a critical role in maintaining the health of that ecosystem. All species interactions, even down to the zooplankton consumption of phytoplankton5, is in a fragile balance that it is extremely vulnerable to human influences, extreme weather events, etc.
Estuaries take in everything from the river systems that feed into them. Estuarine structure is such that they trap and collect any sediments that flow into them. While this typically encompasses river mud, peat, organic sediments from swamp vegetation, and sand from the inlets and barrier islands, this also means that any pollutants, farm runoff, etc. from locations upstream are deposited into the ecosystem. As agriculture and subsequent nitrogen runoff increases on land, the nutrient input into estuaries has similarly increased. Phytoplankton have responded to this increase by increasing their biomass, in turn significantly elevating levels of eutrophication2. The resulting hypoxic conditions is physically burdening on fish, shellfish, and invertebrates living in the estuarine environment3. Studies have shown decreased volumes of life and levels of biodiversity alongside the increasing nutrient inputs2,3,5. As global anthropogenic climate change increases, the frequency of tropical storms has also increased8. Estuarine ecosystems are uniquely vulnerable to these tropical storms and hurricanes, as they lead to rapid increases in runoff and organic matter5, exacerbating the existing artificially high nutrient input. Furthermore, the presence of toxic runoff in these ecosystems has been connected to an increase in fish disease, including lesions, sloughing skin, and bacterial infections5.
1 Pilkey, Orrin H. The North Carolina Shore and Its Barrier Islands: Restless Ribbons of Sand. Duke University Press, 1998.
2 Pinckney, JL, et al. “Annual Cycles of Phytoplankton Community-Structure and Bloom Dynamics in the Neuse River Estuary, North Carolina.” Marine Biology, May 1998.
3 Paerl, Hans W, et al. “Ecosystem Responses to Internal and Watershed Organic Matter Loading: Consequences for Hypoxia in the Eutrophying Neuse River Estuary, North Carolina, USA.” Marine Ecology Progress Series, vol. 166, 28 May 1998.
4 Read, Andrew J, et al. “Abundance of Bottlenose Dolphins in the Bays, Sounds, and Estuaries of North Carolina.” Marine Mammal Science, Jan. 2003, pp. 59-73.
5 Paerl, H. W., et al. “Ecosystem Impacts of Three Sequential Hurricanes (Dennis, Floyd, and Irene) on the United States’ Largest Lagoonal Estuary, Pamlico Sound, NC.” Proceedings of the National Academy of Sciences, vol. 98, no. 10, 2001, pp. 5655-5660., doi:10.1073/pnas.101097398.
6 Gill, J., Norris, K., Potts, P. et al. The buffer effect and large-scale population regulation in migratory birds. Nature 412, 436–438 (2001). https://doi.org/10.1038/35086568.
7 Osborn, Katherine, “Season Fish and Invertebrate Communities in Three Northern California Estuaries” (2017). HSU Theses and Projects 101. https://digitalcommons.humboldt.edu/etd/101.
8 Buis, Alan. “How Climate Change May Be Impacting Storms Over Earth’s Tropical Oceans – Climate Change: Vital Signs of the Planet.” NASA, NASA, 20 Mar. 2020, climate.nasa.gov/blog/2956/how-climatechange-may-be-impacting-storms-over-earthstropical-oceans/.