Introductory Chapter: Estuaries – An Overview of Key Features

1. Overview

Typically bordering between sea and land, estuaries are among the most diversely utilised tidal waterways on the planet. From an oceanographic perspective, an estuary is a coastal area where the hydrodynamics are characterised by the mixing of freshwater from rivers and saltwater from the sea/ocean, which creates a unique brackish environment that is typically rich in nutrients and supports diverse ecosystems. Dyer’s [1] classic definition states: “An estuary is a semi-enclosed coastal body of water which has a free connection to the open sea, extending into the river as far as the limit of tidal influence, and within which seawater is measurably diluted with fresh water derived from land drainage.”

Globally, there are approximately 4500 estuaries (including tidal deltas). The UK coastline has 163 estuaries [2], whilst there are slightly less along the coast of the much larger continental USA. The Lawrence River, 1197 km in length, which connects the Great Lakes to the Atlantic Ocean, is the world’s largest estuary. Estuaries are influenced by both terrestrial and marine processes, including tides, waves, and riverine flows.

2. Dynamical and sedimentary features

Tides play a significant role in estuaries, causing fluctuations in water levels and influencing the mixing of freshwater and saltwater, thus creating a strong gradient in salinity (saltier water closer to the estuary mouth and fresher water inland near riverine sources) that varies with both the tidal cycle (e.g., high and low water) and distance from the river or sea/ocean. Classification by tidal range comprises: microtidal <2 m range (e.g., Mediterranean estuaries); mesotidal <4 m, > 2 m; macrotidal <6 m, > 4 m; and hypertidal >6 m (e.g., Bay of Fundy, Canada; Severn Estuary, UK). Bathymetry (e.g., channel depth and shape) and river flow are also key factors contributing to estuarine circulation and evolution [3]. Estuarial locations are often designated in terms of their salinity structure (and resultant water density) within the water column: highly stratified (salt wedge), partially stratified, and vertically homogeneous (well mixed). The degree of stratification is modified both by the circulation and by mixing processes, including internal mixing across the halocline, such as entrainment (1-way mixing) and turbulent diffusion (2-way mixing). In stratified estuaries, the density differences drive circulation within the estuary known as “gravitational circulation.”

Many estuaries have high sediment loads, often dominated by fine-grained muddy sediments (clay, silt), which are frequently sinks for contaminants and nutrients. Due to tidal asymmetry and tidal straining effects, the net flow within most estuaries means estuaries can act as natural sediment traps, whereby sediment transported by rivers in conjunction with tidal action, and on settling results in accumulation forming mudflats, coastal bars, salt marshes, and other important habitats. In the higher tidal range estuaries, a more mobile suspended turbidity maximum zone can form landward of the inundating salt intrusion.

Within estuaries, the brackish water causes suspended fine-grain sedimentary particles (e.g., clays) to aggregate via electrostatic cohesion through a process known as flocculation. This particle bonding is further enhanced by the presence of sticky biopolymers (e.g., extra-cellular polymeric substances). Flocs are larger and settle quicker than their constituent particles but are less dense due to a Stokes’ Law relationship. Flocculation plays a crucial role in estuarine sediment dynamics and water clarity (e.g., [4]).

3. Biological and ecosystem features

Estuaries are highly nutrient-rich environments. They trap nutrients carried by rivers and the sea, and these nutrients are constantly mixed as they migrate through the brackish water column. This results in ideal habitats for highly productive ecosystems that crucially support a diverse range of flora and the life cycles of many species of fauna. For the former (flora - plant life), coastal estuaries are often fringed by salt marshes, where salt-tolerant plants such as cordgrass (Spartina) and pickleweed (Salicornia) thrive; these plants stabilise the sediment, prevent erosion, and provide a habitat for wildlife. Whilst in more tropical and subtropical regions, mangrove trees dominate estuary shorelines; they have special adaptations, such as aerial roots, to survive in saltwater environments and create shelter for marine species. Found in shallow estuarine waters, seagrasses such as eelgrass (Zostera) form underwater meadows that support a range of fish, crustaceans, and molluscs.

Phytoplankton (cyanobacteria and microalgae) are microscopic plants and are the base of the estuary food web; they perform photosynthesis and form the primary food source for many aquatic organisms. Pertaining to microorganisms, estuaries are rich in bacteria that decompose organic matter, recycle nutrients, and contribute to processes like nitrogen fixation and denitrification. Fungi play a role in breaking down organic matter and can influence the cycling of nutrients within the sediment. Of note are protists—single-celled organisms that are important primary producers and consumers within the estuary ecosystem.

Concerning fauna (animal life), estuaries serve as critical nurseries for many fish species, including commercially important species such as salmon, herring, and flounder. The protected waters and abundant food make them an ideal environment for breeding and feeding grounds. Invertebrates such as crabs, shrimp, and molluscs (e.g., oysters, clams) are abundant in estuaries; they play crucial roles in nutrient cycling and are key food sources for other animals. Estuaries attract a wide variety of bird species, especially waders and migratory birds; these include herons, egrets, and sandpipers, which rely on estuarine habitats for feeding during migration and breeding seasons. Some estuaries support populations of marine mammals such as seals and dolphins; additionally, animals like otters rely on estuaries for food and shelter. Zooplankton—small, aquatic organisms—that drift and feed on phytoplankton and form a crucial part of the estuary food chain; they are a key food source for fish larvae and filter-feeding animals.

4. Geological characteristics

Geologically, the life of an estuary is short. During the Pleistocene: 1st Epoch of Quaternary Period (2.8 million years B.P.), most present-day estuaries were slowly formed during the Holocene interglacial period spanning 11,700–18,000 years B.P. At that time, the sea level was ~100 m below the present level, water was locked up in extensive continental ice sheets, with the shoreline near the edge of the present continental shelf. Ice age glaciers caused river valleys to over-deepen, and as the ice retreated (period known as the Flandrian Transgression), sea level rose at approximately 1 m per century. These drowned river valleys were cut originally by rivers, forming modern estuaries, with present-day sea level reaching ~3000 years B.P.

Since then, minor isostatic changes have occurred: e.g., Scotland, UK, rising at 2–3 mm per year in response to the removal of ice sheets formerly compressing land; whilst in southern England and The Netherlands are sinking at 2 mm per year—these locations were peripheral to the ice sheet cover. Over time, the estuary formation results from a combination of geological processes, sea-level changes (eustatic—variations in the oceanic water volume; isostatic—variations in land level), and hydrodynamic forces. In the short term, the estuary form is constantly altered by the erosion and deposition of sediment. These coastal bodies of water form where freshwater rivers meet the ocean, creating unique environments with varying salinity and sediment dynamics. Estuaries can develop in several ways, depending on the geography and history of the coastline.

5. Classification

There are nominally four major estuary types based on their formation process. (1) Coastal Plain Estuaries (CPE; Drowned River Valleys, Rias) were formed when rising sea levels flooded low-lying river valleys. This typically occurred after the last ice age when melting glaciers caused a significant rise in global sea levels. Sedimentation has not kept pace with inundation and topography; they reflect a river. They tend to have broad, shallow areas (including intertidal mudflats and salt marshes) and are the most common type of estuary (e.g., Chesapeake Bay, USA; Thames Estuary, UK). (2) Bar-built estuaries are similar to CPE, but their sedimentation has kept pace with inundation, promoting the formation of a bar or barrier, such as a sandbar or a spit parallel to the coast, trapping freshwater from the rivers behind it. Bar-built estuaries are typically shallow, and the bar/barrier limits tidal action in the estuary and over time can even result in the formation of a lagoon. (3) Fjord estuaries were incised by glaciers during ice ages, where deep U-shaped valleys were formed and later flooded as the glaciers retreated. Fjords are deep, narrow, and often surrounded by steep cliffs. They often have a shallow sill near the mouth, which can restrict the exchange of fjord water with the sea, creating a stratified water column (e.g., Sognefjord, Norway; Milford Sound, New Zealand). (4) Tectonic estuaries were formed through tectonic plate movements causing land to sink or subside and allowing the sea to flood the area. These estuaries are often deeper and narrower than other types, with irregular shorelines (e.g., San Francisco Bay, USA).

6. Sea level and climate

Global sea level has risen by 170 mm over the past 100 years and is currently rising at 3–4 mm per year. This rise is accelerated due to global warming effects, and a 0.4–0.8 m rise is predicted by the year 2100. A large eustatic change, such as all remaining ice melting, would see the current sea level rise by a further 70 m, and this would create the formation of new estuaries in the upper valleys of present-day rivers. If, on the other hand, there was a reduction in sea level, this would produce much shallower estuaries, and these would quickly fill with sediments from the upper river valley regions. Increased frequency and magnitude of storms due to climate change is resulting in previously low tidal energy estuarine environments (e.g., bordering the Adriatic Sea) starting to mimic characteristics, such as developing turbidity maximum zones, of estuarial locations with greater natural tidal ranges [5].

7. Usage and population

Due to their strategic location at the land-sea interface, estuaries are heavily influenced by human populations. They also act as buffers, protecting inland areas from storm surges and flooding. Many of the world’s largest cities and ports are located near estuaries. Anthropogenically, estuaries are often heavily impacted by human activities, including urbanisation, dredging, agriculture, industrialisation, pollution, and coastal and residential development [6]. Due to their strategic position, estuaries and their surrounding hinterlands have historically served as vital hubs for commerce, industrial activities, and shipping. As economies expand, so does the development of estuaries. Maintaining their ecological health is critical for biodiversity and the services they provide to humans, such as fisheries, tourism, and storm protection. Maintaining the health of estuaries, through managed and sustainable usage, is critical for sustaining the species and services they provide. Estuaries often serve as natural defences against coastal erosion, storm surges, and floods, absorbing and reducing the impact of waves and tides on inland areas, making many natural harbours. This can create commercial opportunities for recreation and tourism.

Collectively, this puts estuaries among both the most developed and stressed areas in many countries. Over the past 35 years, more than 100,000 hectares of estuarine areas have been converted into urban or farming land. The majority of this loss (90%) has taken place in rapidly developing Asian countries. This trend is largely driven by the region’s rapid urban expansion, economic growth, and an increasing demand for agricultural land to support growing populations. Thus, the management of estuaries involves navigating multiple conflicts of interest due to their social, ecological, and economic importance. Estuaries serve as hubs for commercial activities, biodiversity, and recreational use, making them highly valuable but also highly contested spaces. Administering authorities must balance these competing demands while anticipating the repercussions of implemented measures in both a complex and dynamic environment.

8. Conclusions and future

Challenges facing estuary populations include: runoff from agriculture, sewage, and industrial waste that introduces nutrients, heavy metals, and other contaminants into estuaries, leading to issues like eutrophication and harmful algal blooms. Urbanisation, land reclamation, managed realignment, and coastal development reduce the availability of natural estuarine habitats such as salt marshes and mangroves, affecting wildlife populations. Intensive fishing can deplete fish stocks and disrupt the balance of estuarine ecosystems. In terms of climate change: rising sea levels and changing weather patterns pose a threat to estuarine ecosystems. Saltwater intrusion can change the salinity balance, affecting plant and animal life, while stronger storms can increase erosion and sediment transport. Estuaries are biodiversity hotspots, hosting a wide array of species while providing vital ecosystem services to both wildlife and human populations. However, they are also fragile environments that require careful management and conservation to preserve their ecological integrity amidst growing human pressures [7].