Estuaries: Where the River Meets the Sea

Estuaries are commonly described as semi-enclosed bodies of water, situated at the interface between land and ocean, where sea water is measurably diluted by the inflow of freshwater (Hobbie, 2000). They are not only a direct source of commercially important finfish and shellfish species, but are also a critical nursery habitat for commercially important off-shore species. The coupling of physics, biogeochemistry, and ecology occurs at many spatial scales in estuaries (Figure 1) (Geyer et al., 2000). Fisheries production and coastal nutrient enrichment, supplied via rivers and estuaries, have been shown to be positively correlated within many coastal systems around the world (Nixon et al., 1986; Bianchi, 2007).

Figure 1

Schematic showing important linkages between physical (e.g., tidal currents, river discharge, and groundwater) and biological (e.g., fish migrations, larval transport) processes in estuaries.

© 2013 Nature Education Modified from Geyer et al., 2000. All rights reserved.

A general schematic representation of an estuary, as defined by Pritchard (1967), and further modified by Dalrymple et al. (1992) to include more physical and geomorphological processes, is shown in Figure 2. In this diagram, we see a wide range of salinities (0.1 to 32), wave processes that dominate at the mouth of the estuary, tidal processes that occur in the middle region, and river or fluvial processes at the head of the estuary. The relative importance of physical forcing from each of these regions can vary seasonally (e.g., coastal wave energy versus river discharge), and on the event scale (storms, floods, droughts), and ultimately determine the mixing dynamics of both water and sediments in estuaries. Estuarine circulation, river and groundwater discharge, tidal flooding, resuspension events, and exchange flow with adjacent marsh systems all constitute important physical variables that exert some level of control on estuarine biogeochemical cycles and ecological processes.

Figure 2

Classic estuarine zonation depicted from the head region, where fluvial processes dominate, to the mid- and mouth regions where tidal and wave processes are the dominant controlling physical forces, respectively. Differences in the intensity and sources of physical forcing throughout the estuary also result in the formation of distinct sediment facies.

© 2013 Nature Education Modified from Dalrymple et al., 1992. All rights reserved.

Geologically speaking, estuaries are ephemeral features of the coasts. Upon formation, most begin to fill in with sediments and, in the absence of sea level changes, would have life spans of only a few thousand to tens of thousands of years (Schubel & Hirschberg, 1978). Modern estuaries are recent features that only formed over the past 5,000 to 6,000 years during the stable interglacial period of the middle to late Holocene Epoch (0 - 10,000 y BP) - which followed an extensive rise in sea level at the end of the Pleistocene Epoch (1.8 My to 10,000 y BP) (Nichols & Biggs, 1985). A sea level rise of approximately 10 mm y^-1 during this period resulted in many coastal plains being inundated with water and a displacement of the shoreline. The phenomenon of rising (transgression) and falling (regression) sea level over time is referred to as eustacy (Suess, 1906).

While there is considerable debate about the controls of current sea level changes around the world. we can generally conclude that tectonic conditions, regional subsidence rates, and regional climatic changes account for much of this variation. When factoring in these regional differences, rates of sea level change are referred to as relative sea level (RSL) rise or fall. Along the U.S. Atlantic coast we generally see a more consistent rise in sea level; however, sea level rise in northern regions is significantly slower due to isostatic rebound (the upward movement of the Earth's crust following isostatic depression), which occurs on many high latitude coastlines around the world (e.g., Scandinavia). On a global scale, it appears that sea level has risen by 12 to 15 cm over the past century (Emery & Aubrey, 1991); this has primarily been attributed to thermal expansion of sea water in response to an increase in global temperatures (Milliman & Haq, 1996). The Intergovernmental Panel on Climate Change (IPCC) has projected that from 1990 to 2100 there will be a 48 cm rise in sea level with a range of 9 to 88 cm (Church et al., 2001). The highest average sea level rise in the U.S. occurs along the Gulf Coast near the mouth of the Mississippi River (Louisiana coast) (ca. 9.0 mm y^-1), in large part due to high subsidence rates in the region.

Figure 3

Forty-four estuaries along the U.S. coastline that have been diagnosed as having nutrient over-enrichment.

© 2013 Nature Education Modified from Perillo, 1995. All rights reserved.

The physiography (morphology) of estuaries has been shown to vary widely and has an important impact on the physical mixing processes. Pritchard (1952) introduced the first physiographic classification of estuaries, and later modified his estuarine classification system based on the relative importance of the advective flux of salt and water and diffusive flux of salt processes (Pritchard, 1955). Fairbridge (1980) presented a classification system composed of the following seven basic physiographic types: coastal plain (drowned river valley), bar-built, delta, blind, ria, tectonic, and fjord. These categories were developed based on the overall relief of the region and the extent to which circulation is restricted near the mouth of the estuary. A more recent modification of this classification system (Perillo, 1995) uses an approach that better encompasses the spectrum and diversity of estuarine systems (Figure 3). Primary estuaries are formed from terrestrial and/or tectonic processes with minimal changes from the sea. The four categories within primary estuaries are former fluvial valleys, former glacial valleys, river-dominated, and structural. The definitions of these primary estuaries and their respective subcategories are as follows:

1. Former fluvial valleys are formed by flooding of river valleys in the Pleistocene-Holocene during the last interglacial transgression. The two sub-categories are: a) Coastal plain estuaries, which occur on low relief coasts due to high sedimentation and in-filling of a river valley; and b) Rias, which are former river valleys in regions of high relief (mountains and cliffs).

2. Former glacial valleys are formed by flooding of river valleys in the Pleistocene-Holocene during the last interglacial transgression. The two subcategories are as follows: a) Fjords, which are high relief systems formed by glacial scouring; and b) Fjards, which are low relief systems formed by glacial scouring.

3. River-dominated estuaries are formed in high river discharge regions where the valley is presently not drowned by the sea. The two subcategories are as follows: a) Tidal river estuaries, which are associated with large rivers systems that are influenced by tidal action, and where the salt front is usually not well developed at the mouth; and b) Delta-front estuaries, which are found in sections of deltas that are affected by tidal action and/or salt intrusion.

4. Structural estuaries are formed by processes such as faulting, volcanism, postglacial rebound, and isostacy, which have occurred since the Pleistocene.

Secondary estuaries have been modified more by coastal processes than by river discharge, since the time that sea level reached its current position. For example, coastal lagoons are inland bodies of water that run parallel to the coast and are isolated from the sea by a barrier island where one or more small inlets allow for connection with the ocean (Kennish & Paerl, 2010). Subcategories of lagoons are as follows: a) choked lagoons, which have only one long narrow inlet; b) restricted lagoons, which have very few inlets or a wide mouth (partially blocked by a sandbar); and c) leaky lagoons, which have many inlets separated by small barrier islands.

It has been estimated that 61% of the world's population lives along the coastal margin (Alongi, 1998; Bianchi, 2007). Demographic changes in human populations have clearly had detrimental effects on estuaries. Nutrient enrichment is perhaps the most widespread problem in estuaries around the world. For example, 44 estuaries along the entire U.S. coastline have been diagnosed as nutrient over-enriched (Figure 4) (Bricker et al., 1999). The Patuxent River (Maryland, US), a tributary of the Chesapeake Bay, experienced extensive eutrophication from sewage inputs and non-point sources over four decades (1960-2000) (D'Elia et al., 2003). Fortunately, we are beginning to detect measurable improvements in the water quality of some estuaries following scientifically based nutrient reductions and extensive long-term monitoring. During the late 1970s, scientists began to develop a dialog with regional policy makers and local and national funding agencies, which resulted in stable funding for well-structured monitoring programs. Furthermore, based on preliminary indications, it appears that the nitrogen-removal strategy has succeeded in improving water quality in the Patuxent River estuary (D'Elia et al., 2003). This further corroborates the evidence that long-term and scientifically based monitoring programs can result in effective remediation strategies for environmental problems in estuaries.

Figure 4

Morphogenetic classification of estuaries.

© 2013 Nature Education Modified from Bricker et al., 1999. All rights reserved.

Hobbie (2000) summarized the findings of an earlier U.S. National Research Council report that focused on the major effects of human population growth on estuaries. A slightly modified version of this summary is as follows: 1) nutrients, especially nitrogen, have increased in rivers and estuaries, resulting in harmful algal blooms and a reduction in water column oxygen levels; 2) coastal marshes and other intertidal habitats have been severely modified by dredging and filling operations; 3) changes in watershed hydrology, water diversions, and damming of rivers have altered the magnitude and temporal patterns of freshwater flow, sediment, and nutrient discharge to estuaries; 4) many of the commercially important species of fishes and shellfish have been overexploited and/or negatively affected by habitat destruction; 5) extensive growth and industrialization has resulted in high concentrations of both organic (polycyclic aromatic hydrocarbon [PAHs] and polychlorinated biphenyls [PCBs]), and inorganic contaminants (heavy metals) in estuarine sediments and waters; and 6) introduced species have resulted in alterations in habitats and food webs, loss of native species, and a reduction in commercially important species. Model predictions indicate that by 2050, an estimated 8.5 billion people will be living in exoreic watersheds (e.g., watersheds draining to the ocean), an increase of greater than 70% over 1990 population figures(Kroeze and Seitzinger, 1998).