Estuarine Ecohydrology

An Introduction

Eric Wolanski, Michael Elliott

• Publisher: Elsevier Science
• Year: 2015
• ISBN:
  9780444634146
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Estuarine Ecohydrology, Second Edition, provides an ecohydrology viewpoint of an estuary as an ecosystem by focusing on its principal components, the river, the estuarine waters, the sediment, the nutrients, the wetlands, the oceanic influence, and the aquatic food web, as well as models of the health of an estuary ecosystem.

Estuaries, the intersection of freshwater and coastal ecosystems, exhibit complex physical and biological processes which must be understood in order to sustain and restore them when necessary.

This book demonstrates how, based on an understanding of the processes controlling estuarine ecosystem health, one can quantify its ability to cope with human stresses. The theories, models, and real-world solutions presented serve as a toolkit for designing a management plan for the ecologically sustainable development of estuaries.

• Provides a sound knowledge of the physical functioning of an estuary, a critical component of understanding its ecological functioning
• Ideal reference for those interested in marine biology, oceanography, coastal management, and sustainable development
• Describes the essentials behind conceptual and numerical models of the health of an estuary ecosystem and how to use these models to quantify both human impacts and the value of remediation and management measures
• Chapters are written in an accessible way that encourages collaboration between aquatic, marine, and wetland biologists, ecologists, oceanographers, geologists, geomorphologists, chemists, and ecosystem modelers
• Covers the physical, chemical, and biological elements of estuary environments, indicating that the essence of an estuary's functioning lies in its connectivity with the adjacent catchment and the marine/coastal system

1

Introduction

Publisher Summary

An estuary is a semienclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff. The freshwater inflow may occur only for part of the year, and the connection to the sea may be closed for part of the year (e.g., by a sand bar). In some estuaries, the tidal influence may be negligible. Estuaries have been, and still are, amongst the most populated areas worldwide: they are used as transport routes, and, because of their high biological productivity, sustain a high level of food production. Because estuaries link the land and the sea, they are directly or indirectly exposed to many pressures. Those pressures include the need for space for agriculture, housing, and industrial areas; water for irrigation and industrial use and for disposing of waste; and for physical and biological resources, such as aggregates and fish. Materials being put into the estuaries include contaminants and pollutants, sediment, energy, infrastructure; materials removed from the estuaries include salt, water, fish, sediments and ‘space’. Superimposed on these local activities and pressures are the consequences of climate change. The resulting degradation of estuaries also leads to threats to human health. The ecohydrology concept recognizes that the estuarine ecosystem health is driven by links between biology and physics; that human activities in the entire catchment need to be considered; and that the best course of action is to manipulate the system to reinforce its ability to cope with human stresses. The ecosystem does not stop at the tidal limit; it comprises the whole river catchment including the river and its riverine wetlands, the estuarine wetlands, and the coastal waters. Hence, this chapter emphasizes that ecohydrology is intimately linked to ecological engineering, and that a good understanding is required in order to balance and achieve natural and societal needs.

Keywords

Dams

Ecosystem

Eutrophication

Estuary

Food

Pollution

River

Salinity

Sediment

Reclamation

Transport

Watershed

1.1 What is an estuary?

An estuary receives, occasionally, or frequently, an inflow of both freshwater and saltwater; it stores these waters temporarily while mixing them. An estuary is a buffer zone between river (freshwater) and ocean (saltwater) environments that may be affected by tidal oscillations. Many estuaries were established by the flooding of river-eroded or glacially scoured valleys during the Holocene rise of sea level starting about 10,000-12,000 years ago. Because it fills with sediment, and is thus regarded as an ephemeral feature, an estuary has an age, akin to a living organism in evolution. It starts with youth, it matures, and it then becomes old; it can be rejuvenated.

The term ‘estuary’ is derived from the Latin word ‘aestuarium’, this means tidal. This definition is however over-restrictive because estuaries also occur in conditions with no tides such as the rivers discharging into the tideless Baltic Sea and the Danube Delta in the tideless Black Sea.

There have been several definitions of an estuary, such as that of Dionne (1963):

An estuary is an inlet of the sea, reaching into the river valley as far as the upper limit of tidal rise, usually divisible into three sectors: a) a marine or lower estuary, in free connection with the open sea; d) a middle estuary, subject to strong salt and freshwater mixing, and c) an upper or fluvial estuary, characterized by freshwater but subject to daily tidal activity.

There are other definitions of an estuary. For instance Pritchard (1967) defined an estuary as ‘a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with fresh water derived from land drainage’. This definition based on salinity has been accepted since the publication. However, it excludes a number of coastal water bodies such as hypersaline tropical lagoons with no perennial inflows; it also seems to exclude seasonally closed lagoons; it explicitly excludes the Baltic Sea, the Seto Inland Sea in Japan, and other brackish seas.

Dalrymple et al. (1992) proposed a definition of an estuary from the point of view of the fluvial and marine sources of the sediment. Perillo (1995) offered another definition based on the dilution of freshwater with seawater and the presence of euryhaline biological species. An estuary can also be defined as the zone stretching from the tidal limit to the seaward edge of the tidal plume in the open ocean (Kjerfve, 1989). These definitions however do not capture estuaries with features such as periodic closure of their mouths and hypersaline conditions during dry periods, especially these occurring in the Southern Hemisphere such as in South Australia and South Africa. Potter et al. (2010) commented that most estuarine definitions were based on temperate areas, especially in the Northern Hemisphere, and so do not accommodate features such as periodic closure of their mouths and hypersaline conditions during dry periods. They also remarked on the ambiguity as to whether an estuary sensu stricto must be fed by a river. Thus, a more appropriate definition may be that suggested by Potter et al. (2010):

An estuary is a partially enclosed coastal body of water that is either permanently or periodically open to the sea and which receives at least periodic discharge from a river(s), and thus, while its salinity is typically less than that of natural sea water and varies temporally and along its length, it can become hypersaline in regions when evaporative water loss is high and freshwater and tidal inputs are negligible.

The definition of an estuary in this book combines all these definitions; that is, an estuary is a semienclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff, recognising that the freshwater inflow may not be perennial (i.e. it may occur only for part of the year) and that the connection to the sea may be closed for part of the year (e.g. by a sand bar) and that the tidal influence may be negligible (Figure 1.1). The definition includes fjords, fjards, river mouths, deltas, rias, lagoons, tidal creeks, as well as the more classical estuaries. It recognises commonalities with predominantly brackish areas such as the Baltic Sea, and freshwater-poor coastal waters in arid zones.

Figure 1.1 Example of three primary types of estuaries: (a) valley estuaries, (b) estuarine lagoons/lakes, (c) river mouths. Modified from Fairbridge (1980).

It is also difficult to define where an estuary ends and this is usually assumed to be an abrupt coastline break but gradual geomorphological changes show that many estuaries change shape gradually, thus the transition between river, estuary, coastal embayment, and open coast is gradual and not always obvious. Whitfield and Elliott (2011) detail the many types of classification of the estuaries. To accommodate these problems, through the Water Framework Directive, the European Union coined the word ‘Transitional Waters’ as ‘bodies of surface water in the vicinity of river mouths which are partly saline in character as a result of their proximity to coastal waters but which are substantially influenced by freshwater flows’. Some of these ‘transitional waters’ are neither river mouths nor have substantially lowered salinity, and they are neither rivers nor open coasts but are taken to include estuaries, rias, fjords, lagoons, and other types of intermediate water body (Elliott and McLusky, 2002; McLusky and Elliott, 2007).

As shown by recent analyses, estuaries are now considered as ecosystems in their own right rather than either the ends of rivers or inlets of the sea (Elliott and Whitfield, 2011; Whitfield et al., 2012; Basset et al., 2013; and the extensive reviews in Wolanski and McLusky, 2011, and references therein). Therefore, in order to consider their physicochemical and ecological structure and functioning and the management of them in ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• About the Authors
• Preface to the 2nd Edition
• 1: Introduction
• 2: Estuarine water circulation
• 3: Estuarine sediment dynamics
• 4: Tidal wetlands
• 5: Estuarine ecological structure and functioning
• 6: Ecohydrology models
• 7: Ecohydrology solutions
• References
• Index