BETWEEN THE DEVIL AND THE DEEP BLUE SEA: THE DIFFICULTIES OF DEFINING ESTUARIES.By Sally Little (@estuary_ecology)
I am an estuarine ecologist, which means that I study the relationship between organisms and their environment in areas of the coast where freshwater rivers meet the saltwater flood of the tide. Estuaries are interesting because they are naturally dynamic, high-energy environments, characterised by a specific flora and fauna. Physical processes operating on both short (e.g. tidal cycles) and long (e.g. climate and sea level change) timescales form the driving forces for many of the complex processes that occur in these systems. This means that estuaries are sites of continuous change, experiencing chemical (e.g. salinity, dissolved gases and nutrients), sedimentary (e.g. turbidity maximum), hydrological (e.g. tidal and freshwater flow) and morphological variations over daily tidal cycles. Few plants and animals can withstand the extremes of these constantly fluctuating regimes, but those that can, commonly achieve high numbers, making estuaries some of the most important and productive ecosystems in the world.
|The Humber Estuary, UK from the International Space Station - courtesy of @Cmdr_Hadfield|
|A miniature estuary at Sanna on the Ardnamurchan peninsula, North-West Scotland.|
Estuaries (arguably more than any other aquatic ecosystem) are at the pinnacle of the human-environment interface – providing sheltered locations for habitation with access to inland, coastal and offshore resources and thus acting as focal points of human settlement and development throughout history. For example, 10% of the global population (640 million people in the year 2000) live in the lower elevation coastal zone (LECZ; land below 10 m), which covers just 2% of the world’s total land area. This area contains two-thirds of the world’s megacities (population in excess of 10 million people) and more than 10% of the world’s wealth. As such, estuaries are subject to dense populations, development pressures and intensely exploited resources; with issues such as pollution, nutrient enrichment, habitat loss and over-exploitation extremely common in these systems today - pressures which are likely to increase with global population growth.
|The megacity of Shanghai in China is located on the Yangtze River Estuary - the third largest river in the world|
|The megacity of New York has grown around the Hudson River Estuary - the Mahican name of the river (muh-he-kun-ne-tuk) represents its partly estuarine nature as "the river that flows both ways"|
In addition to human pressures, estuaries and coastal zones are particularly vulnerable to climate change (e.g. including eustatic sea level rise, changes in weather patterns and extreme events). It is therefore important to manage both the impact of human activities and future global climate change upon estuarine ecosystems, though this has raised one of the fundamental issues in estuarine research – what is an estuary?
Traditionally, we, as estuarine scientists, have used the ‘expert-view’ definition that “if it looks like an estuary, smells like an estuary and behaves like an estuary, then there is a good chance that it is an estuary”! However, when we increasingly have to provide information to lawyers, planners and policy makers and are required to rigorously defend our terms in courts of law, the repercussions of poor definitions may be legally and economically costly – therefore everything becomes a little more tricky. For example, a recent court appeal case (12 April 2011) between Western Ferries (Clyde) Limited and The Commissioners for Her Majesty’s Revenue and Customs (HMRC) concerning liability to pay corporation tax, highlighted the problem of a lack of a legal definition of an estuary in the UK. Western Ferries asserted that they operated a crossing outside the Clyde estuary and harbour limits, therefore should be taxed under ‘tonnage’ rather than ‘corporation' tax regulations – at stake was a cool £3 million. The judgement considered a variety of definitions of an estuary, from both scientific literature, management directives and evidence from expert witnesses, one of which included the eminent estuarine scientist Dr Donald McLusky. However, even then the definition of an estuary was not clear-cut and proved problematic to establish on a legal basis.
|Gourock and the Firth of Clyde, North-West Scotland|
The issue is that estuaries are extremely difficult to define. That is not to say that there aren’t any definitions, in fact there are over forty definitions of an estuary, the majority of which are based upon physical characteristics and all of which differ based upon the research discipline and geographical location of the defining author. There is however no one definition that provides universal coverage for all the estuaries in the world.
This is the crux of the problem. Whilst estuaries can be characterised in similar ways (e.g. freshwater input, tidally influenced with a salinity gradient), each is physically and biologically different. Often, the further apart estuaries are geographically, the more different they become. The majority of estuaries in the Northern hemisphere, for example, have a tidal range of greater than 4 metres (macrotidal), free connection to the sea, significant freshwater river input and a salinity gradient from fresh to marine waters. In contrast, from a global perspective, very few brackish coastal water bodies match these archetypal classical estuaries of Northern Europe, where the majority of estuarine research has taken place. In Australia and South Africa for example, a growing number of scientists argue that coastal systems such as intermittently open and closed coastal lagoons and lakes be included in the definition of an estuarine ecosystem. In these often microtidal (tidal range <2 m), arid systems, tidal and freshwater input can be negligible giving rise to temporarily open/closed systems, where evaporation can lead to hyperhaline areas (salinity greater than 35). In these systems, the mouth is often marked by physiographic forms (e.g. a sand bar) which serve to close off the estuary from the sea for at least part of the year. However, during these closed phases, these systems have been shown to function normally as estuaries prior to re-opening. Interestingly, even though both South Africa and Australia have legal definitions of an estuary (in contrast to the UK), neither cater for hyperhalinity.
|The temporarily open/closed East Kleinemonde Estuary in South Africa (picture courtesy of Michael J Stone)|
|The temporarily open/closed Brega River Estuary in New South Wales, Australia|