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.
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The Humber Estuary, UK from the International Space Station - courtesy of @Cmdr_Hadfield |
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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.
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The megacity of Shanghai in China is located on the Yangtze River Estuary - the third largest river in the world |
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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.
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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.
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The temporarily open/closed East Kleinemonde Estuary in South Africa (picture courtesy of Michael J Stone) |
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The temporarily open/closed Brega River Estuary in New South Wales, Australia |
To avoid these problems of definition, legislators
are increasingly developing new conservation, socioeconomic and legal definitions
and classification systems for estuaries – the most widely accepted of which
(in Europe at least) is the term ‘transitional waters’ coined by the European
Union within the Water Framework Directive (WFD) to define all waters that are
neither the open coast or true freshwaters (i.e. fjords, fjards, river mouths,
deltas, rias and lagoons as well as the more classical estuaries). Additional legal definitions have been
developed for estuaries worldwide. The
problem with all of these definitions however, is that more often than not,
they do not delimit where an estuary starts and where it ends – an issue which will
be the subject of my next blog!
In California, another Mediterranean climate system, we also have intermittently open estuaries like Australia and South Africa. These lagoons are sometimes called "bar built estuaries"
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