Download (1999) - The conservation of brackish

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS
Aquatic Conser6: Mar. Freshw. Ecosyst. 9: 523 – 527 (1999)
The conser7ation of brackish-water systems: priorities for
the 21st century
R.S.K. BARNES*
Department of Zoology, Uni6ersity of Cambridge, Cambridge, UK
KEY WORDS:
brackish-water; conservation; habitat
There is a marked contrast between the theory and the practice of conservation in terrestrial environments
on the one hand and in most aquatic environments, including those of the coastal marine zone, on the
other. It is effectively axiomatic in the terrestrial sphere that conservation requires active management. It
is not sufficient simply to safeguard an area from external threats, although that could also be important.
Without active intervention, a given terrestrial system is likely to change into another, almost certainly of
lesser conservation interest. The habitat must be manipulated so as to maintain it in a form suitable for
the species and the communities of interest, and the populations of certain species—most especially
various potentially superior competitors and often significant consumers—must be controlled in order to
favour others. Burning, grazing and many other tools are routinely used to prevent natural changes. To
a degree, the same is also true of freshwater habitats: control of water tables, of water quality, of fringing
stands of vegetation and of submerged aquatics are all well-known and much practised measures.
Marine and brackish-water systems are treated differently, however. Through much of the past, these
have been conserved only insofar as they lay within the boundaries of conservation areas declared—and
managed—for other purposes, whether these have been botanical or ornithological. More recently,
however, they have become the focus of conservation interest in their own right with the declaration of
Marine Nature Reserves, or Special Areas of Conservation as a consequence of the ‘Habitats Directive’
(Council Directi6e 92 /43 /EEC; Anon., 1992), etc. In these, in contrast to terrestrial conservation practice,
management measures have largely been restricted to preventing human activities that might impinge
adversely on them, with no attempt being made to manipulate the contained populations.
In part this could be a consequence of the recency of their designation, but it also reflects divergent
philosophical approaches. In essence, it is necessary to interfere in terrestrial systems because most of
them are anthropogenic in origin and are naturally subject to successional changes. In contrast, the sea
is supposed to be a natural system and is thought to be able to maintain itself in the desired state provided
that human interference can be removed or at least controlled. It is essential to prevent natural succession
from occurring on land but the aquatic coastal system is already in its climax state and it is this status quo
that must be maintained, not a degraded version of it. This may (or may not) be true of much of the
coastal sea, but is it also the case in coastal brackish waters including lagoons?
Here the argument is rendered more complex by another system of divergent philosophical approaches.
Some habitats are perceived as being under physical control whilst others are structured by biotic
* Correspondence to: Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.
CCC 1052–7613/99/060523–05$17.50
Copyright © 1999 John Wiley & Sons, Ltd.
Received 14 January 1999
Accepted 17 August 1999
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R.S.K. BARNES
interactions, such as competition and predation. If the nature of the physical environment is such as to
cause periodic heavy mortality, which will serve to depress populations, then those populations will be
below carrying capacity and potential competitive interactions will be unlikely to cause the disappearance
of species of conservation interest. In other words, preserve the natural physically hostile environmental
regime and that regime will manage the habitat.
Brackish waters have traditionally been regarded as being physically hostile. They are, after all, a sort
of no-man’s-land between the major biotic assemblages of the sea and freshwaters. In the first extensive
review of the habitat, Remane (1958), who did much to establish brackish-water biology as an acceptable
discipline, described its ‘special ecological features’ in the form of a series of perjoratives, using
section-heading words such as ‘poverty’, ‘reduction’, ‘depauperization’, ‘deviation’, ‘anomalies’ (i.e.
malformations and reproductive sterility), etc. Other authors have since repeated the tenor of these
attributes. In comparison with freshwaters and the sea, brackish waters are usually portrayed as species
poor, ‘unsaturated’ habitats (Arndt, 1989), and received wisdom, dating back to Lovén’s 1863 lecture to
the first Public Meeting of the Scandinavian Association of Natural Scientists, implicates salinity as the
cause of this under-saturation. Such is implicit in the oft-quoted diagram, first published by Remane
(1934), that shows a species minimum in the brackish-water range of the salinity gradient (Figure 1). The
essential question is therefore: is this evidence of environmental hostility and physical structuring?
Clearly it is the case that very few marine species (mainly a few fish and even fewer decapod
crustaceans) can also inhabit freshwaters, and 6ice 6ersa. Where the two environments abut, it is only to
be expected that one biota largely ceases and another starts. If, however, one was to produce a
Remane-type graph along a transect starting in, e.g. a microtidal sea and finishing on land via a mangrove
swamp, no one would be surprised at its equivalent shape, and it would be unlikely that this interface
would be labelled as hostile per se. (Microtidal sea was carefully chosen here in order to avoid problems
relating to the potential hostility of the intertidal zone.) Thus, while most marine species are stenohaline
and have had no evolutionary reason to become euryhaline, it is only to be expected that their
distributions cease on encountering diluted sea water.
There are, however, other classic explanations for the species poverty of brackish-water habitats besides
the environmental hostility hypothesis (Barnes, 1984), such as: (i) because the habitat contains little
Figure 1. The classic diagram showing relative numbers of species present along a gradient in salinity from freshwater to fully
marine conditions (after Remane, 1934).
Copyright © 1999 John Wiley & Sons, Ltd.
Aquatic Conser6: Mar. Freshw. Ecosyst. 9: 523 – 527 (1999)
BRACKISH-WATER CONSERVATION
525
heterogeneity, being largely a monotonous expanse of soft sediment (the low habitat diversity hypothesis);
(ii) because of the geologically short life of any given brackish-water system and its isolation from other
such systems by expanses of marine environment (the short timescale/isolation hypothesis); and (iii)
because of biological interactions.
Several observations suggest the general falsity of the hostility hypothesis. First, osmoregulation is
energetically much more economic than was once thought (Schmidt-Nielsen, 1975): euryhaline animals
appear to expend only some 1% or less of their metabolic rate in regulating the constancy of their body
fluids, even in the face of a large gradient. Second, the classic reduction in maximum size of, e.g. bivalve
molluscs in estuaries, as documented by Remane (1934), that has been taken to indicate metabolic stress,
includes a powerful confounding variable: age. Reise (1985) and others have shown that as a result of
predation by shore birds, fish, etc. — brackish waters being extensively used as feeding grounds by
migratory crustaceans, fish and birds — bivalve molluscs and other invertebrates in estuaries seldom
achieve old age and hence large size. Thus, e.g. in 1971 the maximum size of cockles Cerastoderma edule
at Hythe in Southampton Water was 24 mm, compared with nearly 40 mm outside the mouth (Barnes,
1973). However, the cause was not decreased growth rate, but simply differing life expectancy; the cockles
at Hythe were no more than 2 years old, whereas the largest ones at the mouth were 9 years old (Barnes,
1973). Third, the large majority of the euryhaline members of the brackish-water fauna that have been
tested can all survive and breed successfully in considerably lower (and higher) salinities than those in
which they have usually been observed to inhabit in the field (Barnes, 1989). Their realised niches are
much smaller than their potential ones, with respect to physical environmental variables.
Nevertheless, a convincing argument can be made for physical structuring of communities near the
heads of estuaries. For example, the discharge of considerably more than the usual quantity of freshwater
into a brackish-water system by the river or rivers entering it (see, e.g. Boesch et al., 1976; Thomason and
Bennett, 1997) will create temporary freshwater conditions near the head and can wipe out the essentially
marine component of the fauna there. These then recolonize after the event, when ‘normal’ salinities have
returned, but their speed of recolonization may well be such that the ground lost is not recovered before
the next local mass mortality occurs. Recovery times of several species has been shown to be slow (e.g.
Simon and Dauer, 1977). In other words, they are continually recolonizing (i.e. advancing) and
continually being killed off (i.e. retreating). How far upstream they happen to have advanced at any
particular point in time (when perhaps someone happens to be mapping their distribution) is purely a
matter of chance, and the only limit to their distribution, other than the fact that they cannot withstand
pure freshwater, is lack of time since the last period of mortality (Barnes, 1994a). This section of the
estuarine salinity gradient inhabited by any given species is labile and will only be in equilibrium when the
gradient itself is unchanging; however, such a state rarely occurs in interface environments. Estuarine
distributions near the freshwater end of the spectrum are, therefore, dynamic and are not explicable in
terms of the precise nature of the physical or biotic variables of the system at any particular point in time.
For all of these species, their distributions with respect to ambient salinity vary from location to location
and from time to time (e.g. Boyden and Little, 1973).
Otherwise, and particularly in non-estuarine brackish waters, the evidence for physical structuring is
much less convincing. Many species are characteristically to be found in greatest abundance or with
greatest frequency in brackish waters (the ‘brackish-water species’, sensu Zenkevitch, 1959 and see
Remane, 1969, Wolff, 1973 and Day, 1981), although virtually all are also known to occur in either
freshwater or the sea (see Barnes, 1989). It has been argued that this is probably a consequence of
interspecific competition: ecologically they may be restricted to brackish habitats in many areas but none
is really restricted (i.e. physiologically) to brackish water. These species also occur in systems that have all
the environmental characteristics of brackish waters except the lowered salinity (i.e. in sheltered, shallow,
detritus- and nutrient-rich areas of soft sediments that support benthic photosynthetic production). This
observation that it is not salinity that restricts the species to brackish waters is the reasoning behind the
Copyright © 1999 John Wiley & Sons, Ltd.
Aquatic Conser6: Mar. Freshw. Ecosyst. 9: 523 – 527 (1999)
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R.S.K. BARNES
renaming of the biota as ‘paralic’ in recent francophone literature (following Perthuisot, 1975, Guélorget
and Perthuisot, 1982, etc.). The characteristic brackish-water fauna is a suite of marine species of differing
degrees of euryhalinity, that penetrate into dilute media to varying degrees, together with a few freshwater
species than can withstand more saline waters.
Near the mouths of some estuaries, circumstantial evidence suggests that the ability of characteristically
estuarine species to withstand low salinities has provided them with an environmental refugium lying
outside the range of potential but more stenohaline competitors. However, in the absence of such
competitors they are as characteristic of full-strength sea water as they are of more dilute media (Barnes,
1994a). There is also concrete evidence of interspecific competitive interactions in brackish-water lagoons:
one possible reason for the rarity of the lagoonal mudsnail Hydrobia neglecta could be competition with
the more abundant and widespread H. ul6ae (Barnes and Gandolfi, 1998). The role of predation in
reducing the abundance (and species-richness) of the soft-sediment macrofauna, including through the
agency of the species using these areas as nursery grounds, was referred to above. Indeed, a glance
through the non-conservation literature reviewing the processes likely to structure most estuarine,
lagoonal and equivalent expanses of soft sediment (e.g. Peterson, 1979; Reise, 1985; Barnes, 1994b;
Ólafsson et al., 1994) soon discloses that the debate is not between physical 6ersus biotic control, but
between various possible types of biological controlling process, particularly top–down predator action
6ersus bottom–up competition.
Leaving aside those brackish-water communities found in areas adjacent to inflowing freshwaters, there
is then abundant evidence that they are not physically structured by salinity, but are instead governed by
biotic processes. In this they are like most terrestrial systems. Therefore, if brackish-water habitats and
their contained organisms are to be conserved, it will not be sufficient simply to prevent adverse human
impact. Dominant competitors and certain types of predator will have to be manipulated, just as they are
on land. Management plans in the 21st century will have to be based on detailed knowledge of the
ecological processes structuring the individual systems. Most of this information remains to be collected,
and if conservation is to succeed much less emphasis will have to be placed on the mapping, recording and
monitoring of the health of existing brackish-water populations and much more emphasis put on
understanding why those populations fluctuate and why certain species are restricted to only a few
individual habitats. Only then can we begin to ask how we can manipulate them to achieve desired aims.
This is especially true in the coastal lagoons that are designated for priority treatment by the ‘Habitats
Directive’, and that contain a suite of rare species scheduled by the British Wildlife and Countryside Act
1981 for statutory protection. A requirement of the ‘Habitats Directive’ is that EU member states shall
replace lagoonal habitat which was lost during the last 50 years and is expected to be lost in the immediate
future. Creation of such new habitats is not the problem. The problem is whether any of the organisms
of interest, if introduced, will survive in the newly created locations. This we as yet have no means of
telling. Until research and associated funding emphasis are placed on establishing how these communities
work, conservation attempts will be little more than trial and error, with few opportunities to learn from
failed attempts. Aquatic conservation knowledge, at least in respect to lagoonal and other brackish-water
habitats, has a long way to go before its base is equivalent to that for terrestrial management plans. And
we do not have all that much time in which to catch up if these habitats are to be safeguarded through
the century, let alone the next millennium.
REFERENCES
Anon. 1992. ‘Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna
and flora’, Official Journal of the European Communities, L206/7, 7 – 50.
Arndt, E.A. 1989. ‘Ecological, physiological and historical aspects of brackish water fauna distribution’, in Ryland,
J.S. and Tyler, P.A. (Eds), Reproduction, Genetics and Distributions of Marine Organisms, Olsen & Olsen,
Fredensborg, 327–338.
Copyright © 1999 John Wiley & Sons, Ltd.
Aquatic Conser6: Mar. Freshw. Ecosyst. 9: 523 – 527 (1999)
BRACKISH-WATER CONSERVATION
527
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Copyright © 1999 John Wiley & Sons, Ltd.
Aquatic Conser6: Mar. Freshw. Ecosyst. 9: 523 – 527 (1999)