by
J.F. Caddy (*) and A. Bakun (*)
INTRODUCTION
The history of land use within catchment basins (e.g. Krug, 1993) and consequent nutrient-loading of river discharges, should play a large part in understanding changes in coastal marine ecosystems, although whole-system studies of these effects are still rare. Estuaries and wetlands (Caddy, 1990) are critical marine habitats of importance to coastal living resources, but also play a large part in reducing land-runoff inputs to coastal and enclosed seas (e.g. Tolmazin, 1985; Cristofor et al., 1993). These critical estuarine ecosystems have been subject to considerable modification in recent decades. At the larger scale of the enclosed and semi-enclosed sea, the “Marine Catchment Basin” or “MCB” concept, proposed originally as a policy framework for semi-enclosed seas, may be useful in encompassing geographical scales of interest to policy-makers and managers, including both the river basins and the semi-enclosed sea they discharge into (UNCED 1991; Caddy 1993). Such a paradigm has special value wherever human activities affecting terrestrial runoff have substantial impacts on a marine system, or are expected to show these effects with further development of the river basin. As such, the MCB provides a more specific framework for treating terrestrial impacts, and provides a framework for those “Large Marine Ecosystems” (LME's: Sherman et al., 1993) when discussing not only man's actions on the marine aquatic system and its living marine resources, but also the repercussions on the estuarine and coastal marine environment, of actions on adjacent land areas that drain into it.
Several recent reviews (e.g. GESAMP, 1988; Howarth, 1993) have focused on the plume-related chemical events and their consequences low in the food chain, but studies on the commercially important components of the ecosystem are less easily located. However, recent experience has indicated not only the key role played by estuarine environments in marine fisheries (e.g. Oglesby et al., 1972; Lenanton and Potter 1987; Houde and Rutherford, 1993; McHugh, 1976), but the potential for significant changes of estuarine faunas under anthropogenic impacts. Such runoff-related changes have also been noted at a larger scale for the ecosystems of coastal and semi-enclosed seas, (e.g. Sutcliffe et al., 1977; Lehtonen and Hilden, 1980; Pucher-Petkovic et al., 1988; Dethlafsen, 1989; Tatara, 1990; Mee, 1992; Caddy, 1993), and these seas often have estuarine characteristics. Environmental changes for these aquatic systems are often accompanied by the replacement of ecosystem components of importance to fisheries by other species better adapted to eutrophic conditions (e.g. Li and Moyle 1981; Caddy and Griffiths, 1990), which may not always be of commercial value. The comparative study of historical data series of environment and living marine resources of coastal areas will however only be successful following proper characterization of the ecosystems concerned, and identification of the main anthropogenic impacts.
Geographical differentiation of the Marine Catchment Basin
In the case of an enclosed or semi-enclosed sea, the Marine Catchment Basin can be seen as including the river basins whose runoff discharges into the sea, their estuaries, and those marine waters actually or potentially affected by riverine runoff. The physical and chemical inputs of outflowing rivers to the immediately adjacent marine coastal waters have been widely discussed (see Martin et al., 1982; Drinkwater, 1986; Jordan et al., 1991). This paper will largely focus on describing some of the effects of three main anthropogenic factors on the living marine resources of coastal water masses, and how these may affect the rational use of commercially-important resources by man.
Experience has shown that long-term accumulation of river inputs can change the nature of semi-enclosed marine areas, and their suitability with respect to particular types of living marine. However, the primary effects of land runoff will be largely confined, as for open oceanic systems, to the coastal current under the influence of the incoming plume. A series of other sources of nutrients (upwelling, tidal mixing, etc.) are described by Caddy and Bakun (1994), and may also be of considerable importance. These sources, and their temporal variations, have often obscured the effects of anthropogenic changes. It is useful however, particularly from a research perspective, to consider a smaller-scale geographical differentiation of the MCB concept that makes it clear that enriched riverine flows are not homogenous or instantaneous in their impact throughout the marine component of the MCB, and that the anthropogenic effects to be looked for are likely to be geographically concentrated.
It is reasonable to suppose that the immediate and most intense anthropogenic impacts on the coastal marine area will be particularly concentrated within the influence of the river plume, and that physical processes contributing to the behaviour of the plume and affecting its biological characteristics are also likely to be of interest. In particular, the influence of Coriolis forces, the vertical upwelling of marine nutrients, and the areas of convergence of water masses, are all relevant to the environment of coastal marine resources.
The behaviour of river “plumes”
The freshwater outflow from a river into the sea is often described as “plume”, but closer study reveals that this is a misleading description of the characteristics of the outflow for many large rivers. As is well known, (see e.g. Mann and Lazier, 1991), Coriolis forces due to the earth's rotation have a predominant effect in causing the river discharge to swing to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere (Figure 2). This effect can be expected to have a very significant differential influence on the living marine resources of the coastal areas to each side of the estuary, and this aspect, although worthy of research from the perspective of pollution-related effects, does not seem to have been explicitly investigated; the characteristics of turbidity and phytoplankton levels within the area of plume influence are easily detectable by remote sensing.
Certainly, the most dramatic effects of nutrient runoff under open marine conditions have involved semi-enclosed systems such as the Black Sea (Caddy and Griffiths, 1990; Mee, 1992). However, even along more open coastlines, large-scale hydrodynamic forces tend to cause river outflow to remain confined against the coast rather than spreading directly offshore. This produces effects resembling the semi-enclosed situation.
Within the banks of the estuary itself, the motion is driven by the upstream fluid pressure head to flow quite directly toward the mouth (although the flow is often relatively intensified toward the side corresponding to the direction of Coriolis deflection). However, as soon as the flow exits the constraint of the banks it begins to be deflected by Coriolis (Figure 2). It thus continues to turn toward the coast until it encounters the constraint of that solid boundary, where it builds up sufficient pressure head to oppose further coastward flow. This initiates a quasi-geostrophic equilibrium where pressure and Coriolis forces are largely in counteractive balance. The flow then continues along the coast, but remains trapped against it. Thus in the Northern Hemisphere, runoff effects tend to dominate the near-coastal ocean area to the right of the river mouth, whereas in the Southern Hemisphere they tend to dominate to the left of the river mouth (Figure 1).
Figure 1. Some physical processes affecting river outflow to the coastal current in relation to coastal water masses.
Figure 2. Illustrating the differential effect of Coriolis forces on plume trajectory in the Northern and Southern Hemispheres
Coriolis effects tend to increasingly dominate over frictional effects as the scale of the process becomes larger. Thus this process of coastal trapping of the runoff-dominated water mass is most effective in very large rivers. In very small runoff features, it may be much less prominent. Likewise, it can be overcome when other dynamical forces become particularly large. For example, during the summer upwelling season off the west coast of North America, the “Plume Waters” of the Columbia River (Landry et al., 1989) may be deflected southward (i.e. to the left of the river mouth) and displaced offshore by the persistent offshore surface transport that is associated with the wind-driven coastal upwelling process (Bakun, 1993). However, during intermittent relaxations of the northerly alongshore wind, the Columbia River Plume collapses rapidly coastward (Smith, 1974), depositing organisms and entrained materials back into the near coastal zone. During the other seasons of the year, the upwelling-producing northerly winds relax, and the Plume then becomes established in a normal position, trapped against the coast to the north (right) of the river mouth (Landry et al., 1989). Likewise, the areas of the western Black Sea most directly affected by eutrophication are Rumanian shelf waters laying to the south (right) of the mouth of the Danube (Mee, 1992). For the River Po, another river discharging high quantities of nutrients, these areas are to the south along the Italian coast in the Italian Adriatic, while for the Rhine they are to the north along the continental margins of the North Sea. These cases illustrate a sub-scale of intensified runoff-related effects which is nested within the larger MCB scale of a semi-enclosed sea.
On yet a smaller sub-scale, the frontal region between the runoff-affected water mass produces a zone of further concentration of effects. Gravity causes the higher density oceanic water to sink beneath the less dense runoff-affected surface water (Figure 3). Mixing processes near the interface produces water of intermediate density, which tends to flow under the less dense surface water type. Thus both surface water types are feeding the formation of a mass of mixed water which slowly sinks at the interface between them. The buoyancy-driven flows which supply this mixed water formation are directed from each of the water types toward the interface, resulting in a zone of convergence that acts to sustain the distinct frontal character of the boundary. Here, positively-buoyant materials, and vertically-migrating planktonic organisms will accumulate in the slowly sinking waters of the convergent frontal zone (Figure 3). Thus, the distribution of food particles tends to become highly concentrated and feeding organisms may realize dramatically increased caloric intake per unit energy expenditure. This process tends to work its way upward through the trophic levels, produce “blooming” in individual growth, in reproductive output, and eventually in local population growth at each trophic level. For this reason, this zone is often extremely important to biological population dynamics (Bakun, in press), including those of important fishery resource species, in runoff-affected regions.
These several nested scales of spatially-stationary, intensive runoff effects mean that particular subareas are characterized by intensified runoff effects, and here the effects of nutrification on coastal fauna may be appreciable. For instance, the conditions necessary for planktonic blooms may be maintained year-round inareas which would otherwise show seasonal peaks of primary production, and these are areas characterized by phytoplankton blooms and red tides (Pingree et al., 1975). Such areas may, for example, be particularly subject to periodic formation of anoxic bottom conditions such as occur in the North Sea off Denmark (Mellergaard and Nielsen, 1990).
Some practical implications of coastal enrichment for fisheries
Many human activities in the terrestrial components of Marine Catchment Basins (e.g. industrial agriculture, deforestation, freshwater extraction and human and industrial waste discharge, plus the effects of artificial changes to river hydraulic characteristics) are mediated through river outflows, but the individual effects of these outflows on the biological components of marine ecosystems can rarely be separated. In studying anthropogenic impacts on the living marine resources, we would expect that in addition to exploitation, the three ***** most important driving functions would be:
increased nutrient levels in the outflow;
higher sediment loadings; and
reductions in freshwater outflow and changes in seasonal flow patterns.
* A further anthropogenic effect has correctly received a high priority for research and monitoring, namely, higher levels of toxic wastes; but this aspect will not be addressed directly in this article, although some of the conclusions we arrive at should have direct relevance there also.
Figure 3. Illustrating physical processes at the boundary of the plume or coastal current where convergence leads to concentration of floating debris and larval stages.
Figure 4 shows diagrammatically a simplified representation of the impacts of these three factors on the environment and resources of the outer estuary and the area of the plume and coastal current under its influence. Evidently the degree to which each factor is of importance depends on the configuration of the river basin and estuary, and the particular human impacts these have been subject to. Perhaps the most important conclusion here is that the effects of the three driving functions will be synergistic, and ignoring one factor is likely to result in an incomplete understanding of the factors concerned.
Increased nutrient loading
Although in general, nutrient loading leads to increased phytoplankton production, it is clear (Howarth, 1993) that estuaries receive more nutrients per surface area than any other ecosystem, and that a significant fraction of the nutrient load of larger rivers has been incorporated into phytoplankton before leaving the estuary (Jordan et al., 1991). As a result, especially for large rivers with broad estuaries, a significant fraction of the nutrients and organic material discharged is incorporated in phytoplankton and associated detritus within the estuary itself.
There is also variability with respect to the particular nutrient which limits production, and while the limiting nutrient may be nitrogen in north-temperature estuarine outflows, phosphorus may be limiting in lakes and in tropical coastal ecosystems. On occasions silicon and iron may also be in short supply: the former leading to a change in balance between diatoms and flagellates (Doering et al., 1989). (See Howarth (1993) for a review of recent literature).
Figure 4 Diagrammatic representation of some interlinkages and synergistic effects between three characteristics of nutrient enriched outflows to coastal eutrophicated seas, and their economic impacts. The boxes with continuous margins represent biotic and abiotic effects: boxes with stippled margins are economically important effects (m = with moderate level of the input variable(s) under consideration).
Red, brown and green tides have been associated with serious losses to both wild fisheries, e.g. in northern Europe (Smayda 1989) and in the New York Bight (Mearns et al., 1982); but also to cage culture (ICES, 1992). In addition, such blooms may not be utilizable by shellfish. These factors are also important in affecting the tourist potential of seaside areas. The consequent occurrence of hypoxic or anoxic conditions may not be confined to eutrophic estuaries with reduced seasonal flow, but has also been noted in open sea conditions within the coastal current (e.g. Mellergaard and Nielsen, 1990). Moderate increases in organic loadings in the estuary and plume area can lead to increased potential for bottom culture of planktivorous or detritivorous shellfish as in the Waddensea (Boddeke, 1978), or in suspended culture. However, further increases in production with reduced or stagnant eutrophic flows, plus excessive extraction of freshwater, can lead to bottom anoxia in estuaries and plume areas, which can lead to oxygen deficiency and fish kills (Mellergaard and Nielsen, 1990) and interact with anadromous fish migration (Otterlind, 1976). In association with high sediment loadings, such anoxic effects can also damage shellfish and benthos populations (Baden et al., 1990), and have been associated with fish disease and cancers (Dethlefsen, 1989), with seasonal die-offs of demersal fish and invertebrates, and with problems in finfish recruitment (Ivanov and Beverton, 1985).
However, as for large lakes (Regier et al., 1988), the nature of production can be changed as “exotic” species take over from native species, and there is likelihood of blooms of noxious species such as Noctiluca (Balkash et al., 1990). The likelihood of toxic dinoflagellates leading to shellfish toxicity (White et al., 1984) is also increased when nutrient balances are changed in coastal waters. Planktivorous medusae and predatory ctenophores such as Mnemiopsis leydei (Volovik et al., 1993; Zaitsev, 1993) probably also increase with increased abundance of planktonic organisms, and in the Azov and Black Seas have lead to catastrophic impacts on small pelagic fish production due to competition for food or egg and larval predation.
Increased sediment loading
The higher turbidity and reduced light penetration associated with both increased phytoplankton blooms and/or increased sediment discharge may adversely affect macrophytes such as sea grasses (e.g. Buckholder et al., 1992). Macroalgae can be destroyed by particulate laden water from sewage outlets (Wilson et al., 1980). Loss of vegetated cover can adversely affect recruitment of species such as penaeid shrimp (Trent et al., 1976; Turner, 1977) and spiny lobsters (Herrnkind et al., 1988), which are dependent on this vegetation for nursery areas and feeding. Areas of gravel and shell bottom valuable for certain species such as oysters may be smothered by siltation through disposal of dredge spoils, adversely affecting species diversity. On the other hand, some species may be favoured by moderate increases in fines, notably some penaeid shrimp and flatfish species (Tatara, 1991).
Other species complexes, such as coral reef areas and their faunas, may be particularly affected by excessive levels of suspended fines and biological loading (e.g. Johannes, 1975; Cortes and Risk, 1985; McManus, 1988). Reefs and outcrops provide physical niches which provide habitat for reptant crustaceans may be occluded or reduced by a rain of sediment, with consequent declines in stock and production of reef fish, spiny lobsters etc. Coral reefs are particularly sensitive to reduction of light levels which affects photosynthesis by xooxanthellae in the tissues of corals, and except for certain specialised forms, are intolerant of high sediment loadings; even moderate nutrient levels promote
growth of smothering algae (Littler and Murray, 1975). Excessive runoff has also been implicated in breakouts of coral predators such as the crown-of-thorns starfish (Birkeland, 1982). In general, pelagic production is more resistant to nutrient inputs than that of the demersal/benthic resources, and one may expect that, with moderate nutrient loading, pelagic production may increase somewhat (Caddy, 1993).
Changes in freshwater runoff
The reduction of the extent of brackish areas and wetlands caused by excessive freshwater extraction upstream will have adverse impacts on marine species which are dependent on brackish habitats for part of their life history (Aleem, 1972; Deegan et al., 1986; Kresler, 1986). Saltwater invasions of the estuary and lower river system may have other negative consequences on estuarine dependent species and those marine species dependent on reduced salinity conditions for part of their life history, and on anadromous and catadromous species. Increases of saltwater intrusion into estuaries may also favour reproduction and abundance increases of medusae with bottom-dwelling stages (Caddy and Griffiths, 1990), and problems with jellyfish invasions may become more serious through their effects as predators on eggs and larvae of fish (e.g. Greve et al., 1993).
Some broad research perspectives
Due to the complexity of effects illustrated in Figure 4, it is difficult to assign any single cause to an impact on production of a commercially important species in marine areas under the influence of anthropogenically-affected estuarine outflows, and carefully planned multidisciplinary studies within well-defined geographical sub-units will be an essential component of efforts to unravel the principle factors responsible for changes in coastal marine ecosystems. However from a biological perspective, the point can be made that in the absence of other river inflows “upstream” with respect to the direction of flow of the coastal current, a comparison of the status of coastal ecosystems to each side ofthe estuary may provide a useful perspective for monitoring programmes and for studies of environmental impacts on coastal fisheries.
Coastal ecosystem effects and Integrated Coastal Area Management
Regarding the need to develop a policy framework for dealing with the effects of river-borne effects on estuarine, coastal and nearshore waters subject to the above anthropogenic, multi-impact phenomenon must be a fairly comprehensive one. A simplified summary of the interface between those activities affecting the brackish and marine ecosystem and the users of the coastal marine environment is illustrated schematically in Figure 5, following the Integrated Coastal Area Management (ICAM) paradigm, which seems the appropriate framework for dealing with multiple interactions of this type (see Clark, 1992). It is clear that although human activities that affect living marine resources in coastal waters are primarily related to population growth in coastal areas and the need to dispose of effluents from urban centres, agricultural runoff and industry, these impacts operate through increased and/or changed primary productivity. This, in turn, interacts through a variety of pathways with other coastal activities such as tourism, aquaculture and coastal development. It is relevant to, and enters the sphere of action of, a wide range of policy areas, including environmental monitoring, fisheries and aquaculture, health and welfare, and coastal employment in general.
Figure 5. A framework for policy development in coastal and nearshore regions subject to various environmental impacts
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