Fauna and flora

Pelagic organisms in estuaries face difficulties of maintaining position in the ebb and flow of water. The net transport of water is downstream, tending to flush pelagic organisms out to sea. On the rising tide there is some danger that they might be carried up into lethally low salinities. Some pelagic animals or larvae adjust their behaviour to reduce these dangers. Some swim mainly on the flood tide and sink to the bottom on the ebb, thereby avoiding being washed too far seawards, e.g. oyster larvae. Some are retained in the estuary because they swim only within a particular range of salinities, being otherwise on the bottom. At the landward end some avoid being carried into fresh water by settling on the bottom if salinity falls below a safe level.

The population of estuaries therefore encounters peculiar and difficult conditions. Nevertheless, a restricted range of species can successfully cope with these problems, and population densities and biomass are often very high (Barnes, 1974; Barnes and Green, 1972; Green, 1968; Perkins, 1974). Shore species which occur both within estuaries and on the open coast, especially the herbivorous fauna, e.g. Littorina littorea, Mytilus edulis and Cerastoderma (Cardium) edule, often grow faster, mature more rapidly and sometimes spawn earlier in the year in estuaries than in more exposed sites. This is probably the result of a combination of shelter, abundance of food and sometimes raised temperature. The end-products of organic decomposition enrich the water with plant nutrients and the rate of primary production in estuaries is often very high. This derives from several sources. The phytoplankton is often rich; there may be an abundant growth of salt marshes; and the surface of mudflats, despite the absence of visible vegetation, is usually highly productive by the photosynthetic activity of micro-organisms (Joint, 1978).

At the seaward end there is always some penetration by marine species, and estuarine shores near the mouth are commonly inhabited by ordinary littoral forms, for example, Semibalanus balanoides, Chthamalus montagui, Elminius modestus, Patella vulgata, Littorina littorea, L. rudis, Mytilus edulis, Ceras-toderma (Cardium) edule, Nucella lapillus, Crangon crangon, Arenicola marina and Carcinus maenas. The distance these extend upstream depends partly upon their powers of osmotic adjustment or osmoregulation, and partly upon the protection afforded by shells, tubes or deep burrows into which some species retire during periods when the salinity falls below a safe level. In mid-estuary, where the widest and most rapid fluctuations of salinity occur, only extremely euryhaline forms can survive. This typically estuarine community (Figure 8.17) often includes Enteromorpha intestinalis, Fucus ceranoides, Corophium volutator, Hydrobia ulvae, Hediste (Nereis) diversicolor, Scrobicularia plana, Macoma baltica, Carcinus maenas, Sphaeroma rugicauda, Gammarus zaddachi, G. duebeni, Balanus improvisus and Pomatoschistus microps. Certain mysids are numerous in brackish water, for example Neomysis integer, and in some estuaries there are often shoals of whitebait, i.e. small herrings and sprats. The plankton sometimes contains dense patches of the copepod Eurytemora hirundoides (Figure 8.16).

In the upper parts of estuaries the gymnoblast hydroid Cordylophora caspia occurs on stones or wooden piers. The prawn Palaemonetes varians is sometimes very numerous in salt-marsh pools. At these levels, part of the population is of freshwater or terrestrial origin. There are often many larvae of midges (Chironomus) and mosquitoes (Aedes), also oligochaetes (Tubifex) and a variety of beetles (Colymbetes, Ochthebius) and bugs (Notonecta, Sigaria).

The distribution of many estuarine species changes seasonally. The flounder Platichthys flesus, a common estuarine fish, migrates upstream in summer but returns to the sea during the colder months for spawning. Carcinus maenas and Crangon vulgaris also move up estuaries in summer and seawards in winter, their movements being influenced by changes of both temperature and salinity because these species osmoregulate less effectively as the temperature falls. Others make the reverse movements. Pandulus montagui goes out to sea in summer and into estuaries in winter.

River mouths where there are wide expanses of sand and mud are important feeding grounds for many species of birds, especially in winter. Among these are the heron (Ardea cinerea), the oystercatcher (Haematopus ostralegus), the curlew (Numenius arquata), the dunlin (Calidris alpina) and a variety of plovers, sandpipers, ducks and geese. However, estuaries are being increasingly subjected to industrial developments, barrage schemes and recreational developments. Over the past few years there has been an increasing awareness of the importance of estuaries to wildlife. In Britain, this is reflected in many new 'estuary initiatives' by statutory and voluntary conservation bodies (Davidson et al., 1991).

Figure 8.17 Distribution of the benthos of the Wansbeck Estuary, Northumberland. (a) Distribution of substrate. (b) Estimated positions of isohalines at high water. At low water the estuary drains and the shallow outflow is virtually fresh (S = 0.6-4.6%o). (c) Horizontal distribution of organisms.

(Data from a one-day students' field trip 1 March 1967.)

Figure 8.17 Distribution of the benthos of the Wansbeck Estuary, Northumberland. (a) Distribution of substrate. (b) Estimated positions of isohalines at high water. At low water the estuary drains and the shallow outflow is virtually fresh (S = 0.6-4.6%o). (c) Horizontal distribution of organisms.

(Data from a one-day students' field trip 1 March 1967.)

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