139736eo.pdf (20MB) - Japan Oceanographic Data Center

acteria, zooplankton and filter feeding invertebrates, which in turn support larger invertebrates andfish, including many of the species of local commercial significance.LAGOONS AND MUD FLATSA third additional habitat which also may be comparatively productive, and thus possiblycritical to the maintenance of coastal stocks of fish and other marine life, occurs in very shallowsoft-bottom areas, in coastal lagoons or in sheltered areas, even in the absence of seagrass beds ormangrove. In such areas, sand-mud bottoms and mud flats can have a high secondary productivity.The finer particles of silt and detritus that settle out in such areas may support large populations ofdetritivorous crustacea and gastropods, and suspension and deposit feeding bivalves and polychaetes.This is particularly so on the estuarine mud flats which occur around the mouths of many temperaterivers. Frequently in the tropics the secondary productivity of such sand-mud bottoms and mud flatsis supported or enhanced by the primary productivity of adjacent seagrass beds or mangrove detritusthat is deposited in low wave-energy areas. Such areas with a very high secondary productivity maybe critical in that they are frequently the nursery grounds for various fish and crustacea; in additionthey often provide vital feeding grounds for migrant or over-wintering populations of wading andshore birds. The extensive mud flats of Kuwait, for example, provide a critical feeding and stagingsite for hundreds of thousands of waders migrating between their breeding grounds in northernEurasia and wintering areas in southern and eastern Africa.It is less widely appreciated, however, that in the tropics in particular intertidal mud andshallow water sand-mud may show a significant primary productivity as a result of the growth of anepibenthic algal lawn, i.e. a thin film of diatoms and blue-green and filamentous green algae. Forexample, in a recent study in Bahrain (IUCN, 1983b), an intertidal mud flat on the east coast had achlorophyll concentration due to the epibenthic algal film of 0.16-0.24 g m-2, a value that approachesth~a typical of oceanic upwelling zones (typically about 0.3 g m-2) and is much higher than that of mosttropical ocean areas (of the order of 0.03 g ma). In fact the primary productivity per unit area of themud flat was estimated to be approximately fifty times higher than that of the surrounding sea wherephytoplankton growth was limited by the high turbidity, high salinity and limited depth. This algalfilm on the mud flat help support large numbers of amphipods and a variety of gastropods, such asMonilea obscura, Rhinoclavis sordidula and Mitrella blanda.Such epibenthic algal films also can occur over mixed coarser sand and/or mud-sand bottomsin shallow subtidal areas up to 1 to 2 m deep, that are sufficiently protected for the bottom not to bedisturbed by wave action. Thus such films are especially observed in coastal lagoons behind bars andbarrier islands, in fringing reef lagoons, and within suitable areas of the long creeks (known locally assham or mersas) that are common on both sides of the Red Sea.However, the primary and secondary productivity of coastal lagoons and enclosed bays andmersas throughout the Indian Ocean is extremely variable. This is so because of the extremes ofsalinity which may occur along many more enclosed shorelines. In some areas rivers and wadis mayflow seasonally into semi-enclosed bays or lagoons, suddenly converting a marine habitat into abrackish or fresh-water one; few organisms can withstand such complete changes in salinity. And,especially within the more arid regions of the Gulf and Red Sea, many semi-enclosed bays and mersashave elevated salinities and temperatures as a result of a hot climate and high rates ofthermo-evapora tion.Thus a recent study in the Red Sea (IUCN, 1984) found that in moderately shallow water (1-3m deep) to the sides of broader mersas, but not particularly enclosed and with some circulation ofwater, salinities can reach 45-48 ppt, compared to 39-40 ppt in the open sea. In very shallow, moreenclosed areas at the innermost parts of mersas salinities up to 50-60 ppt were recorded; the highestfigure obtained was 79 ppt in a rapidly evaporating pond almost isolated from the main part of a mersa.The most saline lagoon areas were completely devoid of marine life but in more equitable areas avariety of fish and invertebrates were common. The invertebrates included mostly crustacea andmolluscs, among which cerithid gastropods of six or more different species of Cerithium andRhinoclavis were the most abundant, occurring in densities of up to 100 m-2, and feeding directly onthe algal film. Other common invertebrates included herbivorous gastropods of the genera Littorinaand Strombus, several predatory gastropods, various crabs, and most significantly juveniles of threespecies of commercial shrimp, of which the most abundant was Penaeus semisulcatus. Such bays and182