Centre for Estuarine and Coastal Ecology, Netherlands Institute of Ecology, Vierstraat 28, 4401 EA Yersek:e, The Netherlands

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1 Publication no Netherlands tnstitu1eof Ecology NIOo-CEMO, Yerseke Biodiversity of Marine Sediments C. Heip Centre for Estuarine and Coastal Ecology, Netherlands Institute of Ecology, Vierstraat 28, 4401 EA Yersek:e, The Netherlands Introduction Biodiversity of the marine environment in general and of marine sediments in particular is poorly known, both in descriptive terms of species richness and its distribution along latitudinal and depth gradients, and in terms of the ecological and evolutionary processes that regulate it. In a recent, otherwise excellent account on species diversity in ecological communities (Ricklefs and Schluter, 1993), the marine benthos is not even mentioned and only two chapters deal with the marine environment at all (Underwood and Petraitis, 1993; i\1cgowan and Walker, 1993). This ignorance or lack of interest is not justified as the marine benthos occupies 70% of the Earth's surface and is an important agent in major global biogeochemical cycles. Overall, at our present state of knowledge, marine biodiversity appears to be low: about 200,000 marine animal species, perhaps 20,000 marine plant species and an even much lower number of marine viruses and microorganisms (bacteria, fungi, protozoans, microalgae) have been scientifically described. Of the 200,000 described animal species, only a few thousand are planktonic, about 130,000 are from hard substrates and about 60,000 from sediments. The marine plant inventory is probably more complete than that of the animals since there are no deep-sea plants. Even if the true number of marine species was an order of magnitude lower than that of the land, on a higher taxon level the marine environment is certainly much richer than the land. Of the 33 animal phyla, only five do not occur in the seas and 13 are endemic to it (Grassle et al., 1991). This implies that genetic, biochemical, and physiological diversity is also much higher in the oceans than on land (Lasserre, 1992). This chapter will deal exclusively with the fauna of marine sediments. Marine sediments cover most of the Earth's surface and although they resemble terrestrial soils in some respects, they also have many unique 1996 CAB INTERNATIONAL. Biodiversity, Science and Development: Touiards a New Partnership (eds F. di Castri and T. Younes) 139

2 Biodiversity of Marine Sediments 141 Table Total number of macrofaunal species on the specified total surface sampled in different deep-sea areas. Number of Surface species sampled Area Author m 2 W. Atlantic, 2100 m Grassle and Maciolek (1992) m 2 W. Atlantic, 1500 m Grassle and Maciolek (1992) m 2 San Diego Trough, 1230 m Jumars (1976) m 2 Rockall Trough, m Gage (1979) m 2 NW Atlantic, 2800 m Rowe et at. (1982) diversity. As an example, the nematode fauna from the continental slope on the west coast of Corsica had a species richness between of per sample of 10cm 2 at depth of m (Soetaert et al., 1991), with many cogeneric species. For macrofauna, Grassle and Maciolek (1992) calculated that the number of species in deep waters typically numbers many hundreds in total sampled areas of 1-2 m 2 (Table 10.1). Local diversity in terrestrial communities is often explained in terms of species interactions, i.e. competition, predation. Related species living together may coexist by partitioning resources (Lack, 1944) and niches can be described as part of a multidimensional space that is defined by resource axes which represent the biotic and abiotic factors along which resources are partitioned (Hutchinson, 1959). This partitioning may be part of a process leading to increased species diversity (McArthur, 1958) because of specialization but there is a limit to this and thus to the number of species in a community. This is the principle of limiting similarity (McArthur and Levins, 1967) and a large body of theory was developed in the 1970s that essentially was based on the notion that equilibrium properties of communities, including the number of coexisting species, were determined largely by species interactions (Schluter and Ricklefs, 1993). Most of the evidence for the importance of species interactions in creating and maintaining local diversity comes from terrestrial studies. The marine studies, mostly from intertidal environments (Connell, 1983; Reise, 1985: Underwood and Petraitis, 1993), are all small-scale. The evidence for competitive interactions in deeper water benthos is circumstantial. One interesting example is the spatial segregation of nematode species in the 5 em top sediment layer of a station on the continental slope off Corsica. Six species of the genus Sabatieria co-occur that all have the same mouth structure and therefore probably the same food. Also six species of the genus Acantholaimus cooccur, but they have different mouth structures and presumably different food. The Sabatieria species potentially competing for the same food source live at different depths in the sediment whereas the Acantholaimus species, that already evolved towards different morphologies, do not (Fig. 10.1) (Soetaert et al., 1995). The surprising consequence of such small-scale segregation is that nematode communities from the same depth layer in different

3 Biodiversity of Marine Sediments 143 stations, tens of kilometres apart in water depths differing hundreds of metres, are more similar than communities living at the same station in two adjacent depth-layers one centimetre from each other. Besides species interactions, disturbance is often invoked as a diversityregulating mechanism in terrestrial environments. Again the importance of regular or irregular disturbances to explain patterns of diversity will be different on local and on large scales. Increased habitat heterogeneity is created by the activity of the biota themselves. Patches consist of small-scale biogenic structures, such as burrows, tubes, feeding pits, trails etc. On the mesoscale, habitats are often not discrete but continuously changing due to some overriding physical factor such as wave energy, current velocity, fronts etc. Grassle and Maciolek (1992) suggest that high overall diversity in the deep sea is maintained by the input of small patches of ephemeral resources and the disturbance that results from the activities of individual animals. In this respect bottom mounds, burrows, feeding pits, other animals such as sponges or xenophy ophoreans have been mentioned. Food input is patchy and rarer as depth increases. Food inputs may be pulsed and tend to accumulate in depressions and burrows made by animals on the deep-sea floor. Rare species tend to be associated with these rare resources. Regional and Global Biodiversity Patterns Trends in diversity on a truly global scale have been documented mainly from the terrestrial environment, but why these trends exist is not really understood (Clarke, 1992). From the marine environment, Thorson (1957) showed a pronounced increase in the species richness of epifauna from hard substrates towards the tropics, but the number of macrofaunal species in sediments appeared to be roughly the same for arctic, temperate and tropical areas. On the other hand, Stehli et al, (1967) clearly demonstrated a diversity trend of bivalve molluscs at species, genus and family level from the tropics to the poles and in a later study (Stehli et al., 1975, cited in Clarke, 1992) demonstrated the same for foraminiferans. Since both groups form calcareous skeletons, Clarke (1992) suggested that perhaps such trends may not exist in other taxa. However, recently, global-scale latitudinal trends in deep-sea epifauna (Gastropoda, Bivalvia and the non-calcareous Isopoda) have been described by Rex et al. (1993) from the Atlantic Ocean. All three taxa showed highly significant latitudinal gradients in the North Atlantic with elevated tropical diversity and depressed diversity in the Norwegian Sea. On a somewhat smaller, regional scale, trends in benthic biodiversity have been described from the whole North Sea (Heip et al., 1992). A clear trend in biodiversity with latitude was found, but this trend was opposite for macrofauna, where diversity increases toward the north, and copepods, where diversity increases toward the south (Fig. 10.2). The higher diversity of copepods in the south (38 species per sample) is easily explained by the presence of sands with a median grain size larger than 200 J1-m, permitting

4 Biodiversity of Marine Sediments 145 the existence of numerous small and slender interstitial (living between sand grains) species. For the macrofauna there is a regular increase of diversity at least between 51 and 58 N, mainly due to polychaetes, but which also exists to a certain degree within the other three main macrofaunal groups (Fig. 10.2). It is more difficult to explain these patterns for the macrofauna than for copepods. Historically, the southern part of the North Sea was dry land until 4000 years ago and colonization of this area occurred both from the south and the north. However, the deeper areas to the north are much richer in species that would easily colonize the newly available North Sea habitats. In the modern situation the existing current.and productivity and sedimentation patterns are all important aspects to explain biodiversity patterns, and human activity, especially the fisheries, is becoming a more and more dominant factor as well. Regional patterns in deep sea biodiversity are also most often explained in ecological and historical terms. In ecological terms the congruity in globalscale patterns of diversity may be explained by coupling between surface and sedimentary processes. Such coupling is probably much more intense than anticipated even a few years ago. It has been shown that the sinking rates of particles are much higher than previously thought and that a response between benthic activity to accumulation on the sediments of detritus derived from primary production at the surface is possible and indeed exists. Surface productivity increases poleward as does benthic biomass and perhaps the often observed inverse relationship between productivity and diversity also holds for the marine benthos. Another reason may be that the carbon flux is more variable in the north. Historical factors may also be important. The low diversity in the Norwegian Sea has been attributed to the Quaternary glaciation and the effects of the sea ice cover. In prosobranch gastropods local diversity is correlated with regional diversity and the dispersal potential of the regional species pool (Stuart and Rex, cited in Rex et al., 1993). Local diversity thus appears to be regulated by colonization from the regional species pool and reflects the historical evolutionary build-up of regional diversity. How Many Species Exist In Marine Sediments? Grassle and Maciolek (1992) published a very extensive study of deep-sea diversity from ten stations along 176 km of the 2100 m isobath (depth contour) off New Jersey and Delaware in the US and four additional stations at 1500 m and 2500 m depth. In these stations a total of 798 species representing 171 families and 14 phyla were identified on a total sampled surface of 21 m 2. Of these species, 460 (58%) were new to science. About 20% of the species were found at all ten 2100 m stations and 34 % occurred at only one station. Of the total soft-sediment fauna 28% of species occurred only once and 11 % only twice. The number of species found rises continuously as more samples and individuals are collected. At a single station species were added at a rate of

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