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In deep-sea systems, in which patterns are less obvious, studies have tended to focus on biodiversity per se. Upper-slope environments are even less well sampled than some deeper areas, so there is a geographic as well as a conceptual discontinuity. Distribution patterns of individual species of shallow-water sedimentary fauna are determined largely by temperature, salinity, depth, surface productivity, and sediment dynamics over broad scales and by biological interactions, sediment geochemistry, and near-bed flow processes at finer scales. Particularly over broad scales, geologic history plays a major role in patterns of distribution Jablonski and Sepkowski , although I will focus on ecological rather than evolutionary scales.

The dependence on temperature, salinity, and depth are easily understood in terms of physiological constraints; many species have specific tolerances to temperature, salinity, and pressure that have to do with osmotic balance and enzyme function. These physiological constraints contribute to the reduced diversity of estuaries and other highly variable, and thus physically challenging, environments.

Given that many organisms derive their nutrition from sediment-associated food particles, higher diversity in sediments of diverse grain size might also be predicted Whitlatch Many species have a complex relationship with the sedimentary environment. Generally speaking, suspension feeders tend to be most abundant in high-energy environments, and deposit feeders are most abundant in depositional areas with fine-grained, muddy sediments. But contrasting these environments in terms of how they determine infau-nal pattern is complex because many important variables vary with flow regime Snelgrove and Butman High-energy environments are typically sandy, with strong bottom flows and horizontal flux of food and perhaps settling larvae.


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Sediment grain size is large, and organic content and microbial content tend to be low. High energy produced by waves and strong currents moves sediments and some organisms. Low-energy environments are often muddy, with weak flows and low horizontal but greater vertical flux of food, fine sediments, and potentially larvae. By mechanisms that are not yet fully understood, these flow-, nutrition-, and substrate-related variables contribute to patterns in species distributions that are fairly consistent in time and space.

The challenge is to determine which mechanisms are most important in creating and maintaining pattern. Understanding how patterns in individual species are maintained is a key prerequisite to understanding biodiversity patterns, and some of the advances made in this area are reviewed below. Most of what is known about shallow-water diversity has been learned from experiments designed to determine the impacts of individual species on other species or from observational data. Among the most relevant of these experiments for understanding regulation of diversity are those that test the impacts of predators on individual species and those that examine the importance of competition in soft-sediment systems.

Indeed, experimental approaches in soft-sediment systems have been heavily influenced by studies of rocky intertidal areas, which have demonstrated the critical importance of keystone predators in maintaining diversity and community structure Paine Data from the majority of studies in soft-sediment systems reviewed by Peterson suggest that interspecific competition is probably not a major structuring force in sedimentary communities but that predation can be important. In reviewing predator exclusion experiments, Peterson found that species richness in sediments tended to increase when predators were excluded.

He also found that species richness in seagrass beds exceeded that in ambient sediments, perhaps because of the predator refuge that seagrasses provide. Numerous studies of changes resulting from foraging predators suggest that foragers have major impacts on densities of dominant taxa but little effect on the relative abundances of species e. But predation effects are not limited to foraging species and their impacts on adult infauna. Indeed, interactions between infaunal adults and settling larvae or recently settled juveniles may be a major structuring force in sedimentary communities.

In reviewing the many studies that have been conducted on adult-juvenile interactions, Olafsson et al. Although suspension feeders can and do filter settling larvae from near-bottom waters, early postsettlement processes may be more important to recruitment success, given the frequency with which deposit feeders have an impact on suspension feeders. Biological disturbances, such as bioturbation, may also enhance diversity Kukert and Smith , although the mechanism is unclear.

Bioturbation is also the basis of the trophic group amensalism hypothesis, in which deposit feeders are suggested to constrain distribution patterns in suspension feeders by resuspending sediment that settles and smothers their larvae and clogs their filtering structures Rhoads and Young Although this hypothesis is not accepted as a general hypothesis for benthic pattern Snelgrove and Butman , the interactions that it describes undoubtedly occur in some instances. Biological structures, such as seagrass blades e. Another factor that may play a major role in establishing pattern is larval supply Figure 1.

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Many benthic invertebrates produce plank-tonic larvae that, depending on the taxon, spend hours to months in the plankton before taking up a benthic existence. A major question in marine ecology is how these planktonic larvae, which are often poor swimmers, are able to settle in a suitable habitat.

Small-scale laboratory experiments that began in the s suggested that larvae have some capacity to choose among sediment types, perhaps based on organic content. But the scales over which habitat selection behavior may be important are limited, given the relatively weak swimming ability of many larvae.

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Consequently, these small-scale stillwater experiments may have limited application to nature Butman The challenge of maintaining pattern. Schematic representation of the processes that affect larval settlement of sedimentary invertebrates. The adult bivalve is shown as a large individual living in mud indicated by solid grey box but unable to live in other sediments indicated by grainy boxes. To maintain populations in an area, the bivalve must complete the cycle indicated by the heavy black circle. Lighter lines indicate sources of mortality. Eggs and sperm are spawned into the water column, where some successful fertilizations will occur but many eggs and sperm will be lost.

The successfully fertilized eggs become small swimming larvae that suffer heavy losses as a result of transport away from suitable habitat, predation, starvation from lack of appropriate food, and exposure to temperatures or salinities beyond their physiological tolerance.

As developing larvae settle toward the bottom, they may encounter hypoxic sediments or predation from suspension-feeding bivalves. Even after the larvae settle and metamorphose into small juveniles, heavy loss is incurred shown in boxes because of predators, physical disturbance, low oxygen, insufficient food, unsuitability of sediment, and disturbance by deposit feeders moving through the sediment.

Once they have grown past the juvenile stage, mortality is greatly reduced. One approach to resolving the importance of habitat selection is to study larval settlement in a laboratory flume. A flume is a recirculating seawater channel that is designed to mimic natural bottom flow but that allows confounding variables such as predators and food supply to be controlled.

Over the past few years, several studies of larval settlement have found that species with well-defined distributions with respect to sediment type are also capable of choosing one type of sediment over another as they settle, even in moving water Table 2. The specific sediment cue to which settling larvae respond is unclear, but organic content is a good candidate for at least some species Butman et al.

In any case, these results suggest that habitat selection probably plays an important role in shallow-water pattern, although passive transport also regulates delivery of larvae to specific areas see Butman How larval ecology relates to maintenance of assemblages and biodiversity remains to be seen. Summary of laboratory flume experiments to determine whether settling larvae of different species are capable of habitat selection. In summary, studies from shallow-water environments offer insights into how distributions of individual species are established and maintained, but they have less to say about biodiversity patterns.

Existing data suggest that rocky intertidal paradigms may not be applicable to soft-sediment systems and that additional experimental work will be needed to evaluate critically the factors that regulate biodiversity. Biodiversity in deep-sea ecosystems has generated much interest e. Why the deep sea is so diverse is a subject of some debate. For some areas of the deep sea, overriding environmental variables, such as low oxygen Levin and Gage , hydrothermal fluid emission Dinet et al.

The suggestion that the long-term stability of most deep-sea environments has allowed evolution of many specialized species Sanders has been questioned based on the lack of evidence for niche specialization and the parabolic diversity-depth relationship that has been observed in some areas Rex The potential impact of predators cropping populations below levels at which competitive exclusion would take place has been questioned based on population attributes of deep-sea species Grassle and Sanders Indeed, if predation effects in the deep sea are similar to those in shallow water, then reduced predation pressure in the deep sea might actually increase diversity.

It has also been hypothesized that small-scale patches of food and disturbance create microhabitats on which different species may specialize and thus avoid competition in a highly food limited environment Grassle and Sanders Indeed, carbon flux to the deep sea is now known to be patchy in many areas e. To test the potential role of food patches in the deep sea, sediments enriched with different types of organic matter were deployed for varying periods of time at m in depth near St. Croix in the US Virgin Islands. Different types of organic matter were found to elicit different colonization responses for different species Figure 2 , depending on the type of organic matter Snelgrove et al.

Thus, small-scale patchiness may enhance deep-sea diversity.

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However, it is important to note that the numbers of species that respond to these types of disturbance are relatively few, and existing data support this mechanism for only a small subset of deep-sea species. It is possible that appropriate patch types have not been identified for other species, but it is also likely that factors such as productivity and evolutionary history come into play in determining biodiversity patterns.

Deep-sea colonization experiments carried out at m depth south of St. Croix, US Virgin Islands. Three polychaete species Capitella spp.

The graphs show the responses of total macrofauna upper left and the three polychaete species to different patch types. Two different time periods were compared to test how the species' response to different patch types would change. All other treatments are densities of animals colonizing 10 cm deep sediment trays with surface areas of cm 2.

Trays were unenriched or enriched with Sargassum spp. Artificially aged Sargassum is algae that was aged before deployment to mimic degradation over a longer time period. Bars denote means, and lines denote 1 SE. Data from Snelgrove et al. Conflicting patterns from different data sets must be resolved to establish any comprehensive paradigm explaining the rich diversity of the deep sea. What is needed is more complete sampling on a global scale, studies that include a broader range of taxa e.

Even though marine sedimentary ecosystems are not well understood, there are good reasons to assume that their loss could affect the planet and human populations directly. For example, marine organisms provide a tremendous reservoir of natural products that could prove invaluable and irreplaceable by synthetic equivalents.

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Ultimately, however, the arguments for preserving marine sedimentary biodiversity that will carry the greatest weight are those of most immediate concern to human populations. In other words, what have marine sedimentary fauna done for you lately? Although I will focus on marine sedimentary environments, there are considerable parallels with freshwater sediments and terrestrial soils e.

Global carbon and geochemical cycling. As a result of the global dominance of marine sedimentary habitats and the importance of sedimentary fauna in local carbon metabolism and burial through their feeding and mixing activities e.