Freshwater Sediments Marine Sediments and Soils

Freshwater sediments, marine sediments, and soils cover the Earth's surface (Table 1.1) and are critical links between the terrestrial, aquatic, and atmospheric realms (Figure 1.1). These below-surface habitats are arguably the most diverse on the planet, teeming with a complex assemblage of species. The profusion of organisms and the composition of the biotic assemblages are integral to the maintenance of below-surface and above-surface habitats, to ecosystem functioning, and to the provision of ecosystem services that are crucial for human well-being (Wall et al. 2001b).

The public, farmers, gardeners, tourist industries, shoreline residents, and fishers are interested in the maintenance of sediments and soils for production of harvestable crops, recreation, and beauty of the landscape. These land users typically have high regard for the ecology of the habitats that they rely on for their livelihoods. They need to know: Will these shared resources—the soils and sediments and their biodiversity—be sustained in the future given increasing human populations and numerous and rapidly occurring changes in the environment? This has not been an easy question for scientists to answer.

Until recent decades, scientists considered the biota in the Earth's soils and sediments to be a "black box": They monitored the physical and chemical components of these environments, but treated the diverse, smaller organisms that comprise the soil and sediment community as an "unknown, undefined" set of functional groups. Now, facing this era of unprecedented anthropogenic disturbance resulting in biodiversity and habitat loss and the spread of invasive species, an urgent question facing both scientists and decision-makers is: Which taxa, and how much biodiversity, must be conserved to maintain or restore essential ecosystem functioning such as plant and animal production, breakdown of organic wastes and nutrient cycling?

Table 1.1. A general comparison of global characteristics of soils, fresh water sediments, and marine sediments.

Table 1.1. A general comparison of global characteristics of soils, fresh

Freshwater

Marine

Parameters

Soils

sediments

sediments

Global coverage

1.2X108 km2

2.5X106 km2

3.5X108 km2

Carbon storage

1500 Gt

0.06 Gt

3800 Gt

Organic content

High

Low

Low

Oxygenation

Oxic

Oxic-anoxic

Oxic-anoxic

Salinity

Low

Low-High

High

Pressure

Low

Moderate

High

Modified from Wall Freckman

et al. 1997.

importance of Three Subsurface Domains for Providing Services importance of Three Subsurface Domains for Providing Services

Figure 1.1. Schematic depiction of relative importance of different ecosystem goods and processes in terrestrial, freshwater, and marine sediments. The circles represent the relative contributions of the ecosystem services provided by soil, freshwater sediments, and marine sediments along the land-sea gradient.

Figure 1.1. Schematic depiction of relative importance of different ecosystem goods and processes in terrestrial, freshwater, and marine sediments. The circles represent the relative contributions of the ecosystem services provided by soil, freshwater sediments, and marine sediments along the land-sea gradient.

The extremely diverse soil- and sediment-dwelling organisms occur all over the Earth and play critical roles in regulating the most vital ecosystem services (Daily et al. 1997; Petersen & Lubchenco 1997; Postel & Carpenter 1997). Identifying the significance of below-surface biological diversity for ecosystem functioning under scenarios of global change is being increasingly recognized as a major research priority (Lake et al. 2000; Smith et al. 2000; Wolters et al. 2000; Wall et al. 2001a). Changes in land-use practices affecting soils, in turn, have impacts for sediments of freshwater lakes, streams, rivers, estuaries, bays, and oceans. Alteration of hydrologic processes, contamination of soils, surface waters, and ground waters, and climate change are but a few examples of pressing problems that cannot be managed sustainably without a more complete understanding of the biota and ecosystem dynamics of soils and sediments. Applying this integrative knowledge to options for management and conservation will be crucial for long-term global sustainability of ecosystems and the welfare of human society (Millennium Ecosystem Assessment 2003).

This book highlights the fascinating biodiversity, ecosystem functions, and critical services provided by this often-assumed static and seemingly nondescript world of soils and sediments. The authors describe examples and present important priorities for research and considerations for sustainable management. This synthesis is an international, transdisciplinary assessment based on detailed knowledge for each of three separate domains—marine sediments, freshwater sediments, and soils—as well as identification of the most critical biota and their functions necessary for sustaining ecosystem services.

Furnishing the needed scientific information for this book has been challenging. We know that soils and sediments in most ecosystems contain enormous biological diversity—tens of millions of bacteria, thousands of fungal species, millions of protozoa and nematodes, up to a million arthropods (Groffman & Bohlen 1999), and thousands of species (World Conservation Monitoring Centre 1992; Wall et al. 2001a,b) per square meter—and that assemblages of these organisms are responsible for ecosystem processes (Tables 1.2 and 1.3). However, specific information is still lacking regarding the importance of many species-specific functions and whether the species involved in these functions are irreplacable. Information on biogeographical distributions and responses to human activities, such as management regimes and global change, is also lacking for most keystone species (e.g., Covich et al. 1999; Levin et al. 2001; Wall et al. 2001a,b).

Below-surface organisms generally have been neglected by researchers, and often have been unrecognized by management and conservation programs relative to above-surface biota. Scientific research as a whole has understandably emphasized the visible: large plants and animals above the Earth's surface (both aquatic and terrestrial). For example, the latitudinal distribution of species, species ranges, and the occurrence of hot spots of biodiversity—all information used for local to national management and policy decisions—are based primarily on the larger, charismatic above-surface organisms. The emphasis on above-surface research continues in universities; there are fewer specialists trained to study the taxonomy, evolutionary biology, and biodiversity of below-surface organisms and their biogeochemical interactions, which limits our understanding in each domain compared with our understanding of above-surface domains. Separate scientific disciplines have developed based on "distinct habitats" in soils, freshwater systems, and marine systems, increasing understanding within each but effectively hindering the study of soils and sediments as an ecological continuum. For example, the biodiversity linkages between the soils of farms and cities in terms of runoff to freshwater sediments, and then to ocean sediments, have typically been neglected. Part of this neglect may be because of the additional complexity and cost involved in organizing interdisciplinary teams, or because of the manner in which research organizations have traditionally focused on specific disciplines. Scientific publications addressing the

Table 1.2. A comparison of the biodiversity (described species) in soil and marine

sediments. Ants and earthworms are not present in marine sediments. Numbers of

species of crustaceans

in soils are for the Isopoda (Bri

usca 1997).

Numbers of d

escribed species

Biota

Soil

Marine sedimentSb

Fungi

18-35,000a

600

Protozoa

l,500a

2,000

Nematodes

5,000a

4,000

Ants

8,800a

Earthworms

3,600a

Crustaceans

5,000c

21,000

a Modified from Brussaard et

al. 1997

b Modified from Snelgrove et

al. 1997

c Modified from Wall et al. 2001a

Table 1.3. Examples of the diverse biota within functional groups are listed for a few ecosystem processes that are similar in soils and sediments. The functional groups play an important role in global transfers and biogeochemical cycling.

Table 1.3. Examples of the diverse biota within functional groups are listed for a few ecosystem processes that are similar in soils and sediments. The functional groups play an important role in global transfers and biogeochemical cycling.

Organisms

Functional groups Ecosystem processes

Vertebrates (gophers, lizards, beavers, whales); Invertebrates (oligochaetaes, polychaetaes, crustaceans, mollusks; echinoderms in sediments; ants, termites in soils), plant roots, macrophytes

Plant roots, algae, diatoms

Stoneflies in fresh water, decapods, millipedes

Bacteria and fungi

Symbiotic (e.g., Rhizobium) and asymbiotic bacteria (e.g., Cyanobacter, Azobacter)

Methanogenic bacteria, denitrifying bacteria

Roots, soil organisms

Bioturbators, ecosystem engineers

Primary producers Shredders

Decomposers

Nitrogen fixers

Trace-gas producers CO2 producers

Soil and sediment alteration and structure, laterally and to greater depths, redistribute organic matter and microbes

Create biomass, stabilize soils and sediments

Fragment, rip and tear organic matter, providing smaller pieces for decay by other organisms

Recycle nutrients, increase nutrient availability for primary production

Biologically fix atmospheric N,

N2O,

Transfer of C, N2 CH4 denitrification Respiration, emission of CO2

See Wall et al. 2001a for detailed listing of numbers of described and estimated species globally in soil taxonomic groups by size.

subject of the relationship of biodiversity and ecosystem functioning in either marine or freshwater sediments make up only about 1/100th of those published on above-surface terrestrial ecosystems. Consequently, taxonomists have described only a fraction of below-surface diversity (e.g., less than 0.1 percent of the marine species may be known; Snelgrove et al. 1997) and there is no site on Earth for which all the soil or sediment species present have been described. Thus a major component of the global ecosystem has been a minor part of analyses that consider how modifications of ecosystems resulting from global changes would affect ecosystem processes and human well-being.

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