Marine Sediments very poor, and we are unable to predict, for example, whether loss of pelagic diversity will impact benthic diversity The problem is that humans impact diversity of marine benthic organisms in many areas through fishing, habitat destruction and modification, pollution, exotic species introductions, and global climate change, but there is often little idea on how different components of the ecosystem will be affected Ecosystem Services and Sedimentary Diversity Although the huge fraction of the Earth’s surface covered by marine sediments and the large numbers of species residing within sediments provide strong motivation to understand the pattern and regulation of benthic diversity, there are also good ecological motivators The oceans provide a variety of ecosystem services (Figure 4), and although we understand little about the role that biodiversity plays in maintaining these services there is very clear evidence that benthic species play critical roles There is a very real chance that ongoing changes in sedimentary biodiversity will result in loss of ecosystem services 111 organic material that passes through organisms undigested will be colonized and broken down by microbes Organisms that ingest the material respire, are preyed on, or die and decompose in the sediment Depending on how decomposition and digestion occur, nitrogen may be released as ammonia, nitrite, nitrate, or nitrogen gas that diffuses or is physically mixed into the water column above In coastal sediments, this cycle is a critical part of regenerating the dissolved nitrogenous compounds that are essential for primary producers; without this regeneration the cycle would end and primary production would cease Carbon is cycled in a similar way Material sinks to the bottom and may eventually be buried (e.g., fossil fuels) or it may be decomposed into carbon dioxide or other carbon compounds The relative rates at which carbon is buried, tied up in living organisms, or respired to carbon dioxide are closely tied to microbial activity and thus to the animals that influence microbial activity through feeding and oxidation of sediments As a result, benthic organisms play a key role in global carbon budgets Sulfur cycling through marine sediments is also dependent on sediment oxygenation and therefore bacterial activity Whether sulfur is buried and stored in sediments or recycled is determined in part by metazoan and microbial activity Nutrient Cycling Benthic marine organisms play a critical role in global cycles of nitrogen, sulfur, and carbon (Giblin et al., 1995; Snelgrove et al., 1997) As organic material sinks to the sediment surface, it carries with it organic carbon and nitrogen This material may be directly ingested by benthic organisms or microbes may colonize it In areas with high sedimentation rates or low densities of organisms, there may be substantial burial loss of this material, but most is consumed or decomposed Even the Pollutant Cycling Because benthic organisms move around and ingest particles, and may themselves be ingested by predators, they can greatly impact the burial fate and mobility of pollutants As sediment particles are bioturbated, so are any pollutants linked to them As a result, benthic organisms can dilute pollutants at the sediment–water interface by mixing them downward By the same process, the continual remixing of sediments by infauna Nutrient regeneration Carbon cycling Filtration Secondary production Pollutant mobilization Pollutant removal/burial Figure Schematic representation of key ecosystem services associated with sedimentary organisms Arrows indicate transfer of material between the water column of the ocean and the sediment