alt=Walrus |thumb|upright=1.5 |Sediment on the left tusk of a [[walrus. Walrus bioturbations in Arctic benthic sediments have large-scale ecosystem effects. While the prevailing categorization is based on the way bioturbators transport and interact with sediments, the various groupings likely stem from the relevance of a categorization mode to a field of study (such as ecology or sediment biogeochemistry) and an attempt to concisely organize the wide variety of bioturbating organisms in classes that describe their function. Examples of categorizations include those based on feeding and motility, feeding and biological interactions, and mobility modes. The most common set of groupings are based on sediment transport and are as follows:
- Gallery-diffusers create complex tube networks within the upper sediment layers and transport sediment through feeding, burrow construction, and general movement throughout their galleries. Gallery-diffusers are heavily associated with burrowing polychaetes, such as Nereis diversicolor and Marenzelleria spp.
- Downward-conveyor species are oriented with their heads towards the sediment-water interface and defecation occurs at depth. However, their ability to transport solutes, such as dissolved oxygen, enhance organic matter decomposition and diagenesis, and alter sediment structure has made them important for the survival and colonization by other macrofaunal and microbial communities. This increase in sediment metabolism and microbial activity further results in enhanced organic matter decomposition and sediment oxygen uptake. Nutrients released from enhanced microbial decomposition of organic matter, notably limiting nutrients, such as ammonium, can have bottom-up effects on ecosystems and result in increased growth of phytoplankton and bacterioplankton.
Burrows offer protection from predation and harsh environmental conditions. For example, gobies, scale-worms, and crabs live in the burrows made by innkeeper worms. Social interactions provide evidence of co-evolution between hosts and their burrow symbionts. This is exemplified by shrimp-goby associations. While thalassinidean shrimps can provide shelter for some organisms and cultivate interspecies relationships within burrows, they have also been shown to have strong negative effects on other species, especially those of bivalves and surface-grazing gastropods, because thalassinidean shrimps can smother bivalves when they resuspend sediment. They have also been shown to exclude or inhibit polychaetes, cumaceans, and amphipods. The presence of bioturbators can have both negative and positive effects on the recruitment of larvae of conspecifics (those of the same species) and those of other species, as the resuspension of sediments and alteration of flow at the sediment-water interface can affect the ability of larvae to burrow and remain in sediments. This effect is largely species-specific, as species differences in resuspension and burrowing modes have variable effects on fluid dynamics at the sediment-water interface.
Biogeochemical effects
Since its onset around 539 million years ago, bioturbation has been responsible for changes in ocean chemistry, primarily through nutrient cycling. Bioturbators played, and continue to play, an important role in nutrient transport across sediments. Bioturbators mix readily available particulate organic phosphorus (P) deeper into ocean sediment layers which prevents the precipitation of phosphorus (mineralization) by increasing the sequestration of phosphorus above normal chemical rates. The sequestration of phosphorus limits oxygen concentrations by decreasing production on a geologic time scale. The negative feedback of animals sequestering phosphorus in the sediments and subsequently reducing oxygen concentrations in the environment limits the intensity of bioturbation in this early environment. In polluted sediments, bioturbating animals can mix the surface layer and cause the release of sequestered contaminants into the water column. Upward-conveyor species, like polychaete worms, are efficient at moving contaminated particles to the surface. Pocket gophers form above-ground mounds, which moves soil from the lower soil horizons to the surface, exposing minimally weathered rock to surface erosion processes, speeding soil formation. Both benthivorous and anadromous fish can affect ecosystems by decreasing primary production through sediment re-suspension,
Lakes and ponds
thumb|Chironomid larvae.
The sediments of lake and pond ecosystems are rich in organic matter, with higher organic matter and nutrient contents in the sediments than in the overlying water. These environments can also be subject to strong season bioturbation effects from anadromous fish. and by mobilization of nutrients. The construction of salmon redds functions to increase the ease of fluid movement (hydraulic conductivity) and porosity of the stream bed. Bioturbators enhance the transport of oxygen into sediments through irrigation and increase the surface area of oxygenated sediments through burrow construction.
The effects of bioturbation on the nitrogen cycle are well-documented. Coupled denitrification and nitrification are enhanced due to increased oxygen and nitrate delivery to deep sediments and increased surface area across which oxygen and nitrate can be exchanged. While both nitrification and denitrification are enhanced by bioturbation, the effects of bioturbators on denitrification rates have been found to be greater than that on rates of nitrification, further promoting the removal of biologically available nitrogen. This increased removal of biologically available nitrogen has been suggested to be linked to increased rates of nitrogen fixation in microenvironments within burrows, as indicated by evidence of nitrogen fixation by sulfate-reducing bacteria via the presence of nifH (nitrogenase) genes. Walruses feed by digging their muzzles into the sediment and extracting clams through powerful suction. In low energy regions (areas with relatively still water), bioturbation is the only force creating heterogeneity in solute concentration and mineral distribution in the sediment. It has been suggested that higher benthic diversity in the deep sea could lead to more bioturbation which, in turn, would increase the transport of organic matter and nutrients to benthic sediments. Incorporation of POC into the food webs of sediment dwelling animals promotes carbon sequestration by removing carbon from the water column and burying it in the sediment. Bioturbation is typically represented as D<sub>B</sub>, or the biodiffusion coefficient, and is described by a diffusion and, sometimes, an advective term. radioisotopes from nuclear fallout, introduced particles including glass beads tagged with radioisotopes or inert fluorescent particles, and chlorophyll a. Biodiffusion models are then fit to vertical distributions (profiles) of tracers in sediments to provide values for D<sub>B</sub>.
Parameterization of bioturbation, however, can vary, as newer and more complex models can be used to fit tracer profiles. Unlike the standard biodiffusion model, these more complex models, such as expanded versions of the biodiffusion model, random walk, and particle-tracking models, can provide more accuracy, incorporate different modes of sediment transport, and account for more spatial heterogeneity.
Evolution
The onset of bioturbation had a profound effect on the environment and the evolution of other organisms. Bioturbation is thought to have been an important co-factor of the Cambrian Explosion, during which most major animal phyla appeared in the fossil record over a short time. The fossil is dated to 555 million years, which places it in the Ediacaran Period. However, this hypothesis requires more precise geological dating to rule out an early Cambrian origin for this specimen.
The evolution of trees during the Devonian Period enhanced soil weathering and increased the spread of soil due to bioturbation by tree roots. Root penetration and uprooting also enhanced soil carbon storage by enabling mineral weathering and the burial of organic matter. to assess the activity that occurred in old sediments. Typically the deeper the fossil, the better preserved and well defined the specimen. Arthropods, in particular are important to the geologic record of bioturbation of Eolian sediments. Dune records show traces of burrowing animals as far back as the lower Mesozoic (250 Million years ago), although bioturbation in other sediments has been seen as far back as 550 Ma. In 1891, geologist Nathaniel Shaler expanded Darwin's concept to include soil disruption by ants and trees. Since the 1980s, the term "bioturbation" has been widely used in soil and geomorphology literature to describe the reworking of soil and sediment by plants and animals.