Bioturbation

In oceanography and limnology, bioturbation is the displacement and mixing of sediment particles by benthic fauna (animals) or flora (plants). The mediators of bioturbation are typically annelid worms (e.g. polychaetes, oligochaetes), bivalves (e.g. mussels, clams), gastropods, holothurians, or any other infaunal or epifaunal organisms. Faunal activities, such as burrowing, ingestion and defecation of sediment grains, construction and maintenance of galleries, and infilling of abandoned dwellings, displace sediment grains and mix the sediment matrix. The sediment-water interface increases in area as a result of bioturbation, affecting chemical fluxes and thus exchange between the sediment and water column. Some organisms may further enhance chemical exchange by flushing their burrows with the overlying waters, a process termed bioirrigation. Benthic flora can affect sediments in a manner analogous to burrow construction and flushing by establishing root structures. Bioturbation is a diagenetic process and acts to alter the physical structure, as well as the chemical nature of the sediment.

Soil bioturbation

In soil science, bioturbation is the physical rearrangement of the soil profile by soil life. Plants and animals exploit the solum for food, and shelter and, in the process, disturb the fabric of the soil and the underlying parent material. Burrowing animals and insects, and plant root systems create passageways for air and water movement, changing soil morphology. A passageway created by an animal that becomes backfilled with soil is known as a krotovina. Invertebrates that burrow and mound soil tend to produce a biomantle topsoil, and as such are primary agents of horizonization. Uprooted trees break up bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates, and disrupt soil horizonation. Bioturbation was initially unrecognized as a pedogenic force. The term didn't exist before 1952, when bioturbation was coined to aid in ichnological assessments. Bioturbation appeared in the soil and geomorphic literature in the early 1980s , and remains a key element of the pedogenic lexicon. Bioturbation is central to the biomantle concept formulated in 1990. The biomantle is the upper part of soil produced largely by biota, dominantly by bioturbation. Biomantles are one-layered when formed in fine fraction materials, and two-layered when formed in mixed fine-and-coarse materials. Bioturbation by burrowing animals results in soil landscapes that are both polygenetic and polytemporal.

Modelling bioturbation

Mathematical models are often used to describe sediment biogeochemistry. Commonly, these models take the form of ordinary differential equations or partial differential equations in which bioturbation appears as a diffusive term. A diffusive description is often adopted to avoid quantifying the plethora of mixing modes resulting from faunal activities. The diffusion coefficient describing the intensity of bioturbation is usually determined by fitting mathematical models to vertical distributions of natural radioactive tracers, radioisotopes resulting from nuclear weapon testing, or introduced particles, such as glass beads tagged with radionuclides.

Evolutionary significance

Bioturbation's importance for soil processes and geomorphology was first realised by Charles Darwin, who devoted his last scientific book to the subject (The Formation of Vegetable Mould through the Action of Worms, 1881). Modern research has provided further insight into the evolutionary and ecological role of bioturbation. In modern ecological theory, bioturbation is recognised as an archetypal example of ‘ecosystem engineering’, modifying geochemical gradients, redistributing food resources, viruses, bacteria, resting stages and eggs. From an evolutionary perspective, recent investigations provide evidence that bioturbation had a key role in the evolution of metazoan life at the end of the Precambrian Era.Bioturbation.