Biology to Environment Links

The basic biological community established under the prevailing environmental conditions has the capacity to modify the sedimentary regime (biomodification). There are several categories of biomodification:

  • by organisms with an ability to stabilise the sediment, (biostabilisation) as shown on intertidal mud and sand flats, for example, by spionid tube beds (e.g. Prionospio elegans, by affecting boundary conditions), microphytobenthic mats (by mucopolysaccharide production), and eelgrass meadows (by sediment binding with rhizome production and by disturbing the sediment-water interface turbulence);
  • by organism behaviour leading to biodestabilisation, which in turn may lead to increased erosion (bioerosion); this may result from excessive reworking (bioturbation) by mobile infaunal organisms (e.g. Macoma balthica) on mudflats;
  • by feeding behaviour increasing the supply of sediment from the water column to the seabed through the production of faeces and pseudofaeces (biosedimentation), for example by suspension feeders such as mussels (Mytilus edulis) on mudflats and cockles (Cerastoderma edule) on sandflats.

Each of these processes modifies the sedimentary regime with the potential of increasing its heterogeneity and thus the number of niches available for colonisation. For example, extensive reworking increases the depth of surface-phenomena such as oxygenated sediments as well as increasing rugosity (surface roughness). Surface roughness disrupts the sediment-water boundary conditions and the ability for organisms to settle although it may also increase erosion.

Heterotrophic marine organisms are predominantly deposit or suspension feeders. Deposit feeders may feed at the surface or at depth within the sediment, resulting in the production of faecal pellets and the movement of organic material from deeper within the sediment to the surface. The vertical and lateral movement of mobile deposit feeders causes the mixing and transport of particles, interstitial water and dissolved gases (Rhoads, 1974). In muddier areas the production of faecal pellets by deposit feeders are of a size which may be ingested or otherwise manipulated by other benthic invertebrates hence increasing sediment reworking. As a consequence, the degree of bioturbation tends to be greater in fine muds dominated by deposit-feeders than in coarse grained substrata (Rhoads, 1974).

The factors most highly correlated with bioturbation are feeding method and location in relation to the sediment-water interface, organism size and degree of mobility, population density, burrowing depth and the density and spacing between animal tubes (Rhoads, 1982). Many of these processes are population size and temperature dependent. In addition, Reichelt (1991) identified three main processes leading to bioturbation: feeding activity, burrow or tube construction and migration within the sediment column due to tidal and diurnal cycles. For example, sedentary deposit-feeding polychaetes often form dense tube aggregations which have a stabilising effect on the sediments. Suspension feeders actively or passively entrap suspended seston which is later deposited at the sediment surface in the form of faecal pellets or un-pelleted pseudofaeces. The upper size limit of particles ingested by suspension feeders is generally smaller than that of deposit feeders (JØrgensen, 1966 in Rhoads, 1974).

Faecal pellets have higher deposition rates than their constituent particles and therefore settle out near the site of production. Deposit feeders may have a more quantitatively significant role in pelletization of the sea floor than suspension feeders or zooplankton (Rhoads, 1974). However, the production of non-pelleted pseudofaeces also contributes to the rate of sedimentation in many mudflat areas.

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