Soft/sandy floor
Soft/sandy floor
Due to the washing out of fine clay and organic silt particles, the granular material of sandy soils is deposited where strong currents and surf act.
The entirety of aquatic organisms living on and in the sandy substrate is referred to as psammon.
Life in sandy soils
The organisms of Psammon can be categorised according to where they live, as well as by food type or size class:
Epipsammon includes mobile organisms such as fish, crustaceans, molluscs and echinoderms that live on the sediment surface.
In contrast to the epipsammon, the organisms of the endopsammonlive buried in the sediment. These include crustaceans, molluscs, cnidarians, annelids and echinoderms, animals such as sponges and hydropolyps (hydrozoa), which, however, are attached to secondary hard structures.
Mesopsammon(sand gap fauna, interstitial fauna) includes species which, due to their small size (0.2 - 2 mm), can crawl almost unhindered through sandy soils and do not have to do any displacement work . They are characterised by ciliated body surfaces for locomotion (e.g. in ciliates, turbellarians or gnathostomulids) or webbed coils (e.g. in nematodes).
In addition to their small size, long, thin body shapes, flexibility and adhesive organs for temporary attachment to sand grains are typical morphological adaptation features.
Almost all groups of marine invertebrates are represented in the meiofauna of sandy soils (turbellarians, polychaetes, nematodes, ciliates, crustaceans (copepods)).
Conditions in the sand gap system
The void space of such sand gap systems is filled with pore water. The granularity of the material determines the permeability of the water, the mobility of organisms and the availability of oxygen. Bioturbation makes a significant contribution to the destabilisation of the sediment and thus also to the O₂ input. The flow of nutrients between the sediment and the water column is also positively influenced by bioturbation.
According to the biological definition, the habitat of the soft bottoms (pelos) begins where the gap space is too narrow for the mesopsammon (sand gap fauna). However, the transition from the sandy sediment bottoms to the silty to muddy soft bottoms is fluid.
Occurrence
With the exception of particularly protected areas near the shore, soft bottoms are only found at greater water depths, as the strength of the water movement is lower here. Consequently, the deposited sediments are finer.
Soft bottoms consist mainly of fine sediments (silt and clay) and organic material such as detritus and, as mentioned above, have no interstitial fauna. They provide an important habitat for many invertebrates and fish and their high biomass production makes them important for the marine food web.
Life forms in soft bottoms/terrigenous mud sediments
Depending on the sedimentation rate, the species forming populations in terrigenous mud sediments differ. At extremely high sedimentation rates of fluvial sediment inputs, non-viscous muds are formed on which sessile species, for example, are quickly buried by the sediment.
Many species exhibit adaptations such as camouflage mechanisms, bodies lying flat on the sediment and dorsally displaced mouth and eye positions, as well as adaptations for locomotion on soft bottoms.