Sandy seabed

The Lagoon bed is made up of slimes and clays mixed with sand, lying on a base of "caranto", a very compact mineralised clay.
Until not a long time ago Venice Lagoon beds were covered with typical vegetation made up of sea phanerogamae, as Zostera noltii andCymodocea nodosa.
The distribution of these species in the different areas of the sea beds depends on the quality and on the dynamic of the water and of the sediments and on their ecological needs.
he dwarf-eel grass lives almost always submerged in dynamical water, while the Cymodocea nodosa, having a very developed radical apparatus, can resist to the strong currents characterising its habitat located nearby the Lagoon inlets.
This type of vegetation protects the sea beds from erosion, it captures and keeps the sediments and provides the young fish fauna and the larvae of the invertebrates with a habitat.
The presence of the sea phanerogamae is then of the utmost importance for the Lagoon ecology even if recently there has been a regression caused by several reasons. The excessive eutrophication of Lagoon water caused by the amount of nutrients coming from the drainage basin, and the sea bed erosion caused by trawling have induced these plants to disappear.
Manila clams (Tapes philippinarum) are a species native to the area between Japan and the Philippines in the Pacific Ocean, which was introduced in the Lagoon area near Chioggia in 1983. Right after its introduction a time of ferocious fishing characterised by the employ of high impact fishing techniques as the hydraulic dredges and the mechanical dregdes broke out.
These techniques involved a direct action on the sea bed, as the capturing of the molluscs implies the raising of the first 5-10 centimetres of sediment, with a consequent suspension of its pollutants and the removal of the seabed vegetation. (dwarf eelgrass, lesser Neptune grass, Posidonia) (from “Aspetti naturalistici della laguna e laguna come risorsa” - Second Part: La laguna come risorsa).
Most of all this last effect had a severe impact on fish fauna, as not only were grass goby's (Zosterisessor ophiocephalus), reproductive phases disturbed, but also the laying of the eggs of the big-scale sand smelt (Atherina boyeri), which takes place on submerged stiff substrates as the leaves of phanerogamae - and the nursery phase of the gilthead sea bream (Sparus aurata), of the bass (Dicentrarchus labrax), of the flat head mullet (Mugil cephalus), of the sole (da “Piano per la gestione delle risorse alieutiche delle lagune della provincia di Venezia”).
The hydraulic dredges were considered illegal by the Court in 1996 and have been therefore substituted by the mechanical dregdes, which has a lower impact on the seabed but that is still being experimented. Clam fishing is allowed only in limited granted areas. (from “Studio e verifica di attrezzi alternativi per la pesca del caparozzolo nella laguna di Venezia”).
The elimination of the phanerogamae from lagoon beds has given space to the diffusion of the seaweed which, having no roots, come easily off the beds and heap up in calm water areas, where they decompose taking away oxygen from the water and causing eutrophication phenomena.