Structural features and taphonomic pathways of a high-biomass epifauna in the northern Gulf of Trieste, Adriatic Sea

Benthic ecologists and palaeoecologists usually associate soft bottoms with infaunal species. On the sublittoral muddy soft bottoms in the inner Gulf of Trieste, however, a complex epifauna is developed, whose biomass, structural features and taphonomy is investigated here. The total biomass amounted to an average of 556 g wet weight/m² and is strongly dominated by suspension feeders, followed by predators and deposit feeders. The biomass can be divided into two major groups: biomass on benthic islands (isolated and small-sized rockgrounds and shellgrounds which are embedded in or lie on the sediment) and sediment dwellers. The former category includes so-called multi-species clumps, which make up 92.5% of the total biomass. The latter category encompasses organisms that inhabit the sediment surface itself. The epigrowth on benthic islands makes up 84.6% of total epifaunal biomass and is exclusively represented by suspension feeders. Serpulids are strongly dominant, followed by ascidians, sponges, anemones and bivalves. The vagile organisms associated with multi-species clumps represent 7.9% of the total epifauna and are also mostly suspension feeders. The echino-derms Ophiothrix quinquetnaculata and Cucumaria planci are the two dominant forms, followed by crustaceans and echinoids. Soft-bottom dwellers represent only 7.5% of total epifaunal biomass and consist mainly of deposit feeders and predators. Three different post-mortem pathways can be recognized for the studied epifauna. Taphonomic loss due to selective preservation is the most important taphonomic process shaping the death assemblage and affects especially the soft-bodied epigrowth on benthic islands. Mineralized epigrowth and shelled soft-bottom dwellers are the main source of benthic islands. Vagile faunas on such islands, in contrast, are strongly affected by disarticulation / fragmentation and their body parts contribute considerably to the sediment composition. The death assemblage does not reflect the trend of low biomass near the shallow stations off the mouth of the Isonzo River and higher biomass at most stations positioned further away from the river and in deeper water (> 10 m), but acccurately reflects the borders of the epifauna, which coincide with the sedimentary facies and the preponderance of the suspension-feeding life habit.