The Influence of Mucous Aggregates on the Microphytobenthic Community in the Northern Adriatic Sea

An extended reappearance of mumus aggregates in the Northern Adriatic Sea in 1991 gave rise to the hypothesis of a possible influence of the mucus, settled on the bottom, on benthic flora and fauna. This work investigates the variations in the microphytobenthic community in three stations of the Gulf of Trieste (AA1, AA2, AA3) during 1991, when mucous aggregates were present, and during the two following years, when no such aggregates were observed. Water samples were collected by Niskin bottle in the bottom layer for nutrient analysis, and sediment samples were collected by divers. The diatom species and cell densities were determined under an inverted microscope. All the data were processed with the aim to evaluate community composition and possible relations with the presence of mucous aggregates in 1991. The results showed a general decrease of benthic diatoms from 1991 to 1993, both in quality and quantity, for all the investigated sites. Statistical analyses on microphytobenthos, hydrological parameters, and nutrient concentrations indicated differences among the considered years. The microphytobenthic community in summer 1991 appeared to be quite different from those of the other years. The density of benthic diatoms was apparently not correlated with nutrient availability in the bottom layer, but was rather influenced by changes in temperature. The presence of a dense microphytobenthic community in 1991 might be explained by a combination of adequate conditions related to the presence of mucous aggregates, including a more undisturbed substratum caused by the interruption of dredging, decreased grazing pressure due to a declined filtering capacity of epifauna, and nutrient‐rich sediment for extra nutrient disposal remineralized at the mucus‐sediment interface. The mucilage aggregates therefore apparently stimulated the microphytobenthic community, in contrast to the effects on the benthic macrofauna, which were perturbed by the mucus.