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

Abstract

Abstract. 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.

Benthic Response to Mucilaginous Aggregates in the Northern Adriatic Sea: Biochemical Indicators of Eutrophication

“…These aggregates tend to settle on the bottom and, as previously observed for organic inputs derived from frontal systems, such large amounts of organic matter, are expected to induce a significant benthic response (Josefson and Conley, 1997). During the summers of 1988-1989-1991 and 1997 floating gelatinous aggregates appeared in large areas of the northern Adriatic basin, causing anoxia and mechanical threats to the benthic system (Welker and Nichetto, 1996). Despite large efforts have been devoted to the study of mucilage's effects on the marine environment, little information is available of sediment benthic response (Fadda et al, 1994).…”

The Role of a Macroalgal Covering Event on the Microphytobenthos Community in Intertidal Sediment (Marano Lagoon, Northern Adriatic Sea)

“…The microphytobenthos abundance (MPB) expressed as cells cm ±3 (± S.D.) was evaluated following Welker & Nichetto's (1996) method. Net oxygen production (NP) and dark respiration (DR) of the local benthic community were determined in the laboratory as changes of oxygen concentration by Clark-type electrode (Hansatech DW1 model) following the procedure recently outlined by Wolfstein et al (2000).…”

Pollution threats in the Mediterranean Sea: An overview

“…Zeolite would also stimulate diatom production of exudates. During the summers of 1988, 1989, 1991 and 1997 floating gelatinous aggregates appeared in large areas of the 20 R. DANOVARO northern Adriatic basin, causing anoxia and mechanical threats to the benthic system (Welker and Nichetto, 1996). The appearance of mucilage aggregates along the Adriatic coasts during the summer of 1997 coincided with evident changes in biochemical features of the benthic system.…”