Mucilage formation in the Northern Adriatic Sea

Ulrich Horstmann

Institut für Meereskunde an der Universität Kiel
Düsternbrooker Weg 20
24105 Kiel
Germany
Email: uhorstmann@ifm.uni-kiel.de

In the frame of an EU - context project investigations on the formation of mucilage in the northern Adriatic Sea were undertaken from 1993 to 1998. In situ measurements during transects across the northern Adriatic Sea, including underwater video observations and mucus sampling by divers, as well as laboratory experiments, lead to the following conclusion on mucus formation.

River water induced eutrophication causes phytoplankton blooms in the north-western part of the Adriatic Sea. Rapid and complete phosphorus uptake and sedimentation of the major part of phytoplankton biomass in summer depletes phosphate from the euphotic zone with a consequent strong excretion of dissolved and particulate organic carbon via phosphorus-limited phytoplankton metabolism.

Colloids and fibrils of phytoplankton-excreted carbohydrates, when maintained for a longer period in the water column, aggregate especially during quiet weather conditions when density discontinuities occur. Colloids form large macroflocks and mucoid stringers. As aggregates are maintained in the water column for weeks, further aggregation and formation of mucoid clouds and carpets at the pycnocline takes place. This material can sink to the sea bottom where it is frequently observed by fishermen, also in years when no mucilagine appears at the sea surface.

During summers when the cyclonic northern Adriatic gyre develops the mucoid material is advected to the north-eastern, northern and finally north-western parts of the Adriatic Sea, where increased concentrations of new and regenerated phosphorus have been measured. Increased nutrient levels, however, promote epi- and endomucoid growth of benthic diatoms and other microalgae. Photosynthesis-induced oxygen bubbles which stick to - and are enclosed in - the mucoid material cause upwelling to the water surface. Wind-driven transport forces the mucilage to accumulate, leading to the mucilagine phenomenon along the beaches.

Satellite images of sea surface temperature and water colour are capable to detect the occurrence of the northern Adriatic gyre and consequently may predict mucilagine events.