Mucilaginous phytoplankton blooms result from the extracellular release of organic material from the cells of microalgae. This extracellular organic carbon is mainly carbohydrates, carboxylic acids, polysaccharides or harmless organic compounds formed by the process of photosynthesis. In fact, microalgae are the most important primary producers and are to the marine ecosystem what plants are to land. Through the process of photosynthesis, they produce oxygen and organic matter, and they themselves are food for zooplankton and are the first step in the food chain. In this way, they make marine life possible, especially at great depths, where they are scattered in an illuminated surface layer, while macroalgae are restricted to a narrow coastal belt where light reaches the bottom. The release of organic matter occurs regularly in healthy phytoplankton, but under certain environmental conditions this phenomenon is more intense and can be observed as mucilage accumulations at the sea surface. This phenomenon can also occur in the bottom layer and cause problems for fishermen in clogging their nets.
The first records related to this phenomenon date back to 1872, and since the end of the 19th century they become an almost regular occurrence in the Adriatic Sea, mainly in the northern part. However, in the last twenty years they occurred almost in the whole Adriatic. Many studies showed that they occur at elevated sea surface temperatures, high light intensity and reduced salinity.
These mucilage themselves are not harmful, but the mucus traps any particles and impurities that are in the water column, so they can cause skin irritation. Mucus is visible in different shapes and dimensions, depending on the intensity of mucilage bloom. A distinction is made between macroaggregates and macroflocs up to 5 cm and large macroaggregates that appear as clouds or bands up to 4 m in diameter. Large macroaggregates spread over large areas of the sea surface or throughout the entire water column.