Consequences of stagnant algal mats for the benthic fauna on shallow soft bottoms at northern Gotland, Baltic Sea

Abstract: Earlier studies have shown that dense drifting algal mats influence the marine benthic fauna negatively. The purpose of this study was to evaluate the effects of drifting filamentous algae on the benthic fauna on soft bottom shallow bays. Sampling of the benthic fauna in areas with drifting algal mats and areas with bare sand was conducted in two shallow bays of northern Gotland, Baltic Sea. An additional experiment was carried out in situ including cages with and without algae, respectively. The opportunity to investigate a drifting algal mat from the start to the final stage of decomposition occurred. The abundance of the benthic fauna increased significantly more in cages without algae compared to the initial data and cages with algae. The number of taxa was also higher in cages without algae when compared to the initial data, cages with algae and under the algal mat. Oxygen levels declined in cages with algae and under the algal mat, but this was most pronounced under the algal mat. Under the drifting algal mat, only species with higher tolerance to reduced oxygen levels were recorded. This study shows that dense algal mats that become stagnant for a longer period of time influence the benthic fauna negatively because of reduced oxygen levels.