Overgrowth patterns of the red algae Furcellaria lumbricalis at an exposed Baltic Sea coast: The results of a remote underwater video data analysis

The exposed coast on the tide-less south-eastern Baltic Sea is generally unsuitable for large perennial, habitat forming plants, such as eelgrass and bladder wrack. In this area the dominant perennial macrophyte, Furcellaria lumbricalis, provides an important substrate for the eggs of commercial fish. This algae is limited to the hard substrates and abundant at depths of 6–10 m. However, the distribution of F. lumbricalis is patchy, probably due to the effects of exposure and abrasion from mobile sediments. It has been proposed that the extent of F. lumbricalis cover relates to substrate size and stability, sediment abrasion, depth and the direction of predominant stormy winds. A remote underwater video equipment was used to obtain records of distribution of these algae. Video footage was segmented into still images from where physical and biological parameters were obtained. F. lumbricalis occurred on cobbles, boulders, and occasionally on pebbles. Cover of algae was greater on boulders surrounded by a stable bottom (boulders) than surrounded by sands and gravels. Cover on cobbles was similar on all bottom types probably due to the movement of this substrate during storms. F. lumbricalis occurred close to the seabed on those substrates on stable bottoms where compared with mobile ones. However, elevation of algae from the seabed and cover patterns depended on depth. According to direction of wind exposure, cover on substrates was greater on sheltered surfaces in comparison to exposed ones, where the F. lumbricalis was dislodged during storms. The distribution of cover of F. lumbricalis may alter according to storm frequency as predicted to take place with changes in climate.