Dominant zoobenthic species in the northwestern coastal sea of Estonia — potential role of abiotic stresses

Most of the Estonian coastal sea in the western part of the Gulf of Finland is characterized by numerous small bays. Some of them are open to the waves and water exchange with the open sea area, while others are exposed to waves from only one direction and have therefore limited water exchange. The coastal sea in the northeastern Baltic Proper is more open to wave and current activity. The bottom material of the coastal sea varies widely from mud and sand to gravel and rocks. Besides, salinity decreases if to move from the northeastern region towards the central Gulf of Finland. These abiotic factors could affect the large-scale spatial distribution of zoobenthic species. The zoobenthos samples were collected during August-November 2007–2009 in 13 locations in the Estonian coastal sea in the area extending from Muuga Bay (the central Gulf of Finland) to Sõrve Peninsula (the northeastern Baltic Proper). Up to 18 triplicate samples were taken within one area. All samples were taken using Van Veen grab with the opening area of 0.025 m2 then sieved through 1.00 mm mesh sieve and preserved in a freezer. Bottom macrofauna was identified to the species level where practicable; such groups as crustaceans and oligochaets were identified to an appropriate higher taxonomic layer. Biomass was determined as wet weight (gww/m2). The average density and biomass of species was determined in each area separately to ascertain dominants. Species abundance and biomass varied considerably within one sampling area. Still we were able to determine dominant species in these areas. Both biomass and abundance showed that two major species Macoma balthica and Mytilus trossulus prevailed. Mytilus trossulus dominated in the open bays at the northeastern Baltic Proper and western Gulf of Finland. Macoma balthica was abundant and showed high biomass in the large bays of the central Gulf of Finland. The analyses confirmed that Mytilus trossulus preferred hard bottoms. Considering natural and anthropogenic stresses to dominant species, this study shows that the presence of harbors and frequent harbor development favors the dominance of Macoma balthica. Mytilus trossulus prevails in the areas that are more exposed to waves as shown by numerical experiments with the wave model SWAN.