Spatial distribution of marine benthic habitats in the Estonian coastal sea, northeastern Baltic Sea

Abstract

INTRODUCTION Annex I of the Habitat Directive (Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora) defines the habitat types of higher importance and in need of conservation within Europe. Together with the Birds Directive this directive forms a solid base for the European nature conservation policy and the Natura 2000 network. In order to implement the requirements of the directive and to resolve the existing needs for local conservation purposes, a benthic habitat classification system EBHAB (Eastern Baltic marine benthic HABitats) was developed in the frame of the multinational EU LIFE project 'Marine Protected Areas in the Eastern Baltic Sea (Baltic MPAs)'. In the current study a detailed inventory of the EBHAB habitat classes was performed within six study areas covering the most valuable nearshore marine areas and including several protected areas, i.e. the Natura 2000 areas in the Estonian coastal range. In general, prior to this inventory the knowledge on the distribution of habitats and associated biota in the coastal areas was very poor including only a few publications each containing a handful sampling sites only (e.g. Martin, 2000; Kotta & Orav, 2001; Kotta et al., 2008, Kovtun et al., 2009). Moreover, the published background information was very scattered and owing to differences in methodologies there was no way to systematize such knowledge. Thus, there was a need to examine the species composition and identify typical communities within their environment. The current inventory allowed us for the first time (1) to summarize the range of large-scale variability of the studied habitats in terms of benthic biodiversity, species composition, and dominance structure; (2) to provide a knowledge base for follow-on biological and ecological studies; and (3) thus, to provide important information for managing our marine resources. MATERIAL AND METHODS Study area Altogether six areas representing various environmental and climatic conditions in the northeastern Baltic Sea were studied in detail with the aim of classifying the observed benthic habitats according to the recently established EBHAB classes (Fig. 1). An overview of the study areas is given below. [FIGURE 1 OMITTED] Eastern and western Gulf of Finland areas The average depth of the Gulf of Finland is 37 m and the maximum depth 123 m. The coastal slope is quite steep and depths reach over 30 m in all bays of the study area. Sand, silt, or sandy clay bottoms dominate in deeper areas, boulders and stones prevail near the coast. The coastline is diverse and disjuncted by peninsulas, many small islands occur in the area. The eastern gulf receives runoff from a huge drainage area and the western gulf is a direct continuation of the Baltic Proper. Therefore the gulf has a permanent east-west gradient of salinity. The salinity range of the study area is 4.5-6.2. The area is strongly influenced by diffuse and point-source nutrient loads. West-Estonian Archipelago Sea The area comprises shallow bays with many small islets and mudflats. The average depth of the area is less than 4 m with the maximum close to 20 m. The bottom morphology of the area is flat, with gentle slopes towards deeps. The whole water basin is semi-exposed. Sand and sandy clay sediments prevail in the entire area of the archipelago. Due to the shallowness and clayey sediments already moderate winds result in strong resuspension of bottom sediments and poor underwater light conditions. Salinity varies from 0 to 7, being the lowest in the estuary of the Kasari River in the eastern part of the area and around 6-7 in the western, open-sea area. Haapsalu and Matsalu bays are the most eutrophied bays within the West-Estonian Archipelago Sea. Western Saaremaa Island Similarly to the previous study area this area consists of small shallow bays with many scattered islets. …