Academic journal article Estonian Journal of Ecology

Spatiotemporal Variability in the Eelgrass Zostera Marina L. in the North-Eastern Baltic Sea: Canopy Structure and Associated Macrophyte and Invertebrate Communities

Academic journal article Estonian Journal of Ecology

Spatiotemporal Variability in the Eelgrass Zostera Marina L. in the North-Eastern Baltic Sea: Canopy Structure and Associated Macrophyte and Invertebrate Communities

Article excerpt

INTRODUCTION

Seagrasses form a diverse ecological group of phanerogams that inhabit the intertidal and shallow sublittoral areas along temperate and tropical coastlines

The paper was presented at the 16th annual seminar of the Marine Biology Department of the Estonian Marine Institute, University of Tartu, held at Laulasmaa, Estonia, on 6 March 2014. (den Hartog, 1970; Larkum et al., 2006). Seagrass beds provide ecosystem services that rank among the most valuable of all ecosystems on the Earth (Costanza et al., 1997); for example, they stabilize the seabed in which they grow (Terrados and Duarte, 2000; Gacia et al., 2003), increase the habitat complexity, and provide shelter and food for a great variety of associated species, many of which are socioeconomically important, thereby increasing ecosystem productivity, biodiversity, and value to humans (Orth, 1992; Bostrom and Bonsdorff, 1997; Heck et al., 2003; Herkul and Kotta, 2009).

The spatial distribution and temporal variability of seagrasses are influenced by multiple environmental variables (Hemminga and Duarte, 2000), including e.g. the nature of the substrate, light (Peralta et al., 2002), temperature (PerezLlorens and Niell, 1993; Marba et al., 1996), salinity (Wortmann et al., 1997), nutrient concentrations in the water column (Orth, 1977) and/or in the sediment (Viaroli et al., 1997). Compared to tropical and subtropical communities temperate seagrass communities show greater seasonality (Duarte et al., 2006).

Out of the 66 known seagrasses only two species are found in the Baltic Sea, and due to diluted salinity conditions only one, Zostera marina L., prevails in the northeastern part of the Baltic Sea. Eelgrass is the most common angiosperm in the temperate waters of the Northern Hemisphere and due to its wide distribution the species is relatively well studied (den Hartog, 1970; Larkum et al., 2006). For the Baltic Sea, however, the information on eelgrass communities originates mainly from Denmark, Sweden, and Finland (see Bostrom et al., 2014 for overview). The brackish Baltic Sea hosts a unique complex of submerged vegetation and usually eelgrass beds are a mixture of 2-6 different higher plant species (Bostrom et al., 2014). The floristic composition and density of the macrophyte community is expected to influence faunal diversity and species abundances (Gustafsson and Bostrom, 2009). This is because macrophytes provide a habitat for many benthic invertebrates. Besides, vegetation is crucial for invertebrates as protection from predation (Puttman, 1986). Quite often associated invertebrates are not very selective and their densities are expected to be related to the overall cover or biomass of the macrophyte community (e.g. Kotta et al., 2000; Worm and Sommer, 2000).

For the north-eastern part of the Baltic Sea, including the Estonian coastal waters, where eelgrass grows at its lowest salinity limit, the information on eelgrass communities is very scarce. There exist only a few publications focusing on the occurrence of eelgrass habitats along the Estonian shores (M6ller and Martin, 2007; Martin et al., 2013, Bostrom et al., 2014). Available data from different mapping studies and national marine monitoring programmes are in most cases only qualitative, reflecting presence or absence of species while detailed information on the growth dynamics and structure of eelgrass communities is missing.

The Estonian coastal sea considerably differs from the northern shores of the Baltic Sea and therefore factors driving the dynamics of eelgrass populations are expected to be different. Specifically, the Estonian coastal region is extended but at the same time is more exposed compared to e.g. Finnish and Swedish shores. Therefore the signs of eutrophication and associated impacts on eelgrass stands are much less severe than in other parts of the Baltic Sea (M6ller and Martin, 2007).

The Estonian coastal sea is primarily physically driven (Herkul et al. …

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