Distribution, structure and function of Nordic eelgrass (Zostera marina) ecosystems: implications for coastal management and conservation

Abstract

This paper focuses on the marine foundation eelgrass species, Zostera marina, along a gradient from the northern Baltic Sea to the north-east Atlantic. This vast region supports a minimum of 1480 km² eelgrass (maximum >2100 km²), which corresponds to more than four times the previously quantified area of eelgrass in Western Europe.Eelgrass meadows in the low salinity Baltic Sea support the highest diversity (4-6 spp.) of angiosperms overall, but eelgrass productivity is low (<2 g dw m^-2 d^-1) and meadows are isolated and genetically impoverished. Higher salinity areas support monospecific meadows, with higher productivity (3-10 g dw m^-2 d^-1) and greater genetic connectivity. The salinity gradient further imposes functional differences in biodiversity and food webs, in particular a decline in number, but increase in biomass of mesograzers in the Baltic.Significant declines in eelgrass depth limits and areal cover are documented, particularly in regions experiencing high human pressure. The failure of eelgrass to re-establish itself in affected areas, despite nutrient reductions and improved water quality, signals complex recovery trajectories and calls for much greater conservation effort to protect existing meadows.The knowledge base for Nordic eelgrass meadows is broad and sufficient to establish monitoring objectives across nine national borders. Nevertheless, ensuring awareness of their vulnerability remains challenging. Given the areal extent of Nordic eelgrass systems and the ecosystem services they provide, it is crucial to further develop incentives for protecting them. © 2014 The Authors. Aquatic Conservation: Marine and Freshwater Ecosystems published by John Wiley & Sons, Ltd.

Keywords: Zostera marina; biological organization; conservation; eelgrass food web; eutrophication; genetic diversity; trajectories.

Figure 1

Study area and position of the six subregions compared. 1. Atlantic (Norway), 2. Skagerrak (Norway and western Sweden), 3. Kattegat/Belt Sea (Denmark and western Sweden, Germany), 4. southern Baltic Sea (Germany, Poland, Lithuania), 5. Baltic Proper (eastern Sweden and Finland), and 6. north-eastern Baltic Sea (Estonia, Latvia). Dashed lines indicate salinity isohalines. Numbered black circles (1–6) within regions 2, 3 and 5, indicate location of meadows included in the faunal study. A distribution map from Iceland is presented in Figure 4, but this region is not included in comparisons of the subregions.

Figure 2

Number of peer reviewed scientific papers dealing with eelgrass ecology for the time period 1890–2010 by country (a) and by topic (b). In (a) international refers to multi-authored papers with contributions from more than a single country. The papers included in the analysis are presented in the Supplementary Material.

Figure 3

Box plots (median and 25th and 75th percentile) with whiskers (maximum and minimum) representing the hydrographical gradients in six subregions of the study area: (a) salinity, (b) total nitrogen, (c) total phosphorus, (d) chlorophyll-a, and (e) Secchi depth.

Figure 4

Distribution of Z. marina in Scandinavia with enlargements (1–5) of the main distribution areas including Iceland.

Figure 5

Box plots (median and 25th and 75th percentile) with whiskers (maximum and minimum) representing eelgrass characteristics in the study area. Eelgrass characteristics under different environmental settings in the six subregions of the study area. (a) shoot density, (b) above-ground biomass, (c) shoot size, and (d) production. Data sources: Region 1: Duarte et al., ; Sivertsen, , Region 2: Sivertsen, ; Holmer et al., , Region 3: Olesen and Sand-Jensen, and references therein; Reusch and Chapman, ; Dist_Stat Data Maasholm, 2001; Jegzentis, ; Reusch et al., ; National Monitoring Programme, Limfjorden, Denmark 1989–2005; Toxicon, ; Gohse-Reimann, , Region 4: Jegzentis, ; Bobsien, , Region 5: Boström et al., , ; Holmer et al., , Region 6: Möller, ; Möller and Martin, , University of Tartu, Estonian Marine Institute database (unpubl.).

Figure 6

Temporal development in eelgrass depth limits in Danish coastal waters. Historical data represent means (± s.e.) of all observations along open coasts (n = 232) and fjords (inner, outer and Limfjorden all together, n = 75) for the period 1880–1930 (Krause-Jensen and Rasmussen, 2009). Data from 1989 to 2009 are nationwide means of deepest observations of 10% eelgrass cover in fjords and open coasts as compiled under the Danish national monitoring and assessment programme and modelled by generalized linear models (Hansen and Petersen, 2011).

Figure 7

Plant and animal characteristics at selected study sites. For location of sites see Figure 1. (a) Habitat complexity in terms of shoot density and biomass of the faunal survey meadows, (b) crustacean grazers (both mesograzers >7 mm and grazers >7 mm) density and biomass, (c) body size distribution of crustacean mesograzers, and (d) relative biomass contribution (%) of different taxa to total biomass of epifauna and fish; G. gr. = gastropod grazers, C. gr. = crustacean grazers, O. cr. = omnivorous crustaceans.