Zostera marina (LINNAEUS 1753), Eelgrass (Angiospermophyta)

Compiled by: Tiia Möller, Estonia

1. Description of the habitat/autecology of the species

Eelgrass Zostera marina is the most widely distributed marine angiosperm in the northern hemisphere (found along the Atlantic and Pacific coasts) (den Hartog, 1970). Eelgrass meadows typically occupy shallow coastal softbottoms and estuaries.

In the Baltic Sea the northern and eastern distribution limits of eelgrass correlate with the 5 psu salinity distribution of surface seawater. Species is usually found at the depth of 2-4 m (range 1-10 m).

2. Distribution (past and present)

The distribution and depth limits of eelgrass have considerably declined in past 100 years. For an overview see also Boström et al., 2003.

In the beginning of 20^th century the eelgrass meadows covered approx 15 % of the Danish marine waters. The reduction started in 1930s with the “wasting disease”, when about 90 % of the whole North European stock disappeared (however, the Zostera meadows in the inner Baltic Sea with lower salinity were not affected). Increased nutrient loading has reduced the vertical recovery and the depth distribution is therefore much shallower than before. Today eelgrass again occurs along most Danish coast but has not reached the former areal extension. The colonisation depths have reduced to 2-3 m in estuaries and 4-5 m in open waters (compared to 5-6 m and 7-8 m depth in 1900). Along the Swedish Skagerrak coast a decline in eelgrass down to 60 % has been observed in the last 10-15 years (Baden et al., 2003).

In Germany (Kiel Bight) the depth limits of Zostera have changed from 6 m (1960s) to less than 2 m (1980s). The decline of Zostera has also been reported for Greifswalder Bodden in the Pomeranian Bight (Schiewer, 2002), but Gosselck (personal communication, 2006) has observed Zostera meadows on the outer coast down to 6 m and in some areas 8 m. Individual plants and small Zostera patches (1 m²) reach 11 m (near Islands Rügen, Hiddensee). In the Greifswalder Bodden, Zostera is observed generelly at 4 m and in some areas at 5 m depth .

In Poland (the Puck Bay) in 1950s, Z. marina formed meadows even at 10 m depth. 1957-1987 these meadows were replaced by filamentous brown algae and Zannichellia palustris. Today Z. marina still persists in a small area in the northern Puck Bay (Kruk-Dowgiallo, 1996 and references therein).

In the southeastern Baltic Sea, along the Lithuanian coasts, eelgrass probably disappeared before any scientific research was made. The seagrass literature from Latvia and Estonia is scarce. In Estonian waters sparse eelgrass patches (depth 2-4.5 m) can be found in the Gulf of Riga, West-Estonian Archipelago Sea and the Gulf of Finland.

For Finnish waters, there is no long-term data for  eelgrass distribution, except for one eelgrass site in southwest Finland, where in timeperiod 1968-1993 no change in density was recorded. Based on a crude areal estimates and extrapolations the total coverage of eelgrass in Finnish waters is proposed to be less than 10 km² (Boström et al., 2003).    

3. Importance (sub-regional, Baltic-wide, global)

Z. marina is one of the most abundant macrophytes on exposed sandy bottoms and in the Baltic Sea it can be regarded as a key-species. Patches and beds of Z. marina stabilize bare substrates and provide habitat for a large set of organisms which can not live in unvegetated bottoms. The three-dimensional structure of Zostera beds (exemplified by a rich sediment infauna) contributes significantly to total biodiversity and abundance. Eelgrass beds are important for fish (and fishery), though due to geographical distribution of eelgrass meadows in the northern Baltic Sea the nursery role for economically important fish species is limited (Boström, 2001 and references therein). Eelgrass leaf canopies dampen water movement and favour the retention of suspended particles, thus contributing to water-quality improvent and shore-line protection from erosion (review by Borum et al., 2004). 

4. Status of threat/decline

Eutrophication of the Baltic Sea has resulted in significant declines of eelgrass meadows in Danish, Swedish and Polish coastal areas. In the northern Baltic, no clear changes in the distribution of eelgrass meadows have been recorded but the long-term changes found in the eelgrass associated invertebrate assemblages are linked to the effects of eutrophication. (Lundberg, 2005 and references therein). Further eutrophication may cause a shift from eelgrass meadows to communities dominated by fast-growing macroalgae and this may result in the loss of valuable habitats and thus loss of overall biodiversity (Boström et al., 2002 and references therein). 

5. Threat/decline factors

Increased nutrient inputs causing eutrophication is a major component to seagrass loss world-wide. This loss is in large part caused by excessive microalgal growth as phytoplankton biomass increases in nutrient-enriched waters and reduces light penetration through the water column to benthic plant communities (short overview given by McGlathery, 2001). Eutrophication can lead to mass occurrence of dense macroalgal canopies, which in turn can lead to decline of the eelgrass. (Hauxwell et al., 2000, overview by McGlathery, 2001). Extreme growth of epiphytes on Zostera may eventually cause local anoxia (Pihl et al., 1995; Baden & Boström, 2001;overview in Borum et al., 2004).

Direct impacts of human activity causing decline of eelgrass include dredging, fishing (trawling), aquaculture, boating, anchoring, oil spill accidents and also other habitat alteration (overview in Borum et al., 2004). 

6. Options for improvement

Continuing national cooperation as well as intensifying local management for decreasing different pollution in the Baltic Sea is the key subject in conservation of Zostera meadows.   

7. References

Baden S. P. & Boström C., 2001. The leaf canopy of seagrass beds: faunal community structure and function in a salinity gradient along the Swedish coast. In: Reise, K. (ed.), Ecological comparisons of sedimentary shores, Ecological Studies 151, Springer-Verlag, Berlin, pp. 213-236.

Baden S. P., Loo L.-O., Pihl L. & Rosenberg L., 1990. Effects of eutrophication on benthic communities including fish: Swedish west coast. Ambio 19: 113-122.

Borum J., Duarte C.M., Krause-Jensen D., Greve T.M. (eds), 2004. European seagrasses: an introduction to monitoring and management. The M&MS project. 95 pp. http://www.seagrasses.org/handbook/european_seagrasses_low.pdf

Boström C., 2001. Ecology of Seagrass Meadows in the Baltic Sea. Environmental and Marine Biology, Department of Biology, Åbo Akademi University, Åbo, Finland. 47 pp.

Boström C., Baden S.P., Krause-Jensen D., 2003. The seagrasses of Scandinavia and the Baltic Sea. In: Green, E. P. & Short, F.T. (eds). World Atlas of Seagrasses. University of California Press. pp. 27-37.

Boström C., Bondsorff E., Kangas P. & Norkko A., 2002. Long-term changes of a brackish water eelgrass (Zostera marina L.) community indicate effects of coastal eutrophication. Est. Coast. Shlef Sci. 55: 795-804.

den Hartog C., 1970. The seagrasses of the world. Verh K Ned Ak Wet Adf. North-Holland, Amsterdam 59: 1-275.

Hauxwell J., Cebrian J., Furlong C., Valiela I., 2000. Macroalgal canopies contribute to eelgrass (Zostera marina) decline in temperate estuarine ecosystems. Ecology 82: 1007-1022.

Kruk-Dowgiallo L. 1996. The role of filamentous brown algae in the degradation of the underwater meadows the Gulf of Gdansk. Oceanological Studies XXV (1-2): 125-135.

Lundberg,C., 2005. Eutrophication in the Baltic Sea - from area-specific biological effects to interdisciplinary consequences. Environmental and Marine Biology, Department of Biology, Åbo Akademi University, Åbo, Finland. 166 pp. 

McGlathery K.J., 2001. Macroalgal blooms contribute to the decline of seagrass in nutrient-enriched coastal waters. J. Phycol. 37: 453-456.

Pihl L., Isaksson I., Wennhage H. & Moksnes P.-O., 1995. Recent increase of filamentous algae in shallow Swedish bays: effects on the community structure of epibenthic fauna and flora. Neth. J. Aquatic Ecol. 29: 349-358.

Schiewer U. 2002. Recent changes in northern German lagoons with special reference to eutrophication. In: Schernewski, G. & U. Schiewer (eds.), Baltic coastal ecosystems, structure, function and coastal zone management. Central and Eastern European Development Studies, Springer Verlag, Berlin, pp. 19-30.