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Susanne Baden, Martin Gullström, Bengt Lundén, Leif Pihl and Rutger Rosenberg

Vanishing Seagrass (Zostera marina, L.) in Swedish Coastal Waters Along the Swedish Skagerrak coast eelgrass (Zostera marina) is a dominant phanerogam on shallow soft bottoms. Eelgrass meadows are important biotopes for many crustacean and fish species being either migratory or stationary. During the 1980s, inventories of the shallow coastal areas with eelgrass have been carried out along the Swedish west coast as a basis for coastal zone management. In the present study we revisited 2000 ha of eelgrass meadows in 5 coastal regions along 200 km of the Skagerrak coast. The inventory was made with the same methods (aquascope) as during the 1980s, but increasing the mapping accuracy by using a Global Positioning System (GPS). The results from this study show that the areal extension of Zostera marina has decreased 58% in 10–15 years with great regional variations. The decline was mainly restricted to the shallow parts of the meadow. The causes and ecological consequences are discussed.

water (13), though eelgrass can withstand salinities ranging from 5.5 to 35‰ and brief periods of temperature exposure to 40°C or encasement in ice (1). Distribution in Scandinavia varies from the fully saline environment in Skagerrak into the brackish Baltic Sea at 61°N with a salinity of 5.5‰ (9). Resident fauna associated with eelgrass on the Swedish Skagerrak coast, many of which exhibit habitat-specific relationships, include the pipefishes, Syngnathus typhle and Nerophis ophidion, black goby, Gobius niger, grass shrimp, Palaemon adspersus, and fifteen-spined stickelback, Spinachia spinachia (4). Eelgrass meadows are also important nursery grounds for juveniles of commercial fish like cod, Gadus morhua, and whiting, Merlangus merlangus (14). In the Baltic Sea, the dominant associated species include the isopod, Idotea baltica, and fishes like perch, Perca fluviatilis, flounder, Pleuronectes flesus and roach, Rutilus rutilus (15). Eels, Anguilla anguilla, are common throughout the eelgrass meadows from the Skagerrak to the Baltic. Inventories of the shallow coastal areas were assessed along the Swedish west coast for coastal zone management purposes. Detailed mapping of sediment habitat types, including presence of phanerogams, were important components for decisions on dredging operations, locations of boat harbors and aquaculture ventures. The aim of the present investigation was to revisit about 2000 ha of Zostera marina inventoried during the 1980s along the Swedish Skagerrak coast using analogous techniques while also increasing mapping accuracy by using a Global Positioning System (GPS) for outlining the seagrass meadows in the field.

THE IMPORTANCE OF SEAGRASS HABITATS Seagrasses are aquatic flowering plants that occur worldwide and may be abundant on soft and sandy bottoms along coastal margins (1). In recent decades, a decrease in aerial extension has occurred mainly along industrialized coasts. This is documented throughout many distributional areas (2) and is one reason why seagrasses are listed in the Rio-declaration (1992/93:13) as habitats worthy of protection. The declaration further directs ratifying states to map marine ecosystems of high biodiversity and production. In this context, seagrass meadows (Chap. 17, part MAPPING OF SEAGRASS DISTRIBUTION D 17.86 d) are especially emphasized. Seagrass meadows have important ecological and economic During the summer of 2000 (mid-July to mid-September), when roles. As a biotope, seagrass generally exhibits high production the standing stock of eelgrass is at a maximum (4), 69 eelgrass and biodiversity of associated fauna (3–7), considerably higher meadows were mapped along the Swedish Skagerrak coast. The than vegetation-free bottoms at the same depth (7). In an attempt mapping was conducted in 5 regions, and compared to those of to estimate the ecological and economic value of various eco- eelgrass coverage at the same locations during the 1980s (Tasystems, Costanza et al. (8) found that coastal ecosystems con- ble 1). Both mapping efforts were undertaken at the same time tribute to more than 30% of the goods and services provided by of the year using equivalent methodology (surveying by the ecosystems of the world. Of these coastal services half are aquascope from a boat). During the 1980s, the distribution of from seagrass and algae beds affecting nutrient fluxes by stor- eelgrass in each area was assessed from a number of transects age, internal cycling, processing, and acquisition. With all limi- perpendicular to the shoreline. Minimum and maximum depth tations involved in these types of calculations the values for distributions were determined by means of a lead-line and the seagrass and algal beds were estimated at USD 19 000 ha–1 yr–1, length of each transect was estimated from depth contours on nautical charts and by various landmarks. With this method, about 10 times the estimated value of tropical rain forests. Zostera marina, L., or eelgrass, is the most abundant seagrass in the Northern Hemisphere and constitutes Table 1. Comparison of eelgrass cover in specific sites along the coastal zone of Skagerrak, an important biotope for many animals Sweden, between different surveys made during the 1980s and the survey made 2000. (4, 9, 10). In Denmark, the vertical Region Mapped Depleted New Net loss of Number of Number of depth distribution has been reduced by eelgrass area eelgrass area eelgrass area eelgrass area reduced enlarged th during the eelgrass eelgrass about 50% during the 20 century from 1980s meadows meadows a historical depth limit of between 5.6 ha ha ha % n n and 11 m depth (11) to a recent distribution down to between 2.5 and 8 m Strömstad 190 120 37 44 12 8 depth in sheltered and exposed areas, Lysekil 167 76 72 2 10 8 Uddevalla 384 303 23 73 6 1 respectively (12). An important limitStenungsund 290 140 86 19 2 1 Kungälv 794 681 33 82 20 1 ing factor for the vertical distribution of eelgrass is light transparency of the 374

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meadow boundaries were estimated to have an error of approximately 10 meters. In contrast to the 1980s, the recent mapping was done with a GPS receiver with an accuracy of about 5 meters. The 2000 mapping was simplified by constraining the investigation to the borders of the 1980s' meadows. Additionally, maximum water depths of the 50 largest eelgrass meadows distributed throughout the investigated area were recorded by Scuba diving. During both investigations, the boundary condition for a meadow was determined as the area where eelgrass covered more than 5% of the bottom. Although some uncertainties exist in estimated changes in eelgrass distribution, reliability is high for the large areas where vegetation was completely absent in 2000. Since all the mapping data from the 1980s were available in analog form, they were scanned and digitized before being used for historical comparisons of seagrass distribution. The geographic coordinate systems used in the 1980s (geographical latitude and longitude) and 2000 (WGS84 derived from the GPS)

were also converted into RT90 (the Swedish National Grid), and maps of the eelgrass changes (Fig. 1) were created using the Geographical Information System (GIS) program ArcView. Final maps of polygons in 3 thematic colors as overlays on topographic maps (original scale 1:100 000, the National Land Survey of Sweden) covering the Swedish Skagerrak coastline represented the losses, increases or unaltered cover of eelgrass. It should be noted that some errors in calculations of historical changes can be attributed to estimates of maximum depth distribution for mapping. THE CHANGE IN EELGRASS COVER We demonstrated a dramatic reduction in eelgrass cover along the Swedish Skagerrak coast. Total net loss was estimated at 58% (1061 ha) of the eelgrass area mapped during the 1980s, and 50 out of 69 eelgrass meadows showed a decline in distribution. The decline was mainly restricted to the shallow part

Figure 1. The change in eelgrass distribution between the 1980s and 2000 along the Swedish Skagerrak coast. The distribution during the 1980s, 2000 and both investigation periods is shown by orange, purple and green colors, respectively. In the location map the white polygons represent the investigated areas and the red polygons delineate the presented maps.



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of the meadows. Regional differences were large when comparing the different parts of the coast (Table 1). Of the 5 investigated regions, the largest degradation of eelgrass occurred in Kungälv (82%) and Uddevalla (73%) (Fig.1), regions that also represented the most extensive seagrass areas mapped during the 1980s. Total net losses in Strömstad and Eelgrass meadows outside the Swedish Stenungsund were 44% and 18%, west coast. The photo to the left shows respectively, whereas there was healthy eelgrass and to the right eelgrass almost no change in Lysekil due covered by ephemeral algae. Excess to the equal areas of depleted and growth of covering algae due to nutrient pollution is considered to be the greatest new or expanded eelgrass beds. threat to the existence of this seagrass. When estimating the net loss of Photo: B. Gustafsson. eelgrass it is critical to compare the relationship between new and depleted eelgrass areas. As can nutrient enrichment and seagrass loss or recovery has not been be seen in Table 1, estimates of new seagrass cover in Lysekil established along the Swedish Skagerrak coast. However, in our (78 ha) and Stenungsund (86 ha) are about the same, as they are study area highest nutrient loads have been recorded in the southfor the 3 other regions (Strömstad, 37 ha; Uddevalla, 23 ha; ern part of the archipelago (Kungälv) coinciding with the areas Stenungsund, 33 ha). Thus, the major reason for the appreciable of greatest loss in seagrass (17). In Scandinavian waters nutridifferences in eelgrass net loss between the 5 regions can be at- ent enrichment is known to facilitate excess growth of ephemtributed to the large regional difference in depleted areas. There eral algae (Fig. 2) on soft bottoms with and without vegetation. is also a considerable variation in number and size of the inves- Cladophora spp. dominate in the beginning of the season with tigated sites in each region. A majority of the revisited eelgrass shifts to dominance by Enteromorpha spp. late in the season (18– meadows occurred in Strömstad (n = 20), Lysekil (n = 18) and 20). In addition to causing a general reduction in light penetraKungälv (n = 21), whereas the number of sites in Uddevalla and tion, the decomposition of these algae especially in the shallow Stenungsund only were 7 and 3, respectively. The general pat- part of the meadows might cause anoxia and formation of hytern of the investigated eelgrass meadows was that Strömstad and drogen-sulfide in the bottom sediment, which could explain the loss of eelgrass and the observations of local fishermen. They Lysekil had many more small areas than the other 3 regions. have described how patches of eelgrass with “cotton” (ephemeral algae) seem to rot during the summer leaving an empty greyCAUSES AND EFFECTS OF SEAGRASS DEPLETION ish-white patch. These observations agree with findings by den From the present investigation it appears that an extensive area Hartog (21) from an intertidal flat at Langstone Harbour, Hampof eelgrass along the Swedish Skagerrak coast has disappeared shire, UK, where a Z. marina and Z. noltii bed disappeared in over the last two decades. This finding is consistent with natu- 1991 after coverage with Enteromorpha radiata and did not reral and human-induced disturbances of seagrasses worldwide (2). appear the year after. Although a total of approximately 60% of the investigated Short and Wyllie-Escheverria (2) found that in the period 1970 to 1982, 50% of seagrass loss worldwide could be attributed to eelgrass has disappeared, local variations are apparent. Reasons natural disturbances like hurricanes, coastal erosion, grazing, and for these variations are presently unknown. Most of the eelgrass diseases, and 50% was caused by anthropogenic perturbations grows in relatively sheltered areas with muddy sediment of varisuch as dredging, oil spills, and reduction in water clarity. In the able organic content (3–24% ash-free dry weight; 22). Accordsubsequent period between 1983 and 1994, about 90 000 ha of ing to Baden and Boström (15), exposure aside from nutrient seagrass loss was documented mainly from the US and Australia. concentrations might also explain local differences in eelgrass On 37 investigated sites, human-induced losses of seagrass in- declines since low exposure promotes more fouling and thus creased to 75% over that time period and two-thirds of these were shadowing of the leaves. The Swedish Skagerrak coast with its thought to be caused by reductions in water transparency. Short extensive archipelago is a popular recreational boating area. Oil and Wyllie-Escheverria (2) regarded declining water transpar- leakage from boat motors (23) could be an additional local stress factor influencing eelgrass disappearance. ency as the most significant threat to global seagrass survival. In Denmark, losses in the eelgrass cover resulted in a 50% reduction in depth distribution over the last century (11, 12). The mean maximum depth of the Skagerrak eelgrass meadows in the APPLICATIONS OF GIS AND REMOTE SENSING present investigation was 3.6 m (SE ± 0.7 m) ranging from 6 m In marine studies information of temporal changes in distribuin one location of Lysekil to 2 m in one location of Stenungsund. tion of habitats such as submerged seagrass meadows can be Assuming similar decreases in depth distribution of eelgrass stored and analyzed with GIS. The result from a GIS analysis meadows along the Swedish Skagerrak coast in the last century provides relevant information for coastal managers in decisionoriginal distributions may have decreased by more than the es- making, policy planning and management of natural resources. With access to the present data on quantified eelgrass changes timated 58% over the last two decades. Recent investigations revealed that the total loss of eelgrass along the Swedish Skagerrak coast, it may be possible to examin 1994 around the island of Funen in Denmark was primarily ine causative parameters like exposure to wind fetch and availdue to summer anoxia and accompanying sulfide release from able data for slight changes in Secchi depth. Since nutrient polthe sediment. However, recolonization took place in 1995–1996, lution is considered a conceivable cause of the dramatic decline probably as a result of the lowest precipitation of the century of seagrass, data for the different drainage areas and land use is resulting in decreased nutrient loads of 67% (N) and 83% (P) of of importance to include in a GIS analysis. Efficient and sustainable coastal zone planning and managemean (1976–1987) values and increased Secchi depths of 1–3 m depending on location (16). This example of a link between ment necessitates improvement of monitoring techniques. When 376



mapping seagrass over large areas a conventional field mapping method is precise but expensive. In many parts of the world the application of remote sensing technology has become a useful tool for conducting baseline mapping and long-term monitoring that assesses environmental change of marine and coastal habitats. The use of airborne remote sensing in surveys of local areas at high spatial resolution is a long-established technique, but for monitoring purposes also requires high costs. An alternative tool for mapping and monitoring of large marine and coastal environments could be a satellite-based remote sensing technique. There are several examples illustrating successful use of satellite remote sensing for studies of coastal and marine habitats outside Sweden, including aquatic vegetation (e.g. 24), coral reefs (e.g. 25), bathymetry (e.g. 26) and particulate content of the water (e.g. 27). At Izembek Lagoon, Alaska, Ward et al. (28) assessed spatial changes in the distribution of Zostera marina meadows using Landsat MSS satellite images. In Swedish waters, satellite remote sensing and mapping of shallow-bottoms of living seagrass may be useful. The present investigation provides a comprehensive field validation for evaluating the poten-

tial of alternative monitoring methods like satellite remote sensing, and also color aerial photography from lower altitudes. CONSERVATION AND MANAGEMENT The first action necessary to reverse the decline of seagrass is to improve water quality (16). Such measures could benefit from parallel efforts to restore seagrass biotopes worldwide (29, 30). If natural recovery is limited due to low vegetative lateral expansion (which is about 0.25 m yr–1 for eelgrass in Danish sheltered waters (31)) restoration could be necessary to speed recovery rates. Recolonization studies of eelgrass carried out in Limfjorden, Denmark, showed that the best transplantation success of eelgrass was to move square blocks measuring 0.2 m on a side from existing meadows in springtime to previously vegetated areas, and space them 1 m apart (31). However, as “prevention is better than cure” we hope that the responsible politicians take powerful measures to avoid a situation in the Swedish Skagerrak coastal zone where extensive transplantation will be necessary.

References and Notes 1. Den Hartog, C. 1970. The seagrasses of the world. Verh. K. Ned. Ak. Wet. Adf. NorthHolland Amsterdam 59, 1–275. 2. Short, F.T. and Wyllie-Echeverria, S. 1996. Natural and human induced disturbance of seagrasses. Environ. Conserv. 23, 17–27. 3. Nienhuis, P.H. and de Bree, B.H.H. 1977. Production and ecology of eelgrass (Zostera marina. L.) in the Grevelingen estuary, the Netherlands, before and after closure. Hydrobiologia 52, 55–66. 4. Baden, S.P. and Pihl, L 1984. Abundance, biomass and production of mobile epibenthic fauna in Zostera marina (L.) meadows, western Sweden. Ophelia 23, 65–90. 5. Heck, Jr K.L. and Thoman, T.A. 1984. The nursery role of sea grass meadows in the upper and lower reaches of Chesapeake Bay. Estuaries 7, 70–92. 6. Edgar, G.J. 1990. Population regulation, population dynamics and competition amongst mobile epifauna associated with seagrass. J. Exp. Mar. Biol. Ecol. 144, 205–234. 7. Boström, C. and Bonsdorff, E. 1997. Community structure and spatial variation of benthic invertebrates associated with Zostera marina (L.) beds in the northern Baltic Sea. J. Sea Res. 37. 153–166. 8. Costanza, R., dÁrge, R., de Groot, R., Farber, S., Grasso, M., Hannon, B., Limburg, K., Naeem, S., O´Neill, R.V., Paruelo, J., Raskin, R.G., Sutton, P. and Belt van den, M. 1997. The value of the world´s ecosystem services and natural capital. Nature 387, 253–260. 9. Phillips, R.C. and Menez, E.G. 1988. Seagrasses. Smithsonian Contr. Mar. Sci. 34, 1– 104. 10. Baden, S.P. 1990. The cryptofauna of Zostera marina (L.): Abundance, biomass and population dynamics. Neth. J. Sea Res. 27, 81–92. 11. Ostenfeld, C.H. 1908. Ålegræssets (Zostera marina´s) vækstforhold og udbredelse i vore farvande. (Growth conditions and distribution of eelgrass (Zostera marina) along the Danish coast.) Beretning fra den Danske Biologiske Station XVI. Centraltrykkeriet, Kjøbenhavn. (In Danish). 12. Rask, N., Bondgaard, M.B.J., Rasmussen, M.B. and Laursen, J.S. 2000. Ålegræs—før og nu. (Eelgrass—past and present). Vand og Jord 2, 51–54. (In Danish). 13. Duarte, C.M. 1995. Submerged aquatic vegetation in relation to different nutrient regimes. Ophelia. 4, 87–112. 14. Fjøsne, K. and Gjøsæter, J. 1996. Dietary composition and the potential food competition between 0-group cod (Gadus morhua L.) and some other fish species in the littoral zone. ICES J. Mar. Sci. 53, 757–770. 15. Baden, S.P. and Boström, C.A. 2001. The leaf canopy of seagrass beds: faunal community structure and function in marine and brackish areas. A review. In: Ecological Comparisons of Sedimentary Shores: Ecological Studies, 151. Reise, K. (ed.). Springer Verlag Series. pp. 213–236. 16. Rask, N., Pedersen, S.E. and Jensen M.H. 1999. Response to lowered nutrient discharge in coastal waters around the island of Funen, Denmark. Hydrobiologia 393, 69–81. 17. Lann, L and Oscarsson, H. 2000. Hur man minskar näringstillförslen till Skagerrak. (How to reduce the nutrient pollution of the Skagerrak.) A Report from the County of Västra Götaland. No. 16, 56 pp. (In Swedish). 18. Thybo-Christensen, M., Rasmussen, M.B. and Blackburn, T.H. 1993. Nutrient fluxes and growth of Cladophora sericea in a shallow Danish bay. Mar. Ecol. Prog. Ser. 100, 273–281. 19. Pihl, L., Magnusson, G., Isaksson, I. and Wallentinus, I. 1996. Distribution and dynamics of ephemeral macroalgae in shallow bays on the Swedish west coast. J. Sea. Res. 35, 169–180. 20. Pihl, L., Svenson, A., Moksnes, P.-O. and Wennhage, H. 1999. Distribution of green algal mats throughout shallow soft bottoms of the Swedish Skagerrak archipelago in relation to nutrient sources and wave exposure. J. Sea Res. 41, 281–294. 21. Den Hartog, C. 1994. Suffocation of a littoral Zostera bed by Enteromorpha radiata. Aquat. Bot. 47, 21–28. 22. Berg, T. 2000, Quantitative Studies of Infauna in Zostera marina Meadows on the Swedish West Coast. Biomass and Abundance Correlated to Organic Content of the Sediment. MSc Thesis. Department of Marine Ecology, Göteborg University, Sweden. 29 pp. 23. Ahlbom, J. and Duus, U. 1999. Mindre gift på drift. (Less toxins adrift.) A Report from the County of Västra. Götaland No. 37. 55 pp. (In Swedish). 24. Ackleson, S.G. and Klemas, V. 1987. Remote sensing of submerged aquatic vegetation in lower Chesapeake Bay: A comparison of Landsat MSS to TM imagery. Remote Sens. Environ. 22, 235–248. 25. Mumby, P.J., Green, E.P., Edwards, A.J. and Clark, C.D. 1997. Coral reef habitat mapping: how much detail can remote sensing provide? Mar. Biol. 130, 193–202. 26. Lundén, B., Wannäs, K. and Shaghude, Y. 2000. Bathymetric mapping in the Zanzibar Channel using Landsat TM. In: Proc. Sixth International Conference on Remote Sensing for Marine and Coastal Environments, Charleston, South Carolina, May 2000. 27. Tassan, S. 1998. A procedure to determine the particulate content of shallow water from Thematic Mapper data. Int. J. Remote Sens. 19, 557–562. Ambio Vol. 32 No. 5, August 2003

28. Ward, D.H., Markon, C.J. and Douglas D.C. 1997. Distribution and stability of eelgrass beds at Izembek Lagoon, Alaska. Aquat. Bot. 58, 229–240. 29. Fonseca, M.S., Thayer, G.W. and Kenworthy, W.J. 1988. Restoration and management of seagrass systems: A review. In: The Ecology and Management of Wetlands, Vol. 2, Management, Use and Value of Wetlands. Timber Press, Portland, Oregon. pp. 353– 368. 30. Orth, R.J., Harwell, M.C. and Fisherman, J.R. 1999. A rapid and simple method for transplanting eelgrass using single, unanchored shoots. Aquat. Bot. 64, 77–85. 31. Christensen, P.B., Sortkjær, O. and McGlathery, K.J. 1995. Transplantation of Seagrass. Report from the Danish National Environmental Research Institute (NERI). 16 pp. 32. Financial support for this study is received from the County of Västra Götaland and World Wide Fund for Nature. We are grateful to Jacques van Montfrans for improving the style of the manuscript. 33. First submitted 12 June 2001. Revised manuscript received 11 Dec. 2001. Accepted for publication 10 Sept. 2002.

Susanne Baden is associate professor at Department of Marine Ecology, Göteborg University. Her research focuses on the ecological and physiological effects of nutrient pollution. Her address: Department of Marine Ecology, Göteborg University, Kristineberg Marine Research Station, SE-450 34 Fiskebäckskil, Sweden. E-mail: [email protected] Martin Gullström is a PhD student at the Department of Marine Ecology, Göteborg University. His research concerns the ecological role of eelgrass. His address: Department of Marine Ecology, Göteborg University, Kristineberg Marine Research Station, SE-450 34 Fiskebäckskil, Sweden. E-mail: [email protected] Bengt Lundén is professor at the Department of Physical Geography at the University of Oslo, but he has his main position at the Department of Physical Geography and Quaternary Geology, Stockholm University. His research concerns remote sensing for geo- and bioscientific applications. His address: Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden. E-mail: [email protected] Leif Pihl is professor in marine fish ecology at the Department of Marine Ecology, Göteborg University. His research concerns the ecology of coastal systems. His address: Department of Marine Ecology, Göteborg University, Kristineberg Marine Research Station, SE-450 34 Fiskebäckskil, Sweden. E-mail: [email protected] Rutger Rosenberg is professor in marine ecology at the Department of Marine Ecology, Göteborg University. His research concerns the marine ecosystems. His address: Department of Marine Ecology, Göteborg University, Kristineberg Marine Research Station, SE-450 34 Fiskebäckskil, Sweden. E-mail: [email protected]


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