Submerged aquatic vegetation
Eelgrass (Zostera marina) is the most widely distributed marine angiosperm in shallow Danish coastal waters. On hard substrates the vegetation is dominated by macroalgae. Growth of short-lived nuisance species of macroalgae is a problem in inner parts of some estuaries.
EELGRASS IN COASTAL WATERS
The following paragraphs summarise the development of Danish eelgrass meadows during the last century. The presentation is based on early reports on eelgrass distribution covering the period 1900-1940, aerial photos from the archives of KMS (The National Survey and Cadastre Agency) covering the period 1945–1990s and data from the national Danish monitoring programme on marine vegetation covering the period 1989–2001.
LONG-TERM CHANGES IN EELGRASS DISTRIBUTION
Today eelgrass again occurs along most Danish coasts (Figure 2.32) but has not reached the former area extension (Olesen 1993, Boström et al. 2003). Comparisons of eelgrass area distribution in two large regions, Øresund and Limfjorden, in 1900 and in the 1990s, suggest that the present distribution area of eelgrass in Danish coastal waters constitutes approximately 20–25% of that in 1900. The area distribution of eelgrass in Limfjorden was estimated at 345 km2 in 1900 (Ostenfeld 1908) and only at 84 km2 in 1994 (based on aerial photography data from the Limfjord counties). In Øresund, eelgrass covered about 705 km2 in 1900 (Ostenfeld 1908) and only about 146 km2 in 1996–2000 (Krause-Jensen et al. 2001). Differences in methodology may, however, influence these comparisons.
The large reduction in area distribution of Danish eelgrass meadows is partly attributed to loss of deep populations. In 1900, colonisation depths averaged 5–6 m in estuaries, and 7–8 m in open waters while in the 1990s, colonisation depths were reduced about 50% to 2-3 m in estuaries and 4–5 m in open waters (Figure 2.33) (Boström et al. 2003). The deep populations are most likely lost as a consequence of eutrophication. Increased nitrogen concentrations stimulate phytoplankton growth and thereby reduce the transparency of the water column and restrict the colonisation depth (Nielsen et al. 2002).
RECENT INTER-ANNUAL FLUCTUATIONS IN EELGRASS DISTRIBUTION
The colonisation depth of eelgrass reflects differences in water quality and physical setting along estuarine gradients. In the inner parts of estuaries, the annual mean colonisation depth ranged between 2.9 and 3.5 m, in outer parts between 3.3 and 4.3 m, and along open coasts between 4.7 and 5.8 m during the period 1989–2001 (Figure 2.34). Eelgrass colonisation depth shows no significant trend between 1989 and 2001 and does not reflect the sligh amelioration of water clarity observed through the same period (Ærtebjerg et al. 2002). Other regulating factors may blur the relationship between light and colonisation depth so that more marked changes in light climate are needed before colonisation depths increase. For example, eelgrass suddenly disappeared from several sites during the warm summers of 1992 and 1994 possibly due to combined exposure to anoxia, sulphide and extreme temperature (Goodman et al. 1995, Terrados et al. 1999).
Eelgrass displays a bell-shaped distribution pattern along the depth gradient with maximum abundance at intermediate depth and lower abundance in shallow and deep water (Figure 2.35). Exposure, desiccation and ice-scour act to reduce eelgrass abundance in shallow water and render the populations extremely dynamic and unpredictable. In the deeper, more protected waters, reductions in eelgrass abundance towards the lower depth limit correlate with light attenuation (Sand-Jensen et al. 1997, Krause-Jensen et al. 2000) and are therefore more directly coupled to changes in eutrophication. The period 1989–2001 showed no significant trend in eelgrass cover at water depths above 2 m, but the cover of shallow populations was significantly reduced in inner estuaries and along open coasts (Henriksen et al. 2001). We have found no obvious explanation for this pattern.
In conclusion, eelgrass responds to several types of disturbances: changes in energy input (light), physical disturbances (e.g. wind, waves, extreme temperature, ice), chemical disturbances (e.g. anoxia, sulphide) and biological disturbances (e.g. the wasting disease). When eelgrass is used as a monitoring parameter to reflect changes in light climate due to eutrophication we should therefore be aware that other factors may affect the response. As the intensity of physical disturbances decline with depth, eelgrass colonisation depth and abundance from intermediate depths towards deeper waters are therefore likely to be better response parameters to eutrophication than the abundance of shallow populations.
SHORT-LIVED NUISANCE MACROALGAE
Information on these algae is relatively scarse and analyses of trends are therefore only possible for the inner part of estuaries at depth intervals of 0–1, 1-2 and 2–4 meter. During the monitoring period (1993–2001) average relative cover varied from 1–20% at 0–1 meter depth, 1–25 % at 1–2 meter depth and 0–20% at 2–4 meter depth. There were no significant changes during the period 1993 to 2001 (Kendalls-tau, p > 0,05) (see figure 2.36).
MACROALGAE ON REEFS IN OPEN WATERS
It has been shown that a good empirical correlation exists between the
supply of the inorganic nutrients (nitrogen and phosphorus in freshwater
inputs) and total cover of macroalgae at deep stations in the Kattegat
during the period 1994 to 2000 (Henriksen
et al. 2001). This means that high supplies of inorganic nutrients
or freshwater leads to a reduced development of benthic vegetation. Exceptions
are stations with an intense grazing pressure from the sea urchin Strongylocentrotus
The total cover of the upright vegetation tended to increase in 2001 relative to the mean of the period 1994–2001 (Table 2.9). In general, algal coverage was low in years with relatively high runoff and high in years of low runoff. Accordingly there was no overall trend in the cover of macroalgae at the monitored reef stations (Figure 2.37).
Danish Environmental Protection Agency & National Environmental Research Institute • updated: