Seagrass an indicator goes astray
Introduction). Losses have occurred all over the world. Also in the Dutch and Lower Saxonian Wadden Sea seagrass beds had been in decline over several decades. A rough aerial survey in 2002 indicated the uneven distribution of seagrass between subregions within the Wadden Sea (Fig. 1). Seagrass was scarce in the south, while it was fairly common in the north. After measures to reduce nutrient emissions have lowered the discharge of nutrients into the coastal waters of the eastern North Sea, seagrass seems to be recovering (Reise et al., 2005).
The European Water Framework Directive requires a good ecological quality for all water bodies by 2015. For the North Sea coast, it is reasonable to use seagrass as an indicator to show when a good status has been reached. By using historical reference conditions from the Northfrisian Wadden Sea it has been estimated that good ecological quality comprises at least a 15 %-share of seagrass beds from the intertidal area. At present, seagrass beds cover about 90 km2, which corresponds to 10 %.
An assessment of seagrass occurrence across such a wide area of difficult accessibility is only feasible from the air. However, green algae constitute a challenge for this method. On satellite images (Brockmann and Stelzer, 2008) and aerial photographs they are easily mistaken for seagrass. To distinguish between the two is essential, however, because contrary to the seagrass the opportunistic green algae respond to nutrient overloads with massive growth. This had happened twenty years ago when algal mats covered 15-20 % of intertidal flats in the Wadden Sea and smothered seagrass and the macrobenthic fauna underneath (Fig. 2) (Reise and Siebert, 1994).
Each year, the Wadden Sea area is flown over at an altitude of 300 to 500 m in June, July and August during low tide exposure. In the course of summer, seagrass continuously grows denser while green algae appear to fluctuate. Both render the sediment surface darkish green. Only textural details indicate whether seagrass or green algae dominate. If a distinction cannot be made at first sight, questionable areas are flown over twice or more to reveal more structural features from different angles. If this does not help either, these problematic areas have to be visited by foot.
During flights notes are taken onto maps 1 : 100.000 made from satellite images to ease orientation over light (sandy) and dark (muddy) tidal flats dissected by branching creeks. Back on the ground, data are transferred to digital maps. Areal sizes covered by seagrass beds and green algal mats are calculated with a Geographical Information System (GIS).
Flying allows for rapid results but cannot replace ground observation entirely. Two species of seagrass occur in the region which differ in their indicatory value and can only be distinguished by experts (Fig. 2 a). Also, the species composition of green algal mats is variable, requires taxonomic expertise and provides information on the causes as well as the effects of massive green algal mats. Therefore, in the month of August, the entire region is surveyed area by area with trained personnel. This is very time consuming and the aim is to visit each area at least once within a period of six years.
The ecological quality will be evaluated over such six year intervals. This can level out fluctuations in seagrass bed area caused by unusual weather conditions or rough seas, and avoid mistaking inherent variability for environmental quality.
Results and outlook
Areal sizes of seagrass beds assessed from the air have increased gradually in the intertidal of the Northfrisian Wadden Sea, particularly since this millennium has commenced (Fig. 3). This may be correlated with declining nutrient inputs. If these could be reduced further and seagrass increases accordingly, the requirement of the EU for a good ecological quality of coastal waters could be fulfilled in the coming years, based among other things on the status of intertidal seagrass beds.
However, in contrast to the south, in the northern region of the Wadden Sea there is no evidence from earlier decades for a retreat in seagrass in response to a nutrient over-supply. May the apparent recovery of seagrass have causes other than the reduction of nutrient input?
According to Weisse and Plüß (2006) storm frequency in winter has increased since the 1960s and since the mid 1990s has decreased again. This is related to climatic oscillations in the Northern Atlantic. High water levels caused by storm surges in summer have declined slightly over the entire period. From field experiments it is known that seagrass is highly susceptible to sediment turnover as this may happen during storm surges (Cabaço und Santos, 2007).
Looking at the spatial pattern of intertidal seagrass (Fig. 4), it is apparent that seagrass predominantly occurs at sites sheltered against the prevailing surf from the south western direction in the Northfrisian region (Reise and Kohlus, 2008). In addition, field investigations have shown that the largest seagrass beds at the Halligen and the island of Pellworm are underlain by solid peat and clay in which seagrass roots get a fair hold. These observations support the assumption that sediment stability may serve as a key factor explaining why there is more seagrass in the Northfrisian region than in all the other regions of the Wadden Sea taken together (Fig. 1).
The pattern in Fig. 3 may also reflect an inverse relationship to storm frequency, which has declined since the mid 1990s. If this interpretation of seagrass occurrences is correct, than its future would be less a function of nutrient supply than of storm activity, and if the latter will increase again, and the last two winters do support just that, then seagrass will not achieve an areal share of 15 % as required for a good ecological quality status by the European Water Framework Directive.
A potential for further expansion of seagrass would be possible along the mainland coast if land reclamation works are phased out. These destroy seagrass beds in two ways. Between the brushwood fences traditionally set up in rectangular fields to calm down wave action, green algae tend to accumulate and then smother seagrass. Secondly, the digging of ditches in a grid-like pattern with the excacated material piled up between parallel ditches is detrimental to seagrass, either directly or by enhanced sedimentation within reclamation fields. These negative effects of land reclamation works on seagrass constitute a dilemma. The works are deemed beneficial for coastal protection if they are successful in building up a foreland to absorb wave energy in front of seawalls. In many cases, however, this aim is not achieved because of an insufficient sediment supply to the area. The net effect is merely an exclusion of seagrass beds and disturbance of other nearshore biota.
In the long run, it appears questionable whether seagrass beds could maintain their spatial share if sea level rise accelerates and storm activity will increase in the wake of global warming. Such a climatic effect is not yet considered in the Water Framework Directive but should better be taken into account.
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Please note that others may also have edited the contents of this article.