Large scale mapping of intertidal areas
Contents
Large scale mapping of intertidal areas
Introduction
The diversity and the changes in habitats over larger temporal and spatial scales is of great interest for regional stakeholders in the German Wadden Sea because of the high political priority of protecting natural developments within the national park areas. For ascertaining the influences of climate change as well as short term disturbances like large scale coastal constructions and accidents, the documentation of changes in habitat diversity can provide essential information on stability properties of the systems and subsystems involved. In addition, the quality assessments in the framework of TMAP (Trilaterial Monitoring and Assessment Plan) of the Netherlands, Germany and Denmark need a sound data basis to document changes in natural resources. Similar conditions hold for the quality assessment and development of monitoring strategies with respect to the EU-Water Framework Directive.
Methodology
The field surveys for large scale mapping were conducted during June and August at three diagnostic sites in the German Wadden Sea: Hörnum Tief (2002), Elbe estuary (2003) and Baltrum/Langeoog (2004) (see Fig. 1, field work see Fig. 3). The sites are situated in areas of clearly different environmental conditions: The East-Frisian tidal basin is comparatively small (about 5 km distance from the mainland to the islands) and highly sheltered by the barrier islands Baltrum and Langeoog. A moderate influence of freshwater (rich in nutrients) exists via two sluices. The Elbe estuary is wide open to north-west winds and characterised by a strongly changing salinity, depending on the tide. The “Hörnum-Tief” basin in the North-Frisian area is sheltered by the islands Sylt, Amrum and Föhr but open to south-west winds. The distance from the mainland to the seaward border of the tidal inlet is about 20 km.
The selection of methods to conduct the in situ mappings of intertidal habitats within the selected sites (overall covering an area of about 200 km2 of tidal flats) was determined by two basic requirements: to achieve a maximum of information and to keep the effort as small as possible. To fulfill these prerequisites, a combination of estimated and measured values was selected to document the characteristics of habitats along a grid of locations (fixed by GPS and depicted by digital photography) with 1 km between. The estimated values, including biotic and abiotic parameters, were summarised on a standardised protocol (transect protocol) and at each location as well as at the habitat noted borders on the way between the locations (see Fig. 2 ).
The measured values were restricted to sediment cores (grain size and water content of sediments, macrofauna species) and shear strength. About 80 parameters were recorded by the protocol, concerning the Elevation/Height (tidal level), the Slope or Inclination, the Surface Structure (form of ripples, etc.), Colour (light sand to dark mud), Sediment (sand, mud etc.), Depth of Layers (clay, gravel, shell particles, etc.), Water Cover (in %), Redox Condition (thickness of the oxidised zone), Macroalgae (Ulva, Fucus, etc., presence and coverage in %), Macrophytes (Zostera, Spartina, presence and coverage in %), Macrofauna (epibenthic species and endobenthic species forming visible life tracks; presence and estimated abundance), and others.
Conclusions on change and stability of habitats within the sites were made by comparing these results to those of a mapping campaign from 1987 until 1992, which covered the entire German Wadden Sea area.
An overview of the mapping results of the three test sites is given in the following, with emphasis on the “Hörnum-Tief” tidal basin.
Backbarrier tidal flats of the islands Baltrum and Langeoog
Compared to the conditions during the field surveys in 1987/1992, the distribution of sediment properties (among others: grain size, water content and shear strength) showed no significant change during 2004. The same holds for the presence (or abundance) and distribution of key organisms/habitats like banks of the blue mussel (Mytilus edulis), meadows of the sand mason (Lanice conchilega) and cockles (Cerastoderma edule), shell mounds and muddy areas. A clear decrease in seagrass distribution, detectable during 1987/1992, continued. In 2004 only a small remainder of these habitat-forming macrophytes existed in a southern nearshore area (Fig. 3).
The protected location of this basin obviously prevented significant changes of morphology and habitat diversity after more than 10 years. Although some strong ice winters during this period eliminated nearly the entire intertidal stocks of the temperature sensitive organisms Lanice conchilega and Cerastoderma edule, their spatial distribution and density in 2004 corresponded to the conditions found during 1987/1992. Only with regard to the seagrass (Z. noltii) there was no resilience: its decrease, documented for the entire tidal flats of Lower Saxony, continued.
Tidal flats of the Elbe estuary
With respect to key species and habitats (for example: Marenzelleria viridis (invader from the USA, during the 1970s by ship ballast water), and the mud crab Corophium volutator (present over wide areas in high abundance: more than 150,000 Ind/m2)) the change in habitats from 1987/1992 until 2003 was negligible. These organisms, settling in silty, stable areas beyond mid tide level, showed no significant change of presence (with regard to abundance) and distribution. On the other hand, severe erosion eliminated about 5 km2 of tidal flats, mainly populated by another key species of this area, the soft shell clam (Mya arenaria). This mussel, living in a depth of 20-40 cm of mainly fine to middle sand occurred in this areas with an average biomass of 60g/m2 ash free dry weight (afdw) during 1987/1992. If we assume a filtration rate of 2 l/h/Individuum, a loss of 300 t afdw and of 300 hl/h filtration capacity results (Fig. 4).
Hörnum-Tief catchment area
During 2002 extended muddy areas were found only in the north-east part of the Hörnum-Tief catchment area. Most tidal flats were characterised by light to dark sands (for sampling locations see Fig. 5). Compared to the conditions found during 1987/92, the amount of sandy habitats especially in the north-east part increased significantly. These findings, affirmed by grain size analyses of the sediment, coincide with further habi-tat conditions documented by the standardised protocol: presence and form of ripples as well as the abundance and distribution of the lugworm (Arenicola marina), well known as a "sand follower" (see Fig. 6).
The spatial distribution of seagrass is more extensive in 2002 compared to 1987/92, but the dominance-structure changed: during 1987/92 Zostera marina dominated, in 2002 Zostera noltii, which probably shows a lower sensitivity to current velocity and disturbances by wave energy (Fig. 7).
The areas of presence of Zostera species remained widely stable. Thus, in a comprehensive observation, it can be assumed that a higher amount of hydrodynamic energy was transported into this tidal basin causing the shift of habitats with regard to the selected examples.
Conclusion
In situ monitoring of habitats provides a high number of results which can be used to assess the state of tidal basin systems. The different results can also be used to examine the consistency and plausibility of hypotheses of change. An evaluation of the vulnerability of habitats to natural or man-made disturbances is a valuable support in decision finding processes like for example the estimation of the resilience of habitat diversity following coastal constructions, the development of response strategies against oil pollution as well as the classification of reference areas (diagnostic sites).
Although the field investigations and analyses are very simple, the total application of the presented large scale mapping methods requires a sound knowledge of the operators with respect to tidal flat ecology. Remote sensing results of sediment structure and other identifiable properties of habitats form a suitable instrument to fulfill the temporal gaps between in situ assessments and help to define the spatial borders of habitat conditions recorded at fixed localities. On the other hand, the results of the field surveys help to find proxy parameters for remote sensing techniques and to classify remote sensing data.
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