Unravelling the relationship between biological diversity and ecosystem processes on the mudflats of a large European estuary

Valuable natural real estate In the new discipline of environmental economics, conventional accounting techniques are used to calculate the value of natural systems to humankind. Ecosystem functions such as food production and water regulation, and natural capital stocks, are given an economic value. This approach, although controversial, gives a ranking of ecosystems which may be useful for conservation purposes. The results of a global analysis by Costanza et al. (1997) showed that marine systems have a higher economic value than terrestrial systems, due to their larger surface area (Figure 1). Within the marine realm, the coastal zone was ranked more important than the open oceans, and estuaries were the second most important type of coastal ecosystem. Indeed, with a total value of $22 832 per hectare, estuaries are the most valuable ecosystem on the planet.

A hot spot of activity at the land-water boundary Estuaries are rated with such a high monetary value because of the wealth of diverse processes which occur in these systems. At the top of the list of these 'ecosystem services' to humans is nutrient cycling. A large amount of dissolved materials, including industrial and human waste materials, are carried into the estuary from the surrounding catchment area. Biogeochemical processes within the water and sediments of an estuary can rapidly capture and process a large fraction of the incoming material. Complex organic molecules are degraded to simpler, inorganic forms which may be released as gases such as CO2 or N2, or transported to the open sea in the form of plant nutrients such as nitrate.

A well-stocked larder The diversity and abundance of microbial life within estuaries provide an important food source for commercially important species, particularly shellfish. Many species of birds depend on the rich food supply of estuarine mudflats and marshes for refuelling during migration, and shallow areas of estuaries are of great importance for the juvenile stages of fish species such as the herring and sole.

Figure 1. Total global flow value (1994 US $ yr-1 x 10⁹).  Click on image to enlarge.

Microscopic life - unseen, but powering the ecosystem Many ecosystem services within estuaries are enhanced or modified by the presence of biofilms. These are thin layers of microscopic cells which colonise intertidal or subtidal sediment surfaces. The majority of the cells in a biofilm are photosynthetic - i.e. they use special pigments to capture the energy of sunlight for growth, just like land plants. Over a half of the primary production in estuaries is due to these microphytobenthic algae. The most common group of cells are the diatoms, a diverse algal group which can be found in all aquatic habitats. At certain times of the year, the diatom biofilms become thick enough to be visible to our eyes as a brown film on the sediment surface. Not surprisingly, such 'blooms' of microphytobenthos are often accompanied by large flocks of birds which feed either on the algae directly [e.g. the shellduck, Tadorna tadorna], or on the animal grazers [e.g. the avocet Recurvirostra avosetta]. Different waders have different techniques for sieving animal morsels from the soft matrix of algae and sediment.

Diatoms - key players in coastal ecology Intertidal benthic diatom communities are highly diverse, both with respect to their taxonomic composition as well as to the spectrum of growth forms present. To date, most studies on estuarine and marine microphytobenthos have focused on the epipelon, which comprises motile pennate diatoms capable of vertical migration in the upper layers of the sediment. They often represent the bulk of the autotrophic biomass and primary production, and play a major role in sediment stabilization through their overproduction of photosynthetic sugars. These substances are excreted and act as a kind of glue which binds sediment particles together. However, far less is known about the biology of the epipsammon, which consists of small species that are attached to sand grains, and the tychoplanktonic component (cells which can grow both in the water and on the sediment). These groups are dominant in sandy and very silty sediments respectively.

Figure 2. A bloom of microscopic diatoms colurs the surface of a mudflat in the Schelde estuary in the Netherlands. (Click on image to enlarge).

Taxonomic diversity in estuarine benthic microphytobenthos is high: excluding imported planktonic and riverine taxa, about 200 taxa were observed in a study of the Westerschelde estuary (The Netherlands). Individual samples contain between 27 and 59 taxa. Due to the inaccessible nature of mudflats, many habitats in this estuary have not yet been sampled, so the true number of benthic diatom species in the Westerschelde estuary is almost certainly higher. Biodiversity and ecosystem function From a functional point of view, it is intriguing that despite their local co-occurrence, the diversity and dynamics of epipelon, epipsammon and tychoplankton appear to be regulated by different processes. Epipelic communities are less diverse than epipsammic communities, and show distinct seasonal succession patterns, with a pronounced spring bloom followed by a summer decline. Epipsammic communities are highly even (i.e. biomass is rather evenly distributed over the species present) and have a remarkably stable species composition throughout the year. Like the epipsammon, the enigmatic tychoplankton displays a stable composition throughout the year. Very little is known about the exact life strategies of epipelic, epipsammic and tychoplanktonic diatoms, but it can be expected that they display great variation in photosynthetic, trophic, survival and reproductive strategies, and hence offer great prospects for studying evolutionary adaptations for survival in the highly dynamic benthic environment. The roles of the different microphytobenthic functional groups in the ecology of the Schelde estuary are now under investigation by scientists from the Netherlands Institute of Ecology, the University of Ghent, and the Technical University of Delft.

With funding from the European Union and from both Dutch and Flemish Academies of Sciences, our primary goal is to quantify the biomass and primary production of microphytobenthic algae throughout the estuary at different times of year, as these parameters are critical to many ecosystem services such as nutrient exchange and sediment stabilisation. Estimates of the concentration of benthic microalgae can now be made from the air, using hyperspectral sensors which are sensitive to the wavelengths reflected by algal pigments. Airborne surveys accurately reveal the location of the most productive intertidal areas, and these sites are investigated further by ground teams. Another optical technique, chlorophyll fluorescence, is used to monitor microalgal photosynthesis in real time. Samples of the sediment surface are then collected and analysed in the laboratory for nutrient concentrations and physical properties. At each collection site, microalgal species are identified using a combination of classical taxonomy (microscopy, isolation into culture) and molecular biological techniques (DNA extraction and sequencing). By comparing the functional data with biodiversity indicators, the 'keystone' species for estuarine ecosystem vitality can be identified.

New insights on estuarine processes With one of the largest wading bird populations in western Europe, and several rare habitat types such as freshwater tidal marshes, the Schelde estuary is a site of international recognition and importance. The estuary is also a site of heavy industry, and is an important commercial shipping transport route, thus coastal zone managers must constantly balance the demands of many conflicting interest groups. Better knowledge of benthic microalgal processes, and the contribution of individual species to these processes, will increase our understanding of the Schelde and other estuarine ecosystems. In the long term, our results may enable predictions to be made as to the functional response of estuaries to climate change, to loss of biodiversity, or to changes in land and water management practices.