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Management

Salt Marshes - A Disregarded Refuge for Brackish Water Meiofauna

Werner Armonies, Wadden Sea Station Sylt of the Biological Institute Helgoland, Sylt, FRG

ABSTRACT

While the terrestrial arthropod fauna of the salt marshes is well known there is little information about their marine counterpart. The marine macrofauna strongly decreases both in abundance and species richness at the landward fringe of the Wadden Sea but the supratidal is still habitat for a highly diverse meiofauna. Up to now, some 300 species are known to occur in the North Sea salt marshes, and a total of some 500 species, half of which are confined to salt marshes, may be expected.

This total is about twice the number of breeding birds in central Europe. Abundance does not markedly deviate from the intertidal, but seasonality is different. This fauna is thought to be a relict of the wide brackish transition zone of the former undiked Wadden Sea and the land, now finding a refuge in the narrow salt marsh belt of the modern coastline.

INTRODUCTION

Due to the intense work done by Heydemann and his students from Kiel University, terrestrial arthropods are the best known faunal components of the coastal salt marshes. As they generally originate from the landward side, both abundance and species richness are highest in the upper salt marshes and strongly decrease towards the sea. There is a corresponding but reversed tendency in the marine fauna, limiting many species to the seaward side of the salt marshes. However, many meiofaunal species (i.e. specimens passing a 1 mm sieve) tolerate wide ranges of salinity fluctuations or even seem to prefer such a variable environment. In a former Wadden Sea without dikes, they were likely to have populated the wide swamp zones between mean high tide level and the highest reaches of the storm tides. Nowadays, they are restricted to a much narrower zone. As the higher reaches of the ancient swamp zones were the first to be diked, meiofauna limited to a low salinity may have lost many species. Therefore today's salt marsh meiofauna may be regarded as an impoverished relict of a formerly richer fauna.

FAUNAL COMPOSITION

Since salt marshes are a marginal biotope of the sea, they are often neglected by marine biologists. In addition, the well rooted sediment is remarkably resistant to the common modes for faunal extraction, and there is no easy way to study this brackish water fauna. As a consequence, our knowledge of the salt marsh meiofauna is rather rudimentary. Currently, in most taxa, there is not much more than a provisional list of species. Compiling the main literature, a total of 311 meiofaunal species is reported to occur in the salt marshes of the North Sea (Tab. 1). Including ăminor" references and unpublished localities, the number of known species may be in the range of 330. This number includes the salt marsh sediment and minor creeks. Larger salt marsh creeks often include sandy sediment with additional species thus connecting the salt marsh meiofauna with the fauna of sheltered beaches. These sandy areas increase the species richness considerably.

However, none of the meiofaunal taxa has been studied along the entire coastline of the North Sea. Therefore, the true number of species will be much higher. A total of 500 species in salt marsh sediment and 700 species including larger creeks, may still represent conservative estimates.

Taxon

Number of
known species

Hydrozoa

Platyhelminthes

Nematoda

Oligochaeta

Polychaeta

Copepoda

Ostracoda

Halacarinae

Metazoa

WSNL 1997-1

W. Armonies

ABUNDANCE AND SEASONALITY

With an annual mean of some 1.25 million of specimens per m2 of sediment surface, meiofaunal abundance does not markedly deviate from the intertidal (Tab. 2). However, as there is no method of extraction working equally well with all meiofaunal taxa in salt-

marsh sediment, this estimate is certainly conservative. Particularly, nematodes and copepods may be underestimated. Salt marsh creeks often show a higher abundance which is mostly due to mass-development of a few species.

Seasonality was only studied in salt marshes near the Island of Sylt. The highest abundance of meiofauna occurred in winter and spring, and a minimum during summer drought, when these salt marshes were not a very suitable habitat for aquatic fauna. In sheep-grazed salt marshes, meiofaunal abundance was lower than in ungrazed ones (Tab. 2). This is thought to be due to a combination of several effects including sediment condensation with restricted oxygen supply, enhanced evaporation, artificial draining and a lack of litter in grazed marshes.

The distribution of species within salt marshes was only studied for platyhelminths. The highest species richness occurred in the lower half of the salt marshes where annual fluctuations of salinity were highest. Many platyhelminths are able to endure periods with adverse conditions (such as summer drought or a too high or low salinity) within protective cysts. They recover as soon as their habitat becomes favorable again. Therefore, a high annual fluctuation of salinity allows many species to fit in during one time or another. Brackish water reed with a constantly low salinity, on the other hand, showed a constantly low species density.

Freshwater species penetrating the salt marshes were not detected, presumably because both habitats are usually separated by a dike.

WSNL 1997-1

W. Armonies

FAUNAL AFFINITIES

As is the case in any habitat, the meiofauna is composed of species indigenous to the salt marshes, marginal populations from bordering habitats, and incidental guests. Species of the latter group frequently occur after storm tides but are unable to permanently survive in the salt marshes. Marginal populations occur in salt marsh creeks (mostly with the ămain" population in intertidal mud) and in sandy areas of the salt marshes (with the ămain" populations in beaches). Members of these populations frequently show a high interannual variability and in most cases it is not known, if the salt marsh populations are self-supporting. Finally, about half of the species (again, only studied for platyhelminths) are restricted to the salt marshes although some of them temporarily show up in the intertidal.

Estuaries seem to have a low importance as alternative refuges for this brackish water fauna. The few studies done in the estuaries of the rivers Ems, Weser, and Elbe just detected a small subset of the salt marsh fauna. Likewise, the brackish waters of the Baltic harbor only about half the species richness observed in the North Sea salt marshes while conversely, all of the Baltic meiofauna species also occur in the salt marshes and beaches of the North Sea. Most of the salt marsh meiofauna are confined to brackish water conditions with clear distribution limits towards both the sea and freshwater. Therefore, both further diking and the seaward salt marsh erosion endanger these species to lose their last North Sea refuge.

WSNL 1997-1

W. Armonies

REFERENCES

Armonies, W., 1986. Free-living Platyhelminthes in sheep-grazed and ungrazed supralittoral salt marshes of the North Sea: abundance, biomass, and their significance in food chains. Neth. J. Sea Res. 20: 385-395.

Hellwig-armonies, M. & W. Armonies, 1987. Meiobenthic gradients with special reference to Platyhelminthes and Polychaeta in an estuarine salt marsh creek - a small-scale model for boreal tidal coasts? Helgoländer Meeresunters. 41: 201-216.

Authors address:

Wattenmeerstation Sylt der
Biologischen Anstalt Helgoland
D - 25992 List / Sylt