270
H. Michaelis and W.J. Wolff
resource of wintering waders (Altenburg et al. 1982; Wolff and Smit 1990;
Wolff 1991; Wolff et al.1993a,b). In the Wadden Sea, biomass has been used as
a general means to characterize the productivity of different coastal areas or
habitats (e.g. Michaelis 1987; Beukema 1989; Essink et al. 1998; Reise and
Lackschewitz 1998). As in the earlier surveys (Altenburg et al. 1982; Wolff and
Smit 1990; Wolff 1991; Wolff et al. 1993a), the present study again points to a
remarkably low biomass on the Banc d' Arguin tidal flats: A mean value of
7.6 g ADW m- 2 without Anadara senilis and of 9.3 g ADW m- 2 with the bivalve
included (Table 12.4). It may be concluded that seasonal differences are
largely lacking and that a low macrozoobenthic standing stock is a general
characteristic of this tropical tidal flat area. Its biomass is only a small fraction
of the values found in the Juist area: A mean total biomass of 32.7 g ADW m- 2
without the mussel beds and of 41.2 g ADW m- 2 with the mussel beds
included. Summer values in that range (30-40 g ADW m- 2 without Mytilus
beds) are also characteristic for other subareas of the Wadden Sea, as shown
by the examples given in Table 12.4.
The vast seagrass beds and the extraordinary clearness of the seawater of
the Banc d' Arguin point to a system in which the phytoplankton is insignificant and the primary production is almost exclusively achieved by seagrasses
and their periphyton (Wolff and Smit 1990). Sevrin-Reyssac (1993) calls the
poverty of plankton in the Banc d' Arguin waters extreme. This is in line with
the observation that benthic suspension feeders are quantitatively unimportant in the tidal flats (Wolff et al.1987, 1993a,b). Though the knowledge on
autecology is insufficient, we conclude that the majority of the species belong
to the deposit and epistrate feeders. In the Juist area, the largest habitat, i. e.
dark sandy flats, is dominated by the suspension feeders Cerastoderma edule
and Mya arenaria followed by the epistrate feeder Macoma balthica (see
Fig. 12.8). In the habitat "light sandy flats" (Fig. 12.7), the first places in biomass contribution are occupied by the epistrate feeder M. balthica, the deposit
and suspension feeder Arenicola marina, and the suspension feeder Cerastoderma edule. Generally, three main sources are considered as being the
nutritive basis of primary consumers in the Wadden Sea: Autochtonous
phytoplankton production, autochthonous microphytobenthos production,
and the import of particulate organic material from the North Sea (van den
Hoek et al. 1979).
In contrast to the Wadden Sea, the main portion of primary production on
the Banc d' Arguin tidal flats is not directly utilizable as a food source. With
the exception of a few sap-sucking isopod species, the benthic invertebrates
do not feed on living seagrass and therefore no invertebrate food webs are
based on this immensely rich supply. Nevertheless, the seagrass material has
to be considered as the main energy source of the Banc d' Arguin system
(Wolff and Smit 1990; Wolff et al. 1993b). The leaves, shed after dying, become
partly displaced but the major part is deposited and decomposed in situ
H. Michaelis and W.J. Wolff
resource of wintering waders (Altenburg et al. 1982; Wolff and Smit 1990;
Wolff 1991; Wolff et al.1993a,b). In the Wadden Sea, biomass has been used as
a general means to characterize the productivity of different coastal areas or
habitats (e.g. Michaelis 1987; Beukema 1989; Essink et al. 1998; Reise and
Lackschewitz 1998). As in the earlier surveys (Altenburg et al. 1982; Wolff and
Smit 1990; Wolff 1991; Wolff et al. 1993a), the present study again points to a
remarkably low biomass on the Banc d' Arguin tidal flats: A mean value of
7.6 g ADW m- 2 without Anadara senilis and of 9.3 g ADW m- 2 with the bivalve
included (Table 12.4). It may be concluded that seasonal differences are
largely lacking and that a low macrozoobenthic standing stock is a general
characteristic of this tropical tidal flat area. Its biomass is only a small fraction
of the values found in the Juist area: A mean total biomass of 32.7 g ADW m- 2
without the mussel beds and of 41.2 g ADW m- 2 with the mussel beds
included. Summer values in that range (30-40 g ADW m- 2 without Mytilus
beds) are also characteristic for other subareas of the Wadden Sea, as shown
by the examples given in Table 12.4.
The vast seagrass beds and the extraordinary clearness of the seawater of
the Banc d' Arguin point to a system in which the phytoplankton is insignificant and the primary production is almost exclusively achieved by seagrasses
and their periphyton (Wolff and Smit 1990). Sevrin-Reyssac (1993) calls the
poverty of plankton in the Banc d' Arguin waters extreme. This is in line with
the observation that benthic suspension feeders are quantitatively unimportant in the tidal flats (Wolff et al.1987, 1993a,b). Though the knowledge on
autecology is insufficient, we conclude that the majority of the species belong
to the deposit and epistrate feeders. In the Juist area, the largest habitat, i. e.
dark sandy flats, is dominated by the suspension feeders Cerastoderma edule
and Mya arenaria followed by the epistrate feeder Macoma balthica (see
Fig. 12.8). In the habitat "light sandy flats" (Fig. 12.7), the first places in biomass contribution are occupied by the epistrate feeder M. balthica, the deposit
and suspension feeder Arenicola marina, and the suspension feeder Cerastoderma edule. Generally, three main sources are considered as being the
nutritive basis of primary consumers in the Wadden Sea: Autochtonous
phytoplankton production, autochthonous microphytobenthos production,
and the import of particulate organic material from the North Sea (van den
Hoek et al. 1979).
In contrast to the Wadden Sea, the main portion of primary production on
the Banc d' Arguin tidal flats is not directly utilizable as a food source. With
the exception of a few sap-sucking isopod species, the benthic invertebrates
do not feed on living seagrass and therefore no invertebrate food webs are
based on this immensely rich supply. Nevertheless, the seagrass material has
to be considered as the main energy source of the Banc d' Arguin system
(Wolff and Smit 1990; Wolff et al. 1993b). The leaves, shed after dying, become
partly displaced but the major part is deposited and decomposed in situ