268
H. Michaelis and w.J. Wolff
12.4 Discussion
Our study of the tropical Banc d' Arguin and the temperate Juist area reveals
similarities as well as considerable differences when habitat division, macrobenthic species richness, standing stock and trophic structure are compared.
Whether these criteria are subject to latitudinal gradients (Reise 1991; Wolff
1991) remains a question to be answered by a large-scale or even global review.
As regards to the habitats present in both areas a remarkably close correspondence exists between the exposed beaches, because the dominant species,
viz. the highly stenotope polychaete Scolelepis squamata, is identical. Between
the sandy flats of the Banc d' Arguin and the light and dark sandy flats of the
Juist area the similarity is confined to the sediment characteristics while
macrozoobenthic standing stock and trophic structure show large differences
(see below). The seagrass habitat of the Banc d' Arguin is not represented in the
Juist area today, although, until the 1970s, it was present to a modest degree,
even with the same dominant species Zostera noltii (Kastler and Michaelis
1999). The mussel beds, shell deposits, and Lanice beds of the Juist area do not
have corresponding habitats on the Banc d' Arguin. The mangrove may be considered as a parallel of the northern pioneer zone in a certain sense as, in both
habitats, the ground is covered by a pioneer vegetation. Microbial mats very
similar to those of the Juist area were also encountered in the high intertidal
zone of the Banc d' Arguin but they remained uninvestigated. Finally, the
supralittoral "high sands" of the Juist area correspond to the tropical sebkha in
certain respects. In the sebkha, an extremely poor benthic fauna was found
(Table l2.2). The high sands, on the other hand, are inhabited by a surprisingly
rich benthic fauna (Table 12.3). It is assumed that the wet and cool northern
climate enables some aquatic animals to survive under these conditions.
A conspicuous difference concerns the spatial distribution of habitats in
both areas. In the Juist area - as generally in the Wadden Sea - there exists an
arrangement which is evidently governed by the exposure to winds, currents
and wave action: The highly exposed, clean sandy habitats face to the west and
north and are gradually followed by more sheltered habitats towards the east
and the south.
In contrast, in the tidal flat area of the Banc d' Arguin, the distribution pattern of muddy sediments is correlated with the presence of the seagrass beds.
They occur over the whole area, even immediately bordering the Atlantic
Ocean (Fig. l2.2). Continuous offshore trade winds are certainly part of the
explanation.
The differences in species richness of the two areas are evident: 132 species
from only 30 stations at the Banc d'Arguin (on average 16 species per station),
78 species from more than 100 stations in the Juist area (on average 10 species
per station). The inclusion of preceding surveys (Einsele et al. 1974; Altenburg
H. Michaelis and w.J. Wolff
12.4 Discussion
Our study of the tropical Banc d' Arguin and the temperate Juist area reveals
similarities as well as considerable differences when habitat division, macrobenthic species richness, standing stock and trophic structure are compared.
Whether these criteria are subject to latitudinal gradients (Reise 1991; Wolff
1991) remains a question to be answered by a large-scale or even global review.
As regards to the habitats present in both areas a remarkably close correspondence exists between the exposed beaches, because the dominant species,
viz. the highly stenotope polychaete Scolelepis squamata, is identical. Between
the sandy flats of the Banc d' Arguin and the light and dark sandy flats of the
Juist area the similarity is confined to the sediment characteristics while
macrozoobenthic standing stock and trophic structure show large differences
(see below). The seagrass habitat of the Banc d' Arguin is not represented in the
Juist area today, although, until the 1970s, it was present to a modest degree,
even with the same dominant species Zostera noltii (Kastler and Michaelis
1999). The mussel beds, shell deposits, and Lanice beds of the Juist area do not
have corresponding habitats on the Banc d' Arguin. The mangrove may be considered as a parallel of the northern pioneer zone in a certain sense as, in both
habitats, the ground is covered by a pioneer vegetation. Microbial mats very
similar to those of the Juist area were also encountered in the high intertidal
zone of the Banc d' Arguin but they remained uninvestigated. Finally, the
supralittoral "high sands" of the Juist area correspond to the tropical sebkha in
certain respects. In the sebkha, an extremely poor benthic fauna was found
(Table l2.2). The high sands, on the other hand, are inhabited by a surprisingly
rich benthic fauna (Table 12.3). It is assumed that the wet and cool northern
climate enables some aquatic animals to survive under these conditions.
A conspicuous difference concerns the spatial distribution of habitats in
both areas. In the Juist area - as generally in the Wadden Sea - there exists an
arrangement which is evidently governed by the exposure to winds, currents
and wave action: The highly exposed, clean sandy habitats face to the west and
north and are gradually followed by more sheltered habitats towards the east
and the south.
In contrast, in the tidal flat area of the Banc d' Arguin, the distribution pattern of muddy sediments is correlated with the presence of the seagrass beds.
They occur over the whole area, even immediately bordering the Atlantic
Ocean (Fig. l2.2). Continuous offshore trade winds are certainly part of the
explanation.
The differences in species richness of the two areas are evident: 132 species
from only 30 stations at the Banc d'Arguin (on average 16 species per station),
78 species from more than 100 stations in the Juist area (on average 10 species
per station). The inclusion of preceding surveys (Einsele et al. 1974; Altenburg