250
M. Sprung et al.
ground water or nitrogen fixation from air (White and Howes 1994).As a logic
consequence, the diet of suspension-feeding organisms shows a strong gradient in the lagoon from mainly vascular plant detritus in the inner part to
mainly phytoplankton in the outer part (Machas and Santos 1999). Mussel
beds take advantage of this planktonic food source growing between the
coastal system and the sea. They are missing in the Ria Formosa system,
probably because this food source is much less developed than in the SyltR0m0Bay.
Particulate organic matter (POC) import from the sea is probably one other
important source for the mussel beds in the Sylt -R0m0 area. Published data
for the Ria Formosa on POC exchange with the sea are lacking. As pointed out,
it seems more plausible that POC should be in the first range exported from
the marshes and seagrass beds to the rest of the lagoon and to the sea in the
form of vascular plant detritus. This subject is controversial and has been discussed as an "outwelling" hypothesis in the literature (e. g. Dame et al. 1986).
Dame (1982) calculated that detritus transport via Spartina leaves should
correspond only in the range of 1 % of the net primary production at North
Inlet (Atlantic coast of the US). However, leaf shedding is not pronounced in
Spartina alterniflora on the West Atlantic coast, whereas each shoot of the
East Atlantic Spartina maritima releases, according to own unpublished observations, between 2 to 4 leaves per month to the system. This may favour
outwelling events, particularly from the Spartina maritima belt in the lower
marsh (Heip et al. 1995).
In both systems, predators can exert a strong top-down effect on secondary
production. This includes bird predation, predation by juvenile fish and by
invertebrate predators. Bird predation seems at first sight more conspicuous
in the Sylt-R0m0 area, which serves as a resting area for many migrant bird
species (Meltofte et al. 1994; Nehls and Scheiffarth 1998). It has been calculated that, on average, 9 % (or 5.1 g C m- 2 a-I) of the secondary production is
removed per year from the tidal flats (Scheiffarth and Nehls 1997; Asmus et al.
1998).
The Ria Formosa is not frequented as a resting place by many migrant bird
species. However, considerable numbers of waders can overwinter in this
lagoon (Batty 1991). For the Tejo estuary situated some 250 km north of the
Ria Formosa, Moreira (1997) calculated that about 12 % of the macrobenthic
invertebrate production is removed by birds, more than half of this value by
gulls. Predation by juvenile fish is important in both systems, which may be
considered nursery areas of many species. This has been particularly
investigated, e.g., for Solea senegalensis in the Ria Formosa (J.P. Andrade
1990) and Pomatoschistus microps in the Sylt -R0m0 area (Herrmann et al.
1998).
Invertebrate predators may shape the intertidal macrobenthic community
to a considerable extent. In the Ria Formosa the small bivalve Abra ovata can
M. Sprung et al.
ground water or nitrogen fixation from air (White and Howes 1994).As a logic
consequence, the diet of suspension-feeding organisms shows a strong gradient in the lagoon from mainly vascular plant detritus in the inner part to
mainly phytoplankton in the outer part (Machas and Santos 1999). Mussel
beds take advantage of this planktonic food source growing between the
coastal system and the sea. They are missing in the Ria Formosa system,
probably because this food source is much less developed than in the SyltR0m0Bay.
Particulate organic matter (POC) import from the sea is probably one other
important source for the mussel beds in the Sylt -R0m0 area. Published data
for the Ria Formosa on POC exchange with the sea are lacking. As pointed out,
it seems more plausible that POC should be in the first range exported from
the marshes and seagrass beds to the rest of the lagoon and to the sea in the
form of vascular plant detritus. This subject is controversial and has been discussed as an "outwelling" hypothesis in the literature (e. g. Dame et al. 1986).
Dame (1982) calculated that detritus transport via Spartina leaves should
correspond only in the range of 1 % of the net primary production at North
Inlet (Atlantic coast of the US). However, leaf shedding is not pronounced in
Spartina alterniflora on the West Atlantic coast, whereas each shoot of the
East Atlantic Spartina maritima releases, according to own unpublished observations, between 2 to 4 leaves per month to the system. This may favour
outwelling events, particularly from the Spartina maritima belt in the lower
marsh (Heip et al. 1995).
In both systems, predators can exert a strong top-down effect on secondary
production. This includes bird predation, predation by juvenile fish and by
invertebrate predators. Bird predation seems at first sight more conspicuous
in the Sylt-R0m0 area, which serves as a resting area for many migrant bird
species (Meltofte et al. 1994; Nehls and Scheiffarth 1998). It has been calculated that, on average, 9 % (or 5.1 g C m- 2 a-I) of the secondary production is
removed per year from the tidal flats (Scheiffarth and Nehls 1997; Asmus et al.
1998).
The Ria Formosa is not frequented as a resting place by many migrant bird
species. However, considerable numbers of waders can overwinter in this
lagoon (Batty 1991). For the Tejo estuary situated some 250 km north of the
Ria Formosa, Moreira (1997) calculated that about 12 % of the macrobenthic
invertebrate production is removed by birds, more than half of this value by
gulls. Predation by juvenile fish is important in both systems, which may be
considered nursery areas of many species. This has been particularly
investigated, e.g., for Solea senegalensis in the Ria Formosa (J.P. Andrade
1990) and Pomatoschistus microps in the Sylt -R0m0 area (Herrmann et al.
1998).
Invertebrate predators may shape the intertidal macrobenthic community
to a considerable extent. In the Ria Formosa the small bivalve Abra ovata can