Bastiaan Knoppers . Bjorn Kjerfve
is not important. Signatures of ()13C have shown uniform values of organic matter in
surface sediments between the central and marginal parts of the lagoons, resembling
those of phytoplankton. The macrophyte fringe seems to serve as a physical and biological filter for biogenic matter transferred from the drainage basin to the lagoon.
However, some export of material from the vegetation belt may occur during sporadic
inundation and wash-out events during the passage of meteorological fronts. Because
of the small areal extent of the marginal wetlands, their impact on primary production of the lagoons is minor, with exception for the Cananeia-Iguape system.
3.12
Pelagic and Benthic Metabolism
In most phytoplankton-based estuaries and coastal lagoons, the relation between sediment oxygen consumption and pelagic primary production varies between 25 and 50%
(Nixon 1982), and heterotrophic metabolism seems to govern these systems on an
annual basis (Smith and Atkinson 1994). Measurements of production and respiration of the pelagic and benthic compartments exist for Barra 1., Guarapina 1., Urussanga 1., Flora 1., and Araruama 1. (Machado and Knoppers 1988; Carmouze et al. 1991;
Carmouze et al. 1993; Knoppers et al. 1996).
The four phytoplankton-based lagoon systems of the state of Rio de Janeiro are
characterized by:
1. a negligible benthic primary production as compared to the pelagic primary production;
2. benthic respiration measuring 25-40% of whole system respiration during winter
to 30-55% during summer, indicating that the pelagic compartment oxidizes a
slightly higher fraction of organic matter compared to the benthic compartment
on an annual basis;
3. marked seasonal shifts between autotrophy and heterotrophy, with autotrophy
dominating during the summer and heterotrophy during the winter;
4. net autotrophy and net heterotrophy being equal on an annual basis.
In contrast, Cananeia-Iguape 1. is more controlled by marine sources and is probably also influenced by the metabolism and export of materials from adjacent mangrove forest (Adaime 1985). The hypersaline and carbonate-rich Araruama 1. is also
different, being oligotrophic, and with pelagic primary production measuring, at most,
10% of the benthic primary production. The total community metabolism was clearly
heterotrophic (Knoppers et al. 1996).
3.13
Nutrient Sources and Primary Production
Nutrient sources to sustain pelagic primary production in coastal lagoons include
input from the atmosphere, rivers, groundwater, and the sea, as well as internal recycling of nutrients between the pelagic and benthic compartments. In addition, nitrogen fixation is a source of nitrogen, and nitrogen is lost due to denitrification, export
to the sea, and accumulation in sediments (Nixon 1982; Seitzinger 1988; Knoppers 1994).
is not important. Signatures of ()13C have shown uniform values of organic matter in
surface sediments between the central and marginal parts of the lagoons, resembling
those of phytoplankton. The macrophyte fringe seems to serve as a physical and biological filter for biogenic matter transferred from the drainage basin to the lagoon.
However, some export of material from the vegetation belt may occur during sporadic
inundation and wash-out events during the passage of meteorological fronts. Because
of the small areal extent of the marginal wetlands, their impact on primary production of the lagoons is minor, with exception for the Cananeia-Iguape system.
3.12
Pelagic and Benthic Metabolism
In most phytoplankton-based estuaries and coastal lagoons, the relation between sediment oxygen consumption and pelagic primary production varies between 25 and 50%
(Nixon 1982), and heterotrophic metabolism seems to govern these systems on an
annual basis (Smith and Atkinson 1994). Measurements of production and respiration of the pelagic and benthic compartments exist for Barra 1., Guarapina 1., Urussanga 1., Flora 1., and Araruama 1. (Machado and Knoppers 1988; Carmouze et al. 1991;
Carmouze et al. 1993; Knoppers et al. 1996).
The four phytoplankton-based lagoon systems of the state of Rio de Janeiro are
characterized by:
1. a negligible benthic primary production as compared to the pelagic primary production;
2. benthic respiration measuring 25-40% of whole system respiration during winter
to 30-55% during summer, indicating that the pelagic compartment oxidizes a
slightly higher fraction of organic matter compared to the benthic compartment
on an annual basis;
3. marked seasonal shifts between autotrophy and heterotrophy, with autotrophy
dominating during the summer and heterotrophy during the winter;
4. net autotrophy and net heterotrophy being equal on an annual basis.
In contrast, Cananeia-Iguape 1. is more controlled by marine sources and is probably also influenced by the metabolism and export of materials from adjacent mangrove forest (Adaime 1985). The hypersaline and carbonate-rich Araruama 1. is also
different, being oligotrophic, and with pelagic primary production measuring, at most,
10% of the benthic primary production. The total community metabolism was clearly
heterotrophic (Knoppers et al. 1996).
3.13
Nutrient Sources and Primary Production
Nutrient sources to sustain pelagic primary production in coastal lagoons include
input from the atmosphere, rivers, groundwater, and the sea, as well as internal recycling of nutrients between the pelagic and benthic compartments. In addition, nitrogen fixation is a source of nitrogen, and nitrogen is lost due to denitrification, export
to the sea, and accumulation in sediments (Nixon 1982; Seitzinger 1988; Knoppers 1994).