The Itamaraca Estuarine Ecosystem, Brazil
75
Total suspended solids (TSS) tend to be lower during the rainy (20ppm)
than in the dry season ( 40 ppm). The exchange of both TSS and particulate
organic carbon (POC) between channel and reef shelf water is principally
controlled by tidal dynamics. During the dry season, a large part of the TSS
remains in the channel and tidal trapping promotes its homogeneous distribution. Reduced dynamics in the channel and "inner sea" favor a net
import of TTS ( 35.12 tons per tidal cycle, POC 2.34 tons per tidal cycle) and
the accumulation of particulate matter within the system. During the rainy
season, despite the large riverine input of suspended matter, the retention
by mangrove swamps and the water exchange between nearshore and shelf
areas appears to remove TSS from the system. The high river discharge
flushes the system and net fluxes cause a net export of TSS (260 tons
per tidal cycle) and POC (6.9 tons per tidal cycle) through the Orange entrance (Broce 1994).
Independent of the season, net longitudinal salt flux and transport of
TSS in the Santa Cruz Channel are largely dominated by advective transport, as is the salt flux towards the ocean. The tidal wave transport is a
secondary mechanism of salt and TSS transport and counteracts advective
transport. Both advection and tidal wave transport contribute to the removal of estuarine TSS. Since cross-sectional shear is much smaller than vertical shear, cross-sectional variability of salinity, TSS, and current distribution contributes little to net longitudinal salt and TSS fluxes in the
Itamaraca system (Medeiros 1991).
Water temperatures in the Santa Cruz channel lack vertical and horizontal gradients but vary between 30.1 °C (28.8-30.9 °C) in the dry season
and 26.8 oc (25.7-27.7 °C) in the rainy season. The Secchi disk depth in the
channel (mean 1.9 m) and at the confluence of tributaries (0.3-0.7 m)
decreases during the rainy season. The pH varies little in both channel
entrances (Orange 7.3, Catuama 8.2; Flores Montes 1996). Dissolved oxygen is near or above saturation levels (95-130 o/o), though lower values
(2.5-4.5 ml 1- 1 ) occur in the central area of the channel during the rainy
season. River discharge is the principal source of nutrients in the Santa
Cruz Channel, followed by sediment resuspension, mangrove litter,
waste input, terrestrial runoff, and atmospheric input. Nitrate
(0.02-6.73 Jlmoll- 1 ) and nitrite (0.001-0.730 Jlmoll- 1 ) concentration tends
to be higher at the mouth of tributaries at low tide during the rainy season
(Flores Montes 1996). Phosphorus levels in the channel are directly related
to river discharge and surface runoff, with average concentrations between
0.71 and 2.4 Jlmoll- 1 during the dry and 0.4 to 0.6 Jlmoll- 1 during the rainy
season (Macedo et al. 1973).
75
Total suspended solids (TSS) tend to be lower during the rainy (20ppm)
than in the dry season ( 40 ppm). The exchange of both TSS and particulate
organic carbon (POC) between channel and reef shelf water is principally
controlled by tidal dynamics. During the dry season, a large part of the TSS
remains in the channel and tidal trapping promotes its homogeneous distribution. Reduced dynamics in the channel and "inner sea" favor a net
import of TTS ( 35.12 tons per tidal cycle, POC 2.34 tons per tidal cycle) and
the accumulation of particulate matter within the system. During the rainy
season, despite the large riverine input of suspended matter, the retention
by mangrove swamps and the water exchange between nearshore and shelf
areas appears to remove TSS from the system. The high river discharge
flushes the system and net fluxes cause a net export of TSS (260 tons
per tidal cycle) and POC (6.9 tons per tidal cycle) through the Orange entrance (Broce 1994).
Independent of the season, net longitudinal salt flux and transport of
TSS in the Santa Cruz Channel are largely dominated by advective transport, as is the salt flux towards the ocean. The tidal wave transport is a
secondary mechanism of salt and TSS transport and counteracts advective
transport. Both advection and tidal wave transport contribute to the removal of estuarine TSS. Since cross-sectional shear is much smaller than vertical shear, cross-sectional variability of salinity, TSS, and current distribution contributes little to net longitudinal salt and TSS fluxes in the
Itamaraca system (Medeiros 1991).
Water temperatures in the Santa Cruz channel lack vertical and horizontal gradients but vary between 30.1 °C (28.8-30.9 °C) in the dry season
and 26.8 oc (25.7-27.7 °C) in the rainy season. The Secchi disk depth in the
channel (mean 1.9 m) and at the confluence of tributaries (0.3-0.7 m)
decreases during the rainy season. The pH varies little in both channel
entrances (Orange 7.3, Catuama 8.2; Flores Montes 1996). Dissolved oxygen is near or above saturation levels (95-130 o/o), though lower values
(2.5-4.5 ml 1- 1 ) occur in the central area of the channel during the rainy
season. River discharge is the principal source of nutrients in the Santa
Cruz Channel, followed by sediment resuspension, mangrove litter,
waste input, terrestrial runoff, and atmospheric input. Nitrate
(0.02-6.73 Jlmoll- 1 ) and nitrite (0.001-0.730 Jlmoll- 1 ) concentration tends
to be higher at the mouth of tributaries at low tide during the rainy season
(Flores Montes 1996). Phosphorus levels in the channel are directly related
to river discharge and surface runoff, with average concentrations between
0.71 and 2.4 Jlmoll- 1 during the dry and 0.4 to 0.6 Jlmoll- 1 during the rainy
season (Macedo et al. 1973).