Weber Friederichs Landim de Souza

Changes in nutrient loads (N, P and Si) in the São Francisco estuary after the construction of dams

The aim of this study was to investigate the post-d am conditions of the loads and yields of dissolved inorganic nitrogen (DIN), orthophosphate (DIP, silicate (DSi) and total phosphorous (TP) in the Lower Sao Franci sco riverestuary (NE-Brazil) after the river was regulated t o a constant flow by the dams. Loads and yields of monthly measurements performed from November 2000 to March 2002 at a gauging station downstream of the dams (8 0 km from the coast) showed 4.1 x 10 3 t/yr and 0.006 t/km 2 /yr of DIN, 0.2 x 10 3 t/yr and 0.002 t/km 2 /yr of DIP, and 448 x 10 3 t/yr and 0.71 t/km 2 /yr of DSi, respectively. Over the last 15 years, D IN loads reduced by 94 % and DSi by 31%. The river turned into an oligotrophic system with p rimary production limited by nitrogen and nutrient yields being among the lowest of Brazilian coastal rivers.

Suspended Matter and Nutrient Gradients of a Small-Scale River Plume in Sepetiba Bay, SE-Brazil

Coastal river plumes represent one of the final sta ges of material transport across the land-sea inter face. Most studies, however have focused on the behavior of me dium to large sized river plumes of coastal-shelf w aters, whereas small sized river plumes acting within estu aries have been neglected. This study addressed the behavior of suspended particulate matter (SPM), dissolved inorg anic nutrients (DIN, DIP and DSi) and Chlorophyll a (Chl. a) of a small sized river plume derived from the close ly lain Sao Francisco and Guandu river channels, se t in the Sepetiba Bay estuary, SE-Brazil. Two surface water sampling campaigns were conducted, one in January 2 (humid summer conditions) and the other in June 200 3 (dry winter conditions). On both occasions, the p lumes dispersed in a SE direction towards the inner porti on of the bay. The “wet” event plume was more turbi d, nutrient rich and dispersed beyond nearshore waters, whereas the “dry” event plume proliferated as a narrow, le ss turbid and more nutrient poor film alongshore. Both exhibi ted a marked degree of patchiness, induced by the d ifferential input of materials from the river sources and resus pension processes from the shallow nearshore bottom . The Sao Francisco river channel was the main source of fres hwater, SPM and nutrients, except for ammonia (NH 4 + -N) derived from domestic effluents of the Guandu river . The mesohaline portion of the estuarine mixing zo ne of the plumes behaved as a slight source for SPM, DSi and DIP, due to bottom resuspension processes. N:P mo lar ratios ranged between 80:1 and 20:1 along the estuarine gr adient, being higher in the summer than in the win ter event, indicating that DIP was the potential nutrient limi ting primary production. Chl. a concentrations incr eased at the outer premises of the plume, suggesting that the sh ort residence times and turbidity of the plume wate rs, hampered primary production nearshore, particularly during t he summer occasion. The small sized plume lacked th e spatial decoupling between the estuarine mixing and turbidi ty zones, generally observed in larger sized coasta l-shelf plumes.

In situ measurements of benthic primary production, respiration and nutrient fluxes in a hypersaline coastal lagoon of SE Brazil

Bentbic oxygen and nutrient ftuxes were measured in a section of the hypersaline carbonate-rich coastal lagoon of Araruama, SE-Brazil. In situ incubations of the sediment surface (Zm - 1.5) were performed at one station with light/dark chambers during september 1993 (early spring period) and april 1995 (earlyautumn period). The carbonate..rich aediments were covered by 1-3 mm thick microalgal mats, dominated by the cyanobaeteria Phormidium sp, Oscillatoria sp, and Lyngbya sp. Benthic net primary production rates were 15.4 ± 0.7 mmolC/m2/d in early spring and 33.8 ± 8.8 mmolC/m2/d in early autumn, total community respiration rates attained 35.3 ± 7.2 and 65.7 ± 16.9 mmolC/m2/d, and pelagic primary production rates 1.7 ± 0.7 and 4.0 ± 1.4 mmolC/m2/d, respectively. Total community metabolism was thus heterauophic and mainly driven by benthic metabolism. The benthic release rates af ammonia were 0.65 ± 0.32 mmolC/m2/day in early spring and 0.58± 0.42mmoVm ldaym early autumn, butwere near to negligible for orthophosphate. Pelagic primaryproduction was limited byphosphorous, in part, by the preferential release of etmmonia over orthophosphate from the sediment-water interface. The benthic primary production and nutrient release rates were within the range of other eury-to hypersaline carbonate-rich environments characterized by non- consolidated algat rats.

A satellite view of riverine turbidity plumes on the NE-E Brazilian coastal zone

Turbidity plumes of Sao Francisco, Caravelas, Doce, and Paraiba do Sul river systems, located along the NE/E Brazilian coast, are analyzed for their dispersal patterns of Total Suspended Solids (TSS) concentration using Landsat images and a logarithmic algorithm proposed by Tassan (1987) to convert satellite reflectance values to TSS. The TSS results obtained were compared to in situ collected TSS data. The analysis of the satellite image data set revealed that each river system exhibits a distinct turbidity plume dispersal pattern. The behavior, dimension and degree of turbidity of the Sao Francisco River plume have been greatly altered by the construction of a cascade of hydroelectric dam reservoirs in its hydrological basin. The plume has lost its typical unimodal seasonal pattern of material dispersion and its turbidity has decreased due to the regulation of river flow by the dams and TSS retainance by the reservoirs. In contrast, the Doce and Paraiba do Sul river plumes are still subject to seasonal pulsations and show more turbid conditions than the SF plume, as dams are less numerous, set in the middle river sections and the natural river flow has been maintained. The Caravelas Coastal System river plume is restricted to near shore shallow waters dominated by resuspension processes. During austral spring and summer when NE-E winds prevail, all plumes generally disperse southward. Short-term northward reversals may occur in winter with the passage of atmospheric cold fronts. The Sao Francisco and Doce river plumes tend to disperse obliquely to the coast and transport materials further offshore, while the Caravelas and Paraiba do Sul plumes tend to disperse mainly parallel to the coast, enhancing TSS retention nearshore.

Influence of hydrological pathways on dissolved organic carbon fluxes in tropical streams

Abstract Water flow pathways and water balance are fundamental components for understanding the dynamics of C in the soil/water interface of small basins. The objective of this study was to describe the seasonal variations and estimate the annual balance of dissolved organic carbon (DOC) by comparing two tropical microbasins (preserved forest—PF and cacao plantation—CP). Twenty‐one weekly collections were conducted from September to December 2012 and from April to June 2013. The calculation of the partial balance considered precipitation (P) as inflow and the stream as outflow. The samples were filtered and analyzed using a TOC analyzer. Overall, the DOC was higher CP compared with FP. The behavior of both venues showed that rainy season caused an increase in concentrations in the overland flow (OF) and in the stream, and a decrease in the precipitation (P) and in the throughfall (T). In the CP, the outflow and the soil were chiefly responsible for the high DOC concentrations in the stream, when compared to the PF, which is the result of constant OM decomposition. Soil composition contributes to the control of DOC consumption in each type of soil. The balances were negative in both microbasins, although losses were higher in the AFS (agroforestry systems) when compared to the PF, especially during rainy seasons (−8.98 and −3.05 kg ha−1 year−1, CP and FP, respectively). Thus, the high annual loss of DOC in the CP of the microbasins during the rainy season indicates changes in ecosystem metabolism due to the vegetation cover and to the interactions with the soil.

Dams Impacts upon the fluxes of biogenic matter

Dams impacts are usually associated with the formation of artificial lakes, population displacements and loss of continental habitats. However, but not less significant, are the impacts upon the fluxes of water and materials to the coastal zone with severe quantitative and qualitative alterations which may cause saline water intrusion and coastal erosion, loss of estuarine and coastal zone fertility and the alterations of regional and global biogeochemical cycles. The present article, presents a brief review on the theme by incorporating Brazilian examples addressed by the work group from the Instituto Nacional de Ciência e Tecnologia de Transferência de Materiais Continente-Oceano (INCT-TMCOcean).