Comportamento biogeoquímico de nutrientes no estuário do Rio Bacanga, Ilha de São Luís-MA

Abstract

The Bacanga River is a sub-estuary of the São Marcos Bay and is located in the metropolitan region of São Luís. It is a region that has multiple uses (fishing, drinking water collection, domestic sewage disposal, recreation and leisure, etc.). The cycle of essential nutrients has been studied in several estuarine systems, interactions with biota (primary production), with suspended and bottom sediments, influence of river runoff, enrichment of coastal waters, fertilization by domestic sewage, constitute the main lines of research in these environments. In the Bacanga estuary, the enrichment of nitrogen and phosphorus has contributed to the eutrophication process of the estuarine zone. On the other hand, the construction of a dam downstream (Figure 2.1), reduced the inflow of coastal waters during the ebb and flow of the tide and, on the other hand, an upstream dam reduced the flow of fresh water to the mixing zone. The main objective of this work was to characterize the biogeochemical behavior of essential nutrients (C, N, P and Si), through a spatio-temporal distribution, interaction with primary production and phytoplankton biomass (chlorophyll “a”) and its relationships with some physical, physical-chemical and chemical parameters. Six field campaigns were carried out during 1997, three in the rainy season (February, April and June) and three in the dry season (August, October and December), with sampling, in each campaign, in twelve collection stations in a profile horizontal and in an extension of 12 km between upstream and downstream. A vertical profile was also carried out at collection stations number 2, 8 and 11 in order to observe the existence of stratification in the estuarine zone. In the field, temperature, conductivity and transparency were measured and water samples were collected for the different physical-chemical and chemical parameters. In the laboratory, classical titrimetric methods, ultraviolet-visible and atomic absorption spectrophotometry were used. The results showed (downstream) the following values ​​for the chemical species: ammonium 4 to 100µM, nitrite 0.6 to 9.7µM, nitrate 1.3 to 7.3µM, urea 2.9µM, phosphate 2 to 14.5µM and silicate 19 to 89µM; and, upstream 0.3 to 3µM ammonium, 0.2 to 1.2µM nitrite, 0.8 to 2.8µM nitrite, 1 to 2µM urea, 0.15 to 1.96µM phosphate and 82 to 196µM silicate. These high values, in the downstream part of the estuarine system, show that the main source of these nutrients is domestic sewage, since the sewage discharge points are concentrated in this area of ​​the estuary. The values ​​found in the upstream part can be considered as natural (background) for the region. Dissolved oxygen ranged from 2 to 5.5ml/L, considering the longitudinal profile, from downstream to upstream, depending on the biogeochemical processes in which it participates. In the downstream part, the values ​​are low (1.9 to 3ml/L) due to weak turbulence and low primary production. On the contrary, in the downstream part, the greater turbulence and the influence of the coastal waters richer in oxygen, justifies those higher values ​​(above 4ml/L). Hydrogen sulfide gas was determined only in the months of April and October and ranged from 1 to 3.4mg/L, also in the longitudinal profile. Primary production showed maximum values ​​(26 to 138mgC/m3/h) in August and considering three depth levels (1, 50 and 100%) of the euphotic zone and chlorophyll “a” from 10 to 44mg/m3 in the longitudinal profile. . This high primary production in this zone of the estuarine system can be explained by several factors, such as low turbulence, the availability of nutrients and sunlight that month. The major ions (Na+, K+, Ca2+, Mg2+ and SO42-) showed the following values: sodium from 12 to 9400mg/L, potassium from 2.6 to 340mg/L, calcium from 10 to 360mg/L, magnesium from 15 to 1143mg/L and sulfate from 5.8 to 2375mg/L, considering the variation between the months of April and October, which correspond to the dry and rainy seasons. The salinity ranged from 0 to 32%, the temperature from 24.5 to 31.3°C, both in the longitudinal profile. This shows that the spring waters are cold and of zero salinity. The vertical profiles showed that there is a thermal, haline and chemical stratification (dissolved gases: oxygen and hydrogen sulphide) that can lead to bottom waters and a deficiency or absence of oxygen. This oxygen deficiency can be induced by thermohaline stratification and by the decomposition of organic matter of natural (mangrove and river runoff) and anthropogenic (domestic sewage) origin. Conservative behavior was evidenced for salinity, temperature, conductivity and major ions and non-conservative for essential nutrients (N, P and Si). Fertilization of Bacanga's estuarine waters occurs mainly through domestic sewage, allowing the environment to present high rates of primary production. However, excessive fertilization can lead to anoxia of the water column with possible death of fish and animals.