Ecological index analysis in the coastal and estuarine waters of the tropical Amazon Region

The Brazilian Amazon Coastal Zone holds the largest estuarine area in the world, located in the Amazon Continental Shelf (ACS). Additionally, it encompasses numerous estuaries and channels, such as Furo do Muriá (FM) and Furo da Laura (FL), which make up the local landscape and favor the primary productivity of water. The objective of this study was using the trophic index (TRIX) and O'Boyle index to determine the trophic state in the ACS area, FL, and FM, compare the results, and relate them to seasonality and river discharge in the sites. In the three estuaries, the applied methodology was the same: temperature, salinity, pH, dissolved oxygen, and saturation rate of dissolved oxygen data were obtained in situ, and the samples were filtered for the analysis of nitrite, nitrate, ammonia, phosphate, and chlorophyll a. The highest TRIX values were found in FL (3.89–3.12) and ACS (4.43–4.44), while for the O'Boyle index, the highest values were found in FL (16.73–100) and FM (43.3–100). Comparing the indices, it was observed that their classification was similar only in the ACS during the period of maximum discharge of the Amazon River (eutrophic), varying slightly in the minimum discharge period (potentially eutrophic for O'Boyle and eutrophicated for TRIX). This difference can be explained by the additional variables considered by the TRIX, such as nutrients and chlorophyll a. The scenario was completely different for FM and FL, where the TRIX showed greater variation owing to the presence of dissolved inorganic nutrients. Both indices can be used to simply and directly reflect the eutrophication conditions of estuarine areas: the joint and multiparametric approach of the indices provides complementary information regarding the dynamics of studied environments through their trophic status.