Nutrient fluctuations in the Quatipuru river: A macrotidal estuarine mangrove system in the Brazilian Amazonian basin

The temporal and spatial variability of dissolved inorganic nutrients (NO3−, NO2−, NH4+, PO43− and DSi), total nitrogen (TN), total phosphorus (TP), nutrient ratios, suspended particulate matter (SPM) and Chlorophyll-a (Chl-a) were evaluated for the macrotidal estuarine mangrove system of the Quatipuru river (QUATIES), east Amazon coast, North Brazil. Temporal variability was assessed by fortnightly sampling at a fixed station within the middle portion of the estuary, from November 2009 to November 2010. Spatial variability was investigated from two field surveys conducted in November 2009 (dry season) and May 2010 (rainy season), along the salinity gradient of the system. The average DIN (NO3− + NO2− + NH4+) concentration of 9 μM in the dry season was approximately threefold greater in comparison to the rainy season. NH4+ was the main form of DIN in the dry season, while NO3− predominated in the rainy season. The NH4+ concentrations in the water column during the dry season are largely attributed to release by tidal wash-out of the anoxic interstitial waters of the surficial mangrove sediments. On the other hand, the higher NO3− levels during the wet season, suggested that both freshwater inputs and nitrification processes in the water column acted in concert. The river PO43− concentrations (DIP < 1 μM) were low and similar throughout the year. DIN was thus responsible for the major temporal and spatial variability of the dissolved DIN:DIP (N:P) molar ratios and nitrogen corresponded, in general, to the prime limiting nutrient for the sustenance of phytoplankton biomass in the estuary. During the dry season, P-limitation was detected in the upper estuary. PO43− adsorption to SPM was detected during the rainy season and desorption during the dry season along the salinity gradient. In general, the average Chl-a level (14.8 μg L−1) was 2.5 times higher in the rainy season than in the dry season (5.9 μg L−1). On average levels reached maxima at about 14 km from the estuaries' mouth, but shifts of the maximum Chl-a zone were subject to a dynamic displacement influenced by the tidal regime and seasonality of freshwater input. Our results showed that the potential phytoplankton productivity in QUATIES was subject to temporal and spatial variability between N and P limitation. The mangrove forests also played a relevant role as a nutrient source as established by the high variability of the nutrient behaviour along the estuarine gradient, consequently affecting the productivity in QUATIES.