Oligotrophic water conditions associated with organic matter regeneration support life and indicate pollution on the western side of Fernando de Noronha Island - NE, Brazil (3°S)

Abstract Oligotrophy has its basis in low inorganic nutrient availability and it is generally known that in equatorial waters inorganic nutrient availability is insufficient to sustain the richness of life observed in some special places at this latitude. Thus, the understanding of the organic dissolved component role in N and P biogeochemical cycles could explain the support of the food chain in oligotrophic equatorial waters. This study was undertaken for the purpose of contributing to the knowledge of nutrient availability and the participation of dissolved organic components in the trophic conditions of an oceanic equatorial island in the South Atlantic. Water and phytoplankton were sampled on the western fringe of the Fernando de Noronha Archipelago in March 2015. Tropical oceanic conditions prevailed in this oceanic domain with high salinity (>36.19) and temperatures (>28.30°C). The oceanic conditions assured a high pH and dissolved oxygen levels (>5.00 mL L-1). Alkalinity showed slight variations, its lowest values being associated with biological interaction. Suspended particulate matter was relatively low and its organic fraction was associated with the productive process and/or pollution contribution, which is confirmed mainly by high DON concentrations (~4.00 µmol L-1) with an association with high total chlorophyll in the primary production process and with urea in the pollution situation. The urea content attained 2.77 µmol L-1 at a waste disposal outlet when a contamination event was observed in the region. The available inorganic nutrients showed low concentrations with an N-ammonium average of 1.99±0.64 µmol L-1, nitrate with an average of 0.13±0.04 µmol L-1, and nitrite with an average of 0.19±0.13 µmol L-1. The DIN varied from 0.74 to 4.32 µmol L-1, principally represented by N-ammonium. DOC showed values of from 53 to 109 µmol L-1 and the DOC:DON ratio confirmed the organic source as being an inorganic N pool of oxidized organic matter and indicated that nitrogen-rich compounds are recycled faster than carbon-rich compounds. Total chlorophyll concentrations reached 0.81 mg m-3 and the noteworthy presence of filamentous cyanobacteria despite low inorganic nitrogen availability lead to a trophic chain initially supported with the contribution of a fixation process.