Linking phytoplankton nitrogen uptake, macronutrients and chlorophyll-a in SW Atlantic waters: The case of the Gulf of SanJorge, Argentina

VARELA DIANA E; PISONI JUAN P.; ESTEVES JOSE L; VARELA DIANA E; WILLIAMS GABRIELA N.; PISONI JUAN P.; ESTEVES JOSE L; SOLIS MIRIAM; WILLIAMS GABRIELA N.; PAPARAZZO FLAVIO E.; SOLIS MIRIAM; PAPARAZZO FLAVIO E.

JOURNAL OF MARINE SYSTEMS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2017 p. 43 - 50

0924-7963

We compared biological and chemical parameters in surface waters of the Gulf of San Jorge to better understandcarbon export and the factors that control phytoplankton production in an area of the Argentinian ContinentalShelf, a vastly under sampled region of the SW Atlantic Ocean. In April of 2012, we estimated new and regeneratedprimary production in the Gulf by measuring nitrate and ammonium uptake, respectively. We also measuredmacronutrient, and in situ chlorophyll-a concentrations, which were compared to chlorophyll-aestimates from remote sensing. Although the Gulf of San Jorge presents high levels of chlorophyll-a and primaryproduction, the relationship between these parameters is not straightforward. Previous studies showed that surfacechlorophyll-a explains only part of the variance in euphotic-zone integrated primary production, and thatsatellite-derived chlorophyll-a underestimates in situ primary production. Our results showed large spatial variabilityin the Gulf, with transitional physico-chemical conditions, such as fronts, that could favor an increase inbiological production. In situ chlorophyll-a concentrations were highest at the mid-shelf station (6.0 mg m−3)and lowest at the northernmost location by an order of magnitude. Remote sensing measurements ofchlorophyll-a underestimated our in situ chlorophyll-a concentrations. Total nitrogen (nitrate + ammonium)uptake showed relatively similar rates throughout the study area (¡Ö130 nM-N d−1), except in the northernmoststation where it was much lower (53 nM-N d−1). This north region had a distinct water mass and maximal levelsof macronutrients (nitrate ¡Ö 6 ¦ÌM, ammonium ¡Ö 1.2 ¦ÌM, phosphate ¡Ö 1.2 ¦ÌM and silicic acid ¡Ö 4 ¦ÌM). For theentire sampling region, chlorophyll-a concentrations strongly correlated with total nitrogen uptake (r = 0.76,n = 8, p b 0.05) and new primary production (r = 0.78, n = 8, p b 0.05). Values of the f-ratio were 0.9 inmid-shelf, and ranged between 0.35 and 0.45 in inner and coastal stations. Our results indicate that highest carbonexportmay occur in the outer part of the Gulf, closer to the mid-shelf region. Further studies will be necessaryto better understand the functioning of this ecosystem, including the impact of fisheries and horizontal transportby currents in the overall CO2 balance.