Nitrogen limitation prevents the effects of iron or dust additions on biological carbon fixation in the Gulf of California

This paper reports the effect of dissolved iron (dFe) and dust additions on the Biological Carbon Fixation (BCF) rates in the surface layer of the central Gulf of California (GC). Two on deck experiments were conducted in the summer of 2016 at an oceanographic station located in the Guaymas Basin. Each experiment consisted in the incubation of natural phytoplankton assemblages collected at three distinct depths within the upper 50 m layer and amended either with iron (FeCl3) or dust leachate, followed by the quantification of BCF rates using the C-13 assimilation method. A very warm (similar to 30 degrees C) water column strongly stratified and with reduced phytoplankton biomass (<1 mg chlorophyll a m(-3)) was indicative of an oligotrophic environment. In addition, the surface levels of NO3- +NO2- (<0.1 mu M), dFe (1.71 +/- 1.04 nM), as well as the Fe:N (78 +/- 47 nmol mu mol(-1)) and N:P (0.04 +/- 0.01) ratios revealed that phytoplankton was under conditions of inorganic nitrogen limitation. BCF rates measured during the experiments ranged between 0.46 +/- 0.04 and 6.24 +/- 0.34 mg C m(-3) h(-1) with the highest values at the maximum of fluorescence and their vertical distribution was associated with the carbon biomass of picophytoplankton and diatoms. Despite the oligotrophic condition that predominated during the cruise, most of the Fe or dust additions did not show a significant effect (p > 0.05) on the magnitude of BCF rates, which was attributed to the fact that N, rather than Fe, was the limiting nutrient for phytoplankton production. However, Fe limitation cannot be completely ruled out as two samples (one for each treatment) showed a significant increase (p < 0.05) in BCF rates. We suggest that atmospheric supply of dust during the summer could play a relevant biological role maintaining high concentrations of dFe (and other bioactive metals) in the surface waters of central GC.