Temporal variability in the nutrient chemistry of the Cariaco Basin

Nutrient data have been collected monthly at the CARIACO time series site in the Cariaco Basin since 1995, providing a unique picture of the cycling of NO3-, NO2-, NH4+, PO43- and SiO2 in this permanently anoxic system underlying a major coastal upwelling zone. Our data indicate that nutrients for phytoplankton growth are primarily supplied by upwelling of subsurface water on a seasonal basis. In addition, coastal runoff seems to supply important amounts of silica and ammonium to surface waters. We saw no indication of local nitrogen fixation in the Cariaco surface waters. In the suboxic zone, our data to date are not of sufficiently high enough resolution to resolve all important features. However, at least partial phosphate removal appears to occur in a zone above the first appearance of sulfide, and associated with intermittent intrusions of oxygenated water. In the suboxic zone, there appear to be thin layers where ammonium and nitrite coexist, potentially permitting anaerobic ammonium oxidation (anammox) to take place. In the deep waters, concentrations of ammonium, phosphate and silica continue to increase at a rate consistent with prior studies. However, in the upper part of the anoxic zone, there is evidence for sulfide removal, probably associated with oxygen intrusions.