Limitation of phytoplankton production by Si and N in the western Atlantic Ocean

Large areas of the western tropical Atlantic Ocean are influenced by the Amazon River plume, in which dissolved inorganic nitrogen (DIN) concentrations are low and nutrient N:P and N:Si ratios are lower than elemental ratios typical of phytoplankton. Accordingly, nutrient concentrations and ratios measured during winter (January to February 2001) and summer (July to August 2001) cruises in the area 2 to 14° N and 57 to 40° W suggest that DIN may limit phytoplankton growth at the majority of the 40 stations sampled. As diatoms are the dominant plankton in the Amazon plume- waters, short-term nutrient enrichment experiments were conducted to determine whether ambient N, P, or Si limited silica production rates (SiP). Results indicated that SiP were substrate limited both above and below the pycnocline at the majority of stations during both high (summer cruise) and low (winter cruise) discharge conditons. DIN concentrations also limited SiP in plumewater stations, but N additions depressed SiP at other stations, possibly using energy to take up the limiting nutrient. Finally, primary production rates were limited by ambient dissolved silicon concentrations at 9 of 13 summer stations. These results indicate the importance of dissolved silicon availability in controlling phytoplankton growth rates in a diatom-dominated river plume system.