Phosphorus-limited bacterioplankton growth in the Sargasso Sea

Oceanic central gyres cover large areas of the earth and contribute significantly to global productivity. Oceanic phytoplankton production is believed to be limited by nitrogen (N) in central gyres and iron (Fe) in high-nutrient low-chlorophyll regions. Bacterioplankton have been less studied but are believed to be limited by organic carbon. We report here that bacterioplankton in the Sargasso Sea were phosphorus (P) limited on cruises in 1992 and 1993. This assertion is supported by measurements of high dissolved and particulate N:P and C:P ratios, high alkaline phosphatase activity and phosphate uptake rates, and bacterioplankton growth rate responses in bioassays where inorganic P was added. Particulate C:P ratios were always higher than the Redfield ratio (106:1) and occasionally greater than 400:1. N:P ratios were 75:1 and 46:1 on 2 cruises and time-series data indicated that ratios were always greater than 24:1 over nearly a 2 yr span. Phosphate concentrations were extremely low in the euphotic zone (<10 nM) and biomass-normalized alkaline phosphatase activities indicated moderate to severe P limitation, with most severe limitation occurring in the spring. Bioassays indicated that heterotrophic bacteria may be P limited in the northwestern Sargasso Sea, especially in the spring. Limitation by P and not dissolved organic carbon may explain why dissolved organic carbon accumulates in the water column at that time.