PHOSPHORUS-LIMITED BACTERIOPLANKTON GROWTH IN THE SARGASSO SEA

Oceanic central gyres cover large areas of the earth and contribute si gnificantly to global productivity. Oceanic phytoplankton production i s believed to be limited by nitrogen (N) in central gyres andiron (Fe) in high-nutrient low-chlorophyll regions. Bacterioplankton have been less studied but are believed to be Limited by organic carbon. We repo rt here that bacterioplankton in the Sargasso Sea were phosphorus (P) limited on cruises in 1992 and 1993. This assertion is supported by me asurements of high dissolved and particulate N:P and C:P ratios, high alkaline phosphatase activity and phosphate uptake rates, and bacterio plankton growth rate responses in bioassays where inorganic P was adde d. 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 4 6:1 on 2 cruises and time-series data indicated that ratios were alway s greater than 24:1 over nearly a 2 yr span. Phosphate concentrations were extremely low in the euphotic zone (<10 nM) and biomass-normalize d alkaline phosphatase activities indicated moderate to severe P Limit ation, with most severe Limitation occurring in the spring. Bioassays indicated that heterotrophic bacteria may be P limited in the northwes tern Sargasso Sea, especially in the spring. Limitation by P and not d issolved organic carbon may explain why dissolved organic carbon accum ulates in the water column at that time.