Nutrient gradients in the western North Atlantic Ocean: Relationship to microbial community structure and comparison to patterns in the Pacific Ocean

Studies of nitrogen and phosphorus dynamics in the oligotrophic surface wat ers of the western North Atlantic Ocean have been constrained because ambie nt concentrations are typically at or below the detection limits of standar d colorometric methods, except during periods of deep vertical mixing. Here we report the application of high-sensitivity analytical methods-determina tions of nitrate plus nitrite (N + N) by chemiluminescence and soluble reac tive phosphorus (SR-P) by the magnesium induced co-precipitation (MAGIC) pr otocol-to surface waters along a transect from the Sargasso Sea at 26 degre esN through the Gulf Stream at 37 degreesN, including sampling at the JGOFS Bermuda Atlantic Time-series Study (BATS) station. The results were compar ed with data from the BATS program, and the HOT station in the Pacific Ocea n, permitting cross-ecosystem comparisons. Microbial populations were analy zed along the transect, and an attempt was made to interpret their distribu tions in the context of the measured nutrient concentrations. Surface concentrations of N+N and SRP during the March 1998 transect separa ted into 3 distinct regions, with the boundaries corresponding roughly to t he locations of the BATS station (similar to 31 degreesN) and the Gulf Stre am (similar to 37 degreesN). Although N+N and SRP co-varied, the [N+N]:[SR- P] molar ratios increased systematically from similar to1 to 10 in the sout hern segment, remained relatively constant at similar to 40-50 between 31 d egreesN and 37 degreesN, then decreased again systematically to ratios < 10 north of the Gulf Stream. Dissolved organic N (DON) and P (DOP) dominated (greater than or equal to 90%) the total dissolved N (TDN) and P (TDP) pool s except in the northern portion of the transect. The [DON]: [DOP] molar ra tios were relatively invariant (similar to 30-60) across the entire transec t. Heterotrophic prokaryotes (operationally defined as "bacteria"), Prochloroc occus, Synechococcus, ultra-and nanophytoplaakton, cryptophytes, and coccol ithophores were enumerated by flow cytometry. The abundance of bacteria was well correlated with the concentration of SRP, and that of the ultra- and nanophytoplankton was well correlated with the concentration of N+N. The on ly group whose concentration was correlated with temperature was Prochloroc occus, and its abundance was unrelated to the concentrations of nutrients m easured at the surface. We combined our transect results with time-series measurements from the BAT S site and data from select depth profiles, and contrasted these North Atla ntic data sets with time-series of N and P nutrient measurements from a sta tion in the North Pacific subtropical gyre near Hawaii [Hawaii Ocean Time-s eries (HOT) site]. Two prominent differences are readily observed from this comparison. The [N + N]: [SRP] molar ratios are much less than 16: 1 durin g stratified periods in surface waters at the BATS site, as is the case at the HOT site year round. However, following deep winter mixing, this ratio is much higher than 16, 1 at BATS. Also, SRP concentrations in the upper 10 0 m at BATS fall in the range 1-10 nM during stratified periods, which is a t least one order of magnitude lower than at the HOT site. That two ecosyst ems with comparable rates of primary and export production would differ so dramatically in their nutrient dynamics is intriguing, and highlights the n eed for detailed cross ecosystem comparisons. (C) 2001 Elsevier Science Ltd . All rights reserved.