This study was conducted as part of two JGOFS transects along 140 degrees W between 12 degrees N and 12 degrees S during February-March 1992 and August-September 1992. Although its purpose was to investigate seasonal variability in nitrogenous nutrient availability and biological utilization in support of primary production, the occurrence of the 1992 El Nino during the first transect permitted us to compare El Nino and post-El Nino conditions. We had hypothesized that an El Nino-related reduction in upwelling of cold nutrient-rich water would lead to a reduction in surface nutrient concentrations and rates of new and primary production in the vicinity of the equator. However, during the height of the El Nino, NO3- concentrations from 2 degrees N to 7 degrees S remained high enough (> 2 mu mol kg(-1)) to preclude nitrogen-limited primary production. Total nitrogen uptake rates measured 6 months after the El Nino were 2.4 times greater than those observed during the El Nino. On both transects, mean values for NH4+ uptake rates were 8 times those for NO3- - uptake. Mean rates of new production integrated to the 1% light depth over the full transects were 4.3 mmol C m(-2) day(-1) during the El Nino, and 9.9 mmol C m(-2) day(-1) 6 months later. Within the 2 degrees N-2 degrees S region, rates of new production were 4.8 and 18.5 mmol C m(-2) day(-1) for the first and second transects, respectively. Ratios of carbon fixed in primary production and nitrogen uptake averaged 7.7 and 5.1 (mole ratio) for the transects during and after the El Nino, respectively. Even though both the rates of primary production and NO3- concentrations were higher after the El Nino, there was a strong suppressing effect of NH4+ concentration on NO3- uptake. On both transects local minima in f-ratios (0.06) were evident within 1 degrees of the equator. The mean f-ratio for 2 degrees N-2 degrees S was slightly lower and less variable (0.06-0.13:(x) over bar=0.11) during the El Nino than after (0.08-0.20; x=0.13). Over a broader meridional band (5-7 degrees N to 5-8 degrees S) f-ratios during the El Nino were similar to values determined in 1988, a non-El Nino year, during the same season. Diel periodicity was evident in NO3- uptake between 2 degrees N and 3 degrees S, reaching 10- and five-fold day vs night enhancement during and after the El Nino, respectively. Following the El Nino, the diel cycle in NO3- uptake was strongly skewed to the early portion of the light day in the most NO3--rich waters. These and other comparisons between the two transects serve to indicate that phytoplankton species assemblages and/or nutritional sufficiency of micro-nutrients were different during and after the El Nino. On both transects plankton nutritional preferences resulted in nitrate-sparing conditions in the vicinity of the equator. In spite of high primary productivity, f-ratio calculations and turnover times for NH4+ suggest that local rates of remineralization were sufficient to meet 87-90% of the nitrogen demand in the 2 degrees N-2 degrees S region, resulting in residence times for NO3- of 305 days during the El Nino and 190 days 6 months later. A potential implication of this condition is a correspondingly low export of the particulate product of photosynthesis to the deep ocean. Water column density structure and nutrient distributions argue for reduced rates of nutrient upwelling during the El Nino event. Altered upper-ocean physics and concomitant changes in plankton community structure and function allowed for more extensive upper-ocean nutrient recycling, and presumably reduced export, of primary production during the El Nino. As a consequence the depletion time of recently upwelled NO3- remained long, and thus this nutrient was conserved during the period of diminished supply from upwelling. While these patterns imply direct regulation of new production by the availability of NH4+, the rule of a micro-nutrient such as Fe that influences (1) the species composition of the phytoplankton assemblage, and associated potential for export from rather than recycling within the euphotic zone or (2) the sensitivity of NO3- uptake to NH4+ presence, cannot at this time be properly evaluated. Significantly higher rates of new production with only a small increase inf-ratio in the period following the El Nino may constitute a more prominent feature in the ENSO cycle of equatorial biological production and export than the El Nino event per se. Whether this is a general feature in the ENSO cycle, or unique to the period of our study, which was one of unusual global atmospheric conditions, has yet to be established. Copyright (C) 1996 Elsevier Science Ltd.
