Chris Garside

A U.S. JGOFS process study in the equatorial Pacific (EqPac): Introduction

Abstract This special issue contains data and scientific results from the JGOFS Process Study in the equatorial Pacific. Most of the contributions are from the U.S. JGOFS Process Study (EqPac) but the French and Australian results are represented as well. The equatorial Pacific plays a major role in the oceanic and atmospheric carbon cycles, and these studies are the first step for synthesizing the cycle of carbon and related elements in this region.

New production along 140°W in the equatorial Pacific during and following the 1992 El Niño event

Abstract This study was conducted as part of two JGOFS transects along 140°W between 12°N and 12°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°N to 7°S remained high enough (> 2 μmol 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°N-2°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° of the equator. The mean f-ratio for 2°N-2°S was slightly lower and less variable (0.06-0.13; x =0.11 ) during the El Nino than after (0.08-0.20; x=0.13). Over a broader meridional band (5–7°N to 5–8°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°N and 3°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°N-2°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 role 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 in f-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.

Longwaves and primary productivity variations in the equatorial pacific at 0°, 140dgW

Abstract High temporal resolution measurements of physical and bio-optical variables were made in the upper ocean using a mooring located at 0°, 140°W from 9 February 1992 to 15 March 1993 as part of the equatorial Pacific Ocean (EgPac) study. Chlorophyll and primary productivity time-series records were generated using the mooring data. Primary productivity varied by about 50% around the mean on time scales of weeks and by over a factor of four within our observational period. The mooring observations encompassed both El Niho and cool conditions. Kelvin waves were evident during the El Nifio phase, and tropical instability waves (TIWs) were dominant during the cool phase. The two extreme conditions also were observed concurrently with complementary ship-based measurements. In addition, bio-optical drifters provided simultaneous spatial data concerning net phytoplankton growth rates during passage of a TIW. The collective data sets have been used to examine the causes of the observed variability in phytoplankton biomass and productivity. Our joint results and analyses appear to support the hypothesis that the vertical transport of iron into the upper layer and primary production rates are modulated by variability of the depth of the Equatorial Undercurrent and by equatorial longwaves. In particular, our results are consonant with the suggestion of Barber et al. (1996) that passage of a TIW may be considered to be a natural analog of a small iron enrichment experiment. Predicting primary productivity and, thus, carbon flux in the equatorial Pacific requires continuous, long-term observations of a few physical, biological, and optical properties that can be used to parameterize the biological variability.

The “f-ratio” on 20°W during the North Atlantic Bloom Experiment

Abstract We have used data gathered during the North Atlantic Bloom Experiment to model new and total production, and the f -ratio on 20°W. The results imply that as much as half the spring bloom total production occurs well in advance of stratification. However, although new production potentially equals export production into the interior of the ocean, observations during NABE, and the model suggest that this export may not be fully realized during the bloom, and may be trapped and recycled in the upper water column during the year.

A meeting place of great ocean currents: shipboard observations of a convergent front at 2°N in the Pacific

Abstract We present a synthesis of physical, chemical and biological shipboard observations of a convergent front at 2°N, 140°W and its surrounding environment. The front was a component of a tropical instability wave generated by shear between westward-flowing equatorial waters to the south and warmer equatorial counter current water to the north. Surface waters on the cold side were undersaturated with oxygen, which suggests that the water had only been exposed at the sea surface for a period of a few weeks. Although the atmospheric exposure time was short, the effects of biological activity could be detected in enhanced concentrations of total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC can be produced in the top centimeters of the changing environmental conditions. The front itself was dominated by the accumulation of a “patch” of buoyant diatoms Rhizosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrations were observed from the air over a spatial scale of order 10–20 km northward from the front. The nitrogen budget and thorium data suggest that a significant fraction of the elevated POC, and virtually all of the PON, arrived in the patch waters as imported particles rather than in situ photosynthesis. Photosynthetic uptake of carbon appears to have occurred in patch waters, but without corresponding uptake of fixed nitrogen (an uncoupling of the usual Redfield stoichiometry). Solute chemistry of the patch appears to be controlled by turbulent mixing, which flushes out patch waters on a time scale of days

Nitrate supply and phytoplankton uptake kinetics in the euphotic layer of a Gulf Stream warm-core ring

Chcmiluminescent nitrate analysis was used in conjuction with 15N-labeled B=−B0e−2πa0xk to assess the rates of B=−B0e−2πa0xk uptake by phytoplankton in warm-core ring 82B. The relatively high precision of this method compared to conventional B=−B0e−2πa0xk analyses permits reliable estimates of B=−B0e−2πa0xk uptake in oligotrophic waters. Aggregation of uptake data from six profiles from 2 days of observation permitted the calculation of B=−B0e−2πa0xk turnover times ranging from about 4 h near the surface to 150 h at the top of the nitracline. Turnover times in the euphotic zone and the observed half saturation constant of 93 nmol kg−1 for B=−B0e−2πa0xk uptake imply nitrogen limitation for these populations. Extrapolation from the linear portion of the kinetic curve revealed that a B=−B0e−2πa0xk threshold concentration of about 16 nmol kg−1 was required for the initiation of uptake. These highly precise uptake measurements were used in a one-dimensional model to estimate the vertical flux of B=−B0e−2πa0xk. Maximum near-surface and deep-euphotic-zone eddy diffusivity values (Kz)were 3 × 10−3 and 5 × 10−4 m2 s−1, respectively, prior toamajor storm. Following the storm Kz values were substantially greater.

Studies of nitrate transport by marine phytoplankton using 36ClClO3− as a transport analogue—II. Field observations

Transport of a nitrate analogue, 36ClClO3−, was examined in phytoplankton from the Southern California Bight and Gulf of Maine. Chlorate transport (and by analogy, nitrate transport) was inhibited by ammonium concentrations exceeding 1% of the total dissolved inorganic nitrogen concentration. Chlorate transport rate was highest in water samples in which net uptake was highest and net uptake or production of ammonium approached zero. Transport of chlorate was considerably less light-dependent than the uptake of nitrate. Chlorate transport appeared to be a constitutive process (no induction period). Natural populations in the mixed layer generally, but not exclusively, showed higher rates of chlorate transport than populations in the nitracline. While it is feasible to estimate new production qualitatively with 36ClClO3−, quantitive estimates are not yet possible. The results suggest that ammonium recycling is tightly coupled between grazers and phytoplankton in the nitrate-depleted mixed layers of the Southern California Bight and Gulf of Maine.