John L. Nevins

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.

Natural isotopic composition of dissolved inorganic nitrogen in the Chesapeake Bay

The natural abundances of 15N in the dissolved inorganic pools of nitrogen in the Chesapeake Bay were measured in the spring and fall of 1984. Changes in the δ15N of NH4+ and the combined pool of (NO3− + NO2−) reflected both seasonal and short-term changes in the estuarine nitrogen cycle. In the spring, oxidation of NH4+ at the head of the bay in the region of the turbidity maximum and in localized regions throughout the bay, led to elevated values of δ15N in the NH4+ pool. The δ15N of the (NO3− + NO2−) pool tended to increase toward the south, enabling an estimate of the isotopic fractionation factor for the consumption of NO3− to be derived; the estimate (1·0070), is similar to literature values of the fractionation factor for NO3− uptake by phytoplankton, supporting previous research suggesting that phytoplankton uptake is the major sink for NO3− in the bay. Denitrification led to elevated values of δ15N in the (NO3− + NO2−) pool in deep water. Over the course of the summer, the δ15N of NH4+ increased throughout the bay. A significant correlation was found between the δ15N of the NH4+ pool and the concentration of NO2− both above and below the pycnocline during the fall cruise, suggesting that the increase in the δ15N of the NH4+ pool was due to the oxidation of NH4+. In the fall, changes were also observed in the δ15N of both the NH4+ and (NO3− + NO2−) pools which were consistent with the occurrence of NH4+ oxidation. From these changes, a fractionation factor for NH4+ oxidation between 1·0120 and 1·0167 was derived, which is similar to values reported in the literature.

Nitrogenous nutrient transformations in the spring and fall in the Chesapeake Bay

Measurements of the standing stock of phytoplankton, bacterioplankton and particulate carbon and nitrogen, and dissolved pools of inorganic nitrogen were made at a series of stations located in the main channel of the Chesapeake Bay during cruises in the spring and fall of 1984. On each cruise, we conducted two transects of the long axis (N-S) of the bay; comparison of these transects revealed that the horizontal and vertical distribution of O2, chlorophyll a, and dissolved inorganic nitrogen can change dramatically on a time scale of days. Experiments were carried out with 15N-labelled substrates (NO2−, NO3−, and NH4+) at a subset of the stations. These experiments were designed to measure the fluxes of nitrogen between the dissolved pools as well as the flux into particles. During the spring cruise, a surprisingly high rate of uptake of NO2− by particles was found. In the fall, uptake by particles followed previously reported patterns more closely. The flux of nitrogen between dissolved inorganic pools was often greater than the flux into particles. The rates of individual reactions varied spatially, and significant activity throughout the water column was often found. The occurrence of typically anaerobic reactions in well-oxygenated waters suggests that anaerobic microzones may be an important site for nitrogen transformations in the bay. Our results also indicate that the rates of a number of microbially-mediated processes were greatly enhanced by storm-induced mixing of the water column during our fall cruise. Such transient enhancements may have a large, and usually undocumented, effect on the nitrogen cycle.

Utilization of nitrogen and phosphorus by primary producers in warm-core ring 82-B following deep convective mixing

Abstract Rates of utilization for NO 3 − , NH 4 + , urea, and PO 4 − by primary producers and the abundances of particulate C, N, and P were determined for the euphotic zone of warm-core ring 82-B in April and early May 1982. The ring had formed in late February, and our study took place between a period of deep convection and the formation of a stable seasonal thermocline. Deep convection enriched the surface waters with NO 3 − and PO 4 − , which remained in sufficient quantities to preclude nutrient limitation of primary production during the course of the study. Concentrations of HN 4 + and urea were always less than or equal to the limit of detection (0.03 μmol kg −1 ). The fraction of total nitrogen taken up as NO 3 − (ƒ ratio) was high (0.62–0.66) in the core region of the ring. During the course of this study, the concentration of NO 3 − decreased near the surface, allowing a NO 3 − gradient to develop, while the concentration of particulate N (PN) and the rate of N uptake increased. However, these processes were not in balance. Only about 16% of the NO 3 − based production remained in the euphotic zone. The PN:Chl a ratio was higher at the bottom of the euphotic zone than at the top. This indicates that detrital PN was being formed faster than it was exported by either sinking or mixing processes. The overall reduction in euphotic zone NO 3 − concentrations was about one third the measured consumption rate. Midway through the study a storm was apparently responsible for redistributing NO 3 − in the euphotic zone, and perhaps mixing NO 3 − up from greater depths. After that event, the mean NO 3 − uptake rate was about twice the rate of NO 3 − disappearance. Upward flux of NO 3 − associated with the relaxation of depressed density surfaces as the ring lost kinetic energy could have supplied NO 3 − at a rate equivalent to 40–110% the mean NO 3 − uptake rate for the period. The highest rates of NO 3 − uptake were observed at the end of the study, and were in excess of the calculated rates for upward flux. This may explain the increasing magnitude of NO 3 − gradients observed durinng the final few days of this study.

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.

Endangered Right Whales Enhance Primary Productivity in the Bay of Fundy

Marine mammals have recently been documented as important facilitators of rapid and efficient nutrient recycling in coastal and offshore waters. Whales enhance phytoplankton nutrition by releasing fecal plumes near the surface after feeding and by migrating from highly productive, high-latitude feeding areas to low-latitude nutrient-poor calving areas. In this study, we measured NH4+ and PO43- release rates from the feces of North Atlantic right whales (Eubalaena glacialis), a highly endangered baleen whale. Samples for this species were primarily collected by locating aggregations of whales in surface-active groups (SAGs), which typically consist of a central female surrounded by males competing for sexual activity. When freshly collected feces were incubated in seawater, high initial rates of N release were generally observed, which decreased to near zero within 24 hours of sampling, a pattern that is consistent with the active role of gut microflora on fecal particles. We estimate that at least 10% of particulate N in whale feces becomes available as NH4+ within 24 hours of defecation. Phosphorous was also abundant in fecal samples: initial release rates of PO43- were higher than for NH4+, yielding low N/P nutrient ratios over the course of our experiments. The rate of PO43- release was thus more than sufficient to preclude the possibility that nitrogenous nutrients supplied by whales would lead to phytoplankton production limited by P availability. Phytoplankton growth experiments indicated that NH4+ released from whale feces enhance productivity, as would be expected, with no evidence that fecal metabolites suppress growth. Although North Atlantic right whales are currently rare (approximately 450 individuals), they once numbered about 14,000 and likely played a substantial role in recycling nutrients in areas where they gathered to feed and mate. Even though the NH4+ released from fresh whale fecal material is a small fraction of total whale fecal nitrogen, and recognizing the fact that the additional nitrogen released in whale urine would be difficult to measure in a field study, the results of this study support the idea that the distinctive isotopic signature of the released NH4+ could be used to provide a conservative estimate of the contribution of the whale pump to primary productivity in coastal regions where whales congregate.