Martine Rodier

Deep-ocean metabolic CO2 production: calculations from ETS activity

Abstract Deep-ocean respiration, in terms of CO 2 production, was calculated from measurements of the respiratory electron transport system in microplankton samples from the north central Pacific Ocean and from the northeastern Sargasso Sea. These two data sets support recent arguments that the Pacific Ocean supports more respiration in its subsurface waters than does the Atlantic. Global new production was calculated from these data sets by calculating the CO 2 production rate for the global open ocean below 200 m and then equating this to the new production. The result is 21.9 Gt C per year, 4–11 times greater than previous calculations.

Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985-1989

Abstract Fourteen transects carried out from 1985 to 1989 permit us to describe the nitrate and chlorophyll a distributions and their temporal variability in the tropical western Pacific (165°E, 20°S-10°N). Sections representative of the moderate 1986–1987 El Nino, the strong 1988–1989 La Nina and an equatorial westerly wind burst were compared to the January 1986 transect, which we define as a “reference” section. Along 165°E, there appears to be four characteristic distributions of nitrate and chlorophyll relative to the thermohaline structure. (1) In the south (20-17°S), a seasonal cycle was observed in both nitrate and chlorophyll distributions. Variability of surface chlorophyll and depth of the deep chlorophyll maximum (DCM) was associated with variations in the vertical mixing and available amount of light. (2) In the north (6–10°N), seasonal and interannual changes remained in the subsurface layer because of the strong stratification. Ekman pumping is one cause of the changes of the DCM depth. (3) In the 2°S–2°N band, the seasonal variability of the DCM depth was associated with variations in precipitation and eastward advection of low salinity water. During the 1986–1987 El Nino, elevation and intensification of the pycnocline and shoaling of the nutrient reservoir were the result of basin-wide changes. Consequences included abnormally low surface chlorophyll concentrations and an increase of the primary production. Intra-El Nino variations of the DCM depth were associated with changes of the thickness of the low salinity surface layer. During the subsequent La Nina, upwelling developed and vertical nitrate and chlorophyll distributions were strongly modified. Surface nitrate and chlorophyll concentrations for this period were the highest of the dataset, and primary production values were about twice the El Nino values. The main differences between upwelling in the western and central Pacific are attributed to the existence of two atmospheric convergence zones in the western basin, and especially to the tendency of the Intertropical Convergence Zone to migrate south of the equator. Intraseasonal Kelvin waves influence the variations of nutrient concentration and phytoplankton biomass. During the December 1989 westerly wind burst, there were strong modification of the thermohaline and nitrate distributions, associated with a geostrophic adjustment, which were not echoed on the chlorophyll distribution. (4) At 10°S, a doming of the different properties was frequently observed. Transient surface enrichments in nitrate or chlorophyll result from combined effects of the divergence between the South Equatorial Counter Current and the southern branch of the South Equatorial Current, Ekman pumping favourable to upwelling and local wind events. In all cases, nitrate and chlorophyll distributions were closely linked to the density structure for which salinity can be the controling factor. Low-frequency variability (seasonal, intraseasonal and interannual scales) of the nitrate and chlorophyll distributions were determined primarily by local or remote physical processes which controlled stability of the water column that governs the vertical displacements of phytoplankton.

Biomass, growth rates and limitation of Equatorial Pacific diatoms

Biomass, growth and species composition of siliceous phytoplankton were studied in the Equatorial Pacific during October 1994. Experiments were carried out in different nutrient conditions along the Equator. An oligotrophic area, with nitrate concentrations as low as 10 nM in the upper layer, was encountered in the western part of the transect (166°E–170°W). The concentration of biogenic silica varied from 10 nmol 1−1 in the surface layer up to 40 nmol 1−1 in the deep chlorophyll maximum located near the nutracline. Biogenic silica production, measured by the 32Si method, showed a similar vertical pattern in the nitrate-depleted water, and the mean assimilation rate for Si was 0.4±0.2 nmol l−1 h−1 (integrated mean value: 63 μmol m−2 h−1). In contrast, nitrate concentration ranged from 2 to 4 μM in the surface layer in the high-nutrient low-chlorophyll (HNLC) area located from 170 to 150°W and biogenic silica increased to 200 nmol 1−1. The Si assimilation rate was 1.7±1.0 nmol 1−1 h−1 (integrated mean value: 162 μmol m-Z h−1). In both areas, 80% of Si biomass was concentrated in larger cells ( > 10 μm). Scanning electron microscopy was used to estimate diatom numbers and cell surface areas. This latter parameter correlates well with biogenic silica and warrants a discussion of the contribution of different species to the total biogenic silica. The measured values for specific uptake rate of Si never reached the optimum uptake or growth rate deduced from environmental parameters and kinetic constants reported in the literature. In addition the mean growth rate (0.9±0.3 doubling per day) for the nitrate-depleted water does not differ from the mean value (0.8±0.2 doubling per day) in the HNLC area. Therefore it can be concluded that diatom growth is severely limited in both regions. This agrees well with the nutrient balance study. In the oligotrophic area, N supply appears to be the limiting factor in the upper layer. In the HNLC region, the results of this study are consistent with the hypothesis that diatom growth might be limited by a micro nutrient such as iron. Carbon production by diatoms was estimated to be 31 ± 4 mmol Cm−2 d−1, which is one-third of the total carbon production of the nutrient-enriched area.

Primary production, new production, and growth rate in the equatorial Pacific: Changes from mesotrophic to oligotrophic regime

[1] Under an apparent monotony characterized by low phytoplankton biomass and production, the Pacific equatorial system may hide great latitudinal differences in plankton dynamics. On the basis of 13 experiments conducted along the 180° meridian (8°S-8°N) from upwelled to oligotrophic waters, primary production was strongly correlated to chlorophyll a (chl a), and the productivity index PI (chl a-normalized production rate) varied independently of macronutrient concentrations. Rates of total ( 14 C uptake) and new ( 15 N-NO 3 uptake) primary production were measured in situ at 3°S in nutrient-rich advected waters and at 0° where the upwelling velocity was expected to be maximal. Primary production was slightly higher at the equator, but productivity index profiles were identical. Despite similar NO 3 concentrations, new production rates were 2.6 times higher at 0° than at 3°S, in agreement with much higher concentrations of biogenic particulate silica and silicic acid uptake rates ( 32 Si method) at the equator. Furthermore, phytoplankton carbon concentrations from flow cytometric and microscopical analyses were used with pigment and production values to assess C:chl a ratios and instantaneous growth rates (μ). Growth rates in the water column were significantly higher, and C:chl a ratios lower at 0° than at 3°S, which is consistent with the more proximate position ofthe equatorial station to the source of new iron upwelling into the euphotic zone. For the transect as a whole, compensatory (inverse) changes of C:chl a and μ in response to varying growth conditions appear to maintain a high and relatively invariant PI throughout the equatorial region, from high-nutrient to oligotrophic waters.

Silicon-nitrogen coupling in the equatorial Pacific upwelling zone

We describe the role of diatoms on nitrogen and silicon cycling in the equatorial Pacific upwelling zone (EUZ) using water column nutrient data from 19 equatorial cruises and particle concentration, new production, and sediment trap data from the U.S. Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac), France JGOFS fluxes in the Pacific (FLUPAC), and U.S. Zonal Flux cruises. Our results suggest that production and sinking of diatoms dominate particulate nitrogen export at silicate concentrations above 4 μM. Below this level, silicate is preferentially retained; while inorganic nitrogen is completely utilized, silicate remains at concentrations of 1–2 μM and is completely exhausted only under nonsteady state conditions. This lower nutrient condition accounts for a majority of particulate nitrogen export in the EUZ with minor loss of particulate silicon. Retention of silicon relative to nitrogen appears due to a combination of new production by nondiatoms, dissolution of silica frustules after grazing, iron limitation, and steady state upwelling. This synthesis supports the argument that diatom production was tightly coupled to new production during the U.S. JGOFS EqPac survey II cruise [Dugdale and Wilkerson, 1998]. However, this compilation suggests EqPac survey II cruise took place during a period of atypically high subsurface nutrients. We conclude that silicon and nitrogen are tightly coupled only at periods of very high nutrient concentration and nonsteady state. In addition, nutrient cycling in the EUZ is consistent at all times with a mechanism of combined iron and grazing control of phytoplankton size classes [Landry et al., 1997].

Estimation of new production in the tropical Pacific

A synthesis of field data from nine cruises and 121 stations in the tropical Pacific (15°N-16°S by 135°W-167°E) was used to develop a statistical model relating areal new production rates (based on 15 NO 3 uptake incubations) to other measured biological and chemical water properties. The large dynamic range of f ratios (new to primary production) measured in the region (0.01-0.46, with a mean of 0.16 ± 0.08) could not be described by any simple function of any of the more than three dozen measured variables tested. Thus the commonly used approach of extrapolating new production using mean f ratios is likely to lead to large uncertainties when used in the tropical Pacific. An alternative approach is examined in which new production is estimated directly by multiple linear regression (MLR) of measured properties. Nearly 80% of variability in new production could be explained with a MLR of four variables together (rates of primary production (or chlorophyll inventories), inventories of ammonium and nitrate, and temperature) better than any single variable alone or any other combination of variables. Each of these variables exhibited effective linearity with respect to new production for this data set, and the robustness of this MLR method to predict new production for other data sets was confirmed by cross validation. These results thus provide a robust, simple tool to extend new production estimates to locations and times where it is not measured directly, using ship-based measurements and potentially remotely sensed data from moorings and satellites.

PCO2, chemical properties, and estimated new production in the equatorial Pacific in January–March 1991

Measurements of the partial pressure of CO2 (PCO2) at the sea surface, dichlorodifluoromethane (F12), salinity, temperature, oxygen, nutrients, wind, and current velocities were made during a cruise (January–March 1991) in the equatorial Pacific from Panama to Noumea via Tahiti. In the western Pacific (140°W to 165°E) the westward South Equatorial Current is well established. Distributions of tracers show extrema near the equator in the eastern Pacific (from 95°W to 140°W), indicating that the upwelling is especially active in this area. The zonal distribution of chemical tracers is not regular because of intrusions of warmer water from the north associated with equatorial long waves. The temporal changes in PCO2 result from thermodynamic changes, biological activity, and gas exchange with the atmosphere. In order to compare the magnitude of these processes, we assess the variations of PCO2 (dPCO2) between two stations as the sum of thermodynamic changes driven by temperature and salinity changes, air-sea exchange computed from observed wind and difference of PCO2 between the sea and the atmosphere, and the biological activity estimated from the nitrate decrease and C:N ratio (106:16). The resulting assessed change in PCO2 is in agreement with the observed change for 42 pairs of stations. Each of these pairs of stations is thus considered as representing a simple water mass advected by the measured currents between the two stations so that daily fluxes can be estimated. The contribution of CO2 outgassing to dPCO2 is low, between −0.2 to −0.0 μatm d−1. The thermodynamical dPCO2 averages 0.7±0.2 μatm d−1 in the mixed layer. The biological dPCO2 (-1.5±0.5 μatm d−1) is the highest in absolute value implying an average value of new production along the equator of 72±25 mmolC m−2 d−1 (0.9±0.3 gC m−2 d−1) for the equatorial Pacific (130°W-165°E). This value is very high and the overestimation could result from the simplistic description of the advection and mixing of water. An attempt to account for these processes by constraining the net heat flux to 100 W m−2 [Weare et al., 1981] reduces the estimate of new production to 58 mmolC m−2 d−1 (0.7 gC m−2 d−1 ). A mean upwelling velocity of 0.5±0.1 m d−1 east of 140°W is calculated, based on F12 undersaturations.

Contrasted geographical distribution of N2 fixation rates and nifH phylotypes in the Coral and Solomon Seas (southwestern Pacific) during austral winter conditions

Biological dinitrogen (N2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (southwestern Pacific) during austral winter conditions. N2 fixation rates were measurable at every station, but integrated (0–150 m) rates were an order of magnitude higher in the Solomon Sea (30 to 5449 µmol N m−2 d−1) compared to those measured in the Coral Sea (2 to 109 µmol N m−2 d−1). Rates measured in the Solomon Sea were in the upper range (100–1000 µmol N m−2 d−1) or higher than rates compiled in the global MARine Ecosystem biomass DATa database, indicating that this region has some of the highest N2 fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from group A (UCYN-A1 and UCYN-A2) and the proteobacteria γ-24774A11 dominated in the Coral Sea and were correlated with N2 fixation rates (p < 0.05), Trichodesmium and UCYN-B dominated in the Solomon Sea and were correlated (p < 0.05) with N2 fixation rates. UCYN-A were totally absent in the Solomon Sea. The biogeographical distribution of diazotrophs is discussed within the context of patterns in measured environmental parameters.

The Western Boundary of the Equatorial Pacific Upwelling: Some Consequences of Climatic Variability on Hydrological and Planktonic Properties

The longitude of the western limit of the equatorial Pacific upwelling is a key parameter for studies of carbon budget and pelagic fisheries variability. Although it is well defined at the surface on the equator by a salinity front and a sharp variation of the partial pressure of CO2, data from two equatorial cruises make it clear that this hydrological limit does not necessarily coincide with the boundary of the nitrate and chlorophyll enriched area. In January-February 1991 during a non-El Niño period, when trade winds and the South Equatorial current (SEC) were favorable to upwelling, the two limits were at the same longitude. Conversely, in September-October 1994 during El Niño conditions, when the equatorial upwelling had stopped, the nitrate and chlorophyll enriched zone was found a few degrees of longitude east of the hydrological boundary (5.5° at the surface and 2.5° for the 50 m upper layer), whereas no such offset was observed for zooplankton biomass. A simple model, based on the HNLC (High Nutrient - Low Chlorophyll) ecosystem functioning, was initialized with nitrate uptake measurements and estimates of upwelling break duration. The model results support the hypothesis that zonal separation of the limits arises from biological processes (i.e. nitrate uptake and phytoplankton grazing) achieved during that upwelling break.

Plankton biomass and production in an open atoll lagoon: Uvea, New Caledonia

Uvea lagoon is an atoll-type one with a discontinuous belt of small islets on its western part and the main island to the east. Its depth increases steadily from east to west. A 2 week cruise in September 1992 aimed to study the ways in which these morphological features influence the functioning of the lagoon pelagic ecosystem. Hydrological parameters present a fair homogeneity, both horizontally and vertically over the whole lagoon, which is due to an efficient mixing and important exchanges with the oligotrophic open ocean. Lack of significant nutrient concentrations (NO3, NO2, NH4, PO4, SiO3) in the water mass is in agreement with low planktonic biomasses: Chlorophyll a (Chl a) concentration is 0.233 mg m−3, and ash-free dry weight is 5.25 and 7.55 mg m−3 for [35–200 μm] and [200–2000 μm] size fractions respectively. These biomass levels are more than twice the concentration of the surrounding open ocean. Total Chl a is dominated by the >1 μm size-fraction, thus contrasting with the dominance of small cells (<1 μm) in the open ocean. Phytoplankton prevails in the [35–200 μm] size-class, indicating the occurrence of microphytobenthos brought by mixing of the water column. The [200–2000 μm] fraction is made up primarily of copepods (61% of the dry weight), appendicularians and radiolarians. Planktonic predators, such as chaetognaths are almost absent. Three different methods dealing with carbon production, i.e., 14C fixation, in-bottle O2 production, and natural O2 variations, lead to a coherent estimate of pelagic primary production: 27.5 mg C m−3 d−1. Half of this production is achieved by <1 μm cells. Zooplankton production, which was assessed by the C/N/P ratios method, is equal to 10.4 mg C m−3 d−1 and its P:B ratio is 114%. On the whole, Uvea lagoon appears to be oligotrophic compared with other ones, because it is wide-open.