Robert Le Borgne

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.

Population and trophic dynamics of Trichodesmium thiebautii in the SE lagoon of New Caledonia. Comparison with T. erythraeum in the SW lagoon

While Trichodesmium erythraeum is prevalent in the semi-closed SW lagoon of New Caledonia, T. thiebautii is dominant in the more open SE lagoon. This led to the comparison of the two species from the results of two high-frequency surveys focused on T. erythraeum (Rodier and Le Borgne, 2008) and T. thiebautii (the present paper). (1) Environmental conditions and triggers of the blooms are the same: calm weather and/or temperature >26 degrees C, and temporary nutrient inputs are required for both; (2) growth rates under favorable conditions are similar (0.14-0.27 d(-1)) but (3) T. thiebautii has lower net ascent rates along the water column during blooming events, due to distinct buoyancy capacities and the resulting vertical distributions; (4) carbon and dinitrogen fixation rates are not significantly different and contribution of Trichodesmium spp. represent less than 35% and 5% of the total phytoplankton carbon and nitrogen requirements, respectively. It is concluded the two species can be mixed in ecological studies, except for their vertical distributions during the bloom periods.

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.

Hydrography and plankton temporal variabilities at different time scales in the southwest lagoon of New Caledonia: A review

The New Caledonia SW lagoon is wide (5-20 nautical miles) and semi-closed. It is influenced by both the open ocean and the high island within a meteorological context subject to seasonal, inter-annual and longer term variations. The short-term variability (>1 day) of meteorological, hydrographical and planktonic parameters is illustrated by a 5-month long time series and is linked to local or remote wind, and precipitation. Seasonal and inter-annual variabilities, inferred from a 10-year long station by spectral analysis, appear clearly for all parameters. Seasonality is the main scale of variability as the island lies near the tropic of Capricorn. Inter-annual variability of a 3-4year periodicity is poorly related to the Southern oscillation index (an equatorial climatic index), stressing the need for a separate tropical index. Long term trends appear on several parameters but their reliability depends on the length of the records. Considering only the longest records (1958-2005), surface temperature appears to have increased since the end of the 1960s in Noumea area. Finally, as a result of greater terrestrial influence, shallower depths, and longer water turnover times close to shore, the temporal variability amplitude decreases from the shore to the barrier reef.

Zonal variability of plankton and particle export flux in the equatorial Pacific upwelling between 165° E and 150° W

Abstract Observations made during a “La Nina” situation (April–May 1996) in the equatorial Pacific upwelling, between 165° E and 150° W, show the classic deepening of hydrological isolines from east to west, resulting in zonal gradients for surface temperature and macronutrients. However, contrasting with such a gradient, no clear zonal variation could be seen for integrated planktonic biomasses and carbon fluxes, namely: chlorophyll a, bacterial abundances, particulate organic phosphorus, mesozooplankton ash-free dry weight, primary production, and the sinking flux of particulate organic carbon (POC). Moreover, mean values of these parameters along the zonal equatorial transect, are not significantly different from those of a 7-day-long time series station made at 0°, 150° W in October 1994 during an El Nino period. Such a steady zonal distribution of planktonic parameters seems to be characteristic of equatorial Pacific upwelling west of the Galapagos Islands so that the spatial distributions of nutrient concentrations and planktonic biomass appear to be uncoupled. This is consistent with the High Nutrient-Low Chlorophyll (HNLC) concept, in which primary production is not controlled directly by macronutrient concentrations. The lack of zonal gradient also suggests that carbon budget of the equatorial Pacific is primarily controlled by oscillations in the zonal and meridian extension of the HNLC area, rather than by values of planktonic biomasses and carbon fluxes within the upwelled water, which are quite constant.

Mesozooplankton biomass and composition in the equatorial Pacific along 180

[1] Latitudinal variations in mesozooplankton biomass and composition were investigated along an equatorial transect (8� S–8� N, 180� ) in October–November 1996. This study also included intensive sampling (3-hour intervals for 48 hours) for diel variations in mesozooplankton vertical distributions at 3� S and the equator. Most of the study took place in the high-nutrient, low-chlorophyll (HNLC) area that stretched between 7� S and 5� N. Mesozooplankton latitudinal distributions were influenced by the passage of a tropical instability wave during the equatorial time series station, which brought lower mesozooplankton biomass from the northeast, contrasting with the lack of a similar effect on concentrations of phytoplankton and particulates. South of the equator, the distributions of the mesozooplankton showed variable patterns with respect to chlorophyll and surface nitrate concentrations that could be ascribed to different states of the HNLC ecosystem. Very low diel variations of mesozooplankton biomass in the 0–50 and 0–100 m depth strata, a shallow vertical distribution, and the dominance of the larger size fraction (500–2000 mm) appear to be typical of the equatorial Pacific HNLC mesozooplankton and contrast with tropical oligotrophic ecosystems. Effects of such characteristics are a low active carbon export and a continuous predatory pressure. INDEX TERMS: 4231 Oceanography: General: Equatorial oceanography; 4855 Oceanography: Biological and Chemical: Plankton; 4880 Oceanography: Biological and Chemical: Trophodynamics; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; KEYWORDS: equatorial Pacific, zooplankton, diel cycles, composition, biomass

Spatial and temporal variability of the phytoplankton carbon to chlorophyll ratio in the equatorial Pacific: A basin‐scale modeling study

[1] The relationship between phytoplankton carbon biomass and chlorophyll is nonlinear because of the complex impacts of light, nutrient conditions, and temperature. A basin-scale ocean circulation-biogeochemistry model implemented with a dynamic phytoplankton model is employed to explore the spatial and temporal variability of the phytoplankton carbon to chlorophyll (C:Chl) ratio in the equatorial Pacific Ocean. The dynamic model computes the phytoplankton C:Chl ratio as a function of light, nitrate, iron, and temperature. The model reproduces well the general features of phytoplankton dynamics in this region, e.g., the deeper chlorophyll maximum (DCM) in the western warm pool and shallower DCM in the upwelling region. The model predicts large spatial and temporal variations of the C:Chl ratio. The mixed layer C:Ch1 ratio increases from 150 in the warm pool, whereas subsurface ratio is ~50 below 100 m. The model produces a weak seasonality in the mixed layer C:Chl ratio but strong interannual variability that is associated with the El Nino-Southern Oscillation (ENSO) events. The warm pool has strong anomalies during the cold phase of the ENSO with a reduced C:Chl ratio and a shoaled DCM. However, the upwelling region reveals strong anomalies during the warm phase of the ENSO, showing an increased C:Chl ratio in the euphotic zone and a deepened DCM. The predicted large spatial and temporal variations of the C:Chl ratio have potential implications for the carbon uptake in the equatorial Pacific.