Aubert Le Bouteiller

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.

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.

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.

A simple and rapid device for measuring planktonic primary production by in situ sampling, and 14C injection and incubation

Abstract A system (LET GO) is described that enables measurements of primary production at sea after in situ incubations, with 14 C being injected at depth immediately after enclosure of the sample. Each incubation cell, about 200 ml, is made of two transparent plexiglass cups facing each other. Mechanical energy to operate the system is provided by the tension of the nylon line between the drifting buoy, which holds the experimental equipment, and a weight at the bottom: when the line is strained, the two cups enclose the water sample, and the 14 C is delivered by a syringe. Absence of metallic or rubber parts ensures that toxicity effects are minimized and that reliable results can be expected. Furthermore, in situ incubations can start 1 or 2 min after arrival on station, leaving the research vessel and winches available for other tasks. These points make it possible to make in situ 14 C incubations during most oceanographic cruises and to increase greatly the acquisition rate of primary production data. The LET GO device has been tested in parallel with the conventional technique. Both techniques showed similar vertical patterns. Carbon fixation measured with the LET GO, however, was greater by a factor 1.3.