Yves Dandonneau

Fluid dynamical niches of phytoplankton types

The biogeochemical role of phytoplanktonic organisms strongly varies from one plankton type to another, and their relative abundance and distribution have fundamental consequences at the global and climatological scales. In situ observations find dominant types often associated to specific physical and chemical water properties. However, the mechanisms and spatiotemporal scales by which marine ecosystems are organized are largely not known. Here we investigate the spatiotemporal organization of phytoplankton communities by combining multisatellite data, notably high-resolution ocean-color maps of dominant types and altimetry-derived Lagrangian diagnostics of the surface transport. We find that the phytoplanktonic landscape is organized in (sub-)mesoscale patches (10–100 km) of dominant types separated by physical fronts induced by horizontal stirring. These physical fronts delimit niches supported by water masses of similar history and whose lifetimes are comparable with the timescale of the bloom onset (few weeks). The resonance between biological activity and physical processes suggest that the spatiotemporal (sub-)mesoscales associated to stirring are determinant in the observation and modeling of marine ecosystems.

Measured and modeled primary production in the northeast Atlantic (EUMELI JGOFS program): the impact of natural variations in photosynthetic parameters on model predictive skill

Use of ocean color satellite data in global biogeochemical studies requires models to predict primary production from the satellite-derived chlorophyll fields. In this paper, measured bio-optical and photo-physiological data are used in place of standard (constant) parameters to adjust a previously published primary production model. In the JGOFS-France program, systematic studies were carried out at three locations in the tropical northeast Atlantic, selected to represent typical EUtrophic, MEsotrophic and oLIgotrophic regimes (EUMELI cruises). During cruise no. 4, these studies included the spectral measurements of the photosynthetically available radiation at sea level and within the water column, the determination of the algal absorption spectra and the determination of the physiological parameters derivable from P versus E experiments (photosynthesis-irradiance responses). The model predictions are compared with in situ determinations made by the 14C technique (JGOFS core parameter). At the three sites, the physical structure (mixed layer and euphotic depths), the algal abundance and community structure, as well as their bio-optical and physiological properties, are very different, so that the predictive performance of the model was tested in trophic conditions that span most of those expected in the global open ocean. The model, when adjusted by entering the actual physiological parameters (chlorophyll-specific absorption of algae, maximum quantum yield, and light saturated carbon fixation rate), provides satisfying results compared to those observed in situ. The relative roles of the physiological parameters are analyzed and sensitivity studies are performed. For global applications, and in the absence of specific information when all seasons and provinces of the world ocean are considered, it will remain necessary for a while to rely on generic models and a selected standard set of physiological properties. The sensitivity studies here presented help in this choice, and a modified set of parameters is proposed and tested. With this set, reconstructed production profiles are close to those determined in the field, and the integrated values are retrieved with no bias and a reduced scatter (18% at one SD) for 17 stations (cruises 3 and 4) and daily production ranging from 0.3 to 2.3 gC m−2.

Cirene: Air—Sea Interactions in the Seychelles—Chagos Thermocline Ridge Region

The Vasco-Cirene program explores how strong air-sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles-Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air-sea flux observations in this region in January–February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles-Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward cur...

A whirling ecosystem in the equatorial Atlantic

[1] The equatorial Pacific and Atlantic oceans exhibit remarkable meridional undulations in temperature and chlorophyll fronts visible from space over thousands of kilometers and often referred to as tropical instability waves. Here, we present new observations of an ecosystem ranging through three trophic levels: phytoplankton, zooplankton and small pelagic fish whirling within a tropical vortex of the Atlantic ocean and associated with such undulations. Cold, nutrient and biologically rich equatorial waters are advected northward and downward to form sharp fronts visible in all tracers and trophic levels. The equatorward recirculation experiences upwelling at depth, with the pycnocline and ecosystem progressively moving toward the surface to reconnect with the equatorial water mass. The observations thus indicate that it is a fully three-dimensional circulation that dominates the distribution of physical and biological tracers in the presence of tropical instabilities and maintains the cusp-like shapes of temperature and chlorophyll observed from space.

Mixing and phytoplankton bloom in the wake of the Marquesas Islands

A persistent phytoplankton bloom was observed during August–December 1998 around the Marquesas Islands in the South Pacific using a spaceborne ocean color sensor. The enhancement of the phytoplankton production is attributed to the island mass effect. The effect consists of a combination of turbulent mixing and advection from the south equatorial current flowing through and around the islands, and iron-enriched waters originating from land drainage and hydrothermal fluxes through old volcanic formations. The enhanced phytoplankton production effects are noticeable a large distance downstream from the island (500 to 1000 km). This island mass effect is an important contributor to the productivity of the region and therefore potentially important for fisheries in the area.

The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.

Diel variations ofin vivo fluorescence in the eastern equatorial Pacific: an unvarying pattern

Abstract Records of in vivo chlorophyll a fluorescence obtained at various periods in upwelled waters of the equatorial Pacific appeared to be strongly dominated by diel variations. High pass filtering of the records by a 24 h running mean, followed by normalization (division by the standard deviation in the −12 h to +12 h interval) produce a normalized diel cycle of in vivo fluorescence, which was almost exactly the same in January, August and December 1991, October 1992 and January 1996. This cycle was characterized by a minimum at 12:00 h, mainly caused by nonphotochemical quenching, and a maximum from 19:00 h to midnight. Fluorescence decreased during the second half of the night in the absence of forcing by solar energy, probably in response to a circadian physiological rhythm. Vertical profiles of fluorescence made during an oceanographic cruise at the equator showed that this physiological rhythm explained the fluorescence cycle at depth, where it was characterized by a minimum at 7:00 h and a maximum at 19:00 h. Knowledge of this diel cycle of fluorescence can help to estimate the chlorophyll content of surface seawater in the equatorial Pacific using under-way records of in vivo fluorescence with a limited number of filtration-extraction based chlorophyll determinations.

Modeled and observed impacts of the 1997–1998 El Niño on nitrate and new production in the equatorial Pacific

The impact of the strong 1997-1998 E1Nifio event on nitrate distribution and new production in the equatorial Pacific is investigated, using a combination of satellite and in situ observations, and an ocean circulation-biogeochemical model. The general circulation model is forced with realistic wind stresses deduced from ERS-1 and ERS-2 scatterometers over the 1993-1998 period. Its outputs are used to drive a biogeochemical model where biology is parameterized as a nitrate sink. We first show that the models capture the essential circulation and biogeochemical equatorial features along with their temporal evolution during the 1997-1998 event, although the modeled variability seems underestimated. In particular, the model fails to reproduce unusual bloom conditions. This is attributed to the simplicity of the biological model. An analysis of the physical mechanisms responsible for the dramatic decrease of the biological equatorial production during E1 Nifio is then proposed. During the growth phase (November 1996 through June 1997), nitrate-poor waters of the western Pacific are advected eastward, and the vertical supply of nitrate is reduced due to nitracline deepening. These processes result in the invasion of the equatorial Pacific by nitrate-poor waters during the mature phase (November 1997 through January 1998). At that time, the central Pacific is nitrate limited and experiences warm pool oligotrophic conditions. As a result, the modeled new production over the equatorial Pacific drops by 40% compared to the mean 1993-1996 values. Then, while E1Nifio conditions are still present at the surface, the nitracline shallows over most of the basin in early 1998. Therefore the strengthening of the trade winds in May 1998 efficiently switches on the nitrate vertical supply over a large part of the equatorial Pacific, leading to a rapid return of high biological production conditions. Strong La Nifia conditions then develop, resulting in a biologically rich tongue extending as far west as 160oE for several months.

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.

Air-sea CO2 fluxes in the equatorial pacific in January-March 1991

The partial pressure of CO[sub 2] in surface seawater and in air were continuously measured during the cruise ALIZE II (January 1991-March 1991) in the equatorial Pacific from Panama to Noumea via Tahiti. It provides a large set of data for estimating the air-sea CO[sub 2] flux in the equatorial Pacific. An average flux is calculated between 2.5 N and 2.5S. This estimation ranges from 5 to 8.5 mmol m[sup [minus]2]d[sup [minus]1] according to the CO[sub 2] exchange coefficient used. 4 refs., 18 figs., 3 tabs.