Marina Lévy

Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime

Using a protocol of numerical experiments where horizontal resolution is progressively increased, we show that small-scale (or sub-mesoscale) physics has a strong impact on both mesoscale physics and phytoplankton production/subduction. Mesoscale and sub-mesoscale physics result from the nonlinear equilibration of an unstable baroclinic jet. The biogeochemical context is oligotrophy. The explicitly resolved sub-mesoscales, at least smaller than one e fth of the internal Rossby radius of deformation, reinforce the mesoscale eddy e eld and contribute to double the primary production and phytoplankton subduction budgets. This enhancement is due to the reinforced mesoscale physics and is also achieved by the small-scale frontal dynamics. This sub-mesoscale physics is associated with density and vorticity gradients around and between the eddies. It triggers a signie cant small-scale nutrient injection in the surface layers, leading to a phytoplankton e eld mostly dominated by e ne spatial structures. It is believed that, depending on wind forcings, this scenario should work alternately with that of Abraham (1998) which invokes horizontal stirring of nutrient injected at large scales. Results also reveal a strong relationship between new production and negative vorticity, in the absence of wind forcing and during the period of formation of the eddies.

The onset of a bloom after deep winter convection in the northwestern Mediterranean sea: mesoscale process study with a primitive equation model

The importance of mesoscale processes for primary production predictions is examined in a process study concerning the onset of the spring bloom after deep winter convection in the northwestern Mediterranean sea. Winter deep convection brings nutrient to the enlightened surface layer, but inhibits photosynthesis; phytoplankton biomasses are very low. As soon as restratification occurs, vertical mixing is blocked and a strong bloom onsets. Coastal Zone Color Scanner images have emphasized a strong mesoscale signal in the sea surface chlorophyll during this period. Mesoscale heterogeneity of the mixed-layer depth, due to the baroclinic instabilities associated with the process of deep water formation, is indeed responsible for the mesoscale variability of primary production. To ascertain interactions between hydrological processes and primary production occurring at mesoscales, a primary production model with a parameterization of production inhibition in situations of deep mixing is embedded in a three-dimensional primitive equation model with explicit mixed-layer physics. The model is initialized with a circular chimney of dense water surrounded by a stratified ocean. Two experiments are performed using different treatments of lateral mixing. In the first experiment, the horizontal diffusion is set to a low level so that mesoscale activity can be explicitly resolved. Surface density meanders of 50 km wavelength develop at the periphery of the chimney. These meanders, and the associated vertical motions, induce the sinking and spreading of the chimney, and subsequent surface restratification. Upward motions are responsible for mesoscale mixed layer shallowing, leading to an enhancement of primary production. Maxima of productivity are obtained at the edge of the chimney, where mesoscale activity is the most intense, in agreement with in situ data. In the second experiment, the horizontal diffusion is set to a high level so that lateral mixing occurs primarly through those terms: explicit mesoscale activity is completely damped. The initial structure of the chimney progressively disappears due to the horizontal diffusion of density across the isopycnals instead of three-dimensional redistribution. Mixed-layer depth and productivity are homogeneous. It is shown that instantaneous primary production can be underestimated by a factor of 4 when mesoscale eddies are not explicitly solved. This finding questions the evolution of large-scale coarse resolution climatic models of the oceanic carbon cycle.

Does the low frequency variability of mesoscale dynamics explain a part of the phytoplankton and zooplankton spectral variability

Observational studies of the last 20 years have revealed a spatial distribution of phytoplankton characterized by a wavenumber spectrum whose slope ranges between k−1 and k−3 (with k being the wavenumber), over horizontal scales ranging from 10 to 100 km. This strong spectral variability can be due either to the physics and/or to the biology. We show in this note that the low frequency variability (LFV) linked to the mesoscale dynamics may explain a significant part of the observed variability. Our numerical results also suggest that the difference between the phytoplankton and zooplankton spectral slopes can depend on the dynamical LFV.

A reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean

Among the tropical oceans, the western Indian Ocean hosts one of the largest concentrations of marine phytoplankton blooms in summer. Interestingly, this is also the region with the largest warming trend in sea surface temperatures in the tropics during the past century—although the contribution of such a large warming to productivity changes has remained ambiguous. Earlier studies had described the western Indian Ocean as a region with the largest increase in phytoplankton during the recent decades. On the contrary, the current study points out an alarming decrease of up to 20% in phytoplankton in this region over the past six decades. We find that these trends in chlorophyll are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers. Future climate projections suggest that the Indian Ocean will continue to warm, driving this productive region into an ecological desert.

Combined effects of mesoscale processes and atmospheric high-frequency variability on the spring bloom in the MEDOC area

A number of processes are proposed to explain the time and space variability of the onset and decay of the spring phytoplankton bloom. This is done in the modeling framework of a case study most representative of the northwestern Mediterranean Sea (MEDOC area). The strategy followed is to isolate the different possible sources of variability (oceanic mesoscale dynamics, spring warming, wind bursts) in a series of process experiments (no flux, warming and wind experiments). The analysis of these experiments provides information for the analysis of a more realistic experiment, forced with daily atmospheric data (high-frequency experiment). On the basis of this study, we propose a categorization of the processes that control the spring bloom, in terms of their impact on the onset and decay of the bloom, and of the time and space scales on which they apply.

Physical and Biogeochemical Controls of the Phytoplankton Seasonal Cycle in the Indian Ocean: A Modeling Study

A three-dimensional primitive equation model Ocean Parallelise (OPA) was coupled to the biogeochemical model Pelagic Interaction Scheme for Carbon and Ecosystem Studies to simulate the ocean circulation and the marine biological productivity through the biogeochemical cycles of carbon and the main nutrients (P, N, Si, Fe). We focus on surface phytoplankton dynamics in the Indian Ocean extending from 30°S to 30°N and from 30°E to 120°E. The seasonal cycle of phytoplankton over the Indian Ocean is generally characterized by two blooms, one during the summer monsoon, the other one during the winter monsoon. Based on the method proposed by Levy et al. (2007), different biogeochemical provinces can be defined during the summer and winter monsoons. The model performed relatively well by simulating the main features of the cumulated increase in chlorophyll, and the time of the bloom onsets is consistent with data. It also reproduced quite well the main biogeochemical provinces in good agreement with data in most of the Arabian Sea (except in the central part), the Bay of Bengal, and in the convergence zone south of the equator. The analysis of the modeled biogeochemical processes has shown that during the blooms onset periods, the most limiting nutrient was nitrogen except some areas around India and the eastern part of the Bay of Bengal where the ecosystem tends toward silicate limitation. These limitations change during the blooms development. The model also highlighted a variety of the critical physical processes (horizontal and vertical advection, turbulent diffusion, mixed layer depth) involved in each biogeochemical province bloom dynamics.

The relevant time scales in estimating the air–sea CO2 exchange in a mid-latitude region

A 1D biogeochemical model simulating the nitrogen and carbon cycles is validated, using continuous observations obtained with the Carioca buoy (sea surface temperature (SST), pCO2; fluorescence) at the DYFAMED station (NW Mediterranean Sea) in 1995–1997, as well as other in situ data. Although the average surface pCO2 is generally slightly over-saturated (878matm), the NW Mediterranean Sea is a weak sink for atmospheric CO2 (0.1570.07 mol C m � 2 yr � 1 ), because the highest fluxes occur in winter and spring during the period of undersaturation when the winds are strong. The seasonal cycle is modulated by synoptic events (Mistral bursts), which can have a strong impact on the behaviour of the system and on the fluxes in winter and spring. While the atmospheric forcing constrains the annual balances and fluxes of tracers and CO2, the winter pre-conditioning of the bloom plays a major role in driving the interannual variability. Sensitivity analyses indicate that atmospheric forcing in this highly variable region should be averaged over periods of no longer than one week to simulate the biological and air–sea CO2 fluxes correctly. Also, because the model used is rather simple, this time scale is an upper limit. The sampling period must be no greater than a few days in order to estimate the CO2 fluxes with an error smaller than 20%. In the NW Mediterranean Sea, it is difficult to use ‘‘satellite proxies’’ (SST; sea colour) to estimate annual air–sea CO2 fluxes because SST does not vary much in winter and during the beginning of the bloom, while chlorophyll can either remain low (winter) or change rapidly (beginning of the bloom). At least several direct observations of pCO2 are needed during winter to define ‘‘initial conditions’’. r 2002 Published by Elsevier Science Ltd.

Phytoplankton diversity and community structure affected by oceanic dispersal and mesoscale turbulence

We explore the role of oceanic dispersal in setting patterns of phytoplankton diversity, with emphasis on the role of mesoscale turbulence, using numerical simulations that resolve mesoscale eddies and a diverse set of phytoplankton types. The model suggests that dispersal of phytoplankton by oceanic transport processes increases phytoplankton diversity at the local scale of O(10–100) km (α-diversity), extends the range of many phytoplankton types, and decreases the ability of rare types to persist in isolated areas. As a consequence, phytoplanktonic assemblages are modified and diversity decreases at the regional scale of O(1000) km (γ-diversity). By progressively accounting for different classes of motion, we show that the increase of α-diversity ensues from vertical mixing of the organisms, dispersal by mean lateral currents, and in slightly larger proportion, dispersal due to eddies. With the progressive inclusion of mechanisms of dispersal, the community becomes dominated by a smaller number of types but with larger degree of coexistence, in larger home range areas. From a resource competition perspective, physical transport can reduce the effective concentration R* of a limiting resource R, thus allowing more types to become equally fit. In addition, mixing of nearby populations allows coexistence of types with unequal fitness. The simulations suggest that mesoscale turbulence plays a particular role, concomitantly providing a means for different phytoplankton types to achieve comparable fitness and extending the exclusion time scale for less competitive types.

Impact of episodic vertical fluxes on sea surface pCO 2

Episodic events like hurricanes, storms and frontal- and eddy-driven upwelling can alter the partial pressure of CO2 (pCO2) at the sea surface by entraining subsurface waters into the surface mixed layer (ML) of the ocean. Since pCO2 is a function of total dissolved inorganic carbon (DIC), temperature (T), salinity and alkalinity, it responds to the combined impacts of physical, chemical and biological changes. Here, we present an analytical framework for assessing the relative magnitude and sign in the short-term perturbation of surface pCO2 arising from vertical mixing events. Using global, monthly, climatological datasets, we assess the individual, as well as integrated, contribution of various properties to surface pCO2 in response to episodic mixing. The response depends on the relative vertical gradients of properties beneath the ML. Many areas of the ocean exhibit very little sensitivity to mixing owing to the compensatory effects of DIC and T on pCO2, whereas others, such as the eastern upwelling margins, have the potential to generate large positive/negative anomalies in surface pCO2. The response varies seasonally and spatially and becomes more intense in subtropical and subpolar regions during summer. Regions showing a greater pCO2 response to vertical mixing are likely to exhibit higher spatial variability in surface pCO2 on time scales of days.

Pathways of anthropogenic carbon subduction in the global ocean

The oceanic uptake of anthropogenic carbon is tightly coupled to carbon subduction, i.e., the physical carbon transfer from the well-ventilated surface ocean to its interior. Despite their importance, pathways of anthropogenic carbon subduction are poorly understood. Here we use an ocean carbon cycle model to quantify the mechanisms controlling this subduction. Over the last decade, 90% of the oceanic anthropogenic carbon is subducted at the base of the seasonally varying mixed layer. Vertical diffusion is the primary mechanism of this subduction (contributing 65% of total subduction), despite very low local fluxes. In contrast, advection drives the spatial patterns of subduction, with high positive and negative local fluxes. Our results suggest that vertical diffusion could have a leading role in anthropogenic carbon subduction, which highlights the need for an accurate estimate of vertical diffusion intensity in the upper ocean to further constrain estimates of the future evolution of carbon uptake.