Javier Ruiz

Mesoscale vertical motion and the size structure of phytoplankton in the ocean.

Phytoplankton size structure is acknowledged as a fundamental property determining energy flow through ‘microbial’ or ‘herbivore’ pathways. The balance between these two pathways determines the ability of the ecosystem to recycle carbon within the upper layer or to export it to the ocean interior. Small cells are usually characteristic of oligotrophic, stratified ocean waters, in which regenerated ammonium is the only available form of inorganic nitrogen and recycling dominates. Large cells seem to characterize phytoplankton in which inputs of nitrate enter the euphotic layer and exported production is higher. But the size structure of phytoplankton may depend more directly on hydrodynamical forces than on the source of available nitrogen. Here we present an empirical model that relates the magnitude of mesoscale vertical motion to the slope of the size–abundance spectrum of phytoplankton in a frontal ecosystem. Our model indicates that the relative proportion of large cells increases withxa0the magnitude of the upward velocity. This suggests that mesoscale vertical motion—a ubiquitous feature of eddies and unstable fronts—controls directly the size structure of phytoplankton in the ocean.

Patterns in the size structure of the phytoplankton community in the deep fluorescence maximum of the Alboran Sea (southwestern mediterranean)

The Alboran Sea (southwestern Mediterranean) exhibits strong horizontal and vertical gradients associated with macroscale and mesoscale physical structures due to the input of surface Atlantic waters into the Mediterranean basin. During the summer of 1992, two anticyclonic and two cyclonic areas were found with fluorescence maxima (DFM) below the seasonal thermocline (ST). Although the depth of the ST is fairly constant, the position and intensity of the DFM is more variable, with a tendency to deepening and smoothing in the anticyclonic gyres. The position of the Atlantic–Mediterranean interface (AMI) can be used as a tracer for cyclonic or anticyclonic dynamics and their potential biological effects. A shallow AMI indicates divergence or upwelling dynamics and coincides with the highest fluorescence intensity, chlorophyll concentration and phytoplankton biovolume in the DFM. Under the conditions typical of the two anticyclonic gyres, the contrary is found. The size structure of phytoplankton shows significant differences between cyclonic and anticyclonic structures. Log-transformed size-abundance spectra can be adequately described by linear models with slopes of −0.78 for cyclonic and −0.93 for anticyclonic structures. The integration of size-abundance spectra indicates that picoplankton biovolume in the DFM is independent of the type of circulation or dynamics, whereas nanoplankton and, particularly, microplankton increase their absolute and relative presence in the DFM under cyclonic or upwelling dynamics.

The boiling-water phenomena at Camarinal Sill, the strait of Gibraltar

Abstract A detailed description of high-amplitude steady topographic internal waves recorded at Camarinal Sill during a survey on the R.V. “Investigador” is presented. These internal waves are generated during the maximum outflow (westward) stage of the tidal current and remain over the sill for more than 4xa0h until the outflow slackens, then being released towards the Mediterranean. Their amplitudes are comparable to the well-known internal bore of Camarinal Sill, occurring during maximum outflow during spring tides. However, they respond to a different physical origin and their spatial features are also quite different. In fact, the favourable hydraulic conditions for the generation of steady waves over the sill inhibit the internal bore generation, and vice versa. Analysis of the observations suggests that steady internal waves are the result of a resonant response of the stratified fluid over the sill to the forcing of the flow on the across-sill topography. An important consequence of the steady internal waves with clear biological implications is the significant mixing phenomena that are induced. Mixing is enabled by an enhancement of the shear at the trough together with a significant induced vertical advection.

Surface distribution of chlorophyll, particles and gelbstoff in the Atlantic jet of the Alboran Sea: from submesoscale to subinertial scales of variability

The surface distribution of light attenuation due to particles (c) as well as chlorophyll-a and gelbstoff fluorescence (Fch and Fcd, respectively) were recorded during an OMEGA (EU funded, MAST III project) cruise in the northwestern Alboran Sea through a high spatial (zonally separated by 10 km and virtually meridionally continuous) and temporal (about 3 days between each of the three repeated surveys made in the zone) resolution sampling design. The distributions obtained for these variables were tightly linked to the physical forcing at the different scales that the sampling design was able to resolve. Low values dominate the quasi permanent anticyclonic gyre occupying the western Alboran Sea, whereas the frontal zone directly affected by the entrance of the Atlantic jet depicts much higher records for c, Fch and Fcd. High geostrophic Froude numbers in the jet, and the subsequent increase in turbulence diffusion of nutrients towards the surface, cannot alone justify this spatial distribution. Instead, high phytoplankton concentration at the jet could also result from the entrainment and advection of water from the upwelling zone at the Spanish coast. However, T–S characteristics suggest that this is neither the most important process for the biological enrichment of the jet, so that other mechanisms such as vertical ageostrophic velocities at the edge of the gyre must also be considered. Due to the time needed for phytoplankton growth, the intense horizontal velocities associated to the jet can decouple the sectors where deep nutrient-rich waters reach the surface from sectors where high values of the recorded variables are observed. The decoupling hinders a differentiation of this fertilization mechanism from other possible alternatives as mixing at the sills in the Strait of Gibraltar. In the third survey, the spatial structure of surface warm waters in the gyre and cold waters in the front became less apparent. ADCP data show a southward migration of the jet in a fluctuation probably related to transient states in the Atlantic jet and western Alboran gyre system. The qualitative response of c, Fch and Fcd to these scales of variability was very similar and close to the changes observed in temperature. However, the values of Fcd varied in a much narrower range than c or Fch (a factor of 2 and 10, respectively), which indicates a distinct control for the abundance of Gelbstoff. This control dumps the range of variability in the western Alboran and its origin is discussed in the context of photobleaching or bacterial degradation of these substances.

Diatom aggregate formation and fluxes: a modeling analysis under different size-resolution schemes and with empirically determined aggregation kernels

A hierarchical analysis is presented for evaluating the accuracy of different formulations to simulate diatom aggregate formation and diagnose its associated carbon fluxes. We find that, for diagnostic purposes, a two-class arrangement of sizes (small particles and aggregates) is an adequate compromise between the accuracy of estimated fluxes and the level of resolution implemented for particle sizes. Ignoring the existence of aggregates severely underestimates fluxes, whereas increasing the size resolution (up to 7 size classes) yields a small improvement in accuracy. We demonstrate that this two-size-class arrangement is also the minimum resolution for accurately modeling the formation of aggregates. This arrangement requires only one aggregation kernel, which we estimate from experimental data without imposing any assumption for the interaction between particles. Although the kernel is robust to changes in the nature of the particles, the simulations obtained with this approach are very sensitive to the initial concentration of aggregates implemented to run the model. This sensitivity to initial conditions does not appear in a model with higher size resolution whose kernel tensor for aggregation has also been derived from experimental mesocosm data through an inverse modeling procedure. Although the level of accuracy achieved in a simulation with this kernel tensor seems promising, its robustness requires further tests in conditions other than a mesocosm experimental design.

What generates daily cycles of marine snow

The recent discovery of daily cycles in the concentration of marine aggregates raises questions regarding the process producing the cycles and whether or not aggregation theory is able to predict them. A model of particle dynamics was used to study these questions. The model incorporated particle aggregation, break-up and sedimentation as well as diel growth and grazing. Three main processes considered to be possible causes for daily cycles of marine aggregates (growth, turbulence and grazing) were tested with the model. The results demonstrated that aggregation theory is able to predict daily cycles of marine aggregates. Neither the daily cycle of growth, nor that of grazing, was able to generate daily cycles alone. However, daily cycles of marine turbulence in the mixed layer caused a clear cyclic behaviour of particulate matter in the model. Therefore, these results suggested that diel variation of turbulence is the best candidate to explain the daily cycles of aggregates observed in the sea.

Vertical patterns of phytoplankton size distribution in the Cantabric and Balearic Seas

Abstract In this paper we analyze plankton size distribution (2–100 μm of equivalent spherical diameter) under different hydrographic structures, such as the seasonal thermocline of the Cantabric and Balearic seas and the oceanographic front present in the Balearic Sea. Nannoplanktonic fraction (2–20 μm) tends to present biomass maxima at shallower depths than the chlorophyll maximum in stratified waters, as well as in the more productive waters near a front. The slope of the normalized size-biomass spectrum was more negative in these zones of phytoplankton biomass maxima indicating a higher proportion of smaller cells. The main hypotheses to explain chlorophyll and biomass maxima as well as phytoplankton accumulation in productive areas are reviewed. According to the observed results, the most suitable mechanism to explain subsurface biomass maxima is the active net growth of the smaller phytoplanktonic cells. The importance of the methodology employed for analyzing the size distribution of plankton communities and its relationship with hydrographic variability are also discussed.