Valeriano Rodríguez

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.

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.

Role of ciliates, flagellates and bacteriophages on the mortality of marine bacteria and on dissolved-DNA concentration in laboratory experimental systems

Several marine bacteriophages, a ciliate (Uronema sp.), and a flagellate (Pseudobodo sp.) were used to study comparatively the grazing and clearance rates of four marine bacteria. Bacteria were fluorescently labelled using rhodamine isothiocyanate. The results obtained indicate that bacteriophages and flagellates caused a significantly higher (P<0.025) decrease in the number of marine bacteria compared to the ciliate Uronema sp. The role of bacteriophages, ciliates and flagellates in the production of dissolved-DNA (D-DNA) was also studied. The interaction between both bacteriophages and protists with marine bacteria are responsible for an increase of the D-DNA concentration, although a direct relationship between D-DNA concentration and predator number was only obtained from experiments performed with Uronema sp.

Relations between chlorophyll, phytoplankton cell abundance and biovolume during a winter bloom in Mediterranean coastal waters

Abstract The relationships between three descriptors of phytoplankton biomass (chlorophyll, cell abundance and biovolume) during a winter bloom in the Mediterranean coastal waters of the Bay of Malaga (Southern Spain) were investigated. The relations observed are non-linear and imply that chlorophyll per cell, average cell volume and chlorophyll per unit cell volume are negative power functions of cell abundance. Non-linearity seems to be the result of time-delays in the initiation of the exponential growth phase for the three variables. The interplay of ontogenetic versus phylogenetic adaptations during that phase is discussed. Biovolume, obtained by image analysis from the integration of both numerical abundance and the size structure of the population, seems to be an ideal way of describing changes in phytoplankton biomass. The need for a transformation of image-analysis volumes to organic content is pointed out.

Estimates of Secondary Production in a Co-Existent Group of Acartia Species (Copepoda, Calanoida)

Secondary production of a congeneric species group of Acartia Dana, 1846, was estimated during a summer-autumn period (from June to October 1989) using a calculation based on the size-frequency distribution. Our study area was Malaga harbour (SE of Spain), a shallow (10 m depth) and semi-isolated ecosystem with restricted circulation. Acartia was the dominant copepod in the water column. Co-occurring species were Acartia grani, Acartia clausi, and Acartia discaudata var. mediterranea. Estimated average production was 29.12 mg dry wt m-3 d-1 (13.10 mg C m-3 3 d-1). The high values of chlorophyll concentration suggest that copepods can always find sufficient food for growth at maximum rate. The results obtained were similar to previous estimates for other Acartia species in coastal areas.

EL PLANCTON DE LAS LAGUNAS DE SIERRA NEVADA: NECESIDAD DE ESTUDIOS TRANSDISCIPLINARES

La correcta valoracion de los procesos ecologicos, como base para el mantenimiento de la integridad ecologica del ecosistema, se ha convertido en una de las tareas de mas dificil cumplimiento en las labores de conservacion de los ecosistemas mediterraneos. La busqueda y desarrollo de herramientas en la deteccion y prevencion de alteraciones ambientales nos ha llevado a centrar la atencion en las lagunas de alta montana del Parque Nacional de Sierra Nevada, ecosistemas que presentan un singular valor ecologico dentro del ambiente mediterraneo, por su caracter oligotrofico, por su situacion a gran altitud y por la adaptacion de sus comunidades a condiciones ambientales extremas. El presente texto describe una propuesta de estudio integral en estos ecosistemas con una aproximacion basada en el analisis de las estructuras de tamano de las comunidades del plancton como herramienta descriptora y predictiva de las respuestas de esas comunidades a los cambios ambientales.