José María Blanco

Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use

Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μ(max) ) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10(6) μm(3) in cell volume (V(cell) ), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μ(max) and carbon-specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V(maxN) ) scales isometrically with V(cell) , whereas nitrogen minimum quota scales as V(cell) (0.84) . Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume-specific V(maxN) compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon-independent, size-related constraints in nutrient uptake, requirement and assimilation.

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.

Physical gradients and spatial variability of the size structure and composition of phytoplankton in the Gerlache Strait (Antarctica)

The patterns of variability of the size structure and functional composition of phytoplankton are closely linked to physical gradients in the coastal waters of the Gerlache Strait (Antarctic Peninsula). Different stratified conditions are found at both extremes of the Strait, separated by a large, rather vertically homogeneous body of water. In the SW extreme close to Bellingshausen Sea, a cold upper layer representative of ice melting influence is dominated by microplankton (20–100mm ESD) biomass, mainly composed of colonies or aggregates of Phaeocystis in bloom stage. In the NE extreme close to Bransfield Strait, a warm surface layer has a 10–20mm nano planktonic community dominated by Cryptomonas and a diverse assemblage of ultraplanktonic (2–10mm) flagellates. In this area, microplankton larger than 40mm ESD is almost absent at the sample volume studied. Also in terms of biomass, mixed central waters are dominated by microplankton, particularly large diatoms. Our results show that diatoms and microplankton are the dominant categories under turbulent conditions, with typical flagellate blooms (Phaeocystis and Cryptomonas) under particularly stratified conditions. In terms of numerical abundance, however, ultraplanktonic flagellates usually dominate the community, giving a bell shaped size-abundance spectra in all the regions and supporting previous suggestions of flagellate dominance even under mixed-layer conditions. r 2001 Published by Elsevier Science Ltd.

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.

The influence of external perturbations on the functional composition of phytoplankton in a Mediterranean reservoir.

The changes in abundance and composition experienced by phytoplankton communities in lakes and reservoirs occur in response to variations in the physical (light climate or energy) and the chemical (nutrient availability or resources) constraints for algal growth. Mediterranean reservoirs are very dynamic systems, subject to frequent changes in the physical environment as a result of water management operations, which suggests that phytoplankton communities might also undergo frequent changes. The phytoplankton community composition, abundance and seasonal dynamics of El Gergal, a medium-size Mediterranean reservoir, is analyzed and interpreted in terms of changes in the nutrient-energy balance. It is demonstrated that the seasonal scale changes in the physical environment trigger the seasonal predictable autogenic dynamics of the phytoplankton community. In addition, frequent short-term external perturbations of the physical environment may also induce allogenic shifts and reversions in the succession. The physical changes occur mainly as a result of variations in the outflows. Results are discussed in terms of phytoplankton functional groups life cycle strategies and water quality management.

Predation and competition effects on the size diversity of aquatic communities

Body size has been widely recognised as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zooplankton and fish assemblages in 13 data sets from freshwater and marine sites with the aim to assess whether there is a general trend in the effect of predation and resource competition on body size distribution across a wide range of aquatic ecosystems. We used size diversity as a measure of the shape of size distribution. Size diversity was computed based on the Shannon-Wiener diversity expression, adapted to a continuous variable, i.e. as body size. Our results show that greater predation pressure was associated with reduced size diversity of prey at all trophic levels. In contrast, competition effects depended on the trophic level considered. At upper trophic levels (zooplankton and fish), size distributions were more diverse when potential resource availability was low, suggesting that competitive interactions for resources promote diversification of aquatic communities by size. This pattern was not found for phytoplankton size distributions where size diversity mostly increased with low zooplankton grazing and increasing nutrient availability. Relationships we found were weak, indicating that predation and competition are not the only determinants of size distribution. Our results suggest that predation pressure leads to accumulation of organisms in the less predated sizes, while resource competition tends to favour a wider size distribution.

Effects of Ultraviolet Radiation on Carbon Fixation in Antarctic Nanophytoflagellates

Carbon fixation in Antarctic nanoflagellates dominated by cryptomonads collected during a summer cruise in 1995 decreased after short‐term exposition (3 h) under both UVA and UVA + UVB radiation compared to white light. The dose applied with artificial lamps was within the range of the natural UV radiation measured at the surface during the cruise. The depletion of C fixation was higher after UVA + UVB than after UVA alone. The inhibition of carbon fixation in the laboratory depended on the time of sample collection and, consequently, on the UV dose received in the natural environment before sampling. Thus, the cells collected in the morning showed 82% of inhibition by UVA + UVB but that collected at noon showed only 72%. The same effect was observed by UVA: 72% of inhibition in the morning samples and 62% at noon. Thus, photoprotection mechanisms seem to be operating during the day protecting the cells against a rise in UV radiation. Red fluorescence (attributed to chlorophyll) per cell, as determined by flow cytometry, was not affected by UV, however, orange fluorescence (attributed to phycoerythrin) increased clearly after UV radiation compared to that in white light. The increment of orange fluorescence was higher after UVA than after UVA + UVB radiation. The rapid increase in fluorescence emission could be due to an uncoupling of energy transfer and it is suggested as a protective mechanism against UV radiation by absorbing UV radiation.

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.