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Author

Ramiro A. Varela

Bio: Ramiro A. Varela is an academic researcher from University of Vigo. The author has contributed to research in topics: Phytoplankton & Bay. The author has an hindex of 9, co-authored 17 publications receiving 322 citations.
Topics: Phytoplankton, Bay, Pycnocline, Upwelling, Hydrography

Papers
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Journal ArticleDOI
TL;DR: The hydrography and dynamics of the Rias Baixas adjacent shelf region is reviewed in this article with the aim to serve as a general ‘state-of-the-art’ reference and to help introduce several topic-related articles in this special volume.

75 citations

Journal ArticleDOI
TL;DR: In this paper, the authors determined photosynthetic electron turnover rates, ETRs, from ca. 100 FRR fluorescence water-column profiles throughout the subtropical and tropical Atlantic during six Atlantic Meridional Transect cruises (AMT 6, May-June 1998, to AMT 11, September-October 2000).
Abstract: Photosynthetic electron transport directly generates the energy required for carbon fixation and thus underlies the aerobic metabolism of aquatic systems. We determined photosynthetic electron turnover rates, ETRs, from ca. 100 FRR fluorescence water-column profiles throughout the subtropical and tropical Atlantic during six Atlantic Meridional Transect cruises (AMT 6, May–June 1998, to AMT 11, September–October 2000). Each FRR fluorescence profile yielded a water-column ETR-light response from which the maximum electron turnover rate ( ETR RCII max ) , effective absorption (σPSII) and light saturation parameter (Ek) specific to the concentration of photosystem II reaction centres (RCIIs) were calculated. ETR RCII max and Ek increased whilst σPSII decreased with mixed-layer depth and the daily integrated photosynthetically active photon flux when all provinces were considered together. These trends suggested that variability in maximum ETR can be partly attributed to changes in effective absorption. Independent bio-optical measurements taken during AMT 11 demonstrated that σPSII variability reflects taxonomic and physiological differences in the phytoplankton communities. ETR RCII max and Ek, but not σPSII, remained correlated with mixed-layer depth and daily integrated photosynthetically active photon flux when data from each oceanic province were considered separately, indicating a decoupling of electron turnover and carbon fixation rates within each province. Comparison of maximum ETRs with 14C-based measurements of Pmax further suggests that light absorption and C fixation are coupled to differing extents for the various oligotrophic Atlantic provinces. We explore the importance of quantifying RCII concentration for determination of ETRs and interpretation of ETR-C fixation coupling.

51 citations

Journal ArticleDOI
TL;DR: In this paper, a three-dimensional, non-linear, baroclinic model is described and tested to study the residual circulation and the thermohaline distribution of the Ria de Vigo (NW Spain) at short time scales and under different wind stress regimes.
Abstract: A three-dimensional, non-linear, baroclinic model is described and tested for the first time to study the residual circulation and the thermohaline distribution of the Ria de Vigo (NW Spain) at short time scales and under different wind stress regimes. Two markedly different realistic scenarios were chosen: northerly upwelling-favourable winds and southerly downwelling-favourable winds. The numerical experiments carried out indicate that the hydrodynamic regime of the Ria de Vigo is mostly a consequence of wind events. As could be expected, moderately strong North winds reinforce the normal (positive) Ria circulation, while winds blowing from the South, when sufficiently strong, reverse the typical circulation pattern and reduce the characteristic outgoing velocities and the flushing time inside the Ria. The temperature and salinity fields generated by the model in both situations were compared with observations and found to be in qualitatively good agreement, supporting the 3D velocity field distribution.

38 citations

Journal ArticleDOI
01 Sep 2005
TL;DR: In this paper, the authors used wind data measured at six meteorological stations to study their spatial representativity on atmosphere-ocean interactions along the western shelf of Galicia and inside the Ria of Vigo.
Abstract: We used wind data measured at six meteorological stations to study their spatial representativity on atmosphere–ocean interactions along the western shelf of Galicia and inside the Ria of Vigo. The correlation as a function of distance and complex empirical orthogonal functions of the wind field were computed, allowing us to divide the region into three sub-domains: the open ocean, the shelf and the Ria of Vigo. Afterwards, wind gradients among stations were computed to determine specific spatial wind features: results showed an expected wind channelling effect inside the Ria of Vigo, but also a zonal onshore negative frictional gradient on the shelf that generates Ekman pumping during upwelling favourable winds. Finally, the Ekman transport estimated from meteorological stations was compared with measured transport from two moorings located inside the Ria of Vigo and on the shelf, and we determined that the meteorological station most representative of the wind field in the shelf is the one installed in a moored buoy on the shelf.

38 citations

Journal ArticleDOI
01 May 2005
TL;DR: The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplank ton, and the entire region under study showed net autotrophic community.
Abstract: Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (101N–101S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From2 1 Nt o 51S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (41mM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated 14 C primary production ranged from � 200 mg Cm � 2 d � 1 in the TTS region to � 1300 mg C m � 2 d � 1 in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m � 2 , respectively). Picophytoplankton were the main contributors (460%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m � 2 d � 1 ) three-fold larger than net production measured in the TTS region (370 mg C m � 2 d � 1 ). The entire region under study showed net autotrophic community

35 citations


Cited by
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Journal ArticleDOI
TL;DR: It is now becoming clearer that in situ values of Fv/Fm and σPSII also contain taxonomic information, and these values have been utilised to examine the effects of physiological stress on the photosynthetic apparatus of phytoplankton in an ever growing number of fluorescence-based studies.
Abstract: Introduction of active chlorophyll a fluo- rescence protocols, in particular fast repetition rate (FRR) fluorometry, to oceanography and limnology 15 yr ago has enabled rapid assessment of photosyn- thetic physiology in situ. The FRR protocol generates simultaneous measurements of Photosystem II (PSII) effective absorption cross sections (termed σPSII) and photochemical efficiency (termed Fv/Fm). Both Fv/Fm and σPSII measurements have been utilised to examine the effects of physiological stress on the photosynthetic apparatus of phytoplankton in an ever growing num- ber of fluorescence-based studies. However, it is now becoming clearer that in situ values of Fv/Fm and σPSII also contain taxonomic information. Here, we present

334 citations

Journal ArticleDOI
TL;DR: The data clearly show that the diatom utilizes a different strategy in the dissipation of excessively absorbed energy compared with the green alga, and in a fluctuating light climate, the differences between green algae and diatoms in the efficiency of biomass production per photon absorbed are caused by the different amount of alternative electron cycling.
Abstract: Summary • The balance of energy flow from light absorption into biomass was investigated under simulated natural light conditions in the diatom Phaeodactylum tricornutum and the green alga Chlorella vulgaris. • The energy balance was quantified by comparative analysis of carbon accumulation in the new biomass with photosynthetic electron transport rates per absorbed quantum, measured both by fluorescence quenching and oxygen production. The difference between fluorescence- and oxygen-based electron flow is defined as ‘alternative electron cycling’. • The photosynthetic efficiency of biomass production was found to be identical for both algae under nonfluctuating light conditions. In a fluctuating light regime, a much higher conversion efficiency of photosynthetic energy into biomass was observed in the diatom compared with the green alga. • The data clearly show that the diatom utilizes a different strategy in the dissipation of excessively absorbed energy compared with the green alga. Consequently, in a fluctuating light climate, the differences between green algae and diatoms in the efficiency of biomass production per photon absorbed are caused by the different amount of alternative electron cycling.

226 citations

Journal ArticleDOI
TL;DR: In this article, a model linking phytoplankton absorption to size classes (PSCs) was developed based on the observation that the absolute value of a ph (443) co-varies with the spectral slope of phytolankton absorbance in the range of 443-510 nm.

193 citations

Journal ArticleDOI
TL;DR: In this paper, a finite difference numerical model is used for the estimation of the Ria de Muros tidal flow velocities and the corresponding power densities, which is validated with in situ velocity measurements performed by means of an acoustic Doppler current profiler (ADCP).

172 citations

Journal ArticleDOI
TL;DR: The photoacclimation response by Symbiodinium was highly variable between algal types for all bio-physical and for many bio-optical measurements; however, a general preference to modifying reaction centre content over effective antennae-absorption was observed.
Abstract: Light is often the most abundant resource within the nutrient-poor waters surrounding coral reefs. Consequently, zooxanthellae (Symbiodinium spp.) must continually photoacclimate to optimise productivity and ensure coral success. In situ coral photobiology is becoming dominated by routine assessments using state-of-the-art non-invasive bio-optical or chlorophyll a fluorescence (bio-physical) techniques. Multiple genetic types of Symbiodinium are now known to exist; however, little focus has been given as to how these types differ in terms of characteristics that are observable using these techniques. Therefore, this investigation aimed to revisit and expand upon a pivotal study by Iglesias-Prieto and Trench (1994) by comparing the photoacclimation characteristics of different Symbiodinium types based on their bio-physical (chlorophyll a fluorescence, reaction centre counts) and bio-optical (optical absorption, pigment concentrations) ‘signatures’. Signatures described here are unique to Symbiodinium type and describe phenotypic responses to set conditions, and hence are not suitable to describe taxonomic structure of in hospiteSymbiodinium communities. In this study, eight Symbiodinium types from clades and sub-clades (A–B, F) were grown under two PFDs (Photon Flux Density) and examined. The photoacclimation response by Symbiodinium was highly variable between algal types for all bio-physical and for many bio-optical measurements; however, a general preference to modifying reaction centre content over effective antennae-absorption was observed. Certain bio-optically derived patterns, such as light absorption, were independent of algal type and, when considered per photosystem, were matched by reaction centre stoichiometry. Only by better understanding genotypic and phenotypic variability between Symbiodinium types can future studies account for the relative taxonomic and physiological contribution by Symbiodinium to coral acclimation.

171 citations