Belén Arbones

Short-time scale development of a Gymnodinium catenatum population in the Ria de Vigo (NW Spain)

Wind direction and fresh water runoff determine the circulation pattern of the Ría de Vigo (NW Spain), which in turn influence the selection and distribution of its phytoplankton populations. Coastal winds with a south–southwesterly component reverse the positive estuarine circulation in the Ría, causing an off‐shore to in‐shore flow of surface waters and, consequently, the outflow of inner waters via deeper layers. We found that this reversal imposed a selective force on the phytoplankton population: diatoms, which could not counteract the sinking movement of the surface waters, were diminished, while dinoflagellates remained in the water column. From the end of September to the beginning of October 1993, an accumulation of Gymnodimium catenatum Graham was observed coinciding with an intrusion of coastal water induced by westerly winds which provoked a reversal in the circulation of the Ría. The slow reestablishment of the positive estuarine circulation pattern, which was due to a weak coastal upwelling and considerable fresh water runoff, allowed the population of G. catenatum to flourish.

Photosynthetic parameters and primary production in the Bransfield Strait: relationships with mesoscale hydrographic structures

Abstract During January 1994, the photosynthetic response (P–E curves) of phytoplankton in the eastern part of the Bransfield Strait (Antarctica) was studied in relation to the mesoscale hydrographic structures in the area. The most important hydrographic features found in the study area were: (i) the Bransfield Strait front which separates surface Bellingshausen waters from surface Weddell Sea water in the northern part of the Strait; (ii) further to the North, the Weddell–Scotia Confluence north of Elephant Island; (iii) to the South, the ice-edge and the associated lenses of melting waters at the south-eastern part of the sampling area. These three structures were associated with zones of shallower mixing depth ( 1 mg Chl m−3). The photosynthetic response of the phytoplankton was also affected by the hydrographic structures. The phytoplankton of the well-mixed Weddell waters showed a lower light saturation parameter (Ek 100 μmol m−2 s−1). There were no differences in the light saturation parameters of phytoplankton samples from the different water bodies found in the region as demonstrated by a t-test for paired comparisons (0.88>P>0.46). The average Ek was 87±26 μmol m−2 s−1 and not significantly different (t-test NS, P=0.83) from the mean irradiance in the upper mixed layer (Zuml) without the Zuml≥150 m stations. The slope of the PmB vs. α relationship was 59±5 μmol m−2 s−1 (r2=0.66, P 1.5 g C m−2 d−1 between King George Island and Elephant Island.

Photosynthesis, primary production and phytoplankton growth rates in Gerlache and Bransfield Straits during Austral summer: cruise FRUELA 95

Bio-opticalparameters, primary production, and phytopl ankton growth rates were determined in the southern part of the Bellingshausen Sea, the western part of the Bransfield Strait, and the Gerlache Strait, during December 1995 and early January 1996. Three water bodies were present at the surface (o100 m); Bellingshausen warm waters (BWW), Bellingshausen cold waters (BCW), and transitional Weddell waters (TWW), which were separated by a thermal front. High chlorophyll concentrations were found in the upper mixed layer (UML) of the BCW waters (5.5774.04 mg m � 3 ), with intermediate values in BWW (2.8571.24 mg m � 3 ) and the lowest values in the TWW (1.5370.94 mg m � 3 ). The broadband photosynthetic parameters ðP B ; a B and EKPARÞ were highest in BWW and lowest in TWW, except for EKPAR; which did not show significant differences between water bodies. Spectralbio-opticalparameters ðaph; f max and EKPURÞ showed a similar distribution to chlorophyll concentrations except EKPUR; which was highest in BWW. A comparison between light-saturation parameters and water-column irradiance suggests that photosynthesis was not light-limited in the mixed layer. However, transitory situations exist where photosynthesis could be close to limitation at the bottom of the photic layer when high chlorophyll concentrations were present in the surface layers. The high maximum quantum yields (0.07370.032 molC (mol photons) � 1 ) also suggest that iron limitation of photosynthesis should not be occurring in the region during the cruise. The mean primary production rates were 1.1170.68 g Cm � 2 d � 1 in the TWW, 2.2870.98 g C m � 2 d � 1 in the BCW, and 2.6870.94 g C � 2 d � 1 in the BWW. The Gerlache Strait and frontal zones were the most productive, with values analogous to those of upwelling areas. Carbon-specific growth rates in UML (0.4370.16 d � 1 ) were similar to those of temperate seas, neither nutrient- nor light-limited. Growth rates in the UML of BCW (0.5470.20 d � 1 ) were higher than those of TWW (0.3770.08 d � 1 ) and BWW (0.3570.09 d � 1 ). The results indicate that factors as microzooplankton grazing or sinking should control the standing

Action spectrum and maximum quantum yield of carbon fixation in natural phytoplankton populations: Implications for primary production estimates in the ocean

Abstract Spectral and non-spectral measurements of the maximum quantum yield of carbon fixation for natural phytoplankton assemblages were compared in order to evaluate their effect on bio-optical models of primary production. Field samples were collected from two different coastal regions of NW Spain in spring, summer and autumn and in a polar environment (Gerlache Strait, Antarctica) during the austral summer. Concurrent determinations were made of spectral phytoplankton absorption coefficient [ a ph ( λ )], white-light-limited slope of the photosynthesis–irradiance relationships ( α B ), carbon uptake action spectra [ α B ( λ )], broad-band maximum quantum yields ( φ m ), and spectral maximum quantum yields [ φ m ( λ )]. Carbon uptake action spectra roughly followed the shape of the corresponding phytoplankton absorption spectra but with a slight displacement in the blue–green region that could be attributed to imbalance between the two photosystems PS I and PS II. Results also confirmed previous observations of wavelength dependency of maximum quantum yield. The broad-band maximum quantum yield ( φ m ) calculated considering the measured spectral phytoplankton absorption coefficient and the spectrum of the light source of the incubators was not significantly different form the averaged spectral maximum quantum yield [ φ max (λ) ] ( t -test for paired samples, P =0.34). These results suggest that maximum quantum yield can be estimated with enough accuracy from white-light P – E curves and measured phytoplankton absorption spectra. Primary production at light limiting regimes was compared using four different models with a varying degree of spectral complexity. No significant differences ( t -test for paired samples, P =0.91) were found between a spectral model based on the carbon uptake action spectra [ α B ( λ ) — model a] and a model which uses the broad-band φ m and measured a ph ( λ ) (model b). In addition, primary production derived from constructed action spectra [ a c B ( λ ) from a ph ( λ ) and α B (model c) was also not significantly different from that derived from total spectral model a ( t -test for paired samples, P =0.60). It was found, however, that primary production at low light regimes can be strongly overestimated (44%) when a ph ( λ ) is derived from chlorophyll concentrations. A white-light model based on broad-band α B (model d), which does not consider phytoplankton light absorption, yields values 17% lower than those of model a. It is concluded that primary production at light-limited conditions can be computed accurately from broad-band maximum quantum yield estimates or from constructed action spectra provided that a ph ( λ ) is measured. However, given that phytoplankton absorption coefficients are necessary for both approaches and as computations based on φ m showed less variability, we suggest that the maximum quantum yield proxy should be used.