Francisco Jiménez-Gómez

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–biological coupling in the Strait of Gibraltar

This study presents a joint analysis of the distributions of some biogeochemical variables and their relation to the hydrodynamics of Gibraltar Strait. It is a synthesis paper that brings together many results obtained during CANIGO project. We show the role of hydrodynamics as a forcing agent for the plankton community structure in the Strait, with emphasis on the two physical processes that we propose as key factors for the coupling: interface position and oscillations, and mixing processes along the Strait. As a general pattern, autotrophic plankton biomass increases at the Strait from southwest to northeast, a tendency that coincides with a gradual elevation of the interface depth in the same direction. The different mechanisms of mixing that take place in the Strait are briefly reviewed: The occurrence of the internal hydraulic jump is an important mechanism of mixing constrained to the spring tide situations, but other processes such as the generation of arrested internal waves of wavelength around 1 km are proposed as a complementary mixing mechanism, particularly during neap tides situations. Both mechanisms, the elevation of the pycnocline and these mixing events, can enhance biological productivity and biomass accumulation on the northeastern sector of the Strait, since phytoplankton cells are there packaged in a water mass with sufficient light and nutrients and smaller advective velocity. There is a clear north–south difference in the biological response to these upwelling episodes in the eastern section, with high nutrient and low chlorophyll in the south and the opposite in the north. The deeper interface and the greater water speed are the proposed reasons for this lower nutrient uptake on the southeastern sector. Finally, the temporal scales of variation of the mixing events, the influence of its periodicity on the productivity of the area and the influence of these upwelling episodes in the nearest Albor! an Sea are discussed. r 2002 Elsevier Science

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.

Distribution of Virus-like Particles in an Oligotrophic Marine Environment (Alboran Sea, Western Mediterranean)

Viruses are abundant in a variety of aquatic environments, often exceeding bacterial abundance by one order of magnitude. In the present study, the spatial distribution of viruses in offshore waters of the Alboran Sea (Western Mediterranean) have been studied to determine the relationships between viruses and host communities in this oligotrophic marine environment. Viral abundance was determined using two methods: (i) epifluorescence light microscopy using the dsDNA binding fluorochrome DAPI, and (ii) direct counts by transmission electron microscopy (TEM). The results obtained were significantly different; the highest viral counts were obtained by mean of TEM analyses. In all the samples tested the number of viruses was exceeded by the bacterial concentrations, with a ratio between viral and bacterial titers varying between 1.4 and 20. VLP (virus-like particle) counts were not significantly correlated (p > 0.001) with chlorophyll a concentration or the abundance of cyanobacteria. However, there was a positive and significant correlation with bacterial abundance (p < 0.001). The analysis of size and morphology of viral particles by TEM and the correlation obtained between the numbers of VLP and bacteria suggest that the majority of the viral particles in the Alboran Sea are bacteriophages. None of the indirect evidence suggested that eukaryotic algae or cyanobacteria were important host organisms in these waters.

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.

Exchange of planktonic biomass through the Strait of Gibraltar in late summer conditions

In order to estimate plankton biomass transport through the Gibraltar Strait, plankton biomass and velocity profiles were measured at three stations located in the eastern side of the Strait as a part of the CANIGO project. Abundance and biomass measurements were carried out for autotrophic (Prochlorococcus, Synechococcus and eukaryotic piconano- and microplankton) and heterotrophic (bacteria and nanoflagellates) organisms, in September 1997. Biomass and velocity decreased from the surface to deeper water. Highest biomass concentration was observed at the northern station ð0:12 g C m � 3 ), whereas maximum mean velocities ð80 cm s � 1 ) were found at the central and southern stations. Biomass transport is estimated with a approach with a 10 -m resolution in the vertical and three subareas of approximately 5 km in the horizontal direction. Estimate of plankton biomass transports towards the Mediterranean and the Atlantic are 5570 and 1140 tonnes C day � 1 ; respectively. The former is co-dominated by heterotrophic bacteria (37%) and autotrophic nanoplankton (42%), while the latter is dominated by heterotrophic organisms like bacteria (75%) and heterotrophic nanoflagellates (14%). The variation during a one-day period of the biomass transport estimate at the central part of the Strait was explored. Also, in order to estimate the influence of spatial distribution of both biomass and velocity in the transport estimates, a comparison of our results with other possible estimates performed with less spatial resolution is carried out. The results confirms that both temporal and spatial resolution are key factors for transport estimates of inhomogeneous distributed variables through the Strait. r 2002 Elsevier Science Ltd. All rights reserved.

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.

Effect of Drought Conditions on Plankton Community and on Nutrient Availability in an Oligotrophic High Mountain Lake

Abstract Natural water level fluctuations (WLFs) are an inherent characteristic of Mediterranean inland waters, which are projected to be amplified by global climate change. La Caldera (Sierra Nevada National Park, Spain) is an oligotrophic high mountain lake (3050 m a.s.l.) that has experienced large fluctuations in water volume (13–100%) during the past 20 years due to irregular annual precipitation patterns (371–1816 mm). Because of the lakes cold and dilute abiotic environment, it is likely susceptible to projected increases in global temperature and represents an ideal sentinel of global change. We analyze the effect of WLFs on water quality and on plankton community in La Caldera to better understand the potential effects of recurrent droughts (3 droughts in a 20-year period) on lake ecology. We have found significantly positive effects of WLFs on total phosphorus (TP) concentrations. There was extreme variability in TP concentrations during three recurrent droughts (1995, 1999, and 2005) reflecting sediment resuspension. However, the data also suggest that this was not the only source of phosphorus. Extremely high P-enriched atmospheric dust inputs could have maintained the abnormally high TP in-lake concentrations measured during 2005. The data indicate that recurrent droughts have reduced lake resistance to TP changes but have increased lake resistance to total nitrogen (TN) changes, which supports the idea that a P-enriched atmospheric dust inputs during 2005. An increase in dissolved inorganic nitrogen (DIN)∶TP mass ratio after 2005 was observed, revealing a higher ecosystem homeostasis of this ratio.

Can a copper sulphate pulse below toxic threshold change plankton communities

More field relevant ecological assessments, apart from single species tests using standard species, are needed to better predict agrochemical effects at higher ecosystems levels. Therefore, an experiment using a non-target aquatic community was used to test the hypothesis of the negative effect of a single pulse of copper sulfate on plankton abundance, structure, richness and diversity endpoints. Microcosms (20 l volume) were established during 21 days of experimentation, using six replicates for controls and with two concentrations of copper sulfate (High treatment, H: 20 µg Cu l−1; and Low treatment, L: 2 µg Cu l−1), both within the copper legal threshold following the Water Framework Directive (2000). The general lineal model found significant differences at the phytoplankton abundance endpoint at the end of the experiment, with an increase of phytoplankton abundance in L treatments dominated by the smaller cell size class. The principal response curve on zooplankton data, despite being insignificant, pointed out some dissimilarities in abundance and structure between treatments and controls: treatments showed lower abundances and were richer in cladocerans and copepods than the control microcosm where rotifers and nauplii were dominant. This indicated that trends change in community structure due to the effects of copper sulfate, and that even if the copper concentrations under study were within legal limits, they showed potential to induce changes in planktonic communities.