Calvo-Diaz Alejandra

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Calvo-Diaz
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Alejandra
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  • Preprint
    Increasing importance of small phytoplankton in a warmer ocean
    ( 2009-03-19) Moran, Xose Anxelu G. ; Lopez-Urrutia, Angel ; Calvo-Diaz, Alejandra ; Li, William K. W.
    The macroecological relationships between marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 μm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric size-scaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the -0.6 to 22ºC range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift towards smaller primary producers in a warmer ocean. Since the fate of photosynthesized organic carbon largely depends on phytoplankton size we anticipate future alterations in the functioning of oceanic ecosystems.
  • Article
    Total and phytoplankton mediated bottom-up control of bacterioplankton change with temperature in NE Atlantic shelf waters
    (Inter-Research, 2010-01-20) Moran, Xose Anxelu G. ; Calvo-Diaz, Alejandra ; Ducklow, Hugh W.
    The regulation of heterotrophic bacterial growth by resource supply (bottom-up control) was temperature-dependent in our analysis of data obtained during 2006 in the euphotic layer of the southern Bay of Biscay (NE Atlantic) continental shelf. The dataset was split into 2 subgroups using 16°C as the boundary between warm and cool waters based on differences in associated physico-chemical conditions, e.g. inorganic nutrient limitation at higher temperatures. The linear regressions between bacterial biomass (BB) and leucine incorporation rates (LIR) were significantly positive in both temperature regimes, thus indicating similar total bottom-up control, albeit with a slightly higher slope in warm waters (0.33 vs. 0.22). However, the relationship of LIR with phytoplankton biomass (chl a), which is an indicator of bottom-up control that is mediated by phytoplankton, was only significant in waters below 16°C. The analysis of bimonthly variations in the BB-LIR and LIR-chl a correlations indicated that the strength of total bottom-up control significantly increased while the role of phytoplankton in supplying DOM to bacteria diminished with mean temperatures over the 12 to 19°C range, suggesting a seasonal switch in the major source of substrates used by bacteria. We show that the abundance of cells with relatively high nucleic acid content (HNA), which are hypothesized to be the most active ones, was positively associated with bacterial production and specific growth rates in cool but not in warm conditions. These results suggest that HNA bacteria are good predictors of bulk activity and production in temperate ecosystems only when the community relies principally on phytoplankton substrates for growth and metabolism.