Papers by Maria Huete-Ortega
This study was aimed at identifying macroecological patterns in the relationship
between phytopla... more This study was aimed at identifying macroecological patterns in the relationship
between phytoplankton cell size, abundance and metabolism in 2 marine ecosystems characterised
by marked differences in resource availability and water-column stability. Several patterns
emerged: (1) nearly isometric size-scaling of phytoplankton carbon fixation rate was described for
both open-ocean and coastal ecosystems (mean slope: 1.17 and 0.90, respectively), supporting the
idea that biomass-specific photosynthesis rates are largely independent of cell size; (2) less steep
values for the size-scaling of abundance (mean slope: −0.73) were found in the coastal ecosystem
compared to the open ocean (mean slope: −1.15); (3) large phytoplankton used more photosynthetic
energy than smaller cells in the coastal ecosystem, but a constant flow of energy along the
size spectrum was found in the open ocean; and (4) phytoplankton biomass turnover rates were 1
order of magnitude higher in the coastal ecosystem than in the open ocean, implying physiological
limitation of phytoplankton growth in the oligotrophic ocean. Bottom-up and top-down mechanisms
and their interaction with nutrient supply dynamics were suggested as major factors determining
the contrasting phytoplankton size abundance distributions observed in coastal and openocean
waters
Ecology Letters, 16, 371-379, 2013
The relationship between phytoplankton cell size and abundance has long been known to follow regu... more The relationship between phytoplankton cell size and abundance has long been known to follow regular, predictable patterns in near steady-state ecosystems, but its origin has remained elusive. To explore the linkage between the size-scaling of metabolic rate and the size abundance distribution of natural phytoplankton communities, we determined simultaneously phytoplankton carbon fixation rates and cell abundance across a cell volume range of over six orders of magnitude in tropical and subtropical waters of the Atlantic Ocean. We found an approximately isometric relationship between carbon fixation rate and cell size (mean slope value: 1.16; range: 1.03–1.32), negating the idea that Kleiber’s law is applicable to unicellular autotrophic protists. On the basis of the scaling of individual resource use with cell size, we predicted a reciprocal relationship between the size-scalings of phytoplankton metabolic rate and abundance. This prediction was confirmed by the observed slopes of the relationship between phytoplankton abundance and cell size, which have a mean value of 21.15 (range: 21.29 to 20.97), indicating that the size abundance distribution largely results from the size-scaling of metabolic rate.
Our results imply that the total energy processed by carbon fixation is constant along the phytoplankton size spectrum in near steady-state marine ecosystems.
To ascertain the response of phytoplankton size classes to changes in environmental forcing, we d... more To ascertain the response of phytoplankton size classes to changes in environmental forcing, we determined size-fractionated biomass, carbon fixation and growth (production/biomass) rates in surface waters along the central Atlantic Ocean (26ºN-24 5ºS). As a result of the enhanced input of nutrients into the euphotic layer and the higher water column stability found at the equatorial upwelling, we observed increases not only in phytoplankton biomass and primary production, but also in turnover rates, suggesting nutrient limitation of phytoplankton physiology in the oligotrophic central Atlantic. The phytoplankton groups analyzed (pico-, small nano-, large nano- and micro-phytoplankton) showed different responses to the equatorial environmental forcing, in terms of carbon biomass, primary production and growth rate. Large nano- and micro-phytoplankton consistently showed higher growth rates and carbon fixation to chl a ratios than smaller phytoplankton. We observed a higher stimulating effect of increased nitrate supply on the small phytoplankton growth rates. This observation can be explained by the dynamics of the equatorial upwelling, where the continuous but small nutrient input into the euphotic layer provide a competitive advantage for smaller cells adapted to oligotrophic conditions. The size-fractionated approach shown here reveals important group-specific differences in the response to environmental forcing, which cannot be appreciated in bulk measurements of the whole community.
Two meridional transects were conducted in the tropical and subtropical Atlantic to describe (i) ... more Two meridional transects were conducted in the tropical and subtropical Atlantic to describe (i) the spatial variability of total and size-fractionated (picophytoplankton and phytoplankton >2 mm) chlorophyll a (chl a) concentration and primary production, (ii) the relative contribution of each phytoplankton size fraction to total biomass and carbon fixation, and (iii) the spatial variability of size-fractionated phytoplankton growth rate (P/B) and assimilation number (P/chl a) in the ocean. The highest chl a for both size fractions was observed in the Western Tropical Atlantic province (WTRA), while the lowest chl a was found in the upper mixed layer (UML) of the South Atlantic Tropical gyre (SATL). A similar pattern was found for carbon fixation. Within the SATL, the highest picophytoplankton contribution to total production was recorded at the Deep Chlorophyll Maximum (DCM), while the contribution of phytoplankton >2 mm was higher in the UML. Additionally, the relative contribution of large phytoplankton to total integrated primary production was higher than its contribution to total biomass. Both size fractions depicted maximum P/B and P/chl a in WTRA surface waters. In the SATL province, phytoplankton >2 mm showed the highest P/B and P/chl a along the UML, while picophytoplankton P/B and P/chl a peaked around the DCM. We suggest that the differential impact of light on small and large phytoplankton may help to explain the contrasting dynamics of the two size classes.
To determine the effects of Saharan dust on the abundance, biomass, community structure, and meta... more To determine the effects of Saharan dust on the abundance, biomass, community structure, and metabolic
activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30uN and 30uS in the
central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged,
different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The
predominant type of metabolic response depended on the ecosystem’s degree of oligotrophy and was modulated
by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial
production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger
with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic
waters, became less responsive to dust as the ecosystem’s degree of oligotrophy increased. Given the divergent
consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the
ecosystem’s degree of oligotrophy, stimulate or weaken biological CO2 drawdown. Thus, the biogeochemical
implications of changing dust fluxes might not be universal, but variable through both space and time.
To ascertain the general patterns in phytoplankton size structure of a temperate, coastal ecosyst... more To ascertain the general patterns in phytoplankton size structure of a temperate, coastal ecosystem, we determined the scaling relationship between total abundance and cell size (size spectrum) for nano- and micro-phytoplankton in a shelf station off NW Iberian Peninsula on a monthly basis during the period 1993–2002. The inverse linear relationship between log abundance and log cell size was persistent throughout the water column and across seasonal and inter-annual time scales. In addition, and despite the high productivity and marked temporal variability in water column structure at our study site, departures from linearity in the size spectra were rare. The slope (–0.96) of the overall size spectrum for the entire time series indicated that roughly equal amounts of biomass were present over different logarithmic size classes in the size range considered. The phytoplankton size spectra had similar average slopes during winter mixing, early upwelling, summer stratification and autumn downwelling, suggesting that, under these oceanographic conditions, both nano- and micro-phytoplankton respond similarly to environmental variability. In contrast, significantly less negative slopes were observed during upwelling relaxation, indicating an increased importance of larger cells. Our results illustrate the utility of individual size distributions to provide a synthetic description of phytoplankton community structure in dynamic, non steady-state marine ecosystems.
We analyzed the relationship between population abundance and cell size in phytoplankton assembla... more We analyzed the relationship between population abundance and cell size in phytoplankton assemblages from
coastal, shelf, and open-ocean environments. Our results show that across the entire size spectrum considered,
population abundance increases over two orders of magnitude from subtropical to coastal regions. We find
a highly significant linear relationship between nutrient concentration and the intercept of the log-log relationship
between population abundance and cell size. In contrast to overall patterns reported mainly for vascular plants
and animals, marine phytoplankton diversity does not show any consistent trend along either latitudinal or
productivity gradients. These results imply that large-scale (biogeographic) variations in phytoplankton standing
stocks are controlled by changes in population abundances rather than by systematic variations in species
richness. These findings provide a mechanistic connection among nutrient availability, population dynamics, and
phytoplankton diversity over macroecological scales.
Talks by Maria Huete-Ortega
Phytoplankton primary production and its distribution amongst different size classes play a criti... more Phytoplankton primary production and its distribution amongst different size classes play a critical role in the functioning of pelagic marine ecosystems both from an ecological and biogeochemical point of view. Around a quarter of global marine primary production occurs in tropical and subtropical latitudes of the world’s oceans. Current knowledge about the variability of size-fractionated primary production in the open ocean is constrained because the limited number of cruises covering large temporal and spatial scales and using the same methodological approaches. Here we report estimates of pico-, nano- and micro-phytoplankton primary production obtained during the 2010 Malaspina circumnavigation Expedition. Size-fractionated carbon fixation rates were estimated with the 14C-uptake technique at 5 depths in 147 stations sampled along the tropical and subtropical Atlantic, Pacific and Indian oceans. The estimates carried out in the Indian and South Pacific oceans highlight because, as far as we know, not many direct observations of the size structure and metabolism of phytoplankton communities have been done before in these oceanic regions. The patterns obtained in the size-distribution of phytoplankton primary production were related to changes in environmental forcings, including the input of nutrients into the euphotic zone through vertical diffusion. These results provide new insights about how nutrient supply dynamics to the upper ocean controls marine primary production and therefore influences global biogeochemical cycles.
Classical allometric theory predicts that individual metabolic rates (R) scale with body size (M)... more Classical allometric theory predicts that individual metabolic rates (R) scale with body size (M) following the ¾-power rule or Kleiber’s law, whereby R is proportional to M3/4 Meta-analysis studies covering around 20 orders of magnitude in body size support the universality of this allometric rule for photosynthetic organisms ranging from unicellular to terrestrial plants. However, recent experimental work questions its applicability for the metabolism of natural phytoplankton assemblages. Thus, in order to test if phytoplankton follow the ¾-power rule, we have determined the size scaling of the cell-specific photosynthesis rate during two cruises in the tropical and subtropical Atlantic ocean. Our results revealed that the power relationship between photosynthesis per cell and cell size has an exponent very close to 1 (isometric scaling) in surface waters and not statistically different from ¾ (allometric scaling) in the deep chlorophyll maximum. These vertical differences observed in phytoplankton community metabolism could reflect the fact that light limitation affects larger cells more severely than smaller cells. Our measurements of the metabolism-size scaling relationship have implications to understand the trophic structure of the community and the energy flow through the ecosystem.
Classical allometric theory predicts that individual metabolic rates (R) scale with body size (M)... more Classical allometric theory predicts that individual metabolic rates (R) scale with body size (M) following the ¾-power rule or Kleiber’s law, whereby R is proportional to M3/4 Meta-analysis studies covering around 20 orders of magnitude in body size support the universality of this allometric rule for photosynthetic organisms ranging from unicellular to terrestrial plants. However, recent experimental work questions its applicability for the metabolism of natural phytoplankton assemblages. Thus, in order to test if the phytoplankton follows the ¾-power rule, we have determined the size scaling of the cell-specific photosynthesis rate during one cruise in the tropical and subtropical Atlantic ocean. Our preliminary results revealed that the power relationship between photosynthesis per cell and cell size has an exponent very close to 1 (isometric scaling) both in surface waters and in the deep chlorophyll maximum. This vertical similarity observed in phytoplankton community metabolism could reflect that other physiological strategies must be compensating the disadvantage of larger cells to capture light under low irradiances. Our measurements of the metabolism-size scaling relationship have implications to understand the trophic structure of the community and the energy flow through the ecosystem.
The knowledge of the size-fractionated phytoplankton abundance and metabolism is essential in ord... more The knowledge of the size-fractionated phytoplankton abundance and metabolism is essential in order to study the functioning of the marine ecosystems both from an ecological and biogeochemical point of view. It is commonly accepted that in pelagic ecosystems picophytoplankton constitute a background component of the phytoplankton community whose biomass and primary production keep relatively constant independently of changes in the environment, and that the geographical and temporal variability observed of these variables is due to the large phytoplankton response to environmental forcing. In addition, due to the higher efficiency in resource acquisition of small phytoplankton, it is known their advantage over larger organisms in those ecosystems with low nutrient concentrations. Here we show the results of simultaneously analysing the size-fractionated 14C uptake and the phytoplankton cell size and abundance in different stations at the tropical and subtropical Atlantic Ocean. Photosynthesis rates were estimated by 1L-volume incubations on deck and 2L seawater samples were concentrated in order to increase the sampling abundance of larger cells. The resulting size distribution of phytoplankton abundance, and the 50% higher photosynthesis rates obtained comparing with those rates estimated during parallel 75mL-volume incubations, suggest some methodological advices for future primary production and biomass estimations, as the commonly accepted sample volume could cause the underestimation of carbon fixation rates and phytoplankton cell abundances. In addition, the analysis of the size-fractionated phytoplankton biomass, primary production and turnover rates from a continuous and discrete point of view have evidenced the absence of the expected phytoplankton metabolism reduction as the cell size increase. Finally, higher photosynthetic efficiencies have been obtained for larger cells and a clear response of all phytoplankton size classes to the nutrient enrichment associated with the equatorial upwelling has been found.
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Papers by Maria Huete-Ortega
between phytoplankton cell size, abundance and metabolism in 2 marine ecosystems characterised
by marked differences in resource availability and water-column stability. Several patterns
emerged: (1) nearly isometric size-scaling of phytoplankton carbon fixation rate was described for
both open-ocean and coastal ecosystems (mean slope: 1.17 and 0.90, respectively), supporting the
idea that biomass-specific photosynthesis rates are largely independent of cell size; (2) less steep
values for the size-scaling of abundance (mean slope: −0.73) were found in the coastal ecosystem
compared to the open ocean (mean slope: −1.15); (3) large phytoplankton used more photosynthetic
energy than smaller cells in the coastal ecosystem, but a constant flow of energy along the
size spectrum was found in the open ocean; and (4) phytoplankton biomass turnover rates were 1
order of magnitude higher in the coastal ecosystem than in the open ocean, implying physiological
limitation of phytoplankton growth in the oligotrophic ocean. Bottom-up and top-down mechanisms
and their interaction with nutrient supply dynamics were suggested as major factors determining
the contrasting phytoplankton size abundance distributions observed in coastal and openocean
waters
Our results imply that the total energy processed by carbon fixation is constant along the phytoplankton size spectrum in near steady-state marine ecosystems.
activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30uN and 30uS in the
central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged,
different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The
predominant type of metabolic response depended on the ecosystem’s degree of oligotrophy and was modulated
by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial
production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger
with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic
waters, became less responsive to dust as the ecosystem’s degree of oligotrophy increased. Given the divergent
consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the
ecosystem’s degree of oligotrophy, stimulate or weaken biological CO2 drawdown. Thus, the biogeochemical
implications of changing dust fluxes might not be universal, but variable through both space and time.
coastal, shelf, and open-ocean environments. Our results show that across the entire size spectrum considered,
population abundance increases over two orders of magnitude from subtropical to coastal regions. We find
a highly significant linear relationship between nutrient concentration and the intercept of the log-log relationship
between population abundance and cell size. In contrast to overall patterns reported mainly for vascular plants
and animals, marine phytoplankton diversity does not show any consistent trend along either latitudinal or
productivity gradients. These results imply that large-scale (biogeographic) variations in phytoplankton standing
stocks are controlled by changes in population abundances rather than by systematic variations in species
richness. These findings provide a mechanistic connection among nutrient availability, population dynamics, and
phytoplankton diversity over macroecological scales.
Talks by Maria Huete-Ortega
between phytoplankton cell size, abundance and metabolism in 2 marine ecosystems characterised
by marked differences in resource availability and water-column stability. Several patterns
emerged: (1) nearly isometric size-scaling of phytoplankton carbon fixation rate was described for
both open-ocean and coastal ecosystems (mean slope: 1.17 and 0.90, respectively), supporting the
idea that biomass-specific photosynthesis rates are largely independent of cell size; (2) less steep
values for the size-scaling of abundance (mean slope: −0.73) were found in the coastal ecosystem
compared to the open ocean (mean slope: −1.15); (3) large phytoplankton used more photosynthetic
energy than smaller cells in the coastal ecosystem, but a constant flow of energy along the
size spectrum was found in the open ocean; and (4) phytoplankton biomass turnover rates were 1
order of magnitude higher in the coastal ecosystem than in the open ocean, implying physiological
limitation of phytoplankton growth in the oligotrophic ocean. Bottom-up and top-down mechanisms
and their interaction with nutrient supply dynamics were suggested as major factors determining
the contrasting phytoplankton size abundance distributions observed in coastal and openocean
waters
Our results imply that the total energy processed by carbon fixation is constant along the phytoplankton size spectrum in near steady-state marine ecosystems.
activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30uN and 30uS in the
central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged,
different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The
predominant type of metabolic response depended on the ecosystem’s degree of oligotrophy and was modulated
by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial
production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger
with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic
waters, became less responsive to dust as the ecosystem’s degree of oligotrophy increased. Given the divergent
consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the
ecosystem’s degree of oligotrophy, stimulate or weaken biological CO2 drawdown. Thus, the biogeochemical
implications of changing dust fluxes might not be universal, but variable through both space and time.
coastal, shelf, and open-ocean environments. Our results show that across the entire size spectrum considered,
population abundance increases over two orders of magnitude from subtropical to coastal regions. We find
a highly significant linear relationship between nutrient concentration and the intercept of the log-log relationship
between population abundance and cell size. In contrast to overall patterns reported mainly for vascular plants
and animals, marine phytoplankton diversity does not show any consistent trend along either latitudinal or
productivity gradients. These results imply that large-scale (biogeographic) variations in phytoplankton standing
stocks are controlled by changes in population abundances rather than by systematic variations in species
richness. These findings provide a mechanistic connection among nutrient availability, population dynamics, and
phytoplankton diversity over macroecological scales.