We used data collected at > 60 stations over a 10 yr period to build the carbon budget of the pla... more We used data collected at > 60 stations over a 10 yr period to build the carbon budget of the plankton community in the euphotic layer of the Eastern North Atlantic Subtropical Gyre (NASE). Autotrophic biomass exceeded microbial heterotrophic biomass by a factor of 1.7. Mean (± SE), integrated chlorophyll a concentration and net particulate primary production (PP) were 17 ± 1 mg m -2 and 271 ± 29 mg C m -2 d -1 , respectively. Protist grazing on phytoplankton represented > 90% of PP. Bacterial production (BP) was 17 ± 3 mg C m -2 d -1 . In vitro O 2 -evolution experiments indicated that net community production was -65 ± 16 mmol O 2 m -2 d -1 , while community respiration (CR) averaged 124 ± 13 mmol O 2 m -2 d -1 , equivalent to 1324 ± 142 mg C m -2 d -1 . However, the sum of the respiration rates by each microbial group, estimated from their biomass and metabolic rates, ranged from 402 to 848 mg C m -2 d -1 . Therefore, CR could not be reconciled with the respiratory fluxes sustained by each microbial group. Comparison between estimated gross photosynthesis by phytoplankton (481 to 616 mg C m -2 d -1 ) and the sum of respiration by each group suggests that the microbial community in the NASE province is close to metabolic balance, which would agree with the observed O 2 supersaturation in the euphotic layer. Taking into account the mean open-ocean values for PP, BP, CR and bacterial growth efficiency, we show that bacteria account for approximately 20% of CR. Our results suggest that the view that bacteria dominate carbon cycling in the unproductive ocean must be reconsidered, or else that in vitro incubations misrepresent the real metabolic rates of one or several microbial groups.
We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in co... more We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world's oceans. Our data set covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, reaching values. 80% when chlorophyll a concentration is. 2 mg L 21 or primary production is. 100 mg C L 21 d 21 , irrespective of water temperature. Conversely, picophytoplankton contribution is substantial (. 40%), at all temperatures, only when chlorophyll a concentration is , 1 mg L 21 or primary production is , 50 mg C L 21 d 21. The temperature–size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon-and size-dependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure.
Due to the covariation between temperature and resource availability in the surface ocean, a corr... more Due to the covariation between temperature and resource availability in the surface ocean, a correct assessment of resource supply is crucial to determine if temperature has a direct effect on phytoplankton size structure. To remove the effect of resources, L opez-Urrutia and Mor an analyzed data subsets with narrow ranges of variation in Chlorophyll a (Chl a) concentration and found that temperature is correlated with Chl a partitioning among size classes, from which they concluded that temperature is an important variable to explain the variability of phytoplankton size structure. Our analysis, however, shows that resource supply varies widely also within these subsets and, importantly, that it is inversely correlated with temperature. Therefore, the relationship between temperature and size structure reflects instead the effect of resources. When groups of samples with similar resource supply conditions are considered, no correlation between temperature and phytoplankton size structure is observed, which invalidates the conclusion of L opez-Urrutia and Mor an. Even within restricted ranges of variation for phytoplankton biomass and production, changes in resource supply alone are sufficient to explain the variability of phytoplankton size structure in the sea.
A coastal station located in the southern Bay of Biscay was sampled on six occasions between 12 J... more A coastal station located in the southern Bay of Biscay was sampled on six occasions between 12 July and 10 September 1991. The vertical distribution of the patterns of photosynthetic carbon incorporation into proteins, polysaccharides, lipids and low molecular weight metabolites, as well as the biochemical composition of particulate matter were determined on each cruise. In addition , the same variables were monitored during microcosm experiments in which sub-surface phytoplankton assemblages were incubated under different rates of increasing irradiance simulating differences in upwelling intensity. During the first three experiments, phytoplankton cells did not show any growth response to the simulated upwelling conditions. When phytoplankton growth was slow or absent and nutrients were still available, the relative synthesis of proteins was high, suggesting that phytoplankton cells tended to maintain the synthesis of proteins rather than storage products under adverse growth-limiting conditions. Sub-surface phytoplankton assemblages had the potential for growth in response to upwelling. Several diatom blooms developed during the last two experiments showing enhanced levels of protein and polysaccharide specific synthesis rates (SSR) and a marked increase in the protein to carbohydrate (P:C) compositional ratio. Parallel sea-truth observations carried out during an upwelling pulse indicated that phytoplankton assemblages under natural conditions underwent similar physiological changes to those found in the experimental microcosms under simulated upwelling. In general, the most remarkable increases in chlorophyll a (Chl a) concentration and macromolecular SSR took place in those microcosms showing higher rates of increasing irradiance. Variations in the patterns of photosynthate partitioning and the P:C ratio were also related to the intensity of the advective process. These results emphasize the importance of the fine variability of the physical field in modulating the physiological responses of phytoplankton to upwelling.
The universal temperature dependence of metabolic rates has been used to predict how ocean biolog... more The universal temperature dependence of metabolic rates has been used to predict how ocean biology will respond to ocean warming. Determining the temperature sensitivity of phytoplankton metabolism and growth is of special importance because this group of organisms is responsible for nearly half of global primary production, sustains most marine food webs, and contributes to regulate the exchange of CO 2 between the ocean and the atmosphere. Phytoplankton growth rates increase with temperature under optimal growth conditions in the laboratory, but it is unclear whether the same degree of temperature dependence exists in nature, where resources are often limiting. Here we use concurrent measurements of phytoplankton biomass and carbon fixation rates in polar, temperate and tropical regions to determine the role of temperature and resource supply in controlling the large-scale variability of in situ metabolic rates. We identify a biogeographic pattern in phytoplankton metabolic rates, which increase from the oligotrophic subtropical gyres to temperate regions and then coastal waters. Variability in phytoplankton growth is driven by changes in resource supply and appears to be independent of seawater temperature. The lack of temperature sensitivity of realized phytoplankton growth is consistent with the limited applicability of Arrhenius enzymatic kinetics when substrate concentrations are low. Our results suggest that, due to widespread resource limitation in the ocean, the direct effect of sea surface warming upon phytoplankton growth and productivity may be smaller than anticipated.
We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in co... more We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world's oceans. Our data set 26 covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is 28 independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% 30 and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, 32 reaching values >80% when chlorophyll a concentration is >2 µg L -1 or primary production is >100 µg C L -1 d -1 , irrespective of water temperature. Conversely, 34 picophytoplankton contribution is substantial (>40%), at all temperatures, only when chlorophyll a concentration is <1 µg L -1 or primary production is <50 µg C L -1 d -1 . The 36 temperature-size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon-and 38 size-dependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative 40 success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure. 42
Short-term experiments indicate that seawater acidification can cause a decrease in the rate of c... more Short-term experiments indicate that seawater acidification can cause a decrease in the rate of calcification by coccolithophores, but the relationship between carbonate chemistry and coccolithophore calcification rate in natural assemblages is still unclear. During the Malaspina 2010 circumnavigation, we measured primary production, calcification, coccolithophore abundance, particulate inorganic carbon (PIC) concentration, and the parameters of the carbonate system, along basin-scale transects in the tropical Atlantic, Indian and Pacific oceans. Euphotic layer-integrated calcification and mean cell-specific calcification in the euphotic layer ranged between 2–10 mgC m 22 d 21 and 5–20 pgC cell 21 d 21 , respectively. We found a significant relationship between primary production and calcification, such that the calcification to primary production (CP/PP) ratio was relatively invariant among ocean basins, with an overall mean value of 0.05 6 0.04. Extrapolating this value to the entire ocean would result in a global pelagic calcification rate of 2.4 PtC yr 21. The mean PIC concentration in surface waters was 1.8 6 1.6 mgC m 23 and its turnover time averaged 20 d. We combined our data of calcification, primary production, and carbonate chemistry from Malaspina 2010 with those obtained during two previous cruises in the northern Arabian Sea. Both the CP/PP ratio and cell-specific calcification were largely constant across a wide range of calcite saturation state (
The abundance, taxonomic composition and patterns of macromolecular synthesis of phy-toplankton w... more The abundance, taxonomic composition and patterns of macromolecular synthesis of phy-toplankton were determined across an upwelling-induced thermal front in the central Cantabrian Sea (southern Bay of Biscay) during July 1993. Enhanced levels of phytoplankton biomass, diatom abundance and photosynthetic rate were measured on the coastal side of the front. Relative carbon (C) incorporation into proteins increased noticeably on the oceanic side, taking values of up to 64%, whereas changes in the relative C incorporation into lipids and low-molecular-weight metabolites followed an opposite trend. Phytoplankton cells on the oceanic side of the front were adapted to the prevailing growth-limiting conditions by maintaining the synthesis of functionally essential mol-ecules—proteins—rather than the synthesis of storage compounds. As a result, the carbon to nitrogen uptake ratio varied from ~5.7 in offshore waters to 8.0 in the nearshore region. Our results suggest that the taxonomic and physiological changes in phytoplankton assemblages as a response to upwelling may result in an increase in the synthesis of organic C relative to the upward flux of nitrate. During coastal upwelling events, phytoplankton cells experience an increase in biomass-specific nutrient uptake rates (e.g. Wilkerson and Dugdale, 1987) and display changes in their patterns of carbon (C) allocation into different biochemical pools (e.g. Barlow, 1984). Moreover, the specific synthesis rates of proteins and polysaccharides have been proved to show up to a 5-fold increase after an advec-tive pulse (Marafi6n et al., 1995). These changes may partially be due to variations in the relative abundance of species (Zimmerman et al., 1987). A combined approach is therefore needed where both taxonomic composition and the physiological state of phytoplankton assemblages are taken into account. Sambrotto etai, (1993) reported elevated C to nitrogen (N) consumption ratios in continental shelf regions (southeast Bering Sea and Gerlache Strait), as well as along sections in the North Atlantic, suggesting that the excess loss of dissolved inorganic C was due to biological processes that recycle N more efficiently than C. In this regard, the 14 C-partitioning technique is particularly useful, as it allows the estimation of phytoplankton N uptake from the measured C incorporation into the protein fraction (DiTullio and Laws, 1983). If upwelling induces a shift in the patterns of carbon partitioning among biomolecules, it may be hypothesized that phy-toplankton in an upwelled water mass could show significant differences in the C:N uptake ratio as compared with microalgae in non-upwelled waters. During July 1993, we monitored the development of a coastal upwelling pulse, taking advantage of the existence of a sharp, well-defined thermal front. We analysed the effects of upwelling on the taxonomic composition of microalgae and their patterns of macromolecular synthesis by comparing the phytoplankton
We have determined the scaling relationship between photosynthesis rate and cell size in natural ... more We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L-power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism–size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient-rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (20.90) than in the oligotrophic open ocean (21.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism–size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem.
Phytoplankton size structure controls the trophic organization of plank-tonic communities and the... more Phytoplankton size structure controls the trophic organization of plank-tonic communities and their ability to export biogenic materials toward the ocean's interior. Our understanding of the mechanisms that drive the variability in phytoplankton size structure has been shaped by the assumption that the pace of metabolism decreases allometrically with increasing cell size. However, recent field and laboratory evidence indicates that biomass-specific production and growth rates are similar in both small and large cells but peak at intermediate cell sizes. The maximum nutrient uptake rate scales isometrically with cell volume and superisometrically with the minimum nutrient quota. The unimodal size scaling of phytoplankton growth arises from ataxonomic, size-dependent trade-off processes related to nutrient requirement , acquisition, and use. The superior ability of intermediate-size cells to exploit high nutrient concentrations explains their biomass dominance during blooms. Biogeographic patterns in phytoplankton size structure and growth rate are independent of temperature and driven mainly by changes in resource supply.
We used data collected at > 60 stations over a 10 yr period to build the carbon budget of the pla... more We used data collected at > 60 stations over a 10 yr period to build the carbon budget of the plankton community in the euphotic layer of the Eastern North Atlantic Subtropical Gyre (NASE). Autotrophic biomass exceeded microbial heterotrophic biomass by a factor of 1.7. Mean (± SE), integrated chlorophyll a concentration and net particulate primary production (PP) were 17 ± 1 mg m -2 and 271 ± 29 mg C m -2 d -1 , respectively. Protist grazing on phytoplankton represented > 90% of PP. Bacterial production (BP) was 17 ± 3 mg C m -2 d -1 . In vitro O 2 -evolution experiments indicated that net community production was -65 ± 16 mmol O 2 m -2 d -1 , while community respiration (CR) averaged 124 ± 13 mmol O 2 m -2 d -1 , equivalent to 1324 ± 142 mg C m -2 d -1 . However, the sum of the respiration rates by each microbial group, estimated from their biomass and metabolic rates, ranged from 402 to 848 mg C m -2 d -1 . Therefore, CR could not be reconciled with the respiratory fluxes sustained by each microbial group. Comparison between estimated gross photosynthesis by phytoplankton (481 to 616 mg C m -2 d -1 ) and the sum of respiration by each group suggests that the microbial community in the NASE province is close to metabolic balance, which would agree with the observed O 2 supersaturation in the euphotic layer. Taking into account the mean open-ocean values for PP, BP, CR and bacterial growth efficiency, we show that bacteria account for approximately 20% of CR. Our results suggest that the view that bacteria dominate carbon cycling in the unproductive ocean must be reconsidered, or else that in vitro incubations misrepresent the real metabolic rates of one or several microbial groups.
We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in co... more We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world's oceans. Our data set covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, reaching values. 80% when chlorophyll a concentration is. 2 mg L 21 or primary production is. 100 mg C L 21 d 21 , irrespective of water temperature. Conversely, picophytoplankton contribution is substantial (. 40%), at all temperatures, only when chlorophyll a concentration is , 1 mg L 21 or primary production is , 50 mg C L 21 d 21. The temperature–size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon-and size-dependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure.
Due to the covariation between temperature and resource availability in the surface ocean, a corr... more Due to the covariation between temperature and resource availability in the surface ocean, a correct assessment of resource supply is crucial to determine if temperature has a direct effect on phytoplankton size structure. To remove the effect of resources, L opez-Urrutia and Mor an analyzed data subsets with narrow ranges of variation in Chlorophyll a (Chl a) concentration and found that temperature is correlated with Chl a partitioning among size classes, from which they concluded that temperature is an important variable to explain the variability of phytoplankton size structure. Our analysis, however, shows that resource supply varies widely also within these subsets and, importantly, that it is inversely correlated with temperature. Therefore, the relationship between temperature and size structure reflects instead the effect of resources. When groups of samples with similar resource supply conditions are considered, no correlation between temperature and phytoplankton size structure is observed, which invalidates the conclusion of L opez-Urrutia and Mor an. Even within restricted ranges of variation for phytoplankton biomass and production, changes in resource supply alone are sufficient to explain the variability of phytoplankton size structure in the sea.
A coastal station located in the southern Bay of Biscay was sampled on six occasions between 12 J... more A coastal station located in the southern Bay of Biscay was sampled on six occasions between 12 July and 10 September 1991. The vertical distribution of the patterns of photosynthetic carbon incorporation into proteins, polysaccharides, lipids and low molecular weight metabolites, as well as the biochemical composition of particulate matter were determined on each cruise. In addition , the same variables were monitored during microcosm experiments in which sub-surface phytoplankton assemblages were incubated under different rates of increasing irradiance simulating differences in upwelling intensity. During the first three experiments, phytoplankton cells did not show any growth response to the simulated upwelling conditions. When phytoplankton growth was slow or absent and nutrients were still available, the relative synthesis of proteins was high, suggesting that phytoplankton cells tended to maintain the synthesis of proteins rather than storage products under adverse growth-limiting conditions. Sub-surface phytoplankton assemblages had the potential for growth in response to upwelling. Several diatom blooms developed during the last two experiments showing enhanced levels of protein and polysaccharide specific synthesis rates (SSR) and a marked increase in the protein to carbohydrate (P:C) compositional ratio. Parallel sea-truth observations carried out during an upwelling pulse indicated that phytoplankton assemblages under natural conditions underwent similar physiological changes to those found in the experimental microcosms under simulated upwelling. In general, the most remarkable increases in chlorophyll a (Chl a) concentration and macromolecular SSR took place in those microcosms showing higher rates of increasing irradiance. Variations in the patterns of photosynthate partitioning and the P:C ratio were also related to the intensity of the advective process. These results emphasize the importance of the fine variability of the physical field in modulating the physiological responses of phytoplankton to upwelling.
The universal temperature dependence of metabolic rates has been used to predict how ocean biolog... more The universal temperature dependence of metabolic rates has been used to predict how ocean biology will respond to ocean warming. Determining the temperature sensitivity of phytoplankton metabolism and growth is of special importance because this group of organisms is responsible for nearly half of global primary production, sustains most marine food webs, and contributes to regulate the exchange of CO 2 between the ocean and the atmosphere. Phytoplankton growth rates increase with temperature under optimal growth conditions in the laboratory, but it is unclear whether the same degree of temperature dependence exists in nature, where resources are often limiting. Here we use concurrent measurements of phytoplankton biomass and carbon fixation rates in polar, temperate and tropical regions to determine the role of temperature and resource supply in controlling the large-scale variability of in situ metabolic rates. We identify a biogeographic pattern in phytoplankton metabolic rates, which increase from the oligotrophic subtropical gyres to temperate regions and then coastal waters. Variability in phytoplankton growth is driven by changes in resource supply and appears to be independent of seawater temperature. The lack of temperature sensitivity of realized phytoplankton growth is consistent with the limited applicability of Arrhenius enzymatic kinetics when substrate concentrations are low. Our results suggest that, due to widespread resource limitation in the ocean, the direct effect of sea surface warming upon phytoplankton growth and productivity may be smaller than anticipated.
We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in co... more We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world's oceans. Our data set 26 covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is 28 independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% 30 and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, 32 reaching values >80% when chlorophyll a concentration is >2 µg L -1 or primary production is >100 µg C L -1 d -1 , irrespective of water temperature. Conversely, 34 picophytoplankton contribution is substantial (>40%), at all temperatures, only when chlorophyll a concentration is <1 µg L -1 or primary production is <50 µg C L -1 d -1 . The 36 temperature-size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon-and 38 size-dependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative 40 success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure. 42
Short-term experiments indicate that seawater acidification can cause a decrease in the rate of c... more Short-term experiments indicate that seawater acidification can cause a decrease in the rate of calcification by coccolithophores, but the relationship between carbonate chemistry and coccolithophore calcification rate in natural assemblages is still unclear. During the Malaspina 2010 circumnavigation, we measured primary production, calcification, coccolithophore abundance, particulate inorganic carbon (PIC) concentration, and the parameters of the carbonate system, along basin-scale transects in the tropical Atlantic, Indian and Pacific oceans. Euphotic layer-integrated calcification and mean cell-specific calcification in the euphotic layer ranged between 2–10 mgC m 22 d 21 and 5–20 pgC cell 21 d 21 , respectively. We found a significant relationship between primary production and calcification, such that the calcification to primary production (CP/PP) ratio was relatively invariant among ocean basins, with an overall mean value of 0.05 6 0.04. Extrapolating this value to the entire ocean would result in a global pelagic calcification rate of 2.4 PtC yr 21. The mean PIC concentration in surface waters was 1.8 6 1.6 mgC m 23 and its turnover time averaged 20 d. We combined our data of calcification, primary production, and carbonate chemistry from Malaspina 2010 with those obtained during two previous cruises in the northern Arabian Sea. Both the CP/PP ratio and cell-specific calcification were largely constant across a wide range of calcite saturation state (
The abundance, taxonomic composition and patterns of macromolecular synthesis of phy-toplankton w... more The abundance, taxonomic composition and patterns of macromolecular synthesis of phy-toplankton were determined across an upwelling-induced thermal front in the central Cantabrian Sea (southern Bay of Biscay) during July 1993. Enhanced levels of phytoplankton biomass, diatom abundance and photosynthetic rate were measured on the coastal side of the front. Relative carbon (C) incorporation into proteins increased noticeably on the oceanic side, taking values of up to 64%, whereas changes in the relative C incorporation into lipids and low-molecular-weight metabolites followed an opposite trend. Phytoplankton cells on the oceanic side of the front were adapted to the prevailing growth-limiting conditions by maintaining the synthesis of functionally essential mol-ecules—proteins—rather than the synthesis of storage compounds. As a result, the carbon to nitrogen uptake ratio varied from ~5.7 in offshore waters to 8.0 in the nearshore region. Our results suggest that the taxonomic and physiological changes in phytoplankton assemblages as a response to upwelling may result in an increase in the synthesis of organic C relative to the upward flux of nitrate. During coastal upwelling events, phytoplankton cells experience an increase in biomass-specific nutrient uptake rates (e.g. Wilkerson and Dugdale, 1987) and display changes in their patterns of carbon (C) allocation into different biochemical pools (e.g. Barlow, 1984). Moreover, the specific synthesis rates of proteins and polysaccharides have been proved to show up to a 5-fold increase after an advec-tive pulse (Marafi6n et al., 1995). These changes may partially be due to variations in the relative abundance of species (Zimmerman et al., 1987). A combined approach is therefore needed where both taxonomic composition and the physiological state of phytoplankton assemblages are taken into account. Sambrotto etai, (1993) reported elevated C to nitrogen (N) consumption ratios in continental shelf regions (southeast Bering Sea and Gerlache Strait), as well as along sections in the North Atlantic, suggesting that the excess loss of dissolved inorganic C was due to biological processes that recycle N more efficiently than C. In this regard, the 14 C-partitioning technique is particularly useful, as it allows the estimation of phytoplankton N uptake from the measured C incorporation into the protein fraction (DiTullio and Laws, 1983). If upwelling induces a shift in the patterns of carbon partitioning among biomolecules, it may be hypothesized that phy-toplankton in an upwelled water mass could show significant differences in the C:N uptake ratio as compared with microalgae in non-upwelled waters. During July 1993, we monitored the development of a coastal upwelling pulse, taking advantage of the existence of a sharp, well-defined thermal front. We analysed the effects of upwelling on the taxonomic composition of microalgae and their patterns of macromolecular synthesis by comparing the phytoplankton
We have determined the scaling relationship between photosynthesis rate and cell size in natural ... more We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L-power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism–size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient-rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (20.90) than in the oligotrophic open ocean (21.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism–size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem.
Phytoplankton size structure controls the trophic organization of plank-tonic communities and the... more Phytoplankton size structure controls the trophic organization of plank-tonic communities and their ability to export biogenic materials toward the ocean's interior. Our understanding of the mechanisms that drive the variability in phytoplankton size structure has been shaped by the assumption that the pace of metabolism decreases allometrically with increasing cell size. However, recent field and laboratory evidence indicates that biomass-specific production and growth rates are similar in both small and large cells but peak at intermediate cell sizes. The maximum nutrient uptake rate scales isometrically with cell volume and superisometrically with the minimum nutrient quota. The unimodal size scaling of phytoplankton growth arises from ataxonomic, size-dependent trade-off processes related to nutrient requirement , acquisition, and use. The superior ability of intermediate-size cells to exploit high nutrient concentrations explains their biomass dominance during blooms. Biogeographic patterns in phytoplankton size structure and growth rate are independent of temperature and driven mainly by changes in resource supply.
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Papers by Emilio Marañón