Phytoplankton responses to changing temperature and nutrient availability are consistent across the tropical and subtropical Atlantic.

Temperature and nutrient supply interactively control phytoplankton growth and productivity, yet the role of these drivers together still has not been determined experimentally over large spatial scales in the oligotrophic ocean. We conducted four microcosm experiments in the tropical and subtropical Atlantic (29°N-27°S) in which surface plankton assemblages were exposed to all combinations of three temperatures (in situ, 3 °C warming and 3 °C cooling) and two nutrient treatments (unamended and enrichment with nitrogen and phosphorus). We found that chlorophyll a concentration and the biomass of picophytoplankton consistently increase in response to nutrient addition, whereas changes in temperature have a smaller and more variable effect. Nutrient enrichment leads to increased picoeukaryote abundance, depressed Prochlorococcus abundance, and increased contribution of small nanophytoplankton to total biomass. Warming and nutrient addition synergistically stimulate light-harvesting capacity, and accordingly the largest biomass response is observed in the warmed, nutrient-enriched treatment at the warmest and least oligotrophic location (12.7°N). While moderate nutrient increases have a much larger impact than varying temperature upon the growth and community structure of tropical phytoplankton, ocean warming may increase their ability to exploit events of enhanced nutrient availability.

Net effect of environmental fluctuations in multiple global-change drivers across the tree of life.

Jensen's inequality predicts that the response of any given system to average constant conditions is different from its average response to varying ones. Environmental fluctuations in abiotic conditions are pervasive on Earth; yet until recently, most ecological research has addressed the effects of multiple environmental drivers by assuming constant conditions. One could thus expect to find significant deviations in the magnitude of their effects on ecosystems when environmental fluctuations are considered. Drawing on experimental studies published during the last 30 years reporting more than 950 response ratios (n = 5,700), we present a comprehensive analysis of the role that environmental fluctuations play across the tree of life. In contrast to the predominance of interactive effects of global-change drivers reported in the literature, our results show that their cumulative effects were additive (58%), synergistic (26%), and antagonistic (16%) when environmental fluctuations were present. However, the dominant type of interaction varied by trophic level (autotrophs: interactive; heterotrophs: additive) and phylogenetic group (additive in Animalia; additive and positive antagonism in Chromista; negative antagonism and synergism in Plantae). In addition, we identify the need to tackle how complex communities respond to fluctuating environments, widening the phylogenetic and biogeographic ranges considered, and to consider other drivers beyond warming and acidification as well as longer timescales. Environmental fluctuations must be taken into account in experimental and modeling studies as well as conservation plans to better predict the nature, magnitude, and direction of the impacts of global change on organisms and ecosystems.

Magnitude of nitrate turbulent diffusion in contrasting marine environments.

Difficulties to quantify ocean turbulence have limited our knowledge about the magnitude and variability of nitrate turbulent diffusion, which constitutes one of the main processes responsible for the supply of nitrogen to phytoplankton inhabiting the euphotic zone. We use an extensive dataset of microturbulence observations collected in contrasting oceanic regions, to build a model for nitrate diffusion into the euphotic zone, and obtain the first global map for the distribution of this process. A model including two predictors (surface temperature and nitrate vertical gradient) explained 50% of the variance in the nitrate diffusive flux. This model was applied to climatological data to predict nitrate diffusion in oligotrophic mid and low latitude regions. Mean nitrate diffusion (~ 20 Tmol N y-1) was comparable to nitrate entrainment due to seasonal mixed-layer deepening between 40°N-40ºS, and to the sum of global estimates of nitrogen fixation, fluvial fluxes and atmospheric deposition. These results indicate that nitrate diffusion represents one of the major sources of new nitrogen into the surface ocean in these regions.

Geographical and Seasonal Thermal Sensitivity of Grazing Pressure by Microzooplankton in Contrasting Marine Ecosystems.

Grazing pressure, estimated as the ratio between microzooplankton grazing and phytoplankton growth rates (g:µ), is a strong determinant of microbial food-web structure and element cycling in the upper ocean. It is generally accepted that g is more sensitive to temperature than µ, but it remains unknown how the thermal dependence (activation energy, E a) of g:µ varies over spatial and temporal scales. To tackle this uncertainty, we used an extensive literature analysis obtaining 751 paired rate estimates of µ and g from dilution experiments performed throughout the world's marine environments. On a geographical scale, we found a stimulatory effect of temperature in polar open-ocean (∼0.5 eV) and tropical coastal (∼0.2 eV) regions, and an inhibitory one in the remaining biomes (values between -0.1 and -0.4 eV). On a seasonal scale, the temperature effect on g:µ ratios was stimulatory, particularly in polar environments; however, the large variability existing between estimates resulted in non-significant differences among biomes. We observed that increases in nitrate availability stimulated the temperature dependence of grazing pressure (i.e., led to more positive E a of g:µ) in open-ocean ecosystems and inhibited it in coastal ones, particularly in polar environments. The percentage of primary production grazed by microzooplankton (∼56%) was similar in all regions. Our results suggest that warming of surface ocean waters could exert a highly variable impact, in terms of both magnitude and direction (stimulation or inhibition), on microzooplankton grazing pressure in different ocean regions.

Temperature fluctuations in a warmer environment: impacts on microbial plankton.

Warming can cause changes in the structure and functioning of microbial food webs. Experimental studies quantifying such impacts on microbial plankton have tended to consider constant temperature conditions. However, Jensen's inequality (or the fallacy of the average) recognizes that organism performance under constant conditions is seldom equal to the mean performance under variable conditions, highlighting the need to consider in situ fluctuations over a range of time scales. Here we review some of the available evidence on how warming effects on the abundance, diversity, and metabolism of microbial plankton are altered when temperature fluctuations are considered. We found that fluctuating temperatures may accentuate warming-mediated reductions in phytoplankton evenness and gross photosynthesis while synergistically increasing phytoplankton growth. Also, fluctuating temperatures have been shown to reduce the positive warming effect on cyanobacterial biomass production and recruitment and to reverse a warming effect on cellular nutrient quotas. Other reports have shown that fluctuations in temperature did not alter plankton responses to constant warming. These investigations have mostly focused on a few phytoplankton species (i.e. diatoms and haptophytes) in temperate and marine ecosystems and considered short-term and transient responses. It remains unknown whether the same responses apply to other species and ecosystems and if evolutionary change in thermally varying environments could alter the magnitude and direction of the responses to warming observed over short-term scales. Thus, future research efforts should address the role of fluctuations in environmental drivers. We stress the need to study responses over different biological organization and trophic levels, nutritional modes, temporal scales, and ecosystem types.

Effect of temperature on the unimodal size scaling of phytoplankton growth.

Contrary to predictions by the allometric theory, there is evidence that phytoplankton growth rates peak at intermediate cell sizes. However, it is still unknown if this pattern may result from the effect of experimental temperature. Here we test whether temperature affects the unimodal size scaling pattern of phytoplankton growth by (1) growing Synechococcus sp., Ostreococcus tauri, Micromonas commoda and Pavlova lutheri at 18 °C and 25 °C, and (2) using thermal response curves available in the literature to estimate the growth rate at 25 °C as well as the maximum growth rate at optimal temperature for 22 species assayed previously at 18 °C. We also assess the sensitivity of growth rate estimates to the metric employed for measuring standing stocks, by calculating growth rates based on in vivo fluorescence, chlorophyll a concentration, cell abundance and biomass (particulate organic carbon and nitrogen content). Our results show that the unimodal size scaling pattern of phytoplankton growth, with a peak at intermediate cell sizes, is observed at 18 °C, 25 °C and at the optimal temperature for growth, and that it prevails irrespective of the standing-stock metric used. The unimodal size scaling pattern of phytoplankton growth is supported by two independent field observations reported in the literature: (i) a positive relationship between cell size and metabolic rate in the picophytoplankton size range and (ii) the dominance of intermediate-size cells in nutrient-rich waters during blooms.

Grazing Pressure Is Independent of Prey Size in a Generalist Herbivorous Protist: Insights from Experimental Temperature Gradients.

Grazing by herbivorous protists contributes to structuring plankton communities through its effect on the growth, biomass, and competitiveness of prey organisms and also impacts the transfer of primary production towards higher trophic levels. Previous evidence shows that heterotrophic processes (grazing rates, g) are more sensitive to temperature than autotrophic ones (phytoplankton growth rates, µ) and also that small cells tend to be more heavily predated than larger ones; however, it remains unresolved how the interplay between changes in temperature and cell size modulates grazing pressure (i.e., g:µ ratio). We addressed this problem by conducting an experiment with four phytoplankton populations, from pico- to microphytoplankton, over a 12 °C gradient and in the presence/absence of a generalist herbivorous protist, Oxyrrhis marina. We found that highest g rates coincided with highest µ rates, which corresponded to intermediate cell sizes. There were no significant differences in either µ or g between the smallest and largest cell sizes considered. The g:µ ratio was largely independent of cell size and C:N ratios, and its thermal dependence was low although species-specific differences were large. We suggest that the similar g:µ found could be the consequence that the energetic demand imposed by rising temperatures would be a more important issue than the mechanical constriction to ingestion derived from prey cell size. Despite the difficulty of quantifying µ and g in natural planktonic communities, we suggest that the g:µ ratio is a key response variable to evaluate thermal sensitivity of food webs because it gives a more integrative view of trophic functioning than both rates separately.

Reconciling models of primary production and photoacclimation [Invited].

Primary production and photoacclimation models are two important classes of physiological models that find applications in remote sensing of pools and fluxes of carbon associated with phytoplankton in the ocean. They are also key components of ecosystem models designed to study biogeochemical cycles in the ocean. So far, these two classes of models have evolved in parallel, somewhat independently of each other. Here we examine how they are coupled to each other through the intermediary of the photosynthesis-irradiance parameters. We extend the photoacclimation model to accommodate the spectral effects of light penetration in the ocean and the spectral sensitivity of the initial slope of the photosynthesis-irradiance curve, making the photoacclimation model fully compatible with spectrally resolved models of photosynthesis in the ocean. The photoacclimation model contains a parameter θm, which is the maximum chlorophyll-to-carbon ratio that phytoplankton can attain when available light tends to zero. We explore how size-class-dependent values of θm could be inferred from field data on chlorophyll and carbon content in phytoplankton, and show that the results are generally consistent with lower bounds estimated from satellite-based primary production calculations. This was accomplished using empirical models linking phytoplankton carbon and chlorophyll concentration, and the range of values obtained in culture measurements. We study the equivalence between different classes of primary production models at the functional level, and show that the availability of a chlorophyll-to-carbon ratio facilitates the translation between these classes. We discuss the importance of the better assignment of parameters in primary production models as an important avenue to reduce model uncertainties and to improve the usefulness of satellite-based primary production calculations in climate research.

Effects of Temperature and Nutrient Supply on Resource Allocation, Photosynthetic Strategy, and Metabolic Rates of Synechococcus sp.

Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (Synechococcus PCC 7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO, rbcL) and light (Photosystem II, psbA) photosynthetic reactions, the content of chlorophyll a, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found that rbcL and psbA abundance increased with nutrient supply, whereas a temperature-induced increase in psbA occurred only in nutrient-replete treatments. Low temperature and abundant nutrients caused increased RuBisCO abundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient-sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates in Synechococcus sp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark-reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.

Generalized size scaling of metabolic rates based on single-cell measurements with freshwater phytoplankton.

Kleiber's law describes the scaling of metabolic rate with body size across several orders of magnitude in size and across taxa and is widely regarded as a fundamental law in biology. The physiological origins of Kleiber's law are still debated and generalizations of the law accounting for deviations from the scaling behavior have been proposed. Most theoretical and experimental studies of Kleiber's law, however, have focused on the relationship between the average body size of a species and its mean metabolic rate, neglecting intraspecific variation of these 2 traits. Here, we propose a theoretical characterization of such variation and report on proof-of-concept experiments with freshwater phytoplankton supporting such framework. We performed joint measurements at the single-cell level of cell volume and nitrogen/carbon uptake rates, as proxies of metabolic rates, of 3 phytoplankton species using nanoscale secondary ion mass spectrometry (NanoSIMS) and stable isotope labeling. Common scaling features of the distribution of nutrient uptake rates and cell volume are found to hold across 3 orders of magnitude in cell size. Once individual measurements of cell volume and nutrient uptake rate within a species are appropriately rescaled by a function of the average cell volume within each species, we find that intraspecific distributions of cell volume and metabolic rates collapse onto a universal curve. Based on the experimental results, this work provides the building blocks for a generalized form of Kleiber's law incorporating intraspecific, correlated variations of nutrient-uptake rates and body sizes.

Temporal variability of diazotroph community composition in the upwelling region off NW Iberia.

Knowledge of the ecology of N2-fixing (diazotrophic) plankton is mainly limited to oligotrophic (sub)tropical oceans. However, diazotrophs are widely distributed and active throughout the global ocean. Likewise, relatively little is known about the temporal dynamics of diazotrophs in productive areas. Between February 2014 and December 2015, we carried out 9 one-day samplings in the temperate northwestern Iberian upwelling system to investigate the temporal and vertical variability of the diazotrophic community and its relationship with hydrodynamic forcing. In downwelling conditions, characterized by deeper mixed layers and a homogeneous water column, non-cyanobacterial diazotrophs belonging mainly to nifH clusters 1G (Gammaproteobacteria) and 3 (putative anaerobes) dominated the diazotrophic community. In upwelling and relaxation conditions, affected by enhanced vertical stratification and hydrographic variability, the community was more heterogeneous vertically but less diverse, with prevalence of UCYN-A (unicellular cyanobacteria, subcluster 1B) and non-cyanobacterial diazotrophs from clusters 1G and 3. Oligotyping analysis of UCYN-A phylotype showed that UCYN-A2 sublineage was the most abundant (74%), followed by UCYN-A1 (23%) and UCYN-A4 (2%). UCYN-A1 oligotypes exhibited relatively low frequencies during the three hydrographic conditions, whereas UCYN-A2 showed higher abundances during upwelling and relaxation. Our findings show the presence of a diverse and temporally variable diazotrophic community driven by hydrodynamic forcing in an upwelling system.

Phytoplankton size diversity and ecosystem function relationships across oceanic regions.

Trait diversity, a key component of biodiversity, mediates many essential ecosystem functions and services. However, the mechanisms behind such relationships at large spatial scales are not fully understood. Here we adopt the functional biogeography approach to investigate how the size composition of phytoplankton communities relates to primary production and export production along a broad latitudinal gradient. Using in situ phytoplankton size distribution data and a trait-based model, we find an increase in the average phytoplankton size, size diversity, primary production and export when moving from low to high latitudes. Our analysis indicates that the interplay between spatio-temporal heterogeneities in environmental conditions and a trade-off between the high affinity for nutrients of smaller cells and the ability to avoid predation by larger cells are the main mechanisms driving the observed patterns. Our results also suggest that variations in size diversity alone do not directly lead to changes in primary production and export. The trade-off thus introduces a feedback that influences the relationship between size diversity and ecosystem functions. These findings support the importance of environmentally mediated trade-offs as crucial mechanisms shaping biodiversity and ecosystem function relationships at large spatial scales.

Nutrient limitation suppresses the temperature dependence of phytoplankton metabolic rates.

Climate warming has the potential to alter ecosystem function through temperature-dependent changes in individual metabolic rates. The temperature sensitivity of phytoplankton metabolism is especially relevant, since these microorganisms sustain marine food webs and are major drivers of biogeochemical cycling. Phytoplankton metabolic rates increase with temperature when nutrients are abundant, but it is unknown if the same pattern applies under nutrient-limited growth conditions, which prevail over most of the ocean. Here we use continuous cultures of three cosmopolitan and biogeochemically relevant species (Synechococcus sp., Skeletonema costatum and Emiliania huxleyi) to determine the temperature dependence (activation energy, Ea) of metabolism under different degrees of nitrogen (N) limitation. We show that both CO2 fixation and respiration rates increase with N supply but are largely insensitive to temperature. Ea of photosynthesis (0.11 ± 0.06 eV, mean ± SE) and respiration (0.04 ± 0.17 eV) under N-limited growth is significantly smaller than Ea of growth rate under nutrient-replete conditions (0.77 ± 0.06 eV). The reduced temperature dependence of metabolic rates under nutrient limitation can be explained in terms of enzyme kinetics, because both maximum reaction rates and half-saturation constants increase with temperature. Our results suggest that the direct, stimulating effect of rising temperatures upon phytoplankton metabolic rates will be circumscribed to ecosystems with high-nutrient availability.

The Size Dependence of Phytoplankton Growth Rates: A Trade-Off between Nutrient Uptake and Metabolism.

Rates of metabolism and population growth are often assumed to decrease universally with increasing organism size. Recent observations have shown, however, that maximum population growth rates among phytoplankton smaller than ∼6 µm in diameter tend to increase with organism size. Here we bring together observations and theory to demonstrate that the observed change in slope is attributable to a trade-off between nutrient uptake and the potential rate of internal metabolism. Specifically, we apply an established model of phytoplankton growth to explore a trade-off between the ability of cells to replenish their internal quota (which increases with size) and their ability to synthesize new biomass (which decreases with size). Contrary to the metabolic theory of ecology, these results demonstrate that rates of resource acquisition (rather than metabolism) provide the primary physiological constraint on the growth rates of some of the smallest and most numerically abundant photosynthetic organisms on Earth.

Cell size as a key determinant of phytoplankton metabolism and community structure.

Phytoplankton size structure controls the trophic organization of planktonic 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.

Resource supply overrides temperature as a controlling factor of marine phytoplankton growth.

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 CO2 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.

Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use.

Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (µ(max) ) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10(6) µm(3) in cell volume (V(cell) ), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both µ(max) and carbon-specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V(maxN) ) scales isometrically with V(cell) , whereas nitrogen minimum quota scales as V(cell) (0.84) . Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume-specific V(maxN) compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon-independent, size-related constraints in nutrient uptake, requirement and assimilation.

Isometric size-scaling of metabolic rate and the size abundance distribution of phytoplankton.

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 -1.15 (range: -1.29 to -0.97), 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.

Decrease in the autotrophic-to-heterotrophic biomass ratio of picoplankton in oligotrophic marine waters due to bottle enclosure.

We investigated the effects of bottle enclosure on autotrophic and heterotrophic picoplankton in North and South subtropical Atlantic oligotrophic waters, where the biomass and metabolism of the microbial community are dominated by the picoplankton size class. We measured changes in both autotrophic (Prochlorococcus, Synechococcus, and picoeukaryotes) and heterotrophic picoplankton biomass during three time series experiments and in 16 endpoint experiments over 24 h in light and dark treatments. Our results showed a divergent effect of bottle incubation on the autotrophic and heterotrophic components of the picoplankton community. The biomass of picophytoplankton showed, on average, a >50% decrease, mostly affecting the picoeukaryotes and, to a lesser extent, Prochlorococcus. In contrast, the biomass of heterotrophic bacteria remained constant or increased during the incubations. We also sampled 10 stations during a Lagrangian study in the North Atlantic subtropical gyre, which enabled us to compare the observed changes in the auto- to heterotrophic picoplankton biomass ratio (AB:HB ratio) inside the incubation bottles with those taking place in situ. While the AB:HB ratio in situ remained fairly constant during the Lagrangian study, it decreased significantly during the 24 h of incubation experiments. Thus, the rapid biomass changes observed in the incubations are artifacts resulting from bottle confinement and do not take place in natural conditions. Our results suggest that short (<1 day) bottle incubations in oligotrophic waters may lead to biased estimates of the microbial metabolic balance by underestimating primary production and/or overestimating bacterial respiration.

Invariant scaling of phytoplankton abundance and cell size in contrasting marine environments.

Scaling relationships such as the variation of population abundance with body size provide links between individual organisms and ecosystem functioning. Previous work, in marine pelagic ecosystems, has focused on the relationship between total phytoplankton abundance and the assemblage mean cell size. However, the relationship between specific population abundance and cell size in marine phytoplankton has received little attention. Here, we show that cell size accounts for a significant amount of variability in the population abundance of phytoplankton species across a cell volume range spanning seven orders of magnitude. The interspecific scaling of population abundance and cell size takes a power exponent near -3/4. Unexpectedly, despite the constraints imposed on large phytoplankton by limited resource acquisition, the size scaling exponent does not differ between contrasting marine environments such as coastal and subtropical regions. These findings highlight the adaptive abilities of individual species to cope with different environmental conditions and suggest that a general rule such as the 'energetic equivalence' constrains the abundance of phytoplankton populations in marine pelagic ecosystems.