Search

While primary productivity is more stable in oceanic regions, it may vary to a great extent with the proximity to coasts, where mesoscale processes may intertwine and shape phytoplankton community composition and biomass. Sometimes, this may lead to the development of anomalous phytoplankton blooms (i.e., episodic blooms that exceed several times the average phytoplankton biomass). A massive bloom observed off the Western Iberian Coast (SW Europe) during March 2009 prompted a full investigation on its spatial and temporal extent, its causes, and its potential impact on the ecosystem. Results revealed that the March 2009 bloom was both novel in terms of biomass in a regional context and one of the largest anomalous blooms until now described in terms of relative magnitude. Its causes were due to a concurrence of long-term (deep winter MLD) and short-term factors (coastal upwelling, sudden changes in the water column, consistent offshore water transport). Its impact on the regional ecosystem is difficult to gauge, although the high concentrations of particulate organic carbon at surface during the bloom period suggests that it may have had a significant local impact. Since climate change is expected to increase the frequency and intensity of extreme weather events, it is possible that anomalous blooms will also become more frequent, expanding their role in shaping carbon export and food webs. These results are crucial for the monitoring of the Western Iberian Coast and are applicable to other complex coastal upwelling regions where phytoplankton biomass and variability have a crucial link to fisheries., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

In contemporary oceans diatoms are an important group of eukaryotic phytoplankton that typically dominate in upwelling regions and at high latitudes. They also make significant contributions to sporadic blooms that often occur in springtime. Recent surveys have revealed global information about their abundance and diversity, as well as their contributions to biogeochemical cycles, both as primary producers of organic material and as conduits facilitating the export of carbon and silicon to the ocean interior. Sequencing of diatom genomes is revealing the evolutionary underpinnings of their ecological success by examination of their gene repertoires and the mechanisms they use to adapt to environmental changes. The rise of the diatoms over the last hundred million years is similarly being explored through analysis of microfossils and biomarkers that can be traced through geological time, as well as their contributions to seafloor sediments and fossil fuel reserves. The current review aims to synthesize current information about the evolution and biogeochemical functions of diatoms as they rose to prominence in the global ocean.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'., (© 2017 The Author(s).)

The North Atlantic is a region of enhanced biogeochemical and climatological importance for the global ocean as it is the site of one of the largest seasonal phytoplankton blooms on the planet. However, there is a lack of understanding of how phytoplankton size influences bloom dynamics and associated nutrient utilization rates, particularly during the decline phase when export to the deep ocean is especially pronounced. Here, we evaluate trends in size‐fractionated carbon, nitrogen, and silicic acid uptake rates in conjunction with environmental parameters to assess these dynamics. In our study, the decline phase of the bloom continued to be highly productive with net primary production (NPP) ranging from 36.4 to 146.6 mmol C m−2 d−1 and approximately 54% of primary production being driven by large phytoplankton cells (≥5 μm) that were primarily utilizing nitrate (mean f‐ratio of 0.77). Entrainment of silicic acid related to deepening of the mixed layer caused by storms increased silicic acid uptake rates to 2.0–5.7 mmol Si m−2 d−1 without concomitant increases in NPP by large cells (silicic acid to carbon uptake ratios averaged 0.12). A companion study in the North Pacific allowed for paired evaluation of these regions. Our results suggest that in highly productive regions where phytoplankton biomass and productivity is distributed across a broad range of cell sizes, such as the North Atlantic, size itself has a stronger influence on nutrient cycling and potential carbon export relative to regions with lower production and a predominance of small (<5 μm) cells, such as the North Pacific. Plain Language Summary: The North Atlantic Ocean experiences a seasonal bloom of phytoplankton. This bloom represents one of the largest removals of anthropogenic carbon to the deep ocean on the planet. Here, we seek to better quantify the mechanisms of this removal by characterizing the amount and composition of phytoplankton present and how much primary production they are engaging in. We sampled during the decline phase of the spring bloom and found the region to be characterized by mostly large phytoplankton primarily utilizing nitrate. When compared to a companion study in the North Pacific, this study confirms the important role cell size and nutrient availability play in determining the degree of productivity in both high and low primary production oceanic regions. Key Points: The decline phase of the North Atlantic spring bloom exhibits transitions in the balance between small and large size‐fractionated primary production dynamicsDuring the bloom decline, diatoms contribute less to primary production but remain a substantial component of phytoplankton biomassPhytoplankton size is a substantial control on ecosystem dynamics in endmember oceanic productivity‐export systems [ABSTRACT FROM AUTHOR]

Introduction: Artificial upwelling has been discussed as a nature-based solution to fertilize currently unproductive areas of the ocean to enhance food web productivity and atmospheric CO2 sequestration. The efficacy of this approach may be closely tied to the nutrient stoichiometry of the upwelled water, as Si- rich upwelling should benefit the growth of diatoms, who are key players for primary production, carbon export and food web efficiency. Methods: With a mesocosm experiment in subtropical waters, we assessed the physiological and functional responses of an oligotrophic phytoplankton community to artificial upwelling under varying Si:N ratios (0.07-1.33). Results: Deep water fertilization led to strongly enhanced primary productivity rates and net autotrophy across Si scenarios. At the community level, Si-rich upwelling50 temporarily increased primary production and consistently enhanced diatom growth, producing up to 10-fold higher abundances compared to Si-deficient upwelling. At the organism level, contrasting effects were observed. On the one hand, silicification and size of diatom cells remained unaffected by Si:N, which is surprising given the direct dependency of these traits on Si. On the other hand, diatom Chlorophyll a density and carbon density were strongly reduced and particulate matter C:N was elevated under Si- rich upwelling. Discussion: This suggests a reduced nutritional value for higher trophic levels under high Si:N ratios. Despite these strong qualitative changes under high Si, diatom cells appeared healthy and showed high photosynthetic efficiency. Our findings reveal great physiological plasticity and adaptability in phytoplankton under artificial upwelling, with Si-dependent trade-offs between primary producer quantity and quality. [ABSTRACT FROM AUTHOR]

IntroductionArtificial upwelling has been discussed as a nature-based solution to fertilize currently unproductive areas of the ocean to enhance food web productivity and atmospheric CO2 sequestration. The efficacy of this approach may be closely tied to the nutrient stoichiometry of the upwelled water, as Si-rich upwelling should benefit the growth of diatoms, who are key players for primary production, carbon export and food web efficiency.MethodsWith a mesocosm experiment in subtropical waters, we assessed the physiological and functional responses of an oligotrophic phytoplankton community to artificial upwelling under varying Si:N ratios (0.07-1.33).ResultsDeep water fertilization led to strongly enhanced primary productivity rates and net autotrophy across Si scenarios. At the community level, Si-rich upwelling50 temporarily increased primary production and consistently enhanced diatom growth, producing up to 10-fold higher abundances compared to Si-deficient upwelling. At the organism level, contrasting effects were observed. On the one hand, silicification and size of diatom cells remained unaffected by Si:N, which is surprising given the direct dependency of these traits on Si. On the other hand, diatom Chlorophyll a density and carbon density were strongly reduced and particulate matter C:N was elevated under Si-rich upwelling.DiscussionThis suggests a reduced nutritional value for higher trophic levels under high Si:N ratios. Despite these strong qualitative changes under high Si, diatom cells appeared healthy and showed high photosynthetic efficiency. Our findings reveal great physiological plasticity and adaptability in phytoplankton under artificial upwelling, with Si-dependent trade-offs between primary producer quantity and quality.

Earth System Models generally predict increasing upper ocean stratification from 21st century warming, which will cause a decrease in the vertical nutrient flux forcing declines in marine net primary productivity (NPP) and carbon export. Recent advances in quantifying marine ecosystem carbon to nutrient stoichiometry have identified large latitudinal and biome variability, with low‐latitude oligotrophic systems harboring pico‐sized phytoplankton exhibiting large phosphorus to carbon cellular plasticity. The climate forced changes in nutrient flux stoichiometry and phytoplankton community composition are thus likely to alter the ocean's biogeochemical response and feedback with the carbon‐climate system. We have added three pico‐phytoplankton functional types within the Biogeochemical Elemental Cycling component of the Community Earth System Model while incorporating variable cellular phosphorus to carbon stoichiometry for all represented phytoplankton types. The model simulates Prochlorococcus and Synechococcus populations that dominate the productivity and sinking carbon export of the tropical and subtropical ocean, and pico‐eukaryote populations that contribute significantly to productivity and export within the subtropical to mid‐latitude transition zone, with the western subtropical regions of each basin supporting the most P‐poor stoichiometries. Subtropical gyre recirculation regions along the poleward flanks of surface western boundary currents are identified as regional hotspots of enhanced carbon export exhibiting C‐rich/P‐poor stoichiometry, preferentially inhabited by pico‐eukaryotes and diatoms. Collectively, pico‐phytoplankton contribute ∼58% of global NPP and ∼46% of global particulate organic carbon export below 100 m through direct and ecosystem processing pathways. Biodiversity and cellular nutrient plasticity in marine pico‐phytoplankton combine to increase their contributions to ocean productivity and the biological carbon pump. Key Points: Simulated Prochlorococcus, Synechococcus, and pico‐eukaryotes contribute ∼60% of marine net primary productivity (NPP)Pico‐phytoplankton cycling contributes half of the marine export production, approaching parity with their contribution to NPPPico‐eukaryotes and diatoms with elevated C:P stoichiometry enhance carbon export at poleward flanks of western boundary currents [ABSTRACT FROM AUTHOR]

This study analyzed the diversity and abundance of diatom frustules including the ancillary parameters using the core top sediments from five locations (21, 19, 15, 13, and 11°N) along the central Arabian Sea (64°E), an area profoundly influenced by atmospheric forcing (monsoons) and oxygen minimum zone (OMZ) with high spatial variability. Significantly higher organic carbon (0.97 ± 0.05%) and diatom frustules (5.92 ± 0.57 × 104 valves g−1) were noticed in the north (21, 19, 15°N) where natural nutrient enrichment via open-ocean upwelling, winter convection, and lateral advection support large diatom-dominated phytoplankton blooms and intense OMZ. Conversely, the south (13, 11°N) depicted significantly lower organic carbon (0.74 ± 0.08%) as well as frustules (4.02 ± 0.87 × 104 valves g−1) as this area mostly remains nutrient-poor dominated by small-medium-sized phytoplankton. The north was dominated by large-sized diatoms like Coscinodiscus that could escape grazing and sink consequently due to higher ballasting. Furthermore, the presence of the intense OMZ in the north might reduce grazing pressure (low zooplankton stock) and mineralization speed facilitating higher phytodetritus transport. Relatively smaller chain-forming centric (Thalassiosira) and pennate diatoms (Pseudo-nitzschia, Fragilaria, Nitzschia, etc.) were found throughout the transect with higher abundance in the south. The euphotic diatom diversity from the existing literature was compared with the frustule diversity from the sediments suggesting not all diatoms make their way to the abyss. Such distinct spatial north–south variability in diatom frustule size as well as abundance could be attributed to cell size, grazing, and water column mineralization rates related to OMZ. [ABSTRACT FROM AUTHOR]

Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc concentrations in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage. Plain Language Summary: Because of the Southern Ocean's unique role in ocean circulation, Antarctic phytoplankton profoundly influence the global carbon cycle. For instance, an increase in the supply of iron—the main nutrient limiting Antarctic phytoplankton—is thought to have lowered CO2 during past ice ages by increasing phytoplankton photosynthesis. However, the potential for other essential elements to limit Southern Ocean productivity is not well known. By accounting for requirements of several nutrients in a global model, we have identified that manganese, an essential cofactor in photosynthesis, can limit phytoplankton growth across the Southern Ocean. The enduring role of manganese deficiency will likely influence the response of Southern Ocean ecosystems to ongoing climate change. Key Points: Mn scarcity in the Southern Ocean limits phytoplankton growth in a global biogeochemical model, especially during austral springThe spatial extent of Mn limitation is sensitive to phytoplankton traits governing photophysiology and metal homeostasisGreater dust deposition to the Southern Ocean expands the role of Mn limitation and restricts carbon export from Fe fertilization [ABSTRACT FROM AUTHOR]

Key Points: Synoptic‐scale winter blooms and mixed‐layer pump events were observed by a BGC‐Argo float in the midlatitude Northwest Pacific OceanThe repetitive mixed‐layer pump episodes generated a very high particulate organic carbon export of ~110 mg C m−2 day−1 in wintertimeThe current BGC‐Argo protocols captured only 20%, or entirely missed, substantial pulsed carbon export driven by mixed‐layer pump events The winter‐ and spring‐time mixed‐layer pump (MLP) significantly augments the global carbon transport from the surface mixed layer to deeper waters when ephemeral surface phytoplankton blooms are repeatedly mixed to depth. Exploiting unusual 190 + daily BGC‐Argo profiling measurements within a recirculation gyre, we show repetitive MLP episodes generating a January–March averaged particulate organic carbon (POC) export of ~110 mg C m−2 day−1 in the midlatitude (31°N) Northwest Pacific. Subsampling this dataset on a 5‐ or 10‐day cycle yielded an order of magnitude less export, or even totally missed all the MLP events. The evidence here supports the need for new strategies if the BGC‐Argo program is to adequately quantify ocean carbon cycling and its effects on biological systems. We propose that a handful of floats be tasked with daily profiling, and machine learning strategies be used to link these data with satellite derived measurements to estimate the synoptic‐scale MLP export. Plain Language Summary: The Biogeochemical‐Argo program (BGC‐Argo) is an essential tool for quantifying carbon export from surface waters to the ocean interior, which is critical for understanding and forecasting changes in atmospheric CO2. Using novel, high‐frequency (1‐day cycle during 9‐ month period) BGC‐Argo data, we estimated the carbon export in the midlatitude Northwest Pacific Ocean stemming from unexpectedly continuous mixed‐layer pump (MLP) events. Such events occur when wintertime phytoplankton blooms developed during calm wind periods then are injected to depth by storms. This process generated remarkably high carbon export over winter, with values ranging from 18% to 30% of that from the massive North Atlantic spring bloom. These events, however, are largely (80%) or entirely missed using the longer standard float cycling intervals (5–10 days) accepted by the BGC‐Argo program. The findings here suggest that some high‐frequency BGC‐Argo observations are necessary for more accurately estimating the global carbon export. [ABSTRACT FROM AUTHOR]

The attenuation coefficient b is one of the most common ways to describe how strong the carbon flux is attenuated throughout the water column. Therefore, b is an essential input variable in many carbon flux and climate models. Marsay et al. (2015, https://doi.org/10.1073/pnas.1415311112) proposed that the median surface water temperature (0–500 m) may be a predictor of b, but our observations from Arctic waters challenge this hypothesis. We found a highly variable attenuation coefficient (b = 0.43–1.84) in cold Arctic waters (<4.1 °C). Accordingly, we suggest that water temperature is not a globally valid predictor of the attenuation coefficient. We advocate instead that the phytoplankton composition and especially the relative abundance of diatoms can be used to parametrize the carbon flux attenuation in local and global carbon flux models. Plain Language Summary: In the surface ocean, microalgae convert dissolved atmospheric CO2 into organic carbon (OC). As OC sinks in the water column, microbes and zooplankton graze on it. It has been suggested that these organisms reduce the amount of sinking OC stronger in warm waters than in cold waters. We found, however, that the amount of sinking OC is sometimes strongly and sometimes weakly reduced in cold Arctic waters (<4 °C). Therefore, we conclude that temperature seems not to be an important factor determining how strong the amount of sinking OC is reduced with depth. We instead advocate that the phytoplankton community composition is useful to predict how strong the amount of sinking OC is reduced with depth. When fast‐sinking algae form algal aggregates or are repackaged into copepod fecal pellets, their OC spends only a short time in the upper water column where hungry grazers are very abundant. The consumers have then much less time to prey on the OC than when the sinking particles consist of slowly sinking algae. Concluding, we argue that it is very important to include the phytoplankton community composition in computer simulations to correctly predict how much OC is stored in the oceans. Key Points: Unlike previous assumptions, carbon flux attenuation can both be weak and strong in cold Arctic waters (<4 °C)Phytoplankton community composition is a more reliable predictor of carbon flux attenuation than temperature alonePhytoplankton and zooplankton abundance and composition should be included in carbon flux models as these parameters can modify b [ABSTRACT FROM AUTHOR]

The long‐term trend of increasing phytoplankton net primary production (NPP) in the Arctic correlates with increasing light penetration due to sea ice loss. However, recent studies suggest that enhanced stormy wind mixing may also play a significant role enhancing NPP. Here, we isolate the role of sea ice and stormy winds (hereafter high‐frequency winds) using an eddy‐permitting ice‐ocean‐biogeochemical model configured for the North Atlantic and the Arctic. In the model, the presence of high‐frequency winds stimulates nutrient upwelling by producing an earlier and longer autumn‐winter mixing period with deeper mixing layer. The early onset of autumn mixing results in nutrients being brought‐up to near‐surface waters before the light becomes the dominant limiting factor, which leads to the autumn bloom. The enhanced mixing results in higher nutrient concentrations in spring and thus a large spring bloom. The model also shows significant iron limitation in the Labrador Sea, which is intensified by high‐frequency winds. The effect of sea ice loss on NPP was found to be regionally dependent on the presence of high‐frequency winds. This numerical study suggests high‐frequency winds play significant role increasing NPP in the Arctic and sub‐Arctic by alleviating phytoplankton nutrient limitation and that the isolated effect of sea ice loss on light plays a comparatively minor role. Plain Language Summary: The amount of phytoplankton (i.e., small floating algae) in the Arctic Ocean has been increasing in response to sea ice losses, which increases light penetration and reduces light limitation on growth. However, the parallel increase in storm activity can also enhance phytoplankton growth by reducing nutrient limitation through increasing surface ocean mixing. Here, we use a numerical ocean—sea ice model with marine biogeochemistry to separate the contribution of stormy winds and sea ice loss on the phytoplankton growth. In the model, stormy winds induce more ocean mixing. This results in a seasonally larger amount of nutrient in surface waters with the capacity of sustaining a larger phytoplankton bloom in spring and autumn if light conditions are optimal for growth. The model shows a significant iron limitation in the Labrador Sea, which is intensified by high‐frequency winds. The contribution of sea ice loss to phytoplankton growth was only significant in the presence of stormy winds. This numerical study suggests high‐frequency winds play significant role increasing phytoplankton growth in the Arctic and sub‐Arctic by alleviating phytoplankton nutrient limitation and that the isolated effect of sea ice loss on light plays a comparatively minor role. Key Points: High‐frequency winds stimulate surface mixing and nutrient upwelling enhancing primary production and biogenic carbon exportThe model shows significant iron limitation in the Labrador Sea, which is intensified by high‐frequency windsSea ice loss contributes to increasing primary production only in Hudson Bay and in the presence of high‐frequency winds [ABSTRACT FROM AUTHOR]

Ongoing and future changes in wind and temperature are predicted to alter upper ocean vertical mixing across the Southern Ocean. How these changes will affect primary production (PP) remains unclear as mixing influences the two controlling factors: light and iron. We used a large ensemble of 1‐D‐biogeochemical model simulations to explore the impacts of changes in mixed‐layer depths on PP in the Southern Ocean. In summer, shoaling mixed‐layer depth always reduced depth‐integrated PP, despite increasing production rates. In winter, shoaling mixed layers had a two‐staged impact: for moderate shoaling PP increased as light conditions improved, but more pronounced shoaling decreased iron supply, which reduced PP. The fraction of PP exported below 100 m also presented a nonmonotonic behavior. This suggests a potential future shift from a situation where reduced winter mixing increases PP and export, to a situation where PP and export may collapse if the ML shoals above a threshold depth. Plain Language Summary: In the Southern Ocean, atmospheric warming associated to climate change is altering the depth at which surface waters are stirred, the so‐called mixed‐layer depth. A change in the mixed‐layer depth impacts the phytoplankton cells that inhabit it by altering their two main limiting factors: iron and light. However, the sign and magnitude of this impact are still not clear. In this work we used mathematical simulations to explain how changes in the seasonal mixed‐layer depth modify the supply of iron and the amount of light, and how these changes impact phytoplankton activity. Our results show that mixed‐layer depth changes in summer and in winter have different impacts. Reducing summer mixed‐layer depth did not change the iron supply, but it reduced the volume of water where phytoplankton thrived. In winter, shallower mixed‐layer depth altered iron and light but in opposed ways. At first, phytoplankton increased its activity as more light became available. However, a continued shallowing of the mixed‐layer depth eventually reduced the iron supply and the phytoplankton activity. Our study proposes a new interpretation on how ongoing changes in the Southern Ocean impact phytoplankton activity and alerts of the presence of threshold depths for the winter mixed layer above which phytoplankton may struggle to survive. Key Points: Moderate shoaling of winter mixed layer causes increase in primary production, but shoaling below a threshold depth can lead to a collapseReversal in this response is due to balancing effects of iron and light limitation and associated with a change in community structure and export efficiencyInterestingly, shoaling summer mixed layer always reduces PP and export because of a reduction of the productive volume [ABSTRACT FROM AUTHOR]

Abstract Copepods fecal pellets (FPs) are important contributors to carbon export in the ocean, but their role in the carbon export is poorly understood in the eutrophic coastal sea, where copepods biomass is always high. Two cruises were carried out in one of the most eutrophic coastal seas in the world — the Changjiang (Yangtze River) estuary during spring and summer in 2011, and copepods fecal pellets carbon (FPC) concentration, their production rates and sinking rates were measured, and a potential export flux of copepods FPs was also calculated. The copepods FPC concentration ranged from 0.21 to 1.25 mg C m−3 (mean = 0.58 ± 0.24 mg C m−3) in spring and 0.46–1.53 mg C m−3 (mean = 0.76 ± 0.25 mg C m−3) in summer, which was significantly lower than that of phytoplankton cells, indicating that copepods FPs only accounted for a minor part of the POC pool in this area. Production rates of copepods FPs fell in the range of the reported values, and the high values always appeared at stations with high phytoplankton biomass. Sinking rates of copepods FPs determined in the lab ranged from 17.82 to 223.30 m d−1 (mean = 73.46 ± 47.56 m d−1) in spring and 34.62–262.33 m d−1 (mean = 92.81 ± 56.80 m d−1) in summer, and they were significantly positive correlated with the FPs volume. The density of copepods FPs was also calculated with the determined sinking rate, and it was higher in summer than that in spring due to the variation of phytoplankton diets from spring to summer. The potential copepods FPs export flux ranged from 17.72 to 60.20 mg C m−2 d−1 (mean = 37.13 ± 16.21 mg C m−2 d−1) in spring and 23.80–166.10 mg C m−2 d−1 (mean = 79.44 ± 48.86 mg C m−2 d−1) in summer, and it was at a comparable level with the export flux of phytoplankton cells during both cruises. This study indicated that copepods FPs comprised a substantial fraction of exported carbon in the Changjiang (Yangtze River) estuary, and they should be taken into account when we study the biological pump in other eutrophic coastal seas around the world. [ABSTRACT FROM AUTHOR]

Copyright of Paleoceanography & Paleoclimatology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We describe the basis of a theory for interpreting measurements of two key biogeochemical fluxes—primary production by phytoplankton (p, μg C · L−1 · day−1) and biological carbon export from the surface ocean by sinking particles (f, mg C · m−2 · day−1)—in terms of their probability distributions. Given that p and f are mechanistically linked but variable and effectively measured on different scales, we hypothesize that a quantitative relationship emerges between collections of the two measurements. Motivated by the many subprocesses driving production and export, we take as a null model that large‐scale distributions of p and f are lognormal. We then show that compilations of p and f measurements are consistent with this hypothesis. The compilation of p measurements is extensive enough to subregion by biome, basin, depth, or season; these subsets are also well described by lognormals, whose log‐moments sort predictably. Informed by the lognormality of both p and f we infer a statistical scaling relationship between the two quantities and derive a linear relationship between the log‐moments of their distributions. We find agreement between two independent estimates of the slope and intercept of this line and show that the distribution of f measurements is consistent with predictions made from the moments of the p distribution. These results illustrate the utility of a distributional approach to biogeochemical fluxes. We close by describing potential uses and challenges for the further development of such an approach. [ABSTRACT FROM AUTHOR]

The total daily phytoplankton loss from the mixed layer can be estimated as the difference between gross primary production (GPP) and the realized change in phytoplankton carbon biomass. Here a modeling approach is used to estimate the total loss rates for five hydrographic domains on the eastern Bering Sea Shelf during a warm (2000–2006) and a cold (2007–2010) period. Model results indicate that the average daily rate of GPP in the mixed layer for all domains is on average slightly higher than in warm years (950±726 mg C m −2 d −1 ) than in cold years (859±640 mg C m −2 d −1 ), but is not significantly different. Similarly, the daily phytoplankton total loss rate from the mixed layer in all domains is on average slightly higher in warm years (961±747 mg C m −2 d −1 ) than in cold years (888±691 mg C m −2 d −1 ), but the difference is not significant. That total loss rates show the same warm vs. cold year pattern as GPP, suggests similar seasonal and latitudinal variations and magnitudes of change for both processes. The annual total loss is compared with the sum of individual process losses (e.g., mixing, grazing, sinking, etc.), with the discrepancy being generally larger than ~15% of the total loss both in warm and cold years. The model results also show that annual respiration is generally greater than losses due to zooplankton grazing and sinking both in warm and cold years. Compared among domains, significant differences ( t -test, P <0.05) between northern and southern domains (defined as North and South of 60°N) are observed for GPP rate, total daily loss rate and each of the individual loss terms in cold years, while values for southern domains are higher than those of northern domains. In warm years there were no significant differences between domains. Furthermore, these results indicate that total loss rates reflect patterns in GPP rate implying a similar metabolic balance within the ecosystem in both warm and cold years. [ABSTRACT FROM AUTHOR]

We construct mass-balance based estimates of carbon (C) export fractions from the water column across the Chukchi Sea shelf. Export is calculated as the difference between phytoplankton drawdown of dissolved inorganic C (DIC) and the accumulation of autochthonous particulate and dissolved organic C in the water column. Organic carbon (C org ) exports of >50% of DIC drawdown are ubiquitous across the shelf, even during, or shortly after, phytoplankton blooms, suggesting widespread and strong pelagic-benthic coupling. Export fractions on the shelf were generally greater in the less-productive Alaska Coastal Water than in the more productive Bering Shelf-Anadyr Water. Additionally, export fractions were greater in 2011 than in 2010, highlighting the significant spatial and inter-annual variability of the fate of C org in this ecologically and biogeochemically important, and rapidly changing, ecosystem. [ABSTRACT FROM AUTHOR]

We employ profiling floats with dissolved oxygen sensors to observe in situ temporal oxygen evolution below the mixed layer, allowing us to characterize net respiration of organic carbon in eight distinct regions over the globe. Export and export efficiency are generally high in locations with strong seasonal variability and low in locations of weak seasonality. Vertically integrated respiration is weakly, yet significantly, correlated with remote observations of chlorophyll, net primary production, and planktonic community size structure. These correlations suggest that regimes of high net primary production and large phytoplankton fuel elevated respiration at depth. Several regions of float-based observations intersect with sites of other detailed observations (e.g., Hawaii and Sargasso Sea), which allows us to compare our results to independent studies. We find that there is good agreement among export production estimates at highly seasonal locations, and that float-based observations may be biased low at weakly seasonal locations. We posit that the reason for the low-latitude discrepancy is the relative steady state of oxygen concentration caused by weak seasonality and shallow wintertime mixed layer depths. [ABSTRACT FROM AUTHOR]

Understanding the influence of anthropogenic forcing on the marine biosphere is a high priority. Climate change-driven trends need to be accurately assessed and detected in a timely manner. As part of the effort towards detection of long-term trends, a network of ocean observatories and time series stations provide high quality data for a number of key parameters, such as pH, oxygen concentration or primary production (PP). Here, we use an ensemble of global coupled climate models to assess the temporal and spatial scales over which observations of eight biogeochemically relevant variables must be made to robustly detect a long-term trend. We find that, as a global average, continuous time series are required for between 14 ( pH) and 32 (PP) years to distinguish a climate change trend from natural variability. Regional differences are extensive, with low latitudes and the Arctic generally needing shorter time series (<~30 years) to detect trends than other areas. In addition, we quantify the 'footprint' of existing and planned time series stations, that is the area over which a station is representative of a broader region. Footprints are generally largest for pH and sea surface temperature, but nevertheless the existing network of observatories only represents 9-15% of the global ocean surface. Our results present a quantitative framework for assessing the adequacy of current and future ocean observing networks for detection and monitoring of climate change-driven responses in the marine ecosystem. [ABSTRACT FROM AUTHOR]

Sinking rates of phytoplankton community with variable taxonomic composition in the offshore Changjiang (Yangtze River) estuary were measured during two cruises in spring and summer, 2011. A homogenous-sample method SETCOL was used to determine the sinking rates. Phytoplankton community was dominated by dinoflagellates in spring and diatoms in summer, and two species Prorocentrum dentatum and Skeletonema dorhnii formed algal blooms in the survey area during the two cruises, respectively. Phytoplankton sinking rates ranged from 0.13 to 1.04 m day − 1 (average = 0.61 ± 0.24 m day − 1 ) in spring and 0.28 to 1.71 m day − 1 (average = 0.80 ± 0.34 m day − 1 ) in summer. In the surface layer, phytoplankton sinking rates at the P. dentatum bloom stations in spring were lower than that at the S. dorhnii bloom stations in summer. No significant correlation was found between phytoplankton sinking rates and most of the environmental parameters during the two cruises, except for temperature and nitrite concentration in summer. A significant correlation was observed between phytoplankton sinking rates and phytoplankton community structure in the surface layers: the higher dominance of diatom in the phytoplankton community corresponded to higher phytoplankton sinking rate. Therefore, the phytoplankton community structure other than the environmental parameters, is the important factor to affect the sinking rates greatly. The consequent carbon flux caused by phytoplankton sinking was estimated, and results suggested that the carbon flux to bottom water during the S. dorhnii bloom (average = 63.13 ± 48.16 mg C m − 2 day − 1 ) in summer was about 2.4 fold of that during the P. dentatum bloom (average = 26.10 ± 26.25 mg C m − 2 day − 1 ) in spring. These findings provide us some insight in understanding the carbon export contributed by marine phytoplankton in the coastal sea, where frequent phytoplankton blooms and high following carbon export occur. [ABSTRACT FROM AUTHOR]

To quantify the balance between new production and vertical nitrogen export of sinking particles, we measured nitrate uptake, net nitrate drawdown, ΔO2/Ar-based net community production, sediment trap flux, and 234Th export at a coastal site near Palmer Station, Antarctica, during the phytoplankton growing season from October 2012 to March 2013. We also measured nitrate uptake and 234Th export throughout the northern western Antarctic Peninsula (WAP) region on a cruise in January 2013. We used a nonsteady state 234Th equation with temporally varying upwelling rates and an irradiance-based phytoplankton production model to correct our export and new production estimates in the complex coastal site near Palmer Station. Results unequivocally showed that nitrate uptake and net community production were significantly greater than the sinking particle export on region-wide spatial scales and season-long temporal scales. At our coastal site, new production (105 ± 17.4 mg N m−2 d−1, mean ± standard error) was 5.3 times greater than vertical nitrogen export (20.4 ± 2.4 mg N m−2 d−1). On the January cruise in the northern WAP, new production (47.9 ± 14.4 mg N m−2 d−1) was 2.4 times greater than export (19.9 ± 1.4 mg N m−2 d−1). Much of this imbalance can be attributed to diffusive losses of particulate nitrogen from the surface ocean due to diapycnal mixing, indicative of a 'leaky' WAP ecosystem. If these diffusive losses are common in other systems where new production exceeds export, it may be necessary to revise current estimates of the ocean's biological pump. [ABSTRACT FROM AUTHOR]

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1-5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method. [ABSTRACT FROM AUTHOR]

Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure, build biomass, and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Sub-Antarctic Zone and northern reaches of the Polar Frontal Zone in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O2/Ar net community oxygen production (NCP) and triple-O2 isotopes gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found that the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed-layer-depth-integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export. [ABSTRACT FROM AUTHOR]

The flux of organic carbon from the surface ocean to mesopelagic depths is a key component of the global carbon cycle and is ultimately derived from primary production (PP) by phytoplankton. Only a small fraction of organic carbon produced by PP is exported from the upper ocean, referred to as the export efficiency (herein e-ratio). Limited observations of the e-ratio are available, and there is thus considerable interest in using remotely sensed parameters such as sea surface temperature to extrapolate local estimates to global annual export flux. Currently, there are large discrepancies between export estimates derived in this way; one possible explanation is spatial or temporal sampling bias in the observations. Here we examine global patterns in the spatial and seasonal variability in e-ratio and the subsequent effect on export estimates using a high-resolution global biogeochemical model. The model used here represents export as separate slow- and fast-sinking detrital material whose remineralization is respectively temperature dependent and a function of ballasting minerals. We find that both temperature and the fraction of export carried by slow-sinking particles are factors in determining e-ratio, suggesting that current empirical algorithms for e-ratio that only consider temperature are overly simple. We quantify the temporal lag between PP and export, which is greatest in regions of strong variability in PP where seasonal decoupling can result in large e-ratio variability. Extrapolating global export estimates from instantaneous measurements of e-ratio is strongly affected by seasonal variability and can result in errors in estimated export of up to ±60%. [ABSTRACT FROM AUTHOR]

Abstract: The influence of environmental factors on size-fractionated phytoplankton production and biomass (chlorophyll a) and community composition was examined in the Hudson Bay system (Hudson Bay, Hudson Strait, and Foxe Basin; HBS) during August 2004 and September 2005 and 2006. Significant variability in the vertical structure of the water column and melt season length was observed between years and between regions of the HBS. Even though there was no year-to-year variability in the phytoplankton production and biomass, we observed significant differences in the phytoplankton size structure and taxonomic composition between mid and late summer. For all years, phytoplankton production and biomass were lower in Hudson Bay (51–1217mg C m−2 d−1; 11–57mg chl a m−2) than in Hudson Strait (675–2740mg C m−2 d−1; 28–97mg chl a m−2). Negative correlation between primary production and stratification strength of the upper water column suggested nutrient limited primary production in Hudson Bay and the south shore of Hudson Strait. Stratification and nitrate concentration also explained the variability in the physiological state (i.e., production:biomass ratio) and size structure of phytoplankton communities between mid and late summer. Daily estimated summer primary production averaged 0.32g C m−2 in Hudson Bay and 1.34g C m−2 in Hudson Strait. Phytoplankton production in the HBS was largely dominated by ultraphytoplankton. On average, only ca. 30% of total production was potentially exported from the euphotic zone. The dominance of flagellate-dominated community may explain the low export of matter and energy toward deeper waters and likely toward the upper trophic levels. [Copyright &y& Elsevier]

Abstract: Northeast Atlantic mode waters (NEAMW) are formed by subduction in a region which is a strong sink of atmospheric CO 2. The mechanisms underlying this sink were thoroughly examined in the frame of the POMME experiment, which covered the 2001 seasonal cycle with particular focus on meso to sub-mesoscales. The biological and physical data collected during POMME were used to initialize, constrain and evaluate a regional, 4.5km-resolution, bio-physical model simulation of the 2001 seasonal cycle in the region of NEAMW formation. We used this model to examine the contribution of sub-mesoscales on the annual budget of carbon export below the mixed-layer and on NEAMW biogeochemical characteristics. This is done by comparing a high-resolution (eddy) simulation at 4.5km resolution with a high-diffusivity (non-eddy) simulation where the only change is a stronger eddy diffusivity. We found that the model mixed-layer depth is more stratified and closer to reality in the eddy simulation. This result confirms with observational data the proposed mechanism of restratification of the upper ocean driven by sub-mesoscales. We found that the phytoplankton bloom and the subduction of NEAMW display strong contrasts at the sub-mesoscale. Nevertheless, the mean intensity of the bloom and of the subsequent biological pump are only marginally modified by sub-mesoscales (less than 5%), while the intensity of the physical pump (subduction of carbon) is moderately reduced (−10%) in the non-eddy experiment. Moreover, the biogeochemical and thermodynamical characteristics of NEAMW are substantially affected, with, in particular, a wider range of densities and biogeochemical characteristics biased toward winter conditions in the eddy simulation. These differences ensued essentially from lateral induction across sub-mesoscale filaments, which is found to contribute to subduction before seasonal stratification, i.e. before the effective subduction period. [Copyright &y& Elsevier]

Abstract: Total 234Th and 238U vertical profiles in the water column were determined along a three-station transect perpendicular to the Eastern Weddell Sea ice shelf. The transect was sampled during pre-bloom (6–7 December 2003) and bloom (19–20 December 2003) conditions to follow changes occurring in the particulate organic carbon (POC) flux during an Antarctic phytoplankton ice-edge bloom. During the pre-bloom period, 234Th:238U ratios>1 (up to 1.3) were measured just above the thermocline at all stations, and throughout the entire mixed layer at the ice-edge. These water column data appear to be the first which support the hypothesis that sea-ice algae could represent an input of 234Th-rich material in polar waters due to cellular uptake of the radionuclide and/or its adsorption onto the extracellular polymeric substances (EPS) produced by these organisms. A simple conceptual model is proposed to link observed 234Th:238U ratios to the input of 234Th-enriched sea-ice algae caused by ice-melting. The extensive, deep-water 234Th deficit observed during the second transect likely results from the combination of vertical particle export and water-seafloor exchange processes. POC steady-state (SS) fluxes out of the photic layer were negative or ∼nil (−11 to 2 mmol m−2 d−1) under pre-bloom conditions, probably due to ice-algae input at the ice-edge and remineralization elsewhere, whereas POC non-steady-state (NSS) fluxes were generally higher (7 to 33 mmol m−2 d−1) when the bloom was developing. We have estimated that these flux values could change by as much as 75% if a sea-ice algae 234Th input term were included in the simple one-box model used. This suggests that failure to consider the sea-ice algae compartment, when using 234Th as a proxy to assess carbon export in seasonally ice-covered regions, could at times lead to significant biases in the interpretation of 234Th-based particle flux estimates. [Copyright &y& Elsevier]

Abstract: It has long been recognised that some oceanic regions have persistently low-chlorophyll levels, even though there are abundant inorganic nutrients. Studies have shown that these high-nutrient low-chlorophyll (HNLC) areas are depleted in iron, an essential micronutrient. In these regions biological production can be enhanced with artificial mesoscale iron fertilisation. However, the ability of iron-induced blooms to efficiently sequester carbon to mesopelagic depths is still an open question. It is hypothesised that sub-Antarctic islands in the HNLC Southern Ocean are also a source of iron and thus fuel the natural phytoplankton blooms observed in their proximity, thereby enhancing levels of particulate organic carbon (POC) export. To test the third part of this hypothesis, POC export was measured in the Southern Ocean region of the Crozet Islands (52°E, 46°S) during the austral summer of 2004/2005 as part of the CROZEX project. Based on satellite imagery, a high-chlorophyll region (maximum concentration=4μgl−1) north and downstream of the islands was distinguished from a low-chlorophyll region (typical concentration=0.3μgl−1) south and upstream of the islands. POC export estimates were obtained by using the naturally occurring particle-reactive radionuclide tracer 234Th. POC export was initially 15mmolCm−2 d−1 in the high-chlorophyll bloom region, compared with 5mmolCm−2 d−1 in the low-chlorophyll, non-bloom region. After a moderately small bloom at the southern control stations (max concentration=0.7μgl−1) the spatial variability in POC export was lost, resulting in equally high levels of POC export (ca. 20mmolCm−2 d−1) throughout the study region. Comparison of 234Th-derived POC export with estimates of new production, calculated from nitrate budgets, revealed evidence for a decoupling of new and export production, with this effect most apparent within the northern bloom area. In addition to methodological issues this apparent decoupling of new and export production could be due to a buildup of dissolved organic nitrogen within the bloom region, thus reducing the amount of POC available for export to mesopelagic depths. [Copyright &y& Elsevier]

Abstract: Annual phytoplankton blooms are observed around most sub-Antarctic islands during austral spring and summer, but are absent in the surrounding high-nutrient low-chlorophyll (HNLC) ocean. The CROZEX study (2004/2005) tested the hypothesis that annual blooms occurring immediately north of the Crozet islands in the Polar Frontal Zone (PFZ) develop because of natural iron fertilisation, while to their south in HNLC waters, there is insufficient iron (Fe) to promote blooms. Size-fractionated nitrogen uptake (ρN) experiments using 15N-NO3 −, NH4 + and urea addressed three major goals. Firstly, measurements of NO3 − uptake (ρNO3 −) aimed to establish whether ρNO3 − responds to natural Fe fertilisation. Secondly, we compared regional ρNO3 − rates, hypothesising that in Fe-fertilised regions, ρNO3 − should exceed that in HNLC regions. Thirdly, by using satellite imagery, we extrapolated ρNO3 − measurements made during a declining bloom to reconstruct seasonal ρNO3 − by the spring bloom. Finally, we estimated the ‘new’ Fe demand required to support ρNO3 −, comparing this with estimated Fe fluxes. Diatoms and colonial Phaeocystis dominated phytoplankton communities north of the islands, while to the south, Phaeocystis was absent. Total ρN was elevated north of the islands (∼400μmolm−2 d−1) relative to south of the islands (∼250μmolm−2 d−1). Nitrate uptake showed a clear response to Fe fertilisation, exhibiting a strong north (∼198μmolm−2 d−1) to south (∼74μmolm−2 d−1) gradient, while neither ρNH4 + nor ρurea showed such significant latitudinal gradients. The N–S integrated f-ratio gradient was 0.47–0.28 while specific N uptake ( d−1) rates were significantly higher in the Fe-fertilised region relative to those in the southern HNLC region. The potential for NH4 + inhibition of ρNO3 − did not appear to be significant. High PON:chl-a ratios combined with relatively low 14C:15N uptake ratios suggested that most phytoplankton were relatively chlorotic and carbon stressed, with the exception of those growing actively within a cyclonic eddy where neither Fe nor light appeared to be limiting. Size-fractionated ρNO3 − and f-ratios exhibited a complex response to NH4 + and Fe availability, with f-ratios in the >20-μm fraction being low (∼0.3) in the HNLC region, but significantly higher (∼0.7) in a localised diatom-dominated bloom in the northern Fe-fertilised region. In contrast, f-ratios in the <2-μm size class were similar everywhere (∼0.44), indicative of Fe-limitation for large-celled diatoms in the southern HNLC region. As a result of Fe-regulated ρNO3 −, new production showed a N–S gradient of ∼24 to ∼15mmolCm−2 d−1, very similar to carbon export determined from NO3 − ‘draw-down’ and from 234Th measurements. The estimated DFe demand required to support seasonal ρNO3 − in the northern region, based on conservative cellular Fe:N quotas, required surface (to 100m) pre spring-bloom DFe concentrations of ∼0.75nmoll−1. Our results support the hypothesis that phytoplankton blooms north of the islands are stimulated by natural Fe fertilisation, with a direct impact on ρNO3 −, particularly for larger cells, resulting in higher new production rates relative to those from the Fe-limited HNLC region south of the Crozet islands. [Copyright &y& Elsevier]

Abstract: We present water column profiles of excess particulate Ba (Baxs, an estimate of biogenic Ba from total particulate Ba after small corrections for lithogenic Ba) along a transect in the Australian sector of the Southern Ocean from the Subantarctic Zone (SAZ) to the Sea Ice Zone during spring (October–December 2001). Surface water Baxs contents appear related to phytoplankton derived particles. Below, in the twilight zone, mesopelagic Baxs records the changes in plankton biomass from the mixed layer over a time scale of a few weeks and confirms its usefulness as an indicator of the carbon remineralisation process. In comparison to the SAZ, the mesopelagic Baxs accumulation is larger and begins at shallower depths south of the Polar Front Zone (PFZ), in the Antarctic Zone (AZ), where diatoms are the dominant component of the phytoplankton community. Summer results from 1998, when mesopelagic Baxs accumulations were larger, also show this latitudinal trend. In contrast, as observed also for deep particulate organic carbon fluxes, the flux of Baxs to moored deep sea sediment traps was larger in the nano-phytoplankton dominated SAZ than the diatom dominated PFZ. Overall, the results suggest relatively high particulate carbon export to the deep sea in the absence of strong remineralisation in the SAZ, and relatively low export to the deep sea in the presence of strong remineralisation further south. Mesopelagic carbon remineralisation is higher in summer than in spring as also observed on deep sediment traps carbon fluxes. Our findings are supported by 234Th and N-uptake proxies from the same transect. This study expands the utility of Ba as an indicator of biogeochemical processes in the twilight zone and supports its usability as a paleoceanographic proxy for deep C export. [Copyright &y& Elsevier]

Marine snow aggregates often dominate carbon export from the surface layer to the deep ocean. Therefore, understanding the formation and properties of aggregates is essential to the study of the biological pump. Previous studies have observed a relationship between phytoplankton growth phase and the production of transparent exopolymer particles (TEP), the sticky particles secreted by phytoplankton that act as the glue during aggregate formation. In this experimental study, we aim to determine the effect of phytoplankton growth phase on properties related to aggregate settling. Cultures of the diatom Thalassiosira weissflogii were grown to four different growth phases and incubated in rotating cylindrical tanks to form aggregates. Aggregate excess density and delayed settling time through a sharp density gradient were quantified for the aggregates that were formed, and relative TEP concentration was measured for cultures before aggregate formation. Compared to the first growth phase, later phytoplankton growth phases were found to have higher relative TEP concentration and aggregates with lower excess densities and longer delayed settling times. These findings may suggest that, although particle concentrations are higher at later stages of phytoplankton blooms, aggregates may be less dense and sink slower, thus affecting carbon export. [ABSTRACT FROM AUTHOR]