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The Arctic warming leads to a decline in sea-ice extent and thickness, rapid warming and freshening of the sea surface which impact the distribution of phytoplankton size composition. Picophytoplankton is an ecologically important component of Arctic pelagic marine ecosystems, and its role may be altered by global warming. In this study, the abundance and biomass, the chlorophyll a (Chl-a) and primary production (PP) of picophytoplankton, and its spatial and temporal distribution were investigated in the Kara Sea during the ice-melt season in July 2019. Picophytoplankton played a major role in the surface PP in the southern and western areas of the Kara Sea. In the surface layer, the contribution of picophytoplankton to total Chl-a increased insignificantly, and the contribution of picophytoplankton to total PP decreased significantly with the time of sea ice retreat. In the euphotic zone, the Chl-a concentration of picophytoplankton and its contribution to total Chl-a decreased with the time of sea ice retreat. The average picophytoplankton biomass determined in the present study (2.72 ± 5.10 mg C m -3 ) corresponded to the biomass estimates in the Arctic. The picophytoplankton community was strongly dominated by eukaryotes, cyanobacteria were only detected at 3 out of 11 stations, with maximum abundances (0.07 × 10 9  cells m -3 ) observed at depths below 15 m. The obtained results contribute significantly to the study of the picophytoplankton dynamics during the ice-melting season in the hard-to-reach Kara Sea., 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 © 2024 Elsevier Ltd. All rights reserved.)

The cellular size and biomass of picophytoplankton were studied by flow cytometer during spring monsoon (March-May of 2015) in equatorial eastern Indian Ocean. We established an empirical relationship between forward scatter and cellular size to address the size and biomass of picophytoplankton. Results indicated that mean cell diameter of Prochlorococcus (0.60 μm) was the smallest, and then followed by Synechococcus (0.98 μm) and picoeukaryotic phytoplankton (1.05 μm). Thereafter, the biomass converted by abundance reached 0.64 μg·C·L -1 for Prochlorococcus, 0.34 μg·C·L -1 for Synechococcus, and 0.20 μg·C·L -1 for picoeukaryotic phytoplankton. Additionally, the distinct biomass contribution of picophytoplankton appeared to be affected by abundance, but not changes in cellular size. Vertically, the cellular sizes of picophytoplankton were remarkably small in upper waters, which was predominantly controlled by the nutrient availability. In contrast, they were larger in deeper waters, which was primarily attributed to the combined effects of low temperature and reduced light availability. Spatially, under the influence of high nutrient concentration induced by the different circulations and coastal upwelling, slightly high carbon biomass of picophytoplankton was observed around the coastal zones of Sri Lanka island and Sumatra, as well as the southern Bay of Bengal., (© 2018 The Authors. MicrobiologyOpen published by John Wiley & Sons Ltd.)

The two smallest size classes of phytoplankton, namely, nano- and picophytoplankton, can play an important role in marine primary production, particularly in warm and nutrient-poor waters. The Arabian Gulf, with its high temperatures, could provide good growth conditions for these two size classes; however, so far, phytoplankton research in the Arabian Gulf has largely focussed on the biggest size class, microphytoplankton. This study presents data on the annual cycle of chlorophyll- a (Chl- a) at three stations in Saudi Arabian waters of the Arabian Gulf, focussing on the contribution of the three phytoplankton size classes to total Chl- a , and identifies environmental parameters affecting these contributions. Whereas dominance of nanophytoplankton was observed during summer, highest contributions of picophytoplankton were found during winter and spring. A close link between nano- and microphytoplankton was noted on the basis of optimum correlation models. The models for both size fractions included photosynthetically active radiation, salinity and phosphate concentration, but with opposing algebraic signs in the two models. By contrast, picophytoplankton was negatively correlated with temperature, indicating that the contribution of this size class could decrease under a future climate-change scenario in the Arabian Gulf. This study reports on the contribution of three phytoplankton size classes to the total chlorophyll- a (Chl- a) concentration at three coastal stations in the Arabian Gulf. It included data on the seasonal cycle for total chlorophyll a , as well as for the three size classes, and indicated physicochemical parameters potentially driving these cycles. It was found that micro- and nanophytoplankton Chl- a are correlated to a similar set of physicochemical parameters, whereas picophytoplankton Chl- a has a distinctly different correlation model. [ABSTRACT FROM AUTHOR]

Picophytoplankton are the smallest, most abundant photosynthetic organisms in the ocean. Knowledge of the diel variability of these tiny microbes has important implications for the structure of microbial food webs and key biogeochemical processes. However, insight into the mechanisms that underlie picophytoplanktonic diel dynamics is limited. By combining a field survey with a published dataset, we found that cell numbers and cell sizes/biomasses of picophytoplankton were tightly synchronized to the day‐night cycle, but they were in a quasi‐antiphase relationship to each other. This pattern is a confirmation and extension of previous studies. Mortality rates showed that Prochlorococcus and Synechococcus were subject to considerable grazing pressure throughout the day and night. The quasi‐antiphase diel cycles in abundance and cell size/biomass are likely determined by the light‐dependent diel behavior of cell growth and division and continuous losses to grazing. This work significantly improves our understanding of autotrophic picoplankton in the oligotrophic ocean. Plain Language Summary: Picophytoplankton are tiny, single‐celled photosynthetic organisms that contribute to almost all primary production in the vast euphotic zones of the oligotrophic ocean. Understanding their roles in that environment is critical but challenging, mainly because of their minuscule size and the complexity of microbial processes and interactions. Time‐series observations based on flow cytometry, a powerful technique that provides information about the numbers and sizes of picophytoplankton cells, have elucidated many ecological and biogeochemical processes associated with picophytoplankton, but some questions remain. A field survey in the northern South China Sea combined with a published dataset revealed that picophytoplankton cell size and biomass tended to decrease (increase) during the night (day) when cell numbers were increasing (decreasing). Such quasi‐antiphase cycles are likely a general feature of near‐steady‐state oligotrophic ecosystems and reflect the cycles of carbon fixation, energy storage, and cell growth during the daytime and cell division and energy depletion during the night. Mortality rates estimated via modified dilution experiments showed that Prochlorococcus and Synechococcus were subject to considerable grazing pressure throughout the day and night. This work significantly improves our understanding of these microorganisms and may have implications for the carbon cycle in oligotrophic marine ecosystems. Key Points: Picophytoplanktonic quasi‐antiphase diel cycles in abundance and cell size/biomass are likely a general feature of the oligotrophic oceanGrazing pressure on Prochlorococcus and Synechococcus is as high during the day as during the night [ABSTRACT FROM AUTHOR]

Seasonal dynamics of phytoplankton production and structural characteristics was studied at two closely located stations in the open part of the Sevastopol Bay mouth, one of which was a mussel farm. The net primary production, chlorophyll a content, total phytoplankton biomass, the quantitative ratio between small and large algal cells, and their seasonal dynamics were the same for both stations. Linear dependencies were observed between the relevant parameters of the stations, with the angular regression coefficients close to 1. The phytoplankton production and biomass peaked during the summer period, reaching 350 ± 43 mg C/(m3 day) and 450 ± 50 mg C/m3, respectively. Chlorophyll a content varied from 0.5 to 3 mg/m3, with the minimum in January‒February and the maximum in summer. The assimilation number values for chlorophyll a during the light period corresponded to those reported for production waters and correlated with the temperatures during the year. Total oxygen consumption by bacterio- and phytoplankton during the summer period was 30–70% of net photosynthesis. The data were obtained on seasonal biomass variations in three groups of algae (Synechococcus, picoeukaryotic phytoplankton, and nanophytoplanktom); on average, picoalgae constituted 30% of the phytoplankton biomass, with maxima during the winter-spring and summer periods. Bacterioplankton abundance was determined using flow cytometry and the SYBR Green I vital fluorochrome. A positive relation between bacterial abundance and water temperature was shown. [ABSTRACT FROM AUTHOR]

The spatial distribution of picophytoplankton abundance, biomass, chlorophyll a and the contribution of picoalgae to the total chlorophyll a was studied in the outer Ob River estuary with the adjacent shelf and in the western part of the Kara Sea. In August–September, the picophytoplankton abundance and biomass varied from 0.1 to 17.3 × 106 cell/L and from 0.06 to 9.20 mg С/m3, respectively. Photosynthetic picoeukaryotes dominated in the plankton picofraction; the contribution of cyanobacteria to the total picophytoplankton biomass did not exceed 11%. The highest contribution of picophytoplankton to the total phytoplankton abundance was observed at a lower (<11 mg/m2) euphotic zone integrated chlorophyll a. The spatial heterogeneity of picoforms contribution was determined by the silicon concentration. [ABSTRACT FROM AUTHOR]

Abstract Seasonal dynamic of Utermöhl fraction of phytoplankton (UFP, cell size > 3 μm) and picophytoplankton (Pico, cell size < 3 μm) were compared in three eutrophic coastal lagoons located in the same geographical area (Western Mediterranean Sea). Considering data on density, biomass, volume and composition of each size class, we focused on three aspects: i) the contribution of UFP and Pico to total phytoplankton; ii) the responses of UFP and Pico to seasonal variations of selected environmental variables; iii) the identification of the main environmental drivers of UFP and Pico seasonal dynamics. Pico contribution to total phytoplankton density was from 20% to 90%, highlighting the potential importance of Pico also in eutrophic ecosystems. Pico contribution to total phytoplankton biomass was lower, from 9% to 16%. Statistically, temperature resulted the strongest environmental factor controlling both the UFP and Pico seasonal variations in all the studied lagoons. In addition, the role of salinity, water transparency and nutrient concentrations was important in relation to the study site and the size class considered. In this work, the effects of temperature, salinity, transparency and nutrient availability on two different phytoplankton size classes have been described in Mediterranean shallow coastal lagoons for the first time, providing new insight for this ecosystem typology worldwide. Graphical abstract Image 1 Highlights • Potential importance of picophytoplankton in eutrophic ecosystems was established. • Temperature was the strongest driver of seasonal cell size structure. • Role of salinity, transparency and nutrients was size- and site-specific. • Synergic interaction of several drivers affected seasonal cell size structure. [ABSTRACT FROM AUTHOR]

Picophytoplankton (Pico) seasonal dynamics and ecology were investigated in three eutrophic coastal lagoons (Calich, Santa Giusta, and Corru S'Ittiri) located in the same geographic region (Sardinia, Mediterranean Sea). Pico dynamics were examined to determine a relationship with variability in environmental conditions characterizing the three sites, and identify which parameters affected observed Pico dynamics. Results showed Pico density reached 10 cells L and biomass values did not exceed 72.8 μg C L. Statistical analyses detected significant differences in hydrographic and nutrient conditions among lagoons, which were also characterized by variable Pico seasonal dynamics and composition. Calich and Santa Giusta were dominated all year around by picocyanobacteria. Maximum picocyanobacteria abundance was observed during summer in both lagoons, associated with the highest temperatures recorded. Picocyanobacteria abundance and nutrient concentrations showed a significant negative and positive correlation in Calich and Santa Giusta, respectively. In contrast, picoeukaryotes dominated Pico assemblages in Corru S'Ittiri, where the highest transparency and salinity and the lowest seasonal variation in salinity were recorded. In Corru S'Ittiri, the maximum Pico abundance was observed in autumn, when lower temperature and the highest nutrient concentrations were observed. Our study provides information on Pico assemblage ecology, which remains scarce in coastal and transitional environments, particularly in Mediterranean climate regimes. [ABSTRACT FROM AUTHOR]

We explored picophytoplankton in the surface (0 m) and bottom (2.3–8.7 m) layers of a shallow (<10 m) eutrophic coastal system (Isahaya Bay, Japan). We found that picophytoplankton (principally Synechococcus) constituted the major phytoplankton in spring and summer. The chlorophyll a (chl. a) concentration in the 0.7–2.0-μm picophytoplankton fraction (hereinafter 'pico-sized chl. a ') and picophytoplankton abundance in Isahaya Bay were higher than those in other eutrophic coastal waters. The pico-sized chl. a concentration and the picophytoplankton abundance counted with an epifluorescence microscope was up to 49.31 μg L−1 and 1.9 × 106 cells mL−1, respectively. Higher contributions of pico-sized chl. a to the total chl. a were evident in summer (up to 63.5%), relative to spring (up to 32.1%), at both depths. Picophytoplankton abundance and the pico-sized chl. a concentration was positively correlated with water temperature and dissolved inorganic phosphorus (DIP) concentrations. Thus, both temperature and DIP may be major controllers of picophytoplankton in Isahaya Bay. The pico-sized chl. a concentration and picophytoplankton cell number at the bottom layer were positively correlated with those in the surface layer, suggesting that picophytoplankton in bottom layers may have sunk from the surface layers. The results imply that the picophytoplankton affects the biogeochemical processes in the bottom of Isahaya Bay more than previously thought. This may be true not only for this estuary but also for other eutrophic coastal seas. • Picophytoplankton contributed considerably to the total phytoplankton biomass in spring and summer of a shallow, eutrophic coastal sea. • Higher contributions of pico-sized chl. a to the total chl. a were evident in summer (up to 63.5%), relative to spring (up to 32.1%), at both depths. • High amount of picophytoplankton may sink from the surface to accumulate in near-bottom layers. [ABSTRACT FROM AUTHOR]

šIn this study, we investigated the food sources of eastern oysters Crassostrea virginica cultivated in Atlantic Canada. Stable isotopes (13Cand 15N) and fatty acid biomarkerswere used to identify these sources under in situ conditions for suspended (~0.5mbelow surface) and bottom(~2m) culture stocks. It was found that particulate organic matter represented the main food source, with major contributions from live phytoplankton. Higher lipid contents were detected in the digestive glands of suspended oysters compared to bottom oysters (p<0.05). Bottom oysters did not show significant preference for detrital or bacterial organic matter. Near-surface waters contained an elevated picophytoplankton biomass (PPP, 0.2-2mm, 1.93 ± 0.16µg l-1, mean ± SEM) compared to nanophytoplankton biomass (NPP, >2mm, 1.05 ± 0.15µg l-1, mean ± SEM). To determine whether the small size PPP was captured and assimilated by C. virginica, feeding trials were conducted in the laboratory using three PPP/NPP diets (20%, 50%, and 80% PPP), consisting of isotopically-labelled (d13C) PPP cells (Nannochloropsis oculata) and non-labelled NPP cells (Tisochrysis lutea). An isotopically-labelled fatty acids analysis indicated PPP assimilation in various tissues (digestive gland, gills, mantle, and abductor muscle), including from oysters fed the reduced (20%) PPP diet. Isotopic enrichment (13C) in the FA22:2 (non-methylene-interrupted orNMI) showed that precursors of NMIs utilized PPP carbon in its biosynthesis process. In conclusion, C. virginica assimilated primarily particulate organic matter (POM), including PPP, which dominated the phytoplankton community in near surface waters. C. virginica can exploit PPP carbon during fatty acid production and further biosynthesis. [ABSTRACT FROM AUTHOR]

We investigated the distribution of several picophytoplankton groups (Prochlorococcus, Synechococcus, and picoeukaryotes) in relation to a warm eddy in the northern South China Sea in summer 2012. An anticydonic eddy centered on 117°E longitude was identified during the sampling period using satellite data and hydrologic mapping. The layer of maximum Prochlorococcus and Synechococcus abundance within the eddy core dropped from 50 to 75 m, which was consistent with the subsurface chlorophyll a maximum. The water-column integrated abundance of Prochlorococcus, Synechococcus, and picoeukaryotes in the eddy core (9.67±0.23,1.56±0.04, and 0.28±0.01 x 103 cells ml-1, respectively) was significantly lower (P<0.05) than that of the reference stations (25.10±2.32,2.71±0.63, and 0.92±0.15 x 103 cells ml-1, respectively), and the abundance of Prochlorococcus in the core was also significantly lower than that at eddy edges (15.75±1.78 x 103 cells ml-1). However, there were no differences in the water-column integrated Chi a between the eddy core and edge. Our findings show that the warm eddy led to the reduced picophytoplankton abundance, especially of Prochlorococcus and picoeukaryotes, causing the layer of maximum picophytoplankton abundance to differ from that of the subsurface Chl a maximum. [ABSTRACT FROM AUTHOR]

'The plankton paradox' view which contradicted competitive exclusion principle has been changing since the last decade. Current outlook based on the evidence from size scaling studies suggests that in a given nutrient regime, phytoplankton that are capable of efficient nutrient intake increases in biomass, perhaps leading to the exclusion of those with inefficient nutrient intake ability. Commonly observed higher biomass of larger cells (e.g., diatoms and dianoflagellates) in eutrophic environment and that of smaller cells (e.g.,picophytoplankton) in oligotrophic environment has been viewed as the competitive success of higher and lower size strata in respective nutrient gradients. We analyse the above view point sceptically, and share the opinion as follows. Conventional approach of using relative biomass as a measure of competitive success in phytoplankton could lead to underestimate the ecological success of smaller phytoplankton in eutrophic and mesotrophic waters. We also suggest that picophytoplankton that require very low nutrient quota can co-exist with larger cells by sharing a negligible part of the community niche, without exerting competition related pressure, irrespective of the nutrient status of the system. Similarly, large cells that are confined to the nutrient rich waters, can avoid competition with smaller algae by their exceptional nutrient storage ability. Consequently, there is a limitation in using relative biomass as a measure of competitive success in phytoplankton community. We also emphasis that in aquatic microbial world where all interactions occur at cellular level, difference in particle size could provide an additional mechanism to prevent competitive exclusion by facilitating resource partitioning and selective predation pressure. • Generally, higher relative biomass (ie. biomass ratio of a particular size fraction in relation with other size fractions/ percentage contribution of biomass) of larger phytoplankton (e.g.,diatoms and dianoflagellates) in eutrophic environment and that of smaller ones (e.g., Picophytoplankton) in oligotrophic environment has been viewed as the competitive success of higher and lower size strata in respective nutrient gradients. • However, difference in particle size and physiology can facilitate positive interaction in phytoplankton to prevent competitive exclusion. • Hence conventional approach of using relative biomass as an index of competitive success in phytoplankton could lead to the underestimation of ecological success of lower size fractions. [ABSTRACT FROM AUTHOR]

Abstract: Temporal and spatial variations in Synechococcus abundance were investigated over an annual cycle (February'10–January'11) along a salinity gradient (0–35) in the tropical Zuari estuary, influenced by south-west monsoons. Synechococcus exhibited salinity preferences with phycoerythrin-rich cells at salinities >2 (Synechococcus-PEI), >20 (Synechococcus-PEII) and <1 (Synechococcus-PEIII) whereas phycocyanin-rich (Synechococcus-PC) dominant at lower salinities. Downstream stratification during monsoon caused Synechococcus group segregation in the surface and near-bottom waters. During monsoon-break and non-monsoon period stabilized waters, increased salinity, temperature, solar radiation and low rainfall favored high Synechococcus abundance whereas unstable waters, increased turbidity and low solar radiation during active monsoon lowered abundance. SYN-PC positively co-related with nitrate and phosphate and SYN-PEI with phosphate. Synechococcus contribution to phytoplankton carbon biomass ranged from 9 to 29%. In monsoonal estuaries, rainfall intensity regulates freshwater runoff which modulates the estuarine environment, creating temporal–spatial niche segregation of Synechococcus groups thereby serving as indicator organisms of the estuarine hydrodynamics. [Copyright &y& Elsevier]

Abstract: During the R/V Xuelong Arctic cruise in summer 2008, we investigated bacterioplankton and picophytoplankton abundance, biomass, spatial distribution, and these variables'' relationship with environmental variables in the Western Canada Basin. The bacterioplankton and picophytoplankton abundances in the upper 200m of the water column were 0.17–8.38×105 cellsml−1 and 0.01–17.71×106 cellsL−1, respectively. The average integrated bacterioplankton and picophytoplankton biomasses in the upper 100m of the water column were 413.3 and 118.8mgCm−2, respectively. Microbial biomass was comparable with previous reports and distributed mainly in the upper 50m of the water column. The bacterioplankton and picophytoplankton biomasses decreased with increasing latitude. Compared with eastern transects, which were strongly influenced by an influx of Pacific water, the western transects had relatively low temperature, high salinity, high nutrients, and high biomass. No significant relationships were detected between assemblages and water temperature or salinity, expect for one between bacterioplankton and salinity in the latitudinal transects. However, a significant negative correlation between picophytoplankton and nutrients and a significant positive correlation between bacterioplankton and picophytoplankton were observed. We suggest that this was mainly caused by the relatively low picophytoplankton biomass, occurrence of heavy stratification (nutricline), and the subsurface chlorophyll maxima (SCM) in study area. Heavy melting of sea ice in summer increases the stratification, which obstructs nutrient supplementation from deep waters, and this might increase the role of microbial assemblages in the upper water column in the Arctic basin area. [Copyright &y& Elsevier]

Abstract: The spatial and temporal variability of phytoplankton were investigated in the southern Adriatic Sea in the period November 2006–June 2008. Three stations were sampled along the Bari–Dubrovnik transect with a seasonal frequency (except summer 2007 and spring 2008) at four depths (surface, maximum fluorescence depth and two intermediate depths), with the aim of analyzing the structure of phytoplankton community and evaluate the standing stocks of the three size fractions (pico-, nano- and microphytoplankton). Abundances were on average 5.41×107±5.97×106 cellsl−1 for picophytoplankton, 1.00×106±6.70×104 cellsl−1 for nanophytoplankton and 2.16×104±7.46×103 cellsl−1 for microphytoplankton. Total biomass ranged from 8.5 to 80.7μgCl−1. The highest values were recorded in April 2007 (38.4±5.9μgCl−1), and the lowest ones in February 2008 (13.8±1.5μgCl−1). Picophytoplankton was generally the most abundant phototrophic fraction (on average, 96% of the total abundances and 49% of the total biomass), while nano- and microphytoplankton provided a minor contribution (4% and less than 1% of total abundance and 35% and 16% of total biomass for nanophytoplankton and microphytoplankton, respectively). A decreasing gradient from west to east was observed for pico- and nanophytoplankton abundances and for total biomass. The low biomass values and the dominance of the autotrophic picoplankton fraction in the whole area reflected a general oligotrophic condition. The easternmost station (AM9), characterized by the lowest phytoplankton abundances and biomass and low nutrient concentrations, resulted in still more oligotrophic condition and probably influenced by the Ionian Surface Water (ISW), coming from the eastern Mediterranean. [Copyright &y& Elsevier]

In July-August 2009, the abundance of picophytoplankton (Pico) in the Velikaya Salma strait varied from 3.4 × 10 to 19.4 × 10 cells/L, while its biomass (B) was 0.8-3.3 mg C/m. In August 2010, Pico abundance was significantly higher (up to 216 × 10 cells/L and 36.8 mg C/m). Pico consisted mainly of cyanobacteria. It constituted 13 (2009) to 28% (2010) of the total phytoplankton biomass. In April 2010, Pico numbers varied from 0.1 × 10 to 0.22 × 10 cells/L and its biomass was 0.05-0.28 mg C/m. Picoeukaryotes were predominant. Pico constituted not more than 2.7% of the phytoplankton biomass. In the ice column, the integrated Pico abundance was 430 × 10 cells/m and the integrated biomass was 365 μg C/m. [ABSTRACT FROM AUTHOR]

We investigated the space–time dynamics of picophytoplankton in South Australian continental shelf waters from February 2008 to January 2009, focusing on localized physical events. We discriminated six picophytoplankton populations by flow cytometry, including Synechococcus (SYN1, SYN2), Prochlorococcus (PROC1, PROC2) and small and large picoeukaryotes (EUKS, EUKL). Local physical events observed included downwelling and dense waters outflowing from a nearby gulf in winter–early spring 2008, upwelling in summer and early spring 2008 and eddy formation in January 2009. Each population responded differently to these events, which resulted in up to four orders of magnitude changes in their abundances. Population-specific hotspots reflected a succession of distinct dominant communities associated with the strength of upwelling events, changes in fluorescence maximum depths and local downwelling and mixing processes. The unexpected high abundances and local dominance of Prochlorococcus in summer reflected the possible influence of eastward and westward current transports and the presence of a High-Light (PROC1)- and Low-Light (PROC2)-adapted ecotypes. This study highlights the role of localized physical events in the dominance of all three picophytoplankton groups that may be critical for the high productivity of the study region, and suggests the importance of hydroclimatic forcing for inter-annual changes in picophytoplankton communities. [ABSTRACT FROM AUTHOR]

Abstract: To ascertain the response of phytoplankton size classes to changes in environmental forcing, we determined size-fractionated biomass, carbon fixation and growth (production/biomass) rates in surface waters along the central Atlantic Ocean (26°N–5°S). As a result of the enhanced input of nutrients into the euphotic layer and the higher water column stability found at the equatorial upwelling, we observed increases not only in phytoplankton biomass and primary production, but also in turnover rates, suggesting nutrient limitation of phytoplankton physiology in the oligotrophic central Atlantic. The phytoplankton groups analysed (pico-, small nano-, large nano- and micro-phytoplankton) showed different responses to the equatorial environmental forcing, in terms of carbon biomass, primary production and growth rate. Large nano- and micro-phytoplankton consistently showed higher growth rates and carbon fixation to chl a ratios than smaller phytoplankton. We observed a higher stimulating effect of increased nitrate supply on the small phytoplankton growth rates. This observation can be explained by the dynamics of the equatorial upwelling, where the continuous but small nutrient input into the euphotic layer provide a competitive advantage for smaller cells adapted to oligotrophic conditions. The size-fractionated approach shown here reveals important group-specific differences in the response to environmental forcing, which cannot be appreciated in bulk measurements of the whole community. [Copyright &y& Elsevier]

The macroecological relationships among marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 μm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature–size relationship with the allometric size-scaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the −0.6 to 22 °C range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift toward smaller primary producers in a warmer ocean. Because the fate of photosynthesized organic carbon largely depends on phytoplankton size, we anticipate future alterations in the functioning of oceanic ecosystems. [ABSTRACT FROM AUTHOR]

Changes in abundance and biomass of Synechococcus were studied during five cruises carried out between November 2004 and September 2005 in the Iskenderun Bay, northeastern Mediterranean. In addition, spatial and temporal variations in physico-chemical factors and Chl a were measured. Abundance ranged from 0.2×104 to 23.09×104 cells ml-1, whereas the biomass ranged from 0.19 1 to 23.01 µgC l-1 in the bay. Synechococcus was found most numerous during September 2005 in the shallower part of the bay while the population was observed least during November 2004 and January 2005. The contribution of <3 µm size fraction to total Chl a ranged between 48% and 74%, being low during the cold period and higher during the warm period. The observed spatial and temporal fluctuations in abundance seemed to be closely related to environmental conditions in the area. Small-sized phytoplankton dominated the bulk of biomass, at least for the summer period. [ABSTRACT FROM AUTHOR]

Abstract: The response of the microbial food web to P-addition was studied during a 10-day Lagrangian experiment in the Eastern Mediterranean during which orthophosphate was added to the surface water of the Cyprus anticyclonic eddy. Very low levels of all microbial populations (heterotrophic bacteria, Synechococcus, Prochlorococcus, autotrophic and heterotrophic nanoflagellates, ciliates) and bacterial production were detected, verifying the extreme oligotrophic character of this area. The microbial biomass was dominated by heterotrophs and the heterotroph/autotroph ratio increased from 1.05 before to 2.8 after the P-addition, on day 4. Bacteria took advantage of the supply of the putative limiting factor (P) increasing their production but not their abundance. The heterotrophic nanoflagellates remained stable in numbers. By contrast, Synechococcus and autotrophic nanoflagellates decreased after the P addition. This is an indication of consumption by ciliates which were the only organisms that showed a significant increase in abundance during the first 4 days after the P addition, relative to the period before the addition as well as to the second phase of the experiment (days 5–9). In particular, the mixotrophic ciliate biomass increased by 50% after the P release. In environmental conditions of general resource scarcity as is the case of the Eastern Mediterranean, the addition of the putative limiting factor (P) was reflected in the increased abundance of microzooplankton shortly after the enrichment as a result of a quick transfer of energy (“heterotrophic by-pass”) through bacteria and heterotrophic nanoflagellates. A “mixotrophic by-pass” of phytoplankton primary producers also occurred, transferring the P-addition driven primary production to higher trophic levels through mixotrophic ciliates. [Copyright &y& Elsevier]

Abstract: Over a one-year cycle, several shifts occur in the phytoplankton community of the Bay of Banyuls-sur-Mer, a French northwestern Mediterranean shallow bay. In order to determine what drives these shifts, seasonal changes in littoral phytoplankton are considered together with environmental parameters. Our methodology combined a survey of the physical structure and nutrient contents of the water column with the complete characterization of the phytoplankton community structure at 3-m and 24-m depths. The sampling did provide a detailed record of the seasonal changes and successions that occur in northwestern Mediterranean littoral waters. Intrusions of low salinity water clearly had a significant impact on phytoplankton community structure. Such water freshening events both freshened the surface water and introduced substantial amounts of nutrient. The ratio between nitrogen and phosphorus changed on these occasions and proved to be a reliable clue for the characterization of phytoplankton community structure. Picophytoplankton predominated when the N:P ratio was below 10:1 under summer warm oligotrophic situations (Synechococcus and Prochlorococcus) and at the start of water nutrient enrichment (picoeukaryotes). As the N:P ratio increased in late winter–early spring period, eukaryotic phytoplankton was promoted at the expense of Synechococcus and Prochlorococcus. Successions in pico-, nano- and microphytoplankton communities appeared mainly to be driven by differences in the ability to acquire nutrients and by differences in species-specific growth rates. Further characterization of picophytoplankton from samples collected during summer when the N:P ratio reached 5:1 suggested the existence of two cyanobacteria sub-populations. This study clearly shows that northwestern littoral Mediterranean waters are not always well mixed and that nutrient availability in the Bay of Banyuls-sur-Mer depends primarily on low salinity water intrusions. [Copyright &y& Elsevier]

Abstract: Climatological variability of picophytoplankton populations that consisted of >64% of total chlorophyll a concentrations was investigated in the equatorial Pacific. Flow cytometric analysis was conducted along the equator between 145°E and 160°W during three cruises in November–December 1999, January 2001, and January–February 2002. Those cruises were covering the La Niña (1999, 2001) and the pre-El Niño (2002) periods. According to the sea surface temperature (SST) and nitrate concentrations in the surface water, three regions were distinguished spatially, viz., the warm-water region with >28°C SST and nitrate depletion (<0.1μmolkg-1), the upwelling region with <28°C SST and high nitrate (>4μmolkg-1) water, and the in-between frontal zone with low nitrate (0.1–4μmolkg-1). Picophytoplankton identified as the groups of Prochlorococcus, Synechococcus and picoeukaryotes showed a distinct spatial heterogeneity in abundance corresponding to the watermass distribution. Prochlorococcus was most abundant in the warm-water region, especially in the nitrate-depleted water with >150×103cellsml-1, Synechococcus in the frontal zone with >15×103cellsml-1, and picoeukaryotes in the upwelling region with >8×103cellsml-1. The warm-water region extended eastward with eastward shift of the frontal zone and the upwelling region during the pre-El Niño period. On the contrary, these regions distributed westward during the La Niña period. These climatological fluctuations of the watermass significantly influenced the distribution of picophytoplankton populations. The most abundant area of Prochlorococcus and Synechococcus extended eastward and picoeukaryotes developed westward during the pre-El Niño period. The spatial heterogeneity of each picophytoplankton group is discussed here in association with spatial variations in nitrate supply, ambient ammonium concentration, and light field. [Copyright &y& Elsevier]

1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers. 2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket. 3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton. [ABSTRACT FROM AUTHOR]

Picophytoplankton biomass and its contribution to total phytoplankton biomass were investigated in relation to the nutrient concentration and total N: total P ratio of the epilinnetic waters of 42 Japanese lakes during the warm season in 1991 (April-October). Picophytoplankton biomass (as chlorophyll a) in meso-, eu-, and hypertrophic lakes was significantly higher than those observed in oligotrophic lakes. However, picophytoplankton biomass increased significantly with increased total P concentrations in all systems excluding hypertrophic lakes. Picophytoplankton contribution to total chlorophyll a content was significantly higher in oligo- and mesotrophis lakes than in eu- and hypertrophic lakes and was inversely correlated with total P concentrations in lake water. Picophytoplankton contribution to the total phytoplankton biomass was positively (r = 0.54, n = 42, n P = 0.0003) correlated with the total N : total P ratio of lake waters. Each lake trophic type with the exception of hypertrophic lakes, showed this trend, although the correlation was not significant. We suggest that picophytoplankton contribution is influenced by the total N : total P ratio rather than lake trophic state; however, picophytoplankton were of little importance in hypertrophic lakes. [ABSTRACT FROM AUTHOR]