Size-fractionated Phytoplankton Research Articles

Size-Fractionated Phytoplankton Biomass in Port Blair Bay, South Andaman Island: Spatial Variability and Environmental Control

Size-Fractionated Phytoplankton Biomass in Port Blair Bay, South Andaman Island: Spatial Variability and Environmental Control

Discrimination of Microplastics and Phytoplankton Using Impedance Cytometry.

Both microplastics and phytoplankton are found together in the ocean as suspended microparticles. There is a need for deployable technologies that can identify, size, and count these particles at high throughput to monitor plankton community structure and microplastic pollution levels. In situ analysis is particularly desirable as it avoids the problems associated with sample storage, processing, and degradation. Current technologies for phytoplankton and microplastic analysis are limited in their capability by specificity, throughput, or lack of deployability. Little attention has been paid to the smallest size fraction of microplastics and phytoplankton below 10 μm in diameter, which are in high abundance. Impedance cytometry is a technique that uses microfluidic chips with integrated microelectrodes to measure the electrical impedance of individual particles. Here, we present an impedance cytometer that can discriminate and count microplastics sampled directly from a mixture of phytoplankton in a seawater-like medium in the 1.5-10 μm size range. A simple machine learning algorithm was used to classify microplastic particles based on dual-frequency impedance measurements of particle size (at 1 MHz) and cell internal electrical composition (at 500 MHz). The technique shows promise for marine deployment, as the chip is sensitive, rugged, and mass producible.

Response of the phytoplankton size fractions along environmental gradients from an oxygen minimum zone in the central Mexican Pacific

The marine phytoplankton community responds to latitudinal, longitudinal (coast to ocean), and vertical environmental gradients, an important subject in Oxygen minimum zones (OMZ). The phytoplankton structure and the effect of environmental gradients along the central Mexican Pacific, an area within an OMZ, were studied, especially the importance of size fractions and taxonomic groups. A combination of various methods and protocols, such as microscopic analysis, flow cytometer, and pigment analysis, were followed in this study. Oceanographic conditions included thermal gradients along the study area and unreported evidence of a weak upwelling in the southern zone (Acapulco). Vertical distribution of chlorophyll a showed subsurface maxima (SCM, 15-45 m depth) in all stations, and deeper chlorophyll-a maxima (DCM, 85-95 m depth) in more oceanic stations. Chain-forming diatoms dominated in the northern zone stations. Prochlorococcus, Synechococcus, and picoeukaryotes abundances ranged between 0.01 to 21.7 cells×104 mL-1, although most samples showed the highest contribution to biomass (47.95 μg C L-1) by picoeukaryotes. Expected tendencies of Prochlorococcus distribution were observed: highest densities coincided with the DCM and divinyl chlorophyll-a distribution at oceanic stations. Fucoxanthin had the highest concentrations, whereas fucoxanthin and zeaxanthin concentrations were higher at SCM depths. We documented the co-dominance of the pico- and microplankton: picoplankton was important at the DCM, related to oligotrophic and more stratified water column, whereas microplankton prevailed in coastal stations, with mixed water column, high nutrient concentrations, and diatoms as the dominant group. Picoeukaryotes abundances were related to the concentration of prasinoxanthin, which suggests an important mamiellophyte component not previously revealed.

Biogeochemical factors regulating photosynthetically dissolved organic carbon produced by phytoplankton in the Taiwan Strait

The distribution and mechanisms of photosynthetically dissolved organic carbon (PDOC) released by marine phytoplankton are frequently neglected and inadequately understood because most studies on carbon sequestration capacity have focused on photosynthetic particulate organic carbon. In this study, percentage extracellular release (PER) and its environmental influencing factors were investigated for 10 cruises in the Taiwan Strait during 2006–2023. The results indicated that the PER increased horizontally from the nearshore to the off-shelf and vertically from the surface to the bottom within the euphotic zone. PER tends to be low in eutrophic waters such as upwellings and estuaries and high in oligotrophic waters. The study revealed that the average contribution of PDOC to total primary productivity (TPP) in the Taiwan Strait could reach 18.2 ± 11.7%, which is similar to the previously estimated global oceanic values. PDOC production satisfied approximately 25% the carbon requirements of heterotropic bacteria (HB). A detailed analysis of the PER combined with model simulations proved that the distribution of the PER in the Taiwan Strait was caused by the joint contribution of irradiance, size-fractionated phytoplankton, and nutrient stoichiometry. Our results contradict the view that the PER is a constant factor that is unaffected by TPP. However, there was a significant negative correlation between the PER and TPP. The PDOC was always lower than the bacterial carbon demand for a broad range of bacterial growth efficiencies, suggesting a weak coupling between phytoplankton exudation and bacterial metabolism. This challenges the idea that there is a well-coupled relationship between bacteria and phytoplankton present on the continental shelf. These findings indicate significant discrepancies in PDOC mechanisms and the quantitative importance of nearshore eutrophic and off-shelf oligotrophic environments. Consequently, it is unwise to use uniform PERs without differentiation under trophic conditions when reevaluating and appraising marine carbon fixation.

Ecological implications and seasonal variability of grazing by marine copepods on phytoplankton: comparison between Acartia omorii and A. steueri in Jangmok Bay, Korea

The grazing impacts of two Acartia species (Acartia omorii and A. steueri) on size-fractionated phytoplankton biomass were measured in Jangmok Bay, Korea (34°59′37.8" N, 128°40′28.2'' E) from January to May 2015. Total chlorophyll (Chl-a) concentrations ranged from 0.66 to 5.18 µg L−1, and micro-phytoplankton (> 20 µm) comprised up to 66% (range, 10.5–65.6%) of the total pigment. The total abundance of Acartia species ranged from 267 to 5931 ind. m−3, and these copepods accounted for 20.8 to 88.0% of the total copepod abundance. The ingestion rates of A. steueri (r2 = 0.904, P = 0.013) and A. omorii (r2 = 0.239, P = 0.046) showed a high correlation with micro-phytoplankton. The average grazing impact of Acartia species on phytoplankton biomass was approximately 6.8 ± 11.8% (range, 0.1–69.0%). Temperature–salinity (T–S) diagram analysis revealed distinct environmental preferences for each species; A. omorii preferred a broader temperature range of 6.2 to 17.1 °C and a salinity range of 31.8 to 33.5, whereas A. steueri was more restricted, preferring temperatures between 6.5 and 12.8 °C and a salinity range of 32.2 to 33.5. These findings not only contribute to our understanding of the ecological roles of these copepod species in marine ecosystems but also highlight the importance of continuous research regarding the mechanisms driving their coexistence and interaction with the coastal food web.

Two-decade satellite observations reveal variability in size-fractionated phytoplankton primary production in the South China Sea

Understanding variations in marine phytoplankton primary production (PP) is crucial for assessing the response of the marine environment to climate change and for quantifying the ocean carbon cycle. However, the spatiotemporal variability of size-fractionated PP in the South China Sea (SCS) remains ambiguous. Our study investigated the monthly, seasonal, and inter-annual variability of size-fractionated PP in the SCS using satellite observations from 2002 to 2022. There were noticeable seasonal and monthly variations in size-fractionated PP, with notably high PP values appearing during the cold season. The disparities in distribution and the distinct fluctuation patterns between size-fractionated PP suggest that total PP alone is not a comprehensive indicator of marine ecosystem health. Over the past two decades in the SCS, there were more pronounced decreases in total, pico-, and nano-PP, whereas micro-PP displayed no significant trend. The most pronounced decline occurred in the northern SCS, contrasted by increases in coastal areas. These size-fractionated PP anomalies showed strong correlations with climate change indices, highlighting the impact of environmental factors on these anomalies, such as sea surface temperature, mixed layer depth, and wind speed. Our findings emphasize the importance considering size-fractionated PP to gain a more nuanced understanding of the ocean carbon cycle and the marine ecosystem's response to climate changes.

Assessing inorganic nitrogen transport in marine phytoplankton assemblages through the 15N-tracer technique and metatranscriptomics

The availability of inorganic nitrogen is considered one of the limiting factors for primary production in the ocean. However, different phytoplankton possess unique strategies to take up and assimilate nitrate and ammonium to cope with environmental changes. To investigate the nitrogen uptake characteristics of different size-fractionated phytoplankton in natural assemblages, 3 research cruises were conducted in the southern East China Sea in 2018 and 2019. The nitrogen uptake characteristics of 2 size-fractionated natural assemblages, microphytoplankton (20-200 µm) and pico-nanophytoplankton (<20 µm), were measured using the 15N-tracer technique. At most of the stations, significantly higher potential maximum uptake rates of ammonium than of nitrate were detected in the pico-nanophytoplankton, indicating that small phytoplankton possess a relatively superior capacity to take up ammonium. By contrast, comparing the potential maximum uptake rate for nitrate between microphytoplankton and pico-nanophytoplankton showed that microphytoplankton exhibit a higher capacity for nitrate uptake as a nitrogen source compared to pico-nanophytoplankton. However, repressed uptake rates of ammonium and nitrate in microphytoplankton were sometimes found at the coastal station even when ambient nitrate concentrations remained high. Metatranscriptomic analysis of nitrogen transporter genes in microphytoplankton suggests that most diatoms utilize regenerated ammonium before nitrate to maintain their populations at the end of blooms. Metatranscriptomic approaches identify the transcriptional responses of dominant diatoms to explain how diatoms regulate their nitrogen transporter genes to cope with environmental changes in natural assemblages.

Diversity and community structure of microzooplankton in the eastern Indian Ocean during the inter-monsoon period

Microzooplankton (MZP) are an important part of the microbial food web and play a pivotal role in connecting the classic food chain with the microbial loop in the marine ecosystem. They may play a more important role than mesozooplankton in the lower latitudes and oligotrophic oceans. In this article, we studied the species composition, dominant species, abundance, and carbon biomass of MZP, including the relationship between biological variables and environmental factors in the eastern equatorial Indian Ocean during the spring intermonsoon. We found that the MZP community in this ocean showed a high species diversity, with a total of 340 species. Among these, the heterotrophic dinoflagellates (HDS) (205 species) and ciliates (CTS) (126 species) were found to occupy the most significant advantageous position. In addition, CTS (45.3%) and HDS (39.7%) accounted for a larger proportion of the population abundance, while HDS (47.1%) and copepod nauplii (CNP) (46.4%) made a larger contribution to the carbon biomass. There are significant differences in the ability of different groups of MZP to assimilate organic carbon. In this sea area, MZP are affected by periodic currents, and temperature is the main factor affecting the distribution of the community. The MZP community is dominated by eurytopic species and CNP. CTS are more sensitive to environmental changes than HDS, among which Ascampbelliella armilla may be a better habitat indicator species. In low-latitude and oligotrophic ocean areas, phytoplankton with smaller cell diameters were found to occupy a higher proportion, while there was no significant correlation between the total concentration of integrated chlorophyll a and the biological variables of MZP. Therefore, we propose that the relationship between size-fractionated phytoplankton and MZP deserves further study. In addition, the estimation of the carbon biomass of MZP requires the establishment of more detailed experimental methods to reflect the real situation of organisms. This study provides more comprehensive data for understanding the diversity and community structure of MZP in the eastern equatorial Indian Ocean, which is also of good value for studying the adaptation mechanism and ecological functions of MZP in low-latitude and oligotrophic ocean ecosystems.

Size-fractionated phytoplankton biomass and primary production in the eastern Indian sector of the Southern Ocean in the austral summer 2018/2019

The Southern Ocean significantly contributes to primary production in the global ocean. In this study, size-fractionated primary production and chlorophyll a (Chl a), which are not well elucidated, were determined in the eastern Indian sector in the austral summer of 2018–2019. The Southern Antarctic Circumpolar Current Front, Southern Boundary of the Antarctic Circumpolar Current (SB), and Antarctic Slope Front (ASF) were set as study areas, and sampling stations were divided into the three areas: to the north of the SB, the area between the SB and ASF, and to the south of the ASF. Primary production was 85.1–385.7 mg C m−2 day−1 north of the SB, 131.4–192.5 mg C m−2 day−1 between the SB and ASF, and 95.6–472.7 mg C m−2 day−1 south of the ASF. The Chl a biomass integrated within the euphotic zone in each area was 19.8–46.1 mg m−2, 10.3–19.9 mg m−2, and 17.0–77.9 mg m−2, respectively. Although no significant difference was observed in the primary production and Chl a biomass among the three areas, the biomass tended to be lower between the SB and ASF than in the other two areas. The large phytoplankton (>10 μm) significantly contributed to the primary production and the Chl a biomass at every station north of the SB, accounting for approximately 65–90%. In the other two areas, there were some stations where large phytoplankton accounted for 60–85%, and others where large phytoplankton did not dominate. The flow to the secondary production was one order of magnitude higher to the north of the SB than to the south of the SB. The time since sea-ice melt possibly plays an important role in determining the dominant phytoplankton size and hence size structure of phytoplankton in the survey area. The features in phytoplankton size structure observed north and south of the SB may not be unique to the areas. High phytoplankton biomass and primary production were also observed in the west of 130°E, which was similar to the results of the previous study. A time lag between the west and east surveys, about one month, may have caused the high-west and low-east patterns.

Bioconcentration, bioaccumulation and biomagnification of mercury in plankton of the Mediterranean Sea

Plankton plays a prominent role in the bioaccumulation of mercury (Hg). The MERITE-HIPPOCAMPE campaign was carried out in spring 2019 along a north-south transect including coastal and offshore areas of the Mediterranean Sea. Sampling of sea water and plankton by pumping and nets was carried out in the chlorophyll maximum layer. Two size-fractions of phytoplankton (0.7–2.7 and 2.7–20 μm) and five of zooplankton (between 60 and >2000 μm) were separated, and their total mercury (THg) and monomethylmercury (MMHg) contents were measured. Bioconcentration of THg was significantly higher in the smallest phytoplankton size-fraction dominated by Synechococcus spp. The bioaccumulation and biomagnification of MMHg in zooplankton was influenced by size, food sources, biochemical composition and trophic level. MMHg was biomagnified in the plankton food web, while THg decreased toward higher trophic levels. Higher MMHg concentrations were measured in oligotrophic areas. Plankton communities in the Southern Mediterranean Sea had lower MMHg concentrations than those in the Northern Mediterranean Sea. These results highlighted the influence of environmental conditions and trophodynamics on the transfer of Hg in Mediterranean plankton food webs, with implications for higher trophic level consumers.

Optimal Assimilation Number of Phytoplankton in the Siberian Seas: Spatiotemporal Variability, Environmental Control and Estimation Using a Region-Specific Model

The maximal value of the chlorophyll-specific carbon fixation rate in the water column or the optimal assimilation number (Pbopt) is an important parameter used to estimate water column integrated primary production (IPP) using models and satellite-derived data. The spatiotemporal variability in the Pbopt of the total and size-fractionated phytoplankton in the Siberian Seas (SSs) and its links with environmental factors were studied based on long-term (1993–2020) field and satellite-derived (MODIS-Aqua) observations. The average value of Pbopt in the SSs was equal to 1.38 ± 0.76 mgC (mg Chl a)–1 h–1. The monthly average values of Pbopt decreased during the growing season from 1.95 mgC (mg Chl a)–1 h–1 in July to 0.64 mgC (mg Chl a)–1 h–1 in October. The average value of Pbopt for small (<3 μm) phytoplankton 1.6-fold exceeded that for large (>3 μm) phytoplankton. The values of Pbopt depend mainly on incident photosynthetically available radiation (PAR). Based on the relationship between Pbopt and PAR, the empirical region-specific algorithm (E0reg) was developed. The E0reg algorithm performed better than commonly used temperature-based models. The application of E0reg for the calculation of Pbopt will make it possible to more precisely estimate IPP in the SSs.

Short-term responses of phytoplankton size-fractionated structure and photosynthetic physiology to thermal effluent in a subtropical coastal bay

Elevated water temperature caused by the thermal discharge from power plants can exert multiple ecological impacts on the phytoplankton community in coastal ecosystems. Most recent studies have focused on the reshaping effects on the community structure; however, the short-term response of phytoplankton physiology to thermal discharge remains unclear. This study conducted research on the scope of thermal discharge from the nuclear power plant and the size-fractionated phytoplankton structure combined with photosynthetic physiology in Daya Bay, China. The thermal discharge significantly affected the surface temperature in the outlet regions, and the thermal plume mainly diffused along the northeast coast of the outfall site, resulting in a significant difference in the surface temperature between the inlet and outlet transects (p<0.05). Elevated surface temperatures decreased the total chlorophyll a concentrations by 33.19% at the outlet regions, with pico-phytoplankton decreasing the most. Chlorophyll a concentrations were higher at sites further away from the outlets, indicating that elevated water temperature might stimulate the rapid growth of phytoplankton, especially nano-phytoplankton which replaced pico-phytoplankton as the dominant group at stations away from the outlets. Significant negative correlations were observed between the photochemical quantum yield (Fv/Fm) and temperature (p<0.05), and the relative electron transport rate (rETR) and temperature (p<0.05). Phytoplankton showed a normal photosynthetic physiological state at most sites with a surface temperature<33°C but was severely affected at the outlet site with a 5°C rise, decreasing from ~0.5 on the inlet transect to 0.07. During the diurnal survey, the high temperatures near the outlet at midday had a compensatory effect on phytoplankton’s light suppression. The results indicated that the physiological state of phytoplankton was clearly influenced by the thermal discharge with diurnal variation, and different size-fractionated phytoplankton groups exhibited heterogeneous responses. The findings may provide further insights into the ecological impacts of thermal discharges as well as global warming in subtropical regions.

Evaluation of Satellite-Derived Size-Fractionated Phytoplankton Primary Production in the South China Sea

Space-borne observations of primary production (PP) are essential for understanding the role of phytoplankton in the biogeochemical cycles on a global or regional scale. In this study, the vertically generalized production model (VGPM) and two size-fractionated PP algorithms (namely U10 and B17) were evaluated in the SCS. Comparison between estimated and in situ measured total PP showed the best performance of U10 with an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 0.65 and mean absolute percentage deviation (MAPD) of 31.11%. B17 produced better accuracy than U10 for PP from pico-phytoplankton (PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pico</sub> ) and micro-phytoplankton (PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">micro</sub> ), whereas U10-derived PP from nano-phytoplankton (PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nano</sub> ) agreed better with in situ measurements than B17-derived PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nano</sub> . Sensitivity analysis showed that PP estimation was most sensitive to chlorophyll-a (Chla), followed by euphotic zone depth (Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eu</sub> ), and photosynthetically available radiation (PAR), and physiological parameters. With regional optimization of Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eu</sub> , phytoplankton size class (PSC), photosynthesis-irradiance curve parameters, and Chla profile modelling, B17 was tuned for the SCS. Its accuracy was greatly improved, as indicated by slopes close to 1, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> s > 0.55, and MAPD < 145%, for all size classes and total PP. Application of the tuned algorithm to satellite produced data demonstrated a decreasing trend for the total PP, PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pico</sub> , and PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nano</sub> while no apparent trend for PP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">micro</sub> . This study represents the feasibility of improving PP estimation from satellite observation for regional applications through retuning to local data. Our findings are useful for understanding the ecological response to climate change in the region and unveil the role of size-fractionated phytoplankton in biogeochemical and carbon cycles on a basin scale.

Seasonal Compositions of Size-Fractionated Surface Phytoplankton Communities in the Yellow Sea

Little information on the phytoplankton community in the Yellow Sea (YS)—especially size-fractionated phytoplankton—is currently available, in comparison to the various physicochemical studies in the literature. Using high-performance liquid chromatography (HPLC), size-fractionated phytoplankton communities were seasonally investigated in the YS in 2019. In the study period, diatoms (55.0 ± 10.2%) and cryptophytes (16.9 ± 9.3%) were the dominant groups. Due to the recent alteration in inorganic nutrient conditions reported in the YS, the contribution of diatoms was lower than in previous studies. The large-sized phytoplankton group (&gt;20 µm) was dominated mostly by diatoms (89.0 ± 10.6%), while the small-sized phytoplankton group (&lt;20 µm) was also dominated by diatoms (41.9 ± 9.1%), followed by cryptophytes (19.2 ± 9.8%). The contributions of small-sized diatoms (&lt;20 µm) have been overlooked in the past, as they are difficult to detect, but this study confirms significant amounts of small-sized diatoms, accounting for 62.3% of the total diatoms in the YS. This study provides an important background for assessing the seasonal variations in different-sized diatom groups in the YS. Further detailed studies on their potential ecological roles should be conducted, in order to better understand marine ecosystems under future warming scenarios.

Phytoplankton and Bacterial Communities’ Patterns in a Highly Dynamic Ecosystem (Central Mediterranean Sea)

The Straits of Messina (Southern Italy, Mediterranean Sea) are a very complex area: they connect two basins (Tyrrhenian and Ionian) with different hydrographic features and is characterised by upwelling and mixing phenomena. The aim of the study was to evaluate if and how the physical and chemical water conditions and hydrodynamics influenced the phytoplankton and bacterial patterns and the functioning of this ecosystem. During a late winter survey, size-fractionated phytoplankton (from 0.2 to 200 μm) biomass (chlorophyll a), cell densities and species composition as well as total picoplankton abundances, morphotype composition, and activity levels of the enzymes leucine aminopeptidase, β-glucosidase, and alkaline phosphatase were investigated. The obtained results showed a marked diversification among the water masses identified within the Straits area. The analyses of the phytoplankton diversity indices, particularly those based on phylogenetic relationships between species (indices of taxonomic diversity and distinctness), confirmed our findings. In conclusion, the patterns of phytoplankton and bacterial communities provide a suitable approach to evaluate how microbial communities respond to changing environmental scenarios. This tool could be applied to other temperate Mediterranean ecosystems.

Oligotrophic Phytoplankton Community Effectively Adjusts to Artificial Upwelling Regardless of Intensity, but Differently Among Upwelling Modes

Artificial upwelling has been proposed as a means of enhancing oceanic CO2 sequestration and/or raising fishery yields through an increase in primary production in unproductive parts of the ocean. However, evidence of its efficacy, applicability and side effects is scarce. Here we present part of the results of a 37-day mesocosm study conducted in oligotrophic waters off the coast of Gran Canaria. The goal was to assess in situ the effects of artificial upwelling on the pelagic community. Upwelling was simulated via two modes: i) a singular deep-water pulse and ii) a recurring supply every 4 days; each mode at four different intensities defined by the total amount of nitrate added: approx. 1.5, 3, 5.7, and 11 µmol L-1. In this study we focus on the phytoplankton response through size-fractionated 14C primary production rates (PP), Chlorophyll a and biomass. We observed increases in PP, accumulated PP, Chlorophyll a and biomass that scaled linearly with upwelling intensity. Upwelling primarily benefitted larger phytoplankton size fractions, causing a shift from pico- and nano- to nano- and microphytoplankton. Recurring deep-water addition produced more biomass under higher upwelling intensities than a single pulse addition. It also reached significantly higher accumulated PP per unit of added nutrients and showed a stronger reduction in percentage extracellular release with increasing upwelling intensity. These results demonstrate that oligotrophic phytoplankton communities can effectively adjust to artificial upwelling regardless of upwelling intensity, but differently depending on the upwelling mode. Recurring supply of upwelled waters generated higher efficiencies in primary production and biomass build-up than a single pulse of the same volume and nutrient load.

Environmental Regulation of Photosynthetically Produced Dissolved Organic Carbon by Phytoplankton Along a Subtropical Estuarine Bay

Photosynthetically produced dissolved organic carbon (PDOC) released by marine phytoplankton has an important significance on the marine carbon cycle and on the growth of heterotopic bacteria (HB). However, PDOC is often neglected in the estimation of global marine primary productivity and carbon sequestration capacity. This work studied the issue of PDOC and its environmental regulation mechanism through field investigation and lab experiments in an estuarine bay during southwest and northeast monsoon. We used the percentage of extracellular release (PER) as a key indicator to evaluate the contribution of PDOC to total primary productivity (TPP). We also compared PER among different seasons and sectors and then analyzed the inter-relationship between PDOC and bacterial carbon demand (BCD), size-fractionated phytoplankton, bacterial production (BP), and TPP. We finally discussed the impact factors of PDOC productivity. The results showed that the average contribution of PDOC to TPP in Qinzhou bay could reach 15% during two seasons, which satisfies about 25% of the carbon requirement by HB in the bay. Multiple factors contribute to the seasonal (SW monsoon 13% &amp;lt; NE monsoon 18%) and sectoral variation (outer bay 26% &amp;gt; middle bay 17% &amp;gt; inner bay 7%) in PER, the most significant of which are salinity and nitrogen to phosphorus ratios. PER is also related to phytoplankton community structure and nutrient limitation, the higher PER in the outer bay is attributed to the dual effect of picophytoplankton being the predominant species and a severe imbalance in the nitrogen to phosphorus ratio, both of which lead to increased phytoplankton PDOC release. On the other hand, the lower PER in the inner bay was mainly due to the dominance of microphytoplankton and nitrogen to phosphorus ratio which is close to the redfield ratio. There is still a non-uniform conclusion on the relationship between PER and TPP, suggesting that this relationship needs to be evaluated not only in a horizontal-independent way but also through seasonal and vertical dimensions. The non-linear relationship between PDOC and BCD found in this study potentially indicated weak coupling between phytoplankton exudation and bacterial metabolism. In addition to phytoplankton exudate, HB is still dependent on other sources of DOC to meet their carbon requirements.

Distinctive phytoplankton size responses to the nutrient enrichment of coastal upwelling and winter convection in the eastern Arabian Sea

Even though the seasonal pattern of phytoplankton biomass distribution in the seas around India is reasonably well understood, there is relatively little information on phytoplankton size classes. The current study shows how phytoplankton of different size classes respond to nutrient enrichment caused by vertical convective mixing during the Northeast Monsoon [(NEM) November-February] and coastal upwelling during the Southwest Monsoon [(SWM) June-September] in the Eastern Arabian Sea (EAS). In the Northeastern Arabian Sea (NEAS), phytoplankton biomass [Chlorophyll-a (Chl-a)] and primary production (PP) were moderate and comparable during both the NEM (Chl-a av. 21 mg m−2 and PP av. 601 mg C m−2 d-1) and the SWM (Chl-a av. 27 mg m−2 and PP av. 516 mg C m−2 d-1). The Southeastern Arabian Sea (SEAS) was oligotrophic (Chl-a av. 12 mg m−2 and PP av. 197 mg C m−2 d-1) during the NEM due to strong surface stratification, but due to coastal upwelling, it turned into a very productive system along the shelf waters during the SWM (Chl-a av. 45 mg m−2 and PP av. 1201 mg C m−2 d-1). Interestingly, the subsurface Chl-a maximum (SCM) was almost absent in the NEAS during both seasons. Similarly, SCM was absent in the entire coastal upwelling zone of the EAS during the SWM. In these areas and situations, Chl-a was found to accumulate in the nutrient-rich surface layers of the shallow euphotic zones.The nutrient enrichment of the coastal upwelling along the shelf waters in the EAS favoured the growth of larger micro-and meso-phytoplankton during the SWM [av. 43.8 mg m−2 (av. 65.9% total Chl-a)], whereas the entire NEAS favoured the growth of nano-phytoplankton during the NEM [av. 13.9 mg m−2 (av. 64.8% total Chl-a)]. Differences in the physical processes enabling the entrainment of nutrients (Nitrate (NO3), Phosphate (PO4) and Silicate (SiO4)] into the euphotic layer could explain the observed differences in the phytoplankton size fractions. The convective mixing in the NEAS during the NEM erodes the bottom of the mixed layer (50–120 m), resulting in only moderate enrichment of NO3 (av. 0.62 ± 0.45 µM) and SiO4 (av. 3.01 ± 0.83 µM), and low level of PO4 (av. 0.49 ± 0.13 µM) in the mixed layer (av. 60 m). This caused N -limitation in the NEAS, which favoured the dominance of nano-phytoplankton. Alternatively, coastal upwelling during the SWM drives deeper subsurface waters (75–150 m) into the shallow surface mixed layer (av. 30 m) in shelf water, causing significant enrichment of NO3 (av. 11.02 ± 3.55 µM), SiO4 (av. 18.34 ± 11.37 µM) and PO4 (av. 1.15 ± 0.21 µM). The enhanced nutrients in the coastal upwelling zones favoured the dominance of larger micro and meso-phytoplankton. However, during the SWM, the oceanic waters of the entire EAS showed low nutrient concentration compared to the shelf waters, which favoured the dominance of nanophytoplankton. In conclusion, our work reveals the large contribution of nano-phytoplankton in Chl-a biomass across the entire EAS, underlining their ecological relevance in both N-limited and N-enriched environments.

Estimation of vertical size-fractionated phytoplankton primary production in the northern South China Sea

Modelling vertical changes of size-fractionated phytoplankton primary production (PP) offers a holistic route to describe marine ecosystems and constitutes an important part of remote sensing to accurately estimate the depth-integrated PP in the global oceans. However, there is no size-fractionated PP model established in the Northern South China Sea (NSCS) yet. In situ measured hyperspectral absorption and irradiance data were used to estimate vertical size-fractionated PP. Results demonstrated that the estimated size-fractionated PP showed good agreement with in-situ measurement. r2s are 0.41, 0.69, and 0.83, and median absolute percentage difference are 23.33%, 25.86%, and 31.05% for pico-, nano-, and micro-PP, respectively. The intercept from the linear relationship between ratio of phytoplankton absorption coefficient (aph(4 3 5)/aph(6 7 6)) and quantum yield of photosynthesis have the biggest impact on estimation of PP based on Monte Carlo approach. The model derived PP profiles showed an exponential attenuation shape curve from the surface to the bottom in coastal water of NSCS. In contrast, a decreasing pattern with increasing depth with a maximum in the subsurface layer or at the chlorophyll maximum depth was observed in the open ocean of NSCS. Such an approach can be utilized to depict the vertical variations of size-fractionated PP, which can serve as supplementary of remote sense to reveal their three-dimensional structure.

Biophysical controls on seasonal changes in the structure, growth, and grazing of the size-fractionated phytoplankton community in the northern South China Sea

Abstract. The size-fractionated phytoplankton growth and microzooplankton grazing are crucial for the temporal change of community size structure, regulating not only trophic transfer but also the carbon cycle of the ocean. However, the size-dependent growth and grazing dynamics on a monthly or an annual basis are less addressed in the coastal ocean. In this paper, the seasonal responses of the size-fractionated phytoplankton growth and grazing to environmental change were examined over 1 year at a coastal site of the northern South China Sea. We found a nanophytoplankton-dominated community with strong seasonal variations in all size classes. Phytoplankton community growth rate was positively correlated to nutrients, with community grazing rate correlating to the total chlorophyll a at the station, reflecting a combined bottom-up and top-down effect on phytoplankton population dynamics. Further analyses suggested that the specific growth rate of microphytoplankton was significantly influenced by phosphate, and that of nanophytoplankton was influenced by light, although picophytoplankton growth was controlled by both nitrate and temperature. In addition, the specific grazing rate of nanophytoplankton was well correlated to phytoplankton standing stock, while that of micro- and pico-compartments was negatively influenced by ciliate abundance and salinity. Finally, a lower grazing impact for micro-cells (38 %) than nano- and pico-cells (72 % and 60 %, respectively) may support size-selective grazing of microzooplankton on small cells at this eutrophic system.