Deep Chlorophyll Maximum Research Articles

Constant abundances of ubiquitous uncultured protists in the open sea assessed by automated microscopy

Protists have fundamental ecological roles in marine environments and their diversity is being increasingly explored, yet little is known about the quantitative importance of specific taxa in these ecosystems. Here we optimized a newly developed automated system of image acquisition and image analysis to enumerate minute uncultured cells of different sizes targeted by fluorescence in situ hybridization. The automated counting routine was highly reproducible, well correlated with manual counts, and was then applied on surface and deep chlorophyll maximum samples from the Malaspina 2010 circumnavigation. The three targeted uncultured taxa (MAST-4, MAST-7 and MAST-1C) were found in virtually all samples from several ocean basins (Atlantic, Indian and Pacific) in fairly constant cell abundances, following typical lognormal distributions. Their global abundances averaged 49, 23 and 7 cells ml-1 , respectively, and altogether the three groups accounted for about 10%-20% of heterotrophic picoeukaryotes. Our innovative high-throughput cell counting routine allows for the first time a direct assessment of the biogeographic distribution of small protists (< 5 μm) and shows the ubiquity in sunlit oceans of three bacterivorous taxa, suggesting their key roles in marine ecosystems.

Seasonal patterns of thermal stratification and primary production in the northern parts of Lake Tanganyika

Abstract Thermal stratification in meromictic Lake Tanganyika weakens during the cool, dry and windy season, allowing intrusions of deeper nutrient-rich waters into the upper mixed layer enhancing primary productivity. The current study examined the seasonal influence of thermal structure on the patterns of primary production in Lake Tanganyika. Two sites (Kigoma Bay and Mahale) were sampled on a monthly basis for 1 year. Water temperature and chlorophyll a fluorescence profiles (0–100 m) were measured using a multisonde. Water samples were taken every 20 m (0–100 m) to measure soluble reactive phosphorus (SRP), chlorophyll a and primary production. Pulse Amplitude Modulated Fluorometry was used for the measurements of primary production. The results show that the dry season coincided with higher wind speeds, elevated SRP and some peaks of chlorophyll a and primary production at both sites. During the wet season, high levels of chlorophyll a and primary production coincided with an increase in the euphotic depth, a deep chlorophyll maximum (DCM) and a contribution from metalimnetic areal production at both sites. Our results suggest that the vertical distribution of primary production in Lake Tanganyika is affected by the lake thermal structure, nutrient availability and the extent of the euphotic zone. In Lake Tanganyika, estimates of areal primary production were significantly affected by DCM and required estimation throughout the entire upper mixed layer and the metalimnion which extended to ~89 m.

Arctic Ocean biogeochemistry in the high resolution FESOM 1.4-REcoM2 model

In the rapidly changing Arctic Ocean, marine primary productivity is tightly linked to the balance between light and nutrient limitation. To capture this balance in ocean general circulation biogeochemical models (OGCBMs), a good representation of the physics is important due to the tight bio-physical coupling in the Arctic. Using a horizontal model resolution of a few kilometers makes it possible to resolve an increasing number of small scale processes, that otherwise need to be parameterized in OGCBMs. Such high resolution is, however, commonly not possible due to computational constrains. Utilizing an unstructured mesh approach, we have run the finite element sea-ice ocean model (FESOM 1.4) coupled to the biogeochemical model REcoM2 in a global configuration with an Arctic-wide resolution of 4.5 km. This resolution is so far unprecedented for a global biogeochemical setup, and here we present an analysis of the mean state of the model. FESOM-REcoM2’s integrated Arctic net primary production (NPP) averages 445 Tg C yr−1 for the years 2011 to 2015, a value that is in the middle of the range compared to estimates from the literature. Most production takes place in the inflow regions of the Nordic and Chukchi Seas, and 32% is associated with the sea ice zone, the latter including the marginal ice zone and below-ice productivity. Light limits production to some degree at all latitudes north of 60°N, with growth becoming nutrient limited following the initial spring bloom in most places. The model reproduces the relatively low surface concentration of dissolved inorganic nitrogen (DIN) that has been observed in the central Arctic Ocean, as well as the low surface DIN concentration towards the end of the growth season further to the south, thereby also capturing widespread subsurface chlorophyll maxima (SCM). The SCMs are found in the whole Arctic except for the areas where sea ice concentration is high the whole year. They have a duration of two weeks to five months. The balance between nutrient and light limitations, both in the vertical and horizontal direction, highlights that decreased light limitation in a future ice free Arctic Ocean will not necessarily induce an increase in NPP due to increasing nutrient limitation, and that further studies of the role of the SCMs are required.

Average nutrient and chlorophyll distributions in the western Mediterranean: RADMED project

Because of its reduced dimensions and its location, surrounded by three continents, the Mediterranean Sea could be especially vulnerable to climate change effects. An increase of the water column stratification could inhibit winter mixing and reduce the frequency and intensity of convection processes which inject nutrients into the photic layer and are responsible for the ventilation of deep waters. In this context, the long-term monitoring of the Mediterranean waters is a basic task. The RADMED project is a monitoring program that covers the waters from the eastern side of the Gibraltar Strait to the Catalan and Balearic Seas. This project was initiated in 2007, merging some previous programs, some of them initiated in 1992. The main objective of this project is to establish average distributions, ranges of variability and long-term trends for physical, and biochemical variables which could be considered as indicative of the environmental state of the sea. The present work analyses nutrient, chlorophyll and oxygen time series from 2007 to 2015 in some cases and from 1992 in other cases. The current analyses show a clear trophic gradient in the RADMED area. Nutrient and chlorophyll concentrations and the intensity of the deep chlorophyll maximum decrease northeastward. The deep chlorophyll maximum depth increases to the northeast. The Balearic and Catalan Seas show a clear seasonal pattern with maximum surface concentrations for nutrients and chlorophyll in winter/spring, associated with winter mixing. On the contrary, the Alboran Sea does not show such a clear seasonal cycle, probably because of the existence of permanent upwelling processes acting along the whole year. The Atlantic Water occupying the upper part of the water column shows a Redfield N:P ratio close to or lower than 16, indicating no phosphorus limitation. Finally, chlorophyll concentrations seem to have increased from 1992 to 2015 in the Alboran Sea, while no long-term changes could be established for the rest of the variables and geographical areas.

Observational Evidence of Subsurface Chlorophyll Response to Mesoscale Eddies in the North Pacific

AbstractOcean eddies were widely reported to strongly impact near‐surface chlorophyll (CHL), but most of previous studies are confined to the ocean surface. Based on large‐scale historical cruise observations with ocean mesoscale eddies detected by altimeters in the North Pacific, we show that the depth‐integrated nitrate and chlorophyll anomalies in the euphotic layer of eddies are less than ±90% for nitrate and ±10% for chlorophyll. These eddy‐driven variations decline from ocean boundaries to the open ocean, due to a combination effect of eddy activity and subsurface nitrate distribution. In the oligotrophic Pacific, CHL anomalies in both cyclonic eddies and anticyclonic eddies negatively correlate with near sea surface temperature anomalies. In contrast, an opposite correlation occurs beneath the subsurface chlorophyll maximum layer (SCML). The integrated CHL results and CHL variability at each sampling depth suggest that the vertical displacement of SCML plays an important role in the near‐surface CHL response to eddies.

Vertical distribution of planktonic ciliates in the oceanic and slope areas of the western Pacific Ocean

Ciliates are important grazers in the planktonic food web, and spatial distribution information is the key to understanding their function. However, our understanding of their vertical distribution in the euphotic zone of oceanic waters is limited. In this study, we investigated the vertical distribution of ciliates in the western Pacific Ocean and the northern South China Sea. The ciliates showed a bimodal distribution, with abundance peaks in surface waters and the deep chlorophyll maximum (DCM) in the western Pacific, but a single surface peak in the northern South China Sea slope. At stations influenced by shelf water, the surface abundance was much greater than in slope waters. Within the ciliates, the vertical distribution of tintinnid species groups I and V had higher abundances overall and showed surface and DCM peaks, respectively. We speculate that aloricate ciliates might also have surface peak and DCM peak groups. The overall vertical distribution patterns showed that the planktonic food web may function differently within the surface waters and the DCM.

Evidence of zooplankton vertical migration from continuous Southern Adriatic buoy current-meter records

From the eleven-year data record at the Southern Adriatic Observatory (E2M3A), we study for the first time in the Adriatic Sea the strength of the backscatter signal from an Acoustic Doppler Current Profiler (ADCP) in relation to zooplankton vertical movement. The signal represents well the general behaviour of the zooplankton to sink at dawn and to emerge at the sunset. The modulation of the signal along the year is linked in particular to the different environmental conditions and the presence of different zooplankton groups. The correlation between the backscatter signal and distinct zooplankton groups (copepods, euphausiids, ostracods, appendicularians, salps) shows that in different seasons and environmental conditions, different groups are responsible for the strength of the signal. Furthermore, the backscatter signal, interpreted in terms of the qualitative composition of the zooplankton community, provides important information on the behaviour and habits of zooplankton in the southern Adriatic. Poorly known aspects of zooplankton behaviour are described in more detail, such as: recovery of zooplankton vertical distribution after mixing events, vertical migration in conditions of increased food availability at depth due to vertical mixing, modulation of vertical migration in dependence of the lunar phase and vertical migration in relation to deep chlorophyll maximum position. In addition, the present study emphasizes the possible influence of gelatinous zooplankton at the open sea on high backscatter values during the warmer part of the year.

Structure, characteristics and possible formation mechanisms of the subsurface chlorophyll maximum in the Yellow Sea Cold Water Mass

The Yellow Sea Cold Water Mass (YSCWM) is the most prominent oceanographic phenomenon in the bottom layer of the Yellow Sea. A subsurface chlorophyll maximum (SCM) has been widely observed in this area, yet it has only been briefly described in previous research. In this study, we present the first in-depth analysis of the spatial distribution and intrinsic characteristics of the SCM in the YSCWM, based on two comprehensive cruises conducted in the central Yellow Sea during August 2011 and July 2013. In general, the spatial distribution of the SCM coincides approximately with the YSCWM area during the stratified seasons, with the southern boundary at around 33.5°N and the western boundary at approximately 122.5°E. The vertical position of the SCM varied between 12 and 29 m and 13–25 m during August 2011 and July 2013 respectively, which is mainly located within or at the bottom of the thermocline and the upper part of the nitracline. The averaged Chl a concentration in the SCM was 1.24 ± 0.27 mg m−3 and 1.92 ± 0.65 mg m−3 respectively during the two cruises. The shallower but stronger intensity characteristics are consistent with the meso-trophic status of the Yellow Sea. Picophytoplankton, composed of Synechococcus and picoeukaryotes, dominated the phytoplankton community in cell abundance and carbon biomass, contributing > 90%. The micro- and nanophytoplankton were almost completely dominated by dinoflagellate species. The phytoplankton community at the SCM depth was not significantly different from that in the surface mixed layer, while the relatively higher photosynthetic yields (Fv/Fm) indicated the active growth of the phytoplankton assemblage in the SCM layer. Significantly lower carbon-to-chlorophyll a ratios estimated in the SCM layer suggest that the main contribution originates from photoacclimation. Our results demonstrate that the active photosynthesis of the phytoplankton combined with their physiological photo-acclimation are the main formation mechanisms of the SCM. We also suggest that the vertical migration of motile dinoflagellates is an additional contributing factor to the SCM in the YSCWM area. A higher vertical sampling resolution and 1-D physical-biological modeling are needed in future studies to confirm our hypothesis and quantify the contributions of the different formation mechanisms in the YSCWM area.

Sensitive Determination of the Dissolved Phosphate Pool for an Improved Resolution of Its Vertical Variability in the Surface Layer: New Views in the P-Depleted Mediterranean Sea

An accurate understanding of the biogeochemistry of dissolved phosphate pool in the upper waters of P-depleted oceanic regions is constrained by the low sensitivity of routine phosphate measurements. In this study, by using the sensitive Liquid Waveguide Capillary Cell method, we report the first extensive cross-basin survey of nanomolar dissolved inorganic phosphate (DIP) and dissolved organic phosphate (DOP) concentration in P-depleted surface waters of the Mediterranean Sea during the stratification period. In the north western Mediterranean Sea (NWMS), DIP above the mixed layer depth (MLD) ranged between 4.9 and 26.5 nM. Along an E-W transect crossing Ionian and Tyrrhenian Seas (E-W transect), DIP above the MLD was lower, ranging between 0.9 and 11.4 nM. Contrarily to the traditional view of a depleted and invariant surface dissolved phosphate pool, a significant vertical variability of DIP and DOP was revealed in upper waters. A positive gradient of DIP was observed above the phosphacline, between the MLD and the deep chlorophyll maximum (DCM) depth, suggesting a potential diffusion of new phosphate to near-surface waters, even under stratified conditions. Interestingly, despite this apparent DIP availability, a significant negative gradient of DOP concentration was observed in the same layer. Finally, the positive gradient in DIP coincided with a significant increase in N:P ratio, suggesting a higher rate of increase of N than of P. The results obtained in this study indicate that acquiring nanomolar DIP data is a sine qua non condition for the comprehension and prediction of the biogeochemical functioning of P-depleted oceanic regions, such as the Mediterranean Sea.

Microbial community structure in the western tropical South Pacific

Abstract. Oligotrophic regions play a central role in global biogeochemical cycles, with microbial communities in these areas representing an important term in global carbon budgets. While the general structure of microbial communities has been well documented in the global ocean, some remote regions such as the western tropical South Pacific (WTSP) remain fundamentally unexplored. Moreover, the biotic and abiotic factors constraining microbial abundances and distribution remain not well resolved. In this study, we quantified the spatial (vertical and horizontal) distribution of major microbial plankton groups along a transect through the WTSP during the austral summer of 2015, capturing important autotrophic and heterotrophic assemblages including cytometrically determined abundances of non-pigmented protists (also called flagellates). Using environmental parameters (e.g., nutrients and light availability) as well as statistical analyses, we estimated the role of bottom–up and top–down controls in constraining the structure of the WTSP microbial communities in biogeochemically distinct regions. At the most general level, we found a “typical tropical structure”, characterized by a shallow mixed layer, a clear deep chlorophyll maximum at all sampling sites, and a deep nitracline. Prochlorococcus was especially abundant along the transect, accounting for 68 ± 10.6 % of depth-integrated phytoplankton biomass. Despite their relatively low abundances, picophytoeukaryotes (PPE) accounted for up to 26 ± 11.6 % of depth-integrated phytoplankton biomass, while Synechococcus accounted for only 6 ± 6.9 %. Our results show that the microbial community structure of the WTSP is typical of highly stratified regions, and underline the significant contribution to total biomass by PPE populations. Strong relationships between N2 fixation rates and plankton abundances demonstrate the central role of N2 fixation in regulating ecosystem processes in the WTSP, while comparative analyses of abundance data suggest microbial community structure to be increasingly regulated by bottom–up processes under nutrient limitation, possibly in response to shifts in abundances of high nucleic acid bacteria (HNA).

Characteristics of phytoplankton communities and their biomass variation in a gas hydrate drilling area in the northern South China Sea

We analyzed the data obtained from field observations on a gas hydrate drilling area in Dongsha of northern South China Sea (SCS) in middle May (before drilling) and early October (after drilling) in 2013. The variation in the phytoplankton communities and biomass as well as the impacts of environmental factors including dissolved methane was studied. Results indicated that the gas hydrate drilling area in Dongsha, SCS exhibited a typical low-nutrients low-chlorophyll a (LNLC) environment accompanied with low phytoplankton abundance. A total of 103 taxa belonging to 52 genera of 5 classes were identified, with diatoms and dinoflagellates dominating the community. Both phytoplankton abundance and chlorophyll a (Chl a) were highest at the subsurface maximum layer. The subsurface chlorophyll maximum (SCM) before and after drilling were stabilized at 75 m (0.30 ± 0.06 mg/m3 and 0.51 ± 0.29 mg/m3, respectively), while the subsurface maximum of abundance after drilling went deeper to 75 m (604.17 ± 313.22 cells/L) from the surface (707.14 ± 243.98 cells/L) before drilling. After drilling, phosphate and Chl a increased significantly, but no significant differences were observed on abundance. Dominant species of diatoms were basically constant with dinoflagellates becoming more apparent in higher occurrence and abundance, while Cyanophyta was diverse after drilling. Redundancy analysis (RDA) and Spearman's correlation analysis both indicated that temperature, pH and phosphates were major factors causing fluctuation in phytoplankton community structure, while dissolved methane had non-significant impact directly. We clearly found both abundance and Chl a increased in particular water layers (between 50 and 75 m) and at stations (DS06, DS08 and DS15) where dissolved methane concentrations were also abnormally high. This study appeared to partly coincide with the findings of natural oil seeps in the Gulf of Mexico, which assumed that the turbulence from the natural oil and gas leaking zone could raise the bottom water through the rising bubbles and bring cold nutrient rich waters to the thermocline from the deep seeps. This plume-generated upwelling could then fuel a bottom-up effect on the photosynthetic species in the upper pelagic waters within the euphotic zone.

Sclerochronological Study of a Glycymeris vangentsumi Population From the Madeira Islands

Despite the positive results obtained using saltwater clam Glycymeris spp. for palaeoenvironmental reconstructions in the extratropical North Atlantic domain, this potential of the species has not been investigated in the subtropical North Atlantic region. The aim of this study was to investigate the driving factors for shell growth in Glycymeris bivalves by analysing growth patterns of G. vangentsumi specimens from an environment that is exposed to relatively minor seasonal contrast in terms of temperature compared to mid/high latitudes around the Madeira Islands. The populations studied comprised a group of sub-fossil shells (n=58) collected near the Desertas Islands, Madeira, in 2014, in a depth range of 80-160 m, together with two live-caught specimens. The shells collected alive were relatively young (<37 years), while the ontogenetic ages of the sub-fossil shells proved to be high (up to 154 years). Compared to those collected at greater depths, the average growth rate of the shells in the shallower environment was higher in the first four years. Subsets of the Madeira (n=16) samples could be arranged into a robust site chronology covering the 1950-2012 period, and these were used to explore shell growth responses to environmental parameters. The composite chronology displayed a negative correlation (r<-0.58, p < 0.01) with the January-April sea surface temperature (SST) around Madeira and a positive correlation (r=0.52, p<0.05) with the March sea surface chlorophyll concentration data of the region, from 1998 to 2012. The results presented here suggest that at this southern collection site, lower water temperature was not a directly limiting environmental variable, and other external environmental effects may exert stronger control on shell growth. These include the effect of late winter ocean mixing on the formation of the deep chlorophyll maximum.

Scratching Beneath the Surface: A Model to Predict the Vertical Distribution of Prochlorococcus Using Remote Sensing

The unicellular cyanobacterium Prochlorococcus is the most dominant resident of the subtropical gyres, which are considered to be the largest biomes on earth. In this study, the spatial and temporal variability in the global distribution of Prochlorococcus was estimated in the Atlantic Ocean using an empirical model based on data from 13 Atlantic Meridional Transect cruises. Our model uses satellite-derived sea surface temperature (SST), remote-sensing reflectance at 443 and 488 nm, and the water temperature at a depth of 200 m from Argo data. The model divides the population of Prochlorococcus into two groups: ProI, which dominates under high-light conditions associated with the surface, and ProII, which favors low light found near the deep chlorophyll maximum. ProI and ProII are then summed to provide vertical profiles of the concentration of Prochlorococcus cells. This model predicts that Prochlorococcus cells contribute 32 Mt of carbon biomass (7.4 × 1026 cells) to the Atlantic Ocean, concentrated mainly within the subtropical gyres (35%) and areas near the Equatorial Convergence Zone (30%). When projected globally, 3.4 × 1027 Prochlorococcus cells represent 171 Mt of carbon biomass, with 43% of this global biomass allocated to the upper ocean (0–45 m depth). Annual cell standing stocks were relatively stable between the years 2003 and 2014, and the contribution of the gyres varies seasonally as gyres expand and contract, tracking changes in light and temperature, with lowest cell abundances during the boreal and austral winter (1.4 × 1013 cells m−2), when surface cell concentrations were highest (9.8 × 104 cells mL−1), whereas the opposite scenario was observed in spring–summer (2 × 1013 cells m−2). This model provides a three-dimensional view of the abundance of Prochlorococcus cells, revealing that Prochlorococcus contributes significantly to total phytoplankton biomass in the Atlantic Ocean, and can be applied using either in situ measurements at the sea surface (r2 = 0.83) or remote-sensing observables (r2 = 0.58).

Nutrient availability and the ultimate control of the biological carbon pump in the western tropical South Pacific Ocean

Abstract. Surface waters (0–200 m) of the western tropical South Pacific (WTSP) were sampled along a longitudinal 4000 km transect (OUTPACE cruise, DOI: 10.17600/15000900) during the austral summer (stratified) period (18 February to 3 April 2015) between the Melanesian Archipelago (MA) and the western part of the SP gyre (WGY). Two distinct areas were considered for the MA, the western MA (WMA), and the eastern MA (EMA). The main carbon (C), nitrogen (N), and phosphorus (P) pools and fluxes provide a basis for the characterization of the expected trend from oligotrophy to ultra-oligotrophy, and the building of first-order budgets at the daily and seasonal timescales (using climatology). Sea surface chlorophyll a well reflected the expected oligotrophic gradient with higher values obtained at WMA, lower values at WGY, and intermediate values at EMA. As expected, the euphotic zone depth, the deep chlorophyll maximum, and nitracline depth deepen from west to east. Nevertheless, phosphaclines and nitraclines did not match. The decoupling between phosphacline and nitracline depths in the MA allows for excess P to be locally provided in the upper water by winter mixing. We found a significant biological “soft tissue” carbon pump in the MA sustained almost exclusively by dinitrogen (N2) fixation and essentially controlled by phosphate availability in this iron-rich environment. The MA appears to be a net sink for atmospheric CO2, while the WGY is in quasi-steady state. We suggest that the necessary excess P, allowing the success of nitrogen fixers and subsequent carbon production and export, is mainly brought to the upper surface by local deep winter convection at an annual timescale rather than by surface circulation. While the origin of the decoupling between phosphacline and nitracline remains uncertain, the direct link between local P upper water enrichment, N2 fixation, and organic carbon production and export, offers a possible shorter timescale than previously thought between N input by N2 fixation and carbon export. The low iron availability in the SP gyre and P availability in the MA during the stratified period may appear as the ultimate control of N input by N2 fixation. Because of the huge volume of water to consider, and because the SP Ocean is the place of intense denitrification in the east (N sink) and N2 fixation in the west (N source), precise seasonal C, N, P, and iron (Fe) budgets would be of prime interest to understand the efficiency, at the present time and in the future, of the oceanic biological carbon pump.

N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

Abstract. We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (February–March 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N2 fixation rates were extremely high at both LD A (593 ± 51 µmol N m−2 d−1) and LD B (706 ± 302 µmol N m−2 d−1), and the diazotroph community was dominated by Trichodesmium. N2 fixation rates were lower (59 ± 16 µmol N m−2 d−1) at LD C, and the diazotroph community was dominated by unicellular N2-fixing cyanobacteria (UCYN). At all stations, N2 fixation was the major source of new N (&gt; 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N2 fixation contributed circa 13–18 % of primary production in the MA region and 3 % in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratio = particulate carbon export ∕ primary production) was maximum at LD A (9.7 %) and was higher than the e ratio in most studied oligotrophic regions (&lt; 5 %), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 % of the PC export at LD A, while their contribution was 5 and &lt; 0.1 % at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper layer appears as a characteristic of the WTSP during the summer season.

Biogeochemical characteristics of suspended particulate matter in deep chlorophyll maximum layers in the southern East China Sea

Abstract. Continental shelves and marginal seas are key sites of particulate organic matter (POM) production, remineralization and sequestration, playing an important role in the global carbon cycle. Elemental and stable isotopic compositions of organic carbon and nitrogen are thus frequently used to characterize and distinguish POM and its sources in suspended particles and surface sediments in the marginal seas. Here we investigated suspended particulate matter (SPM) collected around deep chlorophyll maximum (DCM) layers in the southern East China Sea for particulate organic carbon and nitrogen (POC and PN) contents and their isotopic compositions (δ13CPOC and δ15NPN) to understand provenance and dynamics of POM. Hydrographic parameters (temperature, salinity and turbidity) indicated that the study area was weakly influenced by freshwater derived from the Yangtze River during summer 2013. Elemental and isotopic results showed a large variation in δ13CPOC (−25.8 to −18.2 ‰) and δ15NPN (3.8 to 8.0 ‰), but a narrow molar C ∕ N ratio (4.1–6.3) and low POC ∕ Chl a ratio (&lt; 200 g g−1) in POM, and indicated that the POM in DCM layers was newly produced by phytoplankton. In addition to temperature effects, the range and distribution of δ13CPOC were controlled by variations in primary productivity and phytoplankton species composition; the former explained ∼ 70 % of the variability in δ13CPOC. However, the variation in δ15NPN was controlled by the nutrient status and δ15NNO3- in seawater, as indicated by similar spatial distribution between δ15NPN and the current pattern and water masses in the East China Sea; although interpretations of δ15NPN data should be verified with the nutrient data in future studies. Furthermore, the POM investigated was weakly influenced by the terrestrial OM supplied by the Yangtze River during summer 2013 due to the reduced sediment supply by the Yangtze River and north-eastward transport of riverine particles to the northern East China Sea. We demonstrated that the composition of POM around DCM layers in the southern East China Sea is highly dynamic and largely driven by phytoplankton abundance. Nonetheless, additional radiocarbon and biomarker data are needed to re-evaluate whether or not the POM around the DCM water depths is influenced by terrestrial OM in the river-dominated East China Sea.

Late Quaternary palaeoenvironmental reconstruction of sediment drift accumulation in the Malta Graben (central Mediterranean Sea)

The Malta Graben is a deep tectonic depression in the Sicily Channel, bounded by NW–SE normal faults and filled by thick Pliocene–Quaternary deposits. A previous analysis of a giant piston core (LC09) from the Malta Graben had revealed a wide range of sedimentary features (carbonate turbidites, bioturbated mud and scours), although the chronostratigraphic constraint of the stacking pattern has remained elusive. After establishing a reliable chronological framework based on seven radiocarbon dates for a shorter core from the Malta Graben (ANSIC03-735), a down-core analysis of planktonic foraminifer and coccolith abundance, stable isotopes and sediment grain size was carried out. Since the last glacial maximum, palaeoenvironmental conditions (surface fertility and deep chlorophyll maximum during the last glacial and the Younger Dryas; warm and oligotrophic water masses, with a deep nutricline and intense winter mixing during the Holocene) as well as selected calcareous plankton taxa trends and peaks seem to be similar to those reported for other central and western Mediterranean sites, possibly in spite of a unique response of these areas to late Quaternary climatic fluctuations. Four distinct layers, each tens of centimetres thick, are barren of foraminifers but not of coccoliths. Morphobathymetric data as well as new high-resolution and high-penetration seismic profiles show that prolonged contouritic activity has persisted on the western side of the Malta Graben. It is thus likely that layers barren of foraminifers are due to the overflow of fine-grained (clayey) material beyond drift channel dikes.

Spatial differences in dissolved silicon utilization in Lake Baikal, Siberia: Examining the impact of high diatom biomass events and eutrophication

AbstractRecent research has highlighted how Lake Baikal, Siberia, has responded to the direct and indirect effects of climate change (e.g., ice‐cover duration), nutrient loading, and pollution, manifesting as changes in phytoplankton/zooplankton populations, community structure, and seasonal succession. Here, we combine and compare analyses of chlorophyll a (an estimate of total algal biomass), carotenoid pigments (biomarkers of algal groups), and lake water silicon isotope geochemistry (δ30SiDSi) to differentiate spatial patterns in dissolved silicon (DSi) uptake at Lake Baikal. A total of 15 sites across the three basins (south, central, and north) of Lake Baikal were sampled in August 2013 along a depth gradient of 0–180 m. Strong, significant correlations were found between vertical profiles of photic zone DSi concentrations and δ30SiDSi compositions (r = −0.81, p &lt; 0.001), although these are strongest in the central basin aphotic zone (r = −0.98, p &lt; 0.001). Data refute the hypothesis of DSi uptake by picocyanobacteria. Algal biomass profiles and high surface δ30SiDSi compositions suggest greater productivity in the south basin and more oligotrophic conditions in the north basin. δ30SiDSi signatures are highest at depth (20 m) in central basin sites, indicating greater (10–40%) DSi utilization at deep chlorophyll maxima. DSi limitation occurs in the pelagic central basin, probably reflecting a high diatom biomass bloom event (Aulacoseira baicalensis). Meanwhile in the more hydrologically restricted, shallow Maloe More region (central basin), both high δ30SiDSi compositions and picocyanobacteria (zeaxanthin) concentrations, respectively point to the legacy of an “Aulacoseira bloom year” and continuous nutrient supply in summer months (e.g., localized eutrophication).

Spatial patterns in abundance, taxonomic composition and carbon biomass of nano- and microphytoplankton in Subarctic and Arctic Seas

In the summers of 2007 and 2008, we studied assemblages of nano- and microphytoplankton from the subsurface chlorophyll maximum (SCM) across five broad oceanographic domains in the seas surrounding northern North America. These domains are the eastern Subarctic North Pacific (ESNP), Bering and Chukchi Seas (BE-CH), Beaufort Sea and Canada Basin (BS-CB), Canadian Arctic Archipelago (CAA), and Baffin Bay and Labrador Sea (BB-LS). Average abundance and total carbon biomass (C) of phytoplankton (>2 µm) varied ∼10-fold and ∼20-fold, respectively, across the five domains. In the BE-CH, CAA and BB-LS, diatoms averaged 35–70% and dinoflagellates 11–45% of total phytoplankton C (>2 µm), whereas in the ESNP and BS-CB, unidentified flagellates/coccoids (2–8 µm) represented a greater proportion of total C (27% and 39% respectively) than in the other domains.In the BE-CH and BB-LS, phytoplankton C (>2 µm) was dominated by dinoflagellates of the genus Gymnodinium, centric diatoms including Thalassiosira spp. and Chaetoceros spp., unidentified flagellates/coccoids (2–8 µm), and cryptomonads. In contrast, diatoms such as Thalassiosira spp. and its resting spores dominated C in the CAA, with dinoflagellates being less significant than in the BE-CH and BB-LS. Unidentified flagellates/coccoids (2–8 µm), Gymnodinium spp., and cryptomonads dominated in the ESNP, and particularly in the BS-CB, where diatoms contributed only 18% of the very low levels of total phytoplankton C (>2 µm).Phytoplankton C (>2 µm) to chlorophyll a ratios (phyto C:chl a) averaged only 31 g C g chl a−1 in the oligotrophic BS-CB domain, and 51–150 g C g chl a−1 in the other domains, whereas ratios of biogenic silica to phytoplankton C (>2 µm) (bSi:phyto C) were lowest in the eastern domains. Estimates of phytoplankton C were highly sensitive to the choice of C to cell volume equations (C:vol) adopted in the calculations, particularly in diatom-rich areas.This study highlights how diatoms and dinoflagellates are the main drivers of large-scale variations in C biomass for phytoplankton (> 2 µm), whereas unidentified flagellates/coccoids (2–8 µm) make a significant contribution to C biomass in oligotrophic domains, such as BS-CB, where diatoms and dinoflagellates are less abundant. Reduced surface water density (σT) was associated with deeper SCM layers, and with decreased C biomass of unidentified flagellates/coccoids (2–8 µm). These observations confirm recent studies highlighting the role of surface water stratification caused by melting sea ice in shaping nano- and microphytoplankton assemblages.

Phytoplankton response to the contrasting physical regimes in the eastern Arabian Sea during north east monsoon

Abstract Phytoplankton abundance and composition in two contrasting physical regimes - convective mixing in the northeastern Arabian Sea (NEAS) and Arabian Sea mini warm pool (ASMWP) in the southeastern Arabian Sea (SEAS) - were investigated during the northeast monsoon (NEM) of 2015 and 2017. Observations in 2015 were carried out late during the season, and only one station in the north (at 21°N latitude) fell within the zone of convective mixing where microplankton was dominated by diatoms. In 2017, convective mixing occurred even at 16°N latitude, but the microplankton contribution was low, presumably due to low Si/N ratios. Within the convective mixing regime of the NEAS, chlorophyll (Chl) a concentrations were higher in 2015 (maximum 1080 ng L−1; average 493 ng L−1) than in 2017 (maximum 673 ng L−1; average 263 ng L−1). In contrast, picophytoplankton were dominant in the ASMWP of the SEAS with peak abundance associated with the subsurface chlorophyll maximum. A warm core eddy was present in 2015 in the SEAS where four times higher Prochlorococcus counts were found within the core of the eddy than at its periphery. This study provides the first description of the phytoplankton community in the ASMWP. Our results clearly demonstrate phytoplankton response to the contrasting physical conditions, highlighting the role of bio-physical coupling in the productivity of the Arabian Sea.