Surface Chl Research Articles

A Brand-New Algorithm for Mapping Algal Biomass in Lakes

Column-integrated algal biomass has been recognized as a more logical proxy for the evaluation of lake eutrophication. Here, an algorithm with a 3-step framework is put forward for algal biomass mapping in 3 lakes of China (Lake Hongze, Lake Taihu, and Lake Chaohu). It can be summarized in step 1: inversion of surface chlorophyll a (Chl a ), step 2: inversion of diffuse attenuation coefficient of the photosynthetic active radiation [ K d (PAR)], and step 3: estimation of algal biomass with a pretrained generalized additive model. The proposed algorithm outperforms the result-oriented and process-oriented methods in terms of accuracy in 3 lakes (the root mean square error [RMSE] values for datasets of Lake Hongze, Lake Taihu, and Lake Chaohu were 5.09, 8.21, and 3.90 mg/m 2 , respectively). Validated with match-up satellite data, the algorithm generates acceptable results (RMSE = 5.69 mg/m 2 , mean absolute percentage error = 30.9%, N = 16). Another important discovery is that the extremum of algal biomass of the entire lake (B tot ) does not always coincide with that of total surface Chl a . For example, the maximum total surface Chl a was recorded in 2016, whereas the maximum B tot of Lake Hongze was observed in 2020. For Lake Taihu, 3 peaks of B tot appearing in 2017, 2019, and 2021, respectively, did not coincide with those of total surface Chl a . For Lake Chaohu, the interannual B tot followed a bimodal pattern that differed from the pattern of interannual total surface Chl a . The proposed algorithm plays an indispensable role in broadening the horizon for algal biomass inversion.

Modelling global mesozooplankton biomass using machine learning

Mesozooplankton are a crucial link between primary producers and higher trophic levels and play a vital role in marine food webs, biological carbon pumps, and sustaining fishery resources. However, the global distribution of mesozooplankton biomass and the relevant controlling mechanisms remain elusive. We compared four machine learning algorithms (Boosted Regression Trees, Random Forest, Artificial Neural Network, and Support Vector Machine) to model the spatiotemporal distributions of global mesozooplankton biomass. These algorithms were trained on a compiled dataset of published mesozooplankton biomass observations with corresponding environmental predictors from contemporaneous satellite observations (temperature, chlorophyll, salinity, and mixed layer depth). We found that Random Forest achieved the best predictive accuracy with R2 and RMSE (Root Mean Standard Error) of 0.57 and 0.39, respectively. Also, the global distribution of mesozooplankton biomass predicted by the Random Forest model was more consistent with the observational data than other models. We used the Random Forest model to create a global map of mesozooplankton biomass which serves as a reference for validating process-based ecosystem models. The model outputs confirm that environmental factors, especially surface Chl a, a proxy for prey availability, significantly correlate with the spatiotemporal distribution of mesozooplankton biomass. The scaling relationship between the mesozooplankton biomass and Chl a can be used as an emergent constraint for model validation and development. Moreover, our model predicts that the global mesozooplankton biomass will decrease by 3% by the end of this century under the “business-as-usual” scenarios, potentially reducing fishery production and carbon sequestration. Our study contributes to predicting global mesozooplankton biomass and provides deep insights into the underlying environmental impacts on the distribution of mesozooplankton biomass.

Summer sea ice melting enhances phytoplankton and dimethyl sulfide production

AbstractThe relationships among sea ice melting, phytoplankton assemblages, and the production of climate‐relevant trace gases in the Southern Ocean are gaining increasing attention from the scientific community. This is particularly true for dimethyl sulfide (DMS), which plays an important role in atmospheric chemistry by influencing the formation of sulfated aerosols with radiative impacts and constituting cloud condensation nuclei. In the current study, chlorophyll a (Chl a), DMS and its precursors dimethylsulfoniopropionate (DMSP), were quantified in the Weddell–Scotia Confluence (WSC) during the 2018 record ice extent minimum period. Mixed layer changes were found to be generally associated with spatial variation in sea ice melt, with the depth being six times deeper in ice‐free, well‐mixed regions than in seasonal ice‐melting zones. The surface Chl a concentration increased from ice‐free to ice‐melting regions with elevated sea ice meltwater percentages and drawdown surface nutrient concentrations. The concentrations of surface and depth‐integrated Chl a in the upper 150 m reached maxima in the ice‐melting region with the highest fraction of sea ice meltwater, illustrating that sea ice melting promoted the occurrence of phytoplankton blooms. The DMS and DMSP concentrations in the vicinity of the ice‐melting zone were approximately three times higher than those in the ice‐free waters. The observations of this study show that the regions of ice melting in the WSC were a zone of particularly high sea–air fluxes of DMS, which could significantly contribute to the atmospheric budget of DMS in the polar regions.

Vertical and horizontal variations in phytoplankton chlorophyll a in response to a looping super typhoon

AbstractPrevious studies suggested that the increase in surface chlorophyll a (Chl a) is due to nutrient upwelling or to the upward mixing of the subsurface Chl a maximum layer under the influence of tropical cyclones, while often ignoring the influence of the subsurface Chl a minimum layer and horizontal advection on Chl a. In this study, we show the important roles of the upward mixing of the subsurface Chl a minimum layer, horizontal advection, as well as the upwelling of the subsurface Chl a maximum layer, taking a looping super typhoon “Saola” in the northwest Pacific in August 2023 as an example. The temporal and spatial changes of Chl a and its physical properties were investigated by combining satellite, Argo, reanalysis, and model data. The results indicate that the combined effects of the upwelling of the subsurface Chl a maximum layer caused by wind stress curls and concurrent near‐surface wind mixing were responsible for the surface Chl a increase in the looping area during the typhoon, while the 13% increase in the depth‐integrated Chl a after the typhoon is mainly due to the nutrients brought by upwelling and subsequent biochemical processes. In the edge area affected by the typhoon, the surface Chl a decrease during the typhoon was mainly due to the upward mixing of the subsurface Chl a minimum layer (the effect of upwelling in this area is relatively weak). Furthermore, the horizontal advection led to a continuous surface Chl a decrease in the edge area after the typhoon. These findings could enhance understanding of Chl a dynamics post‐tropical cyclones, aiding marine ecosystem prediction.

The role of subsurface instabilities for increasing chlorophyll concentrations in a warming southern Indian ocean

A warming climate is expected to intensify the stratification of the upper ocean in tropical and subtropical regions, which in turn results in decreases in the primary productivity for these oligotrophic areas. To assess if there is trended change in primary productivity in the southern Indian Ocean (IO) with known striking temperature increase, we use 17-years of satellite chlorophyll (Chl) data and model output to examine the trended changes in Chl. The results exhibited a surprisingly increase in Chl concentrations in part of the southern IO over the gyre area. To investigate the potential mechanisms underlying this Chl increase, we used temperature/salinity observations to re-evaluate stratification in the southern IO. The southern IO experienced basin-wide surface warming over the time series however there was a region of subsurface cooling at 50–100 m around 10°S. In the subtropical IO gyre, subsurface warming occurs at faster rates compare to the surface. Through the calculation of buoyancy frequency (N2), we have confirmed the presence of subsurface instabilities caused by these inhomogeneous trends in the vertical thermohaline structure. This was particularly true over the southern IO gyre, which experienced sustained increase of surface mixing disturbances over the last decade—resulting in a more favorable environment for vertical transport of nutrients into the euphotic zone. A mixed layer nutrient budget analysis suggested that entrainment due to mixed layer deepening is crucial in delivering nutrients into the gyre's upper mixed layer, which fueled phytoplankton activity. This emphasizes the importance of considering subsurface instabilities when interpreting the factors that influence surface Chl variabilities. This study highlights the importance of a three-dimensional framework for examining stratification to assess future marine ecosystem responses to a changing climate.

Hydrography, inorganic nutrients and chlorophyll a linked to sea ice cover in the Atlantic Water inflow region north of Svalbard

Changes in the inflow of Atlantic Water (AW) and its properties to the Arctic Ocean bring more warm water, contribute to sea ice decline, promote borealisation of marine ecosystems, and affect biological and particularly primary productivity in the Eurasian Arctic Ocean. One of the two branches of AW inflow follows the shelf break north of Svalbard, where it dominates oceanographic conditions, bringing in heat, salt, nutrients and organisms. However, the interplay with sea ice and Polar Surface Water (PSW) determines the supply of nutrients to the euphotic layer especially northeast of Svalbard where AW subducts below PSW. In an effort to build up a time series monitoring the key characteristics of the AW inflow, repeat sampling of hydrography, macronutrients (nitrate, phosphate and silicate), and chlorophyll a (chl a) was undertaken along a transect across the AW inflow at 31°E, 81.5°N since 2012 — first during late summer and in later years during early winter. Such time series are scarce but invaluable for investigating the range of variability in hydrography and nutrient concentrations. We investigate linkages between late summer hydrographic conditions and nutrient concentrations along the transect and the preceding seasonal dynamics of surface chl a and sea ice cover in the region north of Svalbard. We find large interannual variability in hydrography, nutrients and chl a, indicating varying levels of nutrient drawdown by primary producers over summer. Sea ice conditions varied considerably between the years, impacting upper ocean stratification, light availability and potential wind-driven mixing, with a strong potential for steering chl a concentration over the productive season. Early winter measurements show variable efficiency of nutrient re-supply through vertical mixing when stratification was low, related to autumn wind forcing and sea ice conditions. While this re-supply elevates nutrient levels sufficiently for primary production, it likely happens too late in the season when light levels are already low, limiting the potential for autumn blooms. Such multidisciplinary observations provide insight into the interplay between physical, chemical and biological drivers in the marine environment and are key to understanding ongoing and future changes, especially at this entrance to the central Arctic Ocean.

Effects of Typhoon Chanthu on Marine Chlorophyll a, Temperature and Salinity

A typhoon is a severe weather process in tropical oceans. Typhoon transit is often accompanied by strong convective weather, such as gales and rainstorms, which threatens fishery, property, and human safety. In this study, the effects of typhoon Chanthu on chlorophyll a (Chl a), temperature, and ocean surface salinity are analyzed using remote sensing data. The results illustrate that before the transit of Chanthu (6–12 September), the mean concentration of Chl a and sea surface salinity (SSS) are low (0.74 mg/m3, 30.59 psu, respectively), while the mean sea surface temperature (SST) is high (29.01 °C). After the typhoon transits (13–30 September), the mean Chl a concentration and salinity increase (1.29 mg/m3, 30.87 psu respectively), while the mean SST decrease (27.43 °C). The Ekman pumping transports nutrients from the deep ocean to the surface layer, promotes the photosynthesis of surface phytoplankton, and increases the concentration of sea surface Chl a. Typhoon Chanthu causes the mixing and entrainment of the upper ocean, which causes the deep cold water of the ocean to rise into the mixed layer and cause the SST to decrease. Severe vertical mixing transports deep high-salt water to the surface, causing SSS to rise. The results of this study have important scientific significance and application value for developing coastal economy, aquaculture, and fishery.

Impact of subsurface chlorophyll maxima on satellite-based Arctic spring primary production estimates

In recent years several regional and pan-Arctic ocean color algorithms, which consider the unique bio-optical properties of the Arctic Ocean, have been developed to accurately extract surface chlorophyll a (Chl a), a key input into net primary production (NPP) algorithms, from spectral remote sensing reflectance (Rrs). However, most satellite derived NPP (NPPSAT) algorithms used in the Arctic do not account for production at deep subsurface chlorophyll maxima (SCMs), a prominent feature in the Arctic Ocean during the summer months, leading to underestimations of NPP during the post-bloom period. In this study, a shallow SCM was observed in early summer (June/July 2016) in Baffin Bay at the time of the sea ice edge bloom, raising the question to what extent this SCM contributes to Rrs and how it is captured in NPPSAT estimates. Radiative transfer simulations based on in situ data of inherent and apparent optical properties and Chl a concentration in the water column were used to examine the effects of heterogeneous vertical Chl a profiles of various strength and depth on spectral reflectance, algorithm-derived surface Chl aSAT and NPPSAT. NPPSAT estimates for varying Chl a profile shapes were then compared to NPPSAT estimates of reference simulations for homogenous Chl a profiles, and to measured NPP calculated using observed Chl a profiles. Results show a significant impact of shallow SCMs on Rrs with the depth of maximum Chl a < 30 m, causing an increase in Chl aSAT by 17 ± 6% relative to a vertically homogenous ocean. Interestingly, SCMs significantly influencing Rrs were found down to the fifth light penetration depth. The increase in Chl aSAT reduced the difference between predicted and measured NPP estimates to −25 ± 12% at stations with a shallow SCM. Furthermore, due to the significant contribution of these shallow SCM stations to regional NPP, regional predicted NPPSAT was only 16% lower than measured NPP. Our results demonstrate that the partial representation of a shallow and very productive SCM in Chl aSAT minimizes errors in satellite-based NPPSAT estimates of the Arctic spring bloom during the sea ice melt period.

Case where a mesoscale cyclonic eddy suppresses primary production: A Stratification-Lock hypothesis

Cyclonic eddies in the ocean often increase primary production and phytoplankton biomass. The main mechanism is the rise of the nutricline in the cyclone's core, which determines the intensification of the ascending flow of deep nutrients into the photic layer. However, some cyclones suppress primary productivity, and the natural processes underlying them are little studied and are of great fundamental interest. In August 2021, a vast cyclonic zone with two mesoscale cyclones occupied the Black Sea deep-water basin. A powerful quasi-tropical atmospheric vortex amplified one of the cyclones, which increased the Ekman pumping and led to strong sea surface cooling by 4 °C. However, the inflow of thermocline water into the euphotic layer and higher nutricline gradients did not lead to elevated depth-integratedconcentration of chlorophyll a (Chl)and phytoplankton biomass. Moreover, the surface Chl and the average primary production was less than in the outside waters. The proposed Stratification-Lock hypothesis explains this phenomenon. A combination of several factors led to its appearance. The drastic water cooling, followed by calm weather and vigorous summer heating, led to strong thermal stratification, dampening turbulent mixing in the near-surface layer. At the same time, the velocity and shear of current in and below the thermocline were low in the entire cyclonic zone, which prevented substantial diapycnal mixing generated at depth. As a result, a 'stratification lock' appeared in the entire water column, suppressing nutrients' upward flux. The proposed hypothesis is consistent with the effect of thermal stratification on primary productivity revealed in the entire cyclonic zone. The higher was the thermal stratification the lower was the depth-integrated total phytoplankton biomass and primary production. Also, diatoms developed in the upper layer in waters with weaker thermal stratification. The described stage lasted two weeks and was interrupted by strong wind mixing, which destroyed the 'stratification lock' and caused an increase in Chl at the sea surface. This stage of the cyclone life cycle occurs under a strictly defined combination of hydro-meteorological conditions, suggesting a relatively rare occurrence. Nevertheless, some evidence of the similar effect of a cyclone on primary productivity can be found in other studies, indicating that this natural mechanism, consisting of a complex weather-current-eddy combination, can operate in various marine areas.

Surface chlorophyll anomalies induced by mesoscale eddy-wind interactions in the northern Norwegian Sea

The substantial productivity of the northern Norwegian Sea is closely related to its strong mesoscale eddy activity, but how eddies affect phytoplankton biomass levels in the upper ocean through horizontal and vertical transport-mixing has not been well quantified. To assess mesoscale eddy induced ocean surface chlorophyll-a concentration (CHL) anomalies and modulation of eddy-wind interactions in the region, we constructed composite averaged CHL and wind anomalies from 3,841 snapshots of anticyclonic eddies (ACEs) and 2,727 snapshots of cyclonic eddies (CEs) over the period 2000-2020 using satellite altimetry, scatterometry, and ocean color products. Results indicate that eddy pumping induces negative (positive) CHL anomalies within ACEs (CEs), while Ekman pumping caused by wind-eddy interactions induces positive (negative) CHL anomalies within ACEs (CEs). Eddy-induced Ekman upwelling plays a key role in the unusual positive CHL anomalies within the ACEs and results in the vertical transport of nutrients that stimulates phytoplankton growth and elevated productivity of the region. Seasonal shoaling of the mixed layer depth (MLD) results in greater irradiance levels available for phytoplankton growth, thereby promoting spring blooms, which in combination with strong eddy activity leads to large CHL anomalies in May and June. The combined processes of wind-eddy interactions and seasonal shallowing of MLD play a key role in generating surface CHL anomalies and is a major factor in the regulation of phytoplankton biomass in the northern Norwegian Sea.

Phytoplankton dynamics as a response to physical events in the oligotrophic Eastern Indian Ocean

The Eastern Indian Ocean (EIO) is considered to exhibit low chlorophyll a (Chl a) concentrations. However, high Chl a levels were occasionally reported and attributed to physical events that frequently occur in this area. Because of limited in situ observations, the underlying formation mechanism of high Chl a through the responses of components within the ecosystem is not fully understood. In this study, we conducted a meridional cruise along the 88°E longitude in the EIO (16.5°N to 20°S) during the fall intermonsoon period (November 8–28th, 2018). For the first time, surface phytoplankton growth and mortality rates were measured using dilution experiments. Sensitive nutrient analysis was also applied to determine the nanomolar nutrient concentrations. Our cruise captured high surface Chl a concentrations (∼298 ng L−1 at 10 m) in the Bay of Bengal, at the Equator, and in the southern EIO, even under consistent nitrogen depletion (dissolved inorganic nitrogen of ∼58 nM). All the blooms observed were in the decay phase, as suggested by the satellite observations of Chl a distributions and the low or negative phytoplankton net growth rate. However, the phytoplankton growth rate was enhanced by ∼4.9 times due to nutrient enrichment in dilution experiments, indicating that the bloom was initiated by nutrient enrichment and ceased by the severe nutrient limitation. The shipboard acoustic Doppler current profiler measurements and satellite observations of sea surface winds, surface currents, and sea surface height collectively suggested that phytoplankton blooms were triggered by nutrient enrichments that were possibly supplied by either the island effect near the Maldives or a cyclonic eddy in the southern EIO. In the former case, the Chl a-rich water generated near the islands was then advected eastward toward the study area by surface currents associated with the Wyrtki Jet. Our study indicated that the EIO was not a stable oligotrophic marine ecosystem with consistently low Chl a concentrations, and was quite dynamic as affected by unique regional physical events.

Response of sea surface temperature, chlorophyll and particulate organic carbon to a tropical cyclonic storm over the Arabian Sea, Southwest India

The response of sea surface temperature (SST), chlorophyll (Chl) and particulate organic carbon (POC) to the tropical cyclonic storm Ockhi between 29th November and 6th December 2017 over the Arabian Sea, Southwest coast of India was studied. The unique observation is that the cyclone overturned northeastwards (clockwise), the wind speed in north-northwestwards (8–12 m s−1) which caused cold patches (< 28.5 °C) in the northwestern Arabian Sea compared to the southwest coast of India. Sea surface cooling (3–4 °C) was observed in concurrence with significant Chl response (0.5–7.5 mg m−3) along the Ockhi cyclone path. The post cyclonic low SST and high Chl from cyclone centre to farther areas suggested the extent of upwelled nutrient rich cold waters for plankton proliferation. POC levels showed a 3-fold increase, affected by significant biomass production due to subsurface phytoplankton and nutrient sources evidenced by deepening of mixed layer depth. The cyclone induced contrasting surface Chl (< 1 mg m−3) and POC (80–160 mg m−3) concentration substantiated the dominance of less labile refractory organic particulates in the offshore waters. The sub surface cooling and Chl bloom was noticed up to 50 m indicating the strong relation of vertical bio-physical structure during tropical cyclones. This study revealed that the tropical cyclones can act in such a way that it improves the positive magnitude of Chl and POC whereas reverses the SST behavior, on either side of the cyclone path within a short period in the oligotrophic waters of Arabian Sea.

Improved Perceptron of Subsurface Chlorophyll Maxima by a Deep Neural Network: A Case Study with BGC-Argo Float Data in the Northwestern Pacific Ocean

Subsurface chlorophyll maxima (SCMs), commonly occurring beneath the surface mixed layer in coastal seas and open oceans, account for main changes in depth-integrated primary production and hence significantly contribute to the global carbon cycle. To fill the gap of previous methods (in situ measurement, remote sensing, and the extrapolating function based on surface-ocean data) for obtaining SCM characteristics (intensity, depth, and thickness), we developed an improved deep neural network (IDNN) model using a Gaussian radial basis activation function to retrieve the vertical profile of chlorophyll a concentration (Chl a) and associated SCM characteristics from surface-ocean data. The annually averaged SCM depth was further incorporated into the bias term and the Gaussian activation function to improve the estimation accuracy of the IDNN model. Based on the Biogeochemical-Argo (BGC-Argo) data acquired for three regions in the northwestern Pacific Ocean, vertical Chl a profiles produced by our improved DNN model using sea surface Chl a and sea surface temperature (SST) were in good agreement with the observations, especially in regions with low surface Chl a. Compared to other neural-network-based models with one hidden layer and a sigmoid activation function, the IDNN model retrieved vertical Chl a profiles well in more eutrophic subpolar regions. Furthermore, the application of the IDNN model to infer vertical Chl a profiles from remote-sensing information was validated in the northwestern Pacific Ocean.

Synoptic Mesoscale to Basin Scale Variability in Biological Productivity and Chlorophyll in the Kuroshio Extension Region

The Kuroshio current separates from the Japanese coast to become the eastward flowing Kuroshio Extension (KE) characterized by a strong latitudinal density front, high levels of mesoscale (eddy) energy, and high chlorophyll a (Chl). While satellite measurements of Chl show evidence of the impact of mesoscale eddies on the standing stock of phytoplankton, there have been very limited synoptic, spatially resolved in situ estimates of productivity in this region. Here, we present underway measurements of oxygen/argon supersaturation (ΔO2/Ar), a tracer of net biological productivity, for the KE made in spring, summer, and early autumn. We find large seasonal differences in the relationships between ΔO2/Ar, Chl, and sea level anomaly (SLA), a proxy for local thermocline depth deviations driven by mesoscale eddies derived from satellite observations. We show that the KE is a pronounced hotspot of high ΔO2/Ar in spring, but corresponding surface Chl values are low and have no correlation with ΔO2/Ar. In summer, there is a hotspot of productivity associated with the Oyashio front, where ΔO2/Ar and Chl are strongly positively correlated. In autumn, ΔO2/Ar and Chl are consistently low throughout the region and also positively correlated. By combining our analysis of the in situ ΔO2/Ar data with complementary Argo, BGC‐Argo, repeat hydrography, and SLA observations, we infer the combination of physical and biological controls that drive the observed distributions of ΔO2/Ar and Chl. We find that the KE and Oyashio currents both act to supply nutrients laterally, fueling regions of high productivity in spring and summer, respectively.

Seasonal and interannual variabilities of chlorophyll across the eastern equatorial Indian Ocean and Bay of Bengal

Seasonal and interannual variabilities of surface and subsurface chlorophyll (Chl) in the eastern equatorial Indian Ocean (IO) and Bay of Bengal were examined based on vertical Chl inferred from satellite data. Four biotic zones were identified by Empirical orthogonal function analysis, which represented different types of mixed layer processes. The areas south of Sri Lanka indicated the most significant Chl increase in the surface and subsurface during summer due to coastal upwelling. Surface Chl highs in the Sri Lanka Dome occurred every summer during the time series. Chl increases were also recorded in the southwestern Bay of Bengal during fall–winter in response to wind-induced divergence and convective mixing. The other two zones in south IO basin between 5 and 15°S exhibited seasonal surface Chl highs in summer and subsurface Chl maximum during fall–winter. Wind-induced divergence plays a major role in the maintenance of this upwelling zone year-round; however, stratification inhibited the surface Chl increase in winter and results in enhanced Chl in the subsurface. The Chl interannual variability was related with Indian Ocean dipole (IOD) associated physical field oscillations. In positive IOD, wind divergence increased significantly along the eastern equatorial IO coast that contributed to Chl enhancement there, it decreased in Bay of Bengal and South IO and acted to reduce the Chl in these regions. In negative IOD, reversals in the forcing anomalies drove increasing Chl in Bay of Bengal and South IO and decreasing of that in the eastern equator. This resulted in significant correlations between IOD index and Chl confined in these regions. There are pronounced variance of phytoplankton carbon biomass in the Bay of Bengal in IOD events, however, less changes of that were seen over the South IO during IOD. Our findings imply that physiological changes in cellular pigmentation are the dominant cause of satellite-observed interannual variations in Chl in the South IO. Our results highlight the role of physiological processes in regulating Chl distributions, and support the use of Chl-based indices as indicators of climatic feedbacks.

Ocean Currents Reconstruction from a Combination of Altimeter and Ocean Colour Data: A Feasibility Study

Measuring the ocean surface currents at high spatio-temporal resolutions is crucial for scientific and socio-economic applications. Since the early 1990s, the synoptic and global-scale monitoring of the ocean surface currents has been provided by constellations of radar altimeters. By construction, altimeter constellations provide only the geostrophic component of the marine surface currents. In addition, given the effective spatial-temporal resolution of the altimeter-derived products (O (100 km) and O (10 days), respectively), only the largest ocean mesoscale features can be resolved. In order to enhance the altimeter system capabilities, we propose a synergistic use of high resolution sea surface Chlorophyll observations (Chl) and altimeter-derived currents’ estimates. The study is focused on the Mediterranean Sea, where the most energetic signals are found at spatio-temporal scales up to 10 km and a few days. The proposed method allows for inferring the marine surface currents from the evolution of the Chl field, relying on altimeter-derived currents as a first-guess estimate. The feasibility of this approach is tested through an Observing System Simulation Experiment, starting from biogeochemical model outputs distributed by the European Copernicus Marine Service. Statistical analyses based on the 2017 daily data showed that our approach can improve the altimeter-derived currents accuracy up to 50%, also enhancing their effective spatial resolution up to 30 km. Moreover, the retrieved currents exhibit larger temporal variability than the altimeter estimates over annual to weekly timescales. Our method is mainly limited to areas/time periods where/when Chl gradients are larger and are modulated by the marine currents’ advection. Its application is thus more efficient when the surface Chl evolution is not dominated by the biological activity, mostly occurring in the mid-February to mid-March time window in the Mediterranean Sea. Preliminary tests on the method applicability to satellite-derived data are also presented and discussed.

Role of riverine inputs, low saline plume advection and mesoscale physical processes in structuring the Chlorophyll a distribution in the western Bay of Bengal during Fall Inter Monsoon

This study delineates the role of small and medium river inputs, Low Saline Plume Advection (LSPA) and eddies in hydrography alteration and Chlorophyll a (Chl. a) in the Western Bay of Bengal. Samples were collected across five transects viz: Hooghly (HO), Mahanadi (MN), Rushikulya (RK), Visakhapatnam (VSKP) and Godavari (GD) during Fall Intermonsoon. Each transect consists of 7 or 8 locations from inshore to offshore. LSPA propagates southward concordance with the East India Coastal Current (EICC) and its southward flow strengthened by a cold-core eddy. LSPA results in the intermittent low salinity in the cross-shore section of HO, MN and RK. Upper layer Chl. a is 2–3 folds higher in inshore and in LSP-influenced locations than in its adjacent stations. The present study identified Double Chlorophyll a Maxima (DoCM) in LSPA-influenced slope regions of MN and RK. DoCM is less known in the BoB. DoCM has both the Surface Chl. a Maxima (SCM) and Subsurface Chl. a Maxima (SSCM). SSCM layer is relatively shallow and intense in slope and offshore regions of MN and RK due to their closeness with cold-core eddy. The present study highlights that freshwater discharge from small and medium rivers impacts hydrobiology around 10–50 km from the shore depends on the magnitude of river influx. LSPA is away from the local inputs and impacts hydrobiology (>500 km) along the path. EICC and eddies together regulated the direction of LSPA. Existing eddies nature alters vertical hydrobiology in slope and offshore regions.

Characterizing the vertical phytoplankton distribution in the Philippine Sea off the northeastern coast of Luzon

The vertical distribution of phytoplankton in the open ocean shows an increase in biomass at a depth referred to as the Subsurface Chlorophyll Maximum (SCM) that contributes significantly to the primary production of the water column. Hence, it is important to understand the dynamics that lead its formation and maintenance. This study examines the SCM in the Philippine Sea off the northeast coast of Luzon, utilizing bio-optical and empirical phytoplankton data from two oceanographic cruises conducted northeast of the island of Luzon in May/June 2011 and April/May 2012. Chlorophyll (Chl) profiles were converted to smoothed chlorophyll functions by using a b-spline basis. In 2011, the mean SCM depth was 97.24 m ± 22.33 m with mean SCM concentration of 0.43 μg/L ± 0.09 μg/L while in 2012, mean SCM was deeper at 115.45 m ± 24.25 m and mean SCM concentration of 0.31 ± 0.09 μg/L. Functional principal component analysis showed that the first principal component (PC) explained variability in the SCM depth, the second PC showed variability in the magnitude of the SCM concentration while the third PC accounted for the presence of multiple peaks. K-means clustering using the principal components resulted in three clusters which represented the offshore stations with the deepest SCM, stations within an observed cyclonic eddy with intermediate SCM and stations with coastal and shelf waters showing shallow SCM. Correlation analyses between Chl and physico-chemical and bio-optical parameters showed that Chl was positively correlated to beam attenuation, a bio-optical property that has been used as an alternative proxy for phytoplankton. This suggests that the observed SCMs represent actual increase in phytoplankton biomass. When the influence of the Kuroshio recirculation gyre was dominant in 2011, cooler temperature in surface waters was seen to significantly increase surface Chl. In 2012, highly saline waters from the tropical North Equatorial Current (NEC) waters appeared to lower the Chl distribution, particularly at the SCM. Phytoplankton abundance was recorded to be higher at the SCM than the surface in both years. In 2011, different species of diatoms dominated all clusters, except at the SCM of the coastal and shelf cluster wherein the dinoflagellate Gyrodinium grossestriatum was dominant. Most dominant species from 2011 were conspicuously absent in 2012 and there was a shift to the diatoms Fragilariopsis (surface), Thalassiosira and Rhizosolenia spp. in all clusters. These provide new insights on the phytoplankton community in relation to the changes in the oceanic circulation from subtropical North Pacific water in 2011 to tropical NEC water in 2012.

Short-Term Response of Chlorophyll a Concentration Due to Intense Wind and Freshwater Peak Episodes in Estuaries: The Case of Fangar Bay (Ebro Delta)

Estuaries and coastal bays are areas of large spatio-temporal variability in physical and biological variables due to environmental factors such as local wind, light availability, freshwater inputs or tides. This study focuses on the effect of strong wind events and freshwater peaks on short-term chlorophyll a (Chl a) concentration distribution in the small-scale and microtidal, Fangar Bay (Ebro Delta, northwestern Mediterranean). The hydrodynamics of this bay are primarily driven by local wind episodes modulated by stratification in the water column. Results based on field-campaign observations and Sentinel-2 images revealed that intense wind episodes from both NW (offshore) and NE-E (onshore) caused an increase in the concentration of surface Chl a. The mechanisms responsible were horizontal mixing and the bottom resuspension (also linked to the breakage of the stratification) that presumably resuspended Chl a containing biomass (i.e., micropyhtobentos) and/or incorporated nutrients into the water column. On the other hand, sea-breeze was not capable of breaking up the stratification, so the chlorophyll a concentration did not change significantly during these episodes. It was concluded that the mixing produced by the strong winds favoured an accumulation of Chl a concentration, while the stratification that causes a positive estuarine circulation reduced this accumulation. However, the spatial-temporal variability of the Chl a concentration in small-scale estuaries and coastal bays is quite complex due to the many factors involved and deserve further intensive field campaigns and additional numerical modelling efforts.

Impact of air emissions from shipping on marine phytoplankton growth

With the rapid expansion of maritime traffic, increases in air emissions from shipping have exacerbated numerous environmental issues, including air pollution and climate change. However, the effects of such emissions on marine biogeochemistry remain poorly understood. Here, we collected ship-emitted particles (SEPs) from the stack of a heavy-oil-powered vessel using an onboard emission test system and investigated the impact of SEPs on phytoplankton growth over the northwest Pacific Ocean (NWPO). In SEP microcosm experiments conducted in oceanic zones with different trophic statuses, the phytoplankton response, as indicated by chlorophyll a (Chl a), has been shown to increase with the proportion of SEP-derived nitrogen (N) relative to N stocks (PSN) in baseline seawater, suggesting that SEPs generally promote phytoplankton growth via N fertilisation. Simulations using an air quality model combined with a ship emission inventory further showed that oxidised N (NOx) emissions from shipping contributed ~43% of the atmospheric N deposition flux in the NWPO. Air emissions from shipping (e.g. NOx and sulphur dioxide) also indirectly enhanced the deposition of reduced N that existed in the atmosphere, constituting ~15% of the atmospheric N deposition flux. These results suggest that the impact of airborne ship emissions on atmospheric N deposition is comparable to that of land-based emissions in the NWPO. Based on the ship-induced PSN in surface seawater calculated by modeling results and World Ocean Atlas 2013 nutrient dataset, and the well-established quantitative relationship between Chl a and PSN obtained from microcosm experiments, we found a noticeable change in surface Chl a concentrations due to N deposition derived from marine traffic in the NWPO, particularly in the coastal waters of the Yellow Sea and open oceans. This work attempts to establish a direct link between marine productivity and air emissions from shipping.