Surface Phytoplankton Blooms Research Articles

Reservoir Drawdown Highlights the Emergent Effects of Water Level Change on Reservoir Physics, Chemistry, and Biology

AbstractWater level drawdowns are increasingly common in lakes and reservoirs worldwide as a result of both climate change and water management. Drawdowns can have direct effects on physical properties of a waterbody (e.g., by altering stratification and light dynamics), which can interact to modify the waterbody's biology and chemistry. However, the ecosystem‐level effects of drawdown remain poorly characterized in small, thermally stratified reservoirs, which are common in many regions of the world. Here, we intensively monitored a small eutrophic reservoir for 2 years, including before, during, and after a month‐long drawdown that reduced total reservoir volume by 36%. During drawdown, stratification strength (maximum buoyancy frequency) and surface phosphate concentrations both increased, contributing to a substantial surface phytoplankton bloom. The peak in phytoplankton biomass was followed by cascading changes in surface water chemistry associated with bloom degradation, with sequential peaks in dissolved organic carbon, dissolved carbon dioxide, and ammonium concentrations that were up to an order of magnitude higher than the previous year. Dissolved oxygen concentrations substantially decreased in surface waters during drawdown (to 41% saturation), which was associated with increased total iron and manganese concentrations. Combined, our results illustrate how changes in water level can have cascading effects on coupled physical, chemical, and biological processes. As climate change and water management continue to increase the frequency of drawdowns in lakes worldwide, our results highlight the importance of characterizing how water level variability can alter complex in‐lake ecosystem processes, thereby affecting water quality.

Changjiang and Kuroshio contributions to oxygen depletion on the Zhejiang Coast

In recent decades, intensified anthropogenic activities have resulted in increasing occurrence of hypoxia in the East China Sea. Kuroshio, as a natural factor, also threatens the oxygen content over the continental shelf. There have been many studies investigating the different contributions of Changjiang and Kuroshio to oxygen depletion over the continental shelf. This study revisited this issue and further investigated the mechanisms controlling the different role of Changjiang and Kuroshio in oxygen depletion and focused mainly on the Zhejiang Coast. A coupled high-resolution physical-biogeochemical model was used to investigate the connections between the variations in nutrients, chlorophyll, stratification, and oxygen and the delivery of Changjiang diluted water and Kuroshio subsurface water over the shelf, especially on the Zhejiang Coast in the summer of 2017. The distinct features of hypoxia off the Changjiang estuary (severe but transient) and that along the Zhejiang Coast (mild but prolonged) are caused by the different dynamic environments and nutrients sources. North of 30˚N, intense oxygen depletion and bottom hypoxia are typically under the constraint of Changjiang diluted water. While the impacts of upwelled materials associated with the Kuroshio subsurface water enhance southward with the simultaneously weakened impacts from the Changjiang diluted water. Besides confirming the support of upwelling on surface phytoplankton bloom along the Zhejiang Coast, this study detected subsurface chlorophyll maximum immediately underneath the main pycnocline offshore of the Zhejiang Coast during upwelling. This indicated that the upwelled oceanic nutrients were transported further offshore along isopycnals and also fertilized phytoplankton growth at the subsurface. The exacerbation of either anthropogenic or natural factors could potentially intensify oxygen depletion along the Zhejiang Coast.

Changes in Nutrient Concentration Resulting From Floods and Their Impact on the Estuary–Sea Continuum

AbstractEvaluating the contribution of rivers to coastal seas in the Earth system and accurately predicting marine environmental responses to climate change are crucial for mitigating and addressing future natural disasters, especially those related to extreme hydrological events, such as fluvial floods. The Changjiang (Yangtze) River Estuary (CRE) and the East China Sea continuum, known for high environmental stress, complex marine dynamic processes, and frequent flood events, were selected to determine the compound impacts of a severe flood that persisted for the whole of July 2017. We conducted three cruise surveys before and after this flood event. Our surveys revealed that the phosphate concentration in the channel of the CRE declined from 2 to <1 μM during the flood, while the nitrate concentration remained stable. We applied a coupled physical‐biogeochemical model to determine the spatio‐temporal influence of floodwater (freshwater from upstream during flood events). During the initial rising discharge stage, surface phytoplankton blooms triggered by enhanced stratification consumed phosphate and significantly increased the N:P ratio in the sea. The ratio was restored as the blooms diminished, but remained elevated due to lower phosphate floodwater in the northeastward area within the low‐salinity river plume. Model experiments also showed that phytoplankton biomass and N:P ratio in the flood plume region were sensitive to the riverine phosphate concentration inputs during the flood event.

Contrasting sea ice conditions shape microbial food webs in Hudson Bay (Canadian Arctic)

The transition from ice-covered to open water is a recurring feature of the Arctic and sub-Arctic, but microbial diversity and cascading effects on the microbial food webs is poorly known. Here, we investigated microbial eukaryote, bacterial and archaeal communities in Hudson Bay (sub-Arctic, Canada) under sea-ice cover and open waters conditions. Co-occurrence networks revealed a <3 µm pico‒phytoplankton-based food web under the ice and a >3 µm nano‒microphytoplankton-based food web in the open waters. The ice-edge communities were characteristic of post-bloom conditions with high proportions of the picophytoplankton Micromonas and Bathycoccus. Nano‒ to micro‒phytoplankton and ice associated diatoms were detected throughout the water column, with the sympagic Melosira arctica exclusive to ice-covered central Hudson Bay and Thalassiosira in open northwestern Hudson Bay. Heterotrophic microbial eukaryotes and prokaryotes also differed by ice-state, suggesting a linkage between microbes at depth and surface phytoplankton bloom state. The findings suggest that a longer open water season may favor the establishment of a large phytoplankton-based food web at the subsurface chlorophyll maxima (SCM), increasing carbon export from pelagic diatoms to deeper waters and affect higher trophic levels in the deep Hudson Bay.

Occurrence of Surface Phytoplankton Bloom as the Kuroshio Current Passes an Island

AbstractWhen the Kuroshio Current passes by islands, a surface phytoplankton bloom is often observed in the wake of the island. To investigate its causes, a typical example from the Kuroshio region, in which a geostrophic current (1 m/s) passes around a cylindrical island, was simulated using a three‐dimensional biogeochemical model. At the surface, concentrations of dissolved inorganic nitrogen, phytoplankton, and zooplankton increased in the wake. In the euphotic zone, phytoplankton mainly grew in cyclonic eddies, where nutrients originated from the deep ocean. The sum of biogeochemical processes related to the phytoplankton increased immediately behind the island and had a maximum of 0.045 mmol N m−3 day−1 at 50 km downstream, which gradually decreased to a low value further from the island. Among four types of phytoplankton, the pico‐phytoplankton‐dominated the bloom behind the island, which was also observed in the Tokara Strait. A tracer run indicated that the hydrodynamic processes directly contributed to 50% of the increase in the phytoplankton concentration on the surface in summer and a decrease in winter, depending on the appearance of a subsurface chlorophyll‐a maximum. With an increase in island size, the size of eddies, distance between eddies, and surface phytoplankton concentration also increased. For the first 300 km behind the island, no obvious zooplankton growth was observed, but a box eNEMURO result indicated that trophic transfer from phytoplankton to zooplankton emerged further away, which provides a possible explanation of Kuroshio Paradox.

Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

Abstract. Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these is that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighbouring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. We identify the onset of the surface phytoplankton growth as concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic carbon derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favours carbon uptake by phytoplankton lasting a few days. Simultaneously, the air–sea net heat flux switches from negative to positive, linked to changes in the latent air–sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the surface phytoplankton bloom. We conclude that decreases in the mixing- and mixed-layer depths lead to the onset of the phytoplankton growth due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 d between 2016 and 2019 as between 38 and 191 mmol C m−2. These results have important implications, as biological processes play a major role in the seasonal evolution of surface pCO2 and thereby the rate of reduction in atmospheric CO2 by exchange at the air–sea interface.

Variability of surface and subsurface phytoplankton blooms in a seasonal coastal upwelling system

Understanding how phytoplankton respond to their physical environment is key to predicting shifts in bloom dynamics under a changing climate. This research investigates phytoplankton dynamics in a seasonal coastal upwelling system of the eastern Great Australian Bight (GAB), South Australia, using >10 years of in-situ mooring and CTD observations together with >18 years of satellite and wind data. Earlier studies concluded that upwelling in the region is driven by interactions between upwelling-favourable winds associated with synoptic weather events during austral summer months (Jan–March) and the formation of a pool of nutrient-rich shelf-bottom water on the Lincoln Shelf, known as the Kangaroo Island (KI) Pool. Surface phytoplankton blooms in the upwelling centre, which form off the southern tip of the Eyre Peninsula, are characterized by chlorophyll-a levels of ∼1–2 mg/m3. Our analysis of satellite chl-a data indicates the absence of surface phytoplankton blooms in the upwelling centre in more than a third of all upwelling seasons, despite the existence of statistically similar upwelling-favourable winds. The KI Pool is located ∼100 km upstream from the upwelling centre in a marine region that generally displays very low surface phytoplankton levels. Analysis of water-column data indicates strong interannual variability the KI Pool that is correlated with the intensity of surface phytoplankton blooms in the upwelling centre. The analysis also reveals the existence of substantial (∼1 mg/m3 chl-a) subsurface phytoplankton blooms at depths between ∼20 and 70 m in shallower waters of the Lincoln Shelf, where nutrient-rich shelf-bottom water of the KI Pool reaches into the euphotic zone. Our data suggest that subsurface phytoplankton blooms, forming at depths that cannot be detected by satellites, represent a major driver of the upwelling ecosystem of the eastern GAB.

Comparison of physical and biological responses to tropical cyclones between the low and middle latitude zones of the western North Pacific

Upper ocean conditions vary considerably between latitudes, which results in different responses of the ocean to tropical cyclones (TCs). During the TC season, many TCs in the western North Pacific (WNP) extend northward from the low latitude zone (LLZ) to the middle latitude zone (MLZ). We studied the differences in the influence of TCs on the LLZ and MLZ by comparing the changes in the mixed-layer depth (MLD), sea surface temperature (SST), and chlorophyll-a (Chl-a) concentrations before and after 32 typical TCs that occurred from 2006 to 2019 in these two regions. Although the mean intensity and transit time of these TCs in the MLZ were lower than in the LLZ, the mean MLD deepening was greater, the SST cooling was more significant, and the surface Chl-a concentration increases were more pronounced in the MLZ after TCs. Differences in the responses of these two WNP regions to TCs were directly related to the differences in their upper ocean conditions. Compared to the LLZ, the MLZ had a shallower MLD, nutricline, and subsurface chlorophyll maxima depth, lower temperature, and higher nutrient concentrations in the subsurface layers. The upper ocean conditions in this region were relatively more susceptible to alteration after TCs, and the subsurface water, nutrients, and phytoplankton were more readily brought to the surface, largely contributing to the SST cooling and surface phytoplankton bloom. In particular, as the latitude increases, the degree of response of MLD, SST and surface Chl-a concentration after TCs may be more pronounced. These findings indicate that differences in responses to TCs in ocean areas at different latitudes were mainly due to differences in upper ocean physical and biochemical structure rather than to intensity and transit time of TCs.

Spatial Distributions of Riverine and Marine Dissolved Organic Carbon in the Western Arctic Ocean: Results From the 2018 Korean Expedition

AbstractSeasonal primary production and river discharge increases in the Arctic Ocean exert a significant influence on the dissolved organic carbon (DOC) cycle. To improve our knowledge of the spatial heterogeneity of DOC source and concentration in the rapidly changing Arctic Ocean, we investigated the distributions of riverine and marine DOC in the western Arctic Ocean during the summer of 2018. Although the surface bulk DOC concentration indicated no clear distinction in its distribution between the Chukchi Borderland (CBL)/northern Chukchi Sea (NCS) and East Siberian Sea (ESS)/Mendeleyev Ridge (MR) regions, the estimated riverine DOC concentration (28 ± 4.2 μM C) and its contribution (40 ± 5.7%) in the surface layer of the CBL/NCS region were higher than those (19 ± 5.6 μM C and 26 ± 8.5%) in the ESS/MR region, which was attributed to the accumulation of freshwater, strong stratification, and a longer residence time in the CBL/NCS region. In contrast, although marine DOC was the dominant DOC component in both the CBL/NCS and ESS/MR regions, the higher marine DOC concentration (54 ± 8.1 μM C) and its contribution (73 ± 8.2%) in the East Siberian shelf/slope region were consistent with high bacterial abundance, which was associated with extremely high surface phytoplankton blooms sustained by nutrient supply from the deep layer, suggesting that the supply of bioavailable DOC resulted in active bacterial activities. Overall, the spatial differences in water properties between the two regions had large influences on the regional distributions of riverine and marine DOC.

Phytoplankton distribution related to different winter conditions in 2016 and 2017 in the open southern Adriatic Sea (eastern Mediterranean)

Phytoplankton community structure and dynamics were investigated in the open southern Adriatic Sea during two winter-spring seasons (2016 and 2017) under different oceanographic and meteorological conditions. The principal environmental factor was a pronounced inflow of the Levantine Intermediate Water into the Adriatic, favoured by the cyclonic circulation of the Northern Ionian Gyre. As a consequence, high salinity values of about 38.8–38.9, were registered. Fifteen research cruises were undertaken in the same sampling area situated in the northern part of the Southern Adriatic Pit (SAP) with a maximum depth of 1200 m. Two specific circumstances were encountered: (i) high abundances of phytoplankton in the deep layer associated with strong downward flow in 2016, and (ii) an intense surface phytoplankton bloom in March 2017. This particular event occurred following the strong vertical convective mixing, which increased nutrient availability in the euphotic layer. High in situ Chl-a concentrations (max. 1.65 mg m−3) and phytoplankton abundances higher than 105 cells L−1 are not common in the habitually oligotrophic open southern Adriatic Sea, and this study attempts to determine the driving force of that phenomenon. The phytoplankton community in SAP involved nanoflagellates and diatoms as the most abundant taxonomic groups during the March 2017 bloom. Presence of some coastal phytoplankton taxa suggests the influx of coastal water masses and transport of species either longitudinally, from the northern to the Southern Adriatic in concomitance of the Northern Adriatic Dense Water spreading, or transversal as a response to the strong mesoscale activity in the study region, comprising intense shelf - open sea interaction.

Increasing Hydrostatic Pressure Impacts the Prokaryotic Diversity during Emiliania huxleyi Aggregates Degradation

In the dark ocean, the balance between the heterotrophic carbon demand and the supply of sinking carbon through the biological carbon pump remains poorly constrained. In situ tracking of the dynamics of microbial degradation processes occurring on the gravitational sinking particles is still challenging. Our particle sinking simulator system (PASS) intends to mimic as closely as possible the in situ variations in pressure and temperature experienced by gravitational sinking particles. Here, we used the PASS to simultaneously track geochemical and microbial changes that occurred during the sinking through the mesopelagic zone of laboratory-grown Emiliania huxleyi aggregates amended by a natural microbial community sampled at 105 m depth in the North Atlantic Ocean. The impact of pressure on the prokaryotic degradation of POC and dissolution of E. huxleyi-derived calcite was not marked compared to atmospheric pressure. In contrast, using global O2 consumption monitored in real-time inside the high-pressure bottles using planar optodes via a sapphire window, a reduction of respiration rate was recorded in surface-originated community assemblages under increasing pressure conditions. Moreover, using a 16S rRNA metabarcoding survey, we demonstrated a drastic difference in transcriptionally active prokaryotes associated with particles, incubated either at atmospheric pressure or under linearly increasing hydrostatic pressure conditions. The increase in hydrostatic pressure reduced both the phylogenetic diversity and the species richness. The incubation at atmospheric pressure, however, promoted an opportunistic community of “fast” degraders from the surface (Saccharospirillaceae, Hyphomonadaceae, and Pseudoalteromonadaceae), known to be associated with surface phytoplankton blooms. In contrast, the incubation under increasing pressure condition incubations revealed an increase in the particle colonizer families Flavobacteriaceae and Rhodobacteraceae, and also Colwelliaceae, which are known to be adapted to high hydrostatic pressure. Altogether, our results underline the need to perform biodegradation experiments of particles in conditions that mimic pressure and temperature encountered during their sinking along the water column to be ecologically relevant.

Using the Red Band Difference Algorithm to Detect and Monitor a Karenia spp. Bloom Off the South Coast of Ireland, June 2019

During the months of May, June, July and August 2019 the Red Band Difference algorithm was tested over Irish waters to assess its suitability for the Irish harmful algal bloom alert system. Over the 4 weeks of June an extensive localised surface phytoplankton bloom formed in the Celtic Sea, south of Ireland. Satellite imagery from the Sentinel-3a’s Ocean and Land Colour Instrument, processed using the Red Band Difference algorithm detected the bloom in surface shelf waters and helped monitor its movement. Daily satellite images indicated that the bloom appeared at the sea surface on the 2nd June 2019 and peaked in size and surface abundance in offshore shelf waters within 4 weeks, remnants remained at the surface into July. A particle tracking approach was used to replicate oceanic circulation patterns in the vicinity of the observed algal bloom and estimate its trajectory. The initial horizontal distribution of particles in the tracking model were based on a satellite imagery polygon of the bloom when it first appeared in surface waters. Good agreement was observed between satellite imagery of the bloom and the particle tracking model. In situ sampling efforts from a research cruise and the national inshore phytoplankton monitoring programme confirmed that Karenia mikimotoi was the causative organism of the bloom. This pilot study shows great potential to use the Red Band Difference algorithm in the existing Irish harmful algal bloom alert system. In addition, satellite ocean colour data combined with particle tracking model estimates can be a useful tool to monitor high biomass harmful algal bloom forming species, such as Karenia mikimotoi, in surface coastal waters around Ireland and elsewhere.

Atlantic‐Origin Cold Saline Water Intrusion and Shoaling of the Nutricline in the Pacific Arctic

AbstractAtlantic‐origin cold saline water has previously not been considered an important contributor to the nutrient supply in the Pacific Arctic due to the effective insulation by the overlying Pacific‐origin waters that separate the surface mixed layer from the deeper Atlantic Water. Based on hydrographic observations in the northwestern Chukchi Sea from 2015 to 2017, we demonstrate that the intrusion of Atlantic‐origin cold saline water into the halocline boundary between Pacific and Atlantic‐origin waters in 2017 lifted Pacific‐origin nutrients up to the surface layer. We find that the cyclonic atmospheric circulation in 2017 was considerably strengthened, leading to lateral intrusions of two bodies of cold halocline water from the Eurasian marginal seas into the northwestern Chukchi Sea. Our results reveal that the intrusions of cold halocline waters caused unprecedented shoaling of the nutricline and anomalously high surface phytoplankton blooms in typically highly oligotrophic surface waters in the region during summer.

Upper-ocean physical and biological features associated with Hudhud cyclone: A bio-physical modelling study

Abstract The east coast of India is highly prone towards the devastation by the cyclonic storms. The Bay of Bengal (BoB) basin in the northeastern Indian Ocean is characterized to provide adequate conditions to the cyclogenesis. The cyclogenesis is more active during pre- and post-monsoon seasons over the BoB. The post-monsoon season shows highest storm occurrence in the region due to presence of warm and freshwater in the upper-ocean which helps to maintain deep barrier layer and supports cyclogenesis. The very severe cyclonic storm, Hudhud was a post-monsoonal storm occurred during 07th–14th October 2014 over the BoB. Hudhud made landfall near Visakhapatnam coast at noon of 12th October 2014. The present study utilized a three dimensional coupled bio-physical model for the analysis of biogeochemical response of the upper-ocean to Hudhud cyclone. The different surface and subsurface physical and biological features along the track of Hudhud has depicted a cyclone-generated surface phytoplankton bloom and its associated decline of surface dissolved oxygen. The study aims at a better understanding of the spatial and temporal variability of the upper-ocean parameters in response to the passage of tropical cyclone (TC) Hudhud. It is found that the intense cyclonic wind stress caused upwelling in the wake of cyclone track about 1550 km from its genesis location. The excess surface freshwater flux due to precipitation establishes stronger stratification in the coastal region. This enhanced stratification found to restrict the supply of nutrients to the euphotic zone and, hence, limiting the surface primary productivity in the coastal region.

Intense deposition and rapid processing of seafloor phytodetritus in a glaciomarine fjord, Andvord Bay (Antarctica)

The West Antarctic Peninsula (WAP) region encompasses numerous fjords known to be hotspots of benthic biodiversity and biomass. These ecosystems are undergoing rapid climate warming and oceanographic change; consequent glacial melt and retreat may dramatically alter benthic communities and ecosystem functions in WAP fjords. In addition, there is extreme variability in seasonal productivity and phytodetrital food input to the deep shelf benthos of the WAP. Here we document the flux and utilization of phytodetritus at 530 m depth in Andvord Bay, a WAP glaciomarine fjord, using seafloor time-lapse images spanning a 10-month period, from December 2015 to September 2016. To explore the relationship between sea-surface conditions and detrital export, we developed a color-based method to quantify seafloor phytodetritus cover in time-lapse images, and correlated phytodetritus cover with sea-ice cover and surface wind speed. During the most rapid period of phytodetritus accumulation in early January 2016, approximately 3 cm of phytodetritus was deposited on the fjord floor over six days, representing a large input of organic carbon to the benthos. The timing of this rapid export event was not related to overlying wind conditions and occurred when the fjord was ice-free. To assess the response of the megafaunal community, we measured fecal-pellet production by the dominant surface deposit-feeder, the ampharetid polychaete Amythas membranifera. The deposit-feeding rate of A. membranifera increased substantially during high seafloor phytodetritus cover and returned to background levels for the rest of the autumn and winter. Ampharetids and mobile megafauna were observed to feed throughout the entire time-series, including beneath winter sea ice, suggesting that the delivery of phytodetritus from surface phytoplankton blooms in spring/summer sustains these abundant populations year-round with a sediment food bank. A. membranifera can reach densities of >37 m−2 and, as a population, processes a volume equivalent to the top 1.05 cm of sediment annually. The maximum individual feeding rates measured are comparable to some temperate intertidal deposit feeders. Nonetheless, phytodetritus on 31–73% of the seafloor was consumed by macrofauna or microbial populations not visible in time-lapse photographs, suggesting that recently deposited phytodetritus is processed primarily by these smaller size classes. This study demonstrates the seasonal coupling and subsequent decoupling of detritivore activity in response to a massive detrital deposition event in a deep, subpolar WAP fjord. This work indicates that both megafaunal deposit feeders and smaller size classes play important roles in processing labile organic matter in WAP fjord ecosystems.

Metabolic response of prokaryotic microbes to sporadic hypoxia in a eutrophic subtropical estuary

Coastal eutrophication and consequent oxygen depletion (hypoxia) occurs worldwide due to increased human activity. The paucity of genomic information of microbes in hypoxia prone coastal waters have hindered our understanding of microorganism related causation and adaption to the environment. Here, using metagenomic approach, we investigated microbial metabolic capability in heavily polluted Pearl River estuary. Our results highlighted the possible roles of microbial metabolic activity in the formation of bottom water hypoxia by revealing enriched organic degradation related microbial genes in the bottom layer beneath surface phytoplankton bloom. Microbial nitrate reduction in hypoxia layer was low, possibly due to the low pH and fluctuating oxygen level. On contrary, high abundance of sulfate-reducing, and antibiotic and metal resistance related genes were detected in bottom and surface layers, respectively, indicating microbial adaptation to oxygen depletion and pollution. Our study provides gene level information on the interactive relations between microbial functions and environmental stress.

Detection of Phytoplankton Blooms in the Turbid Coastal Waters Using Satellite-Derived Fluorescence Line Height off Kakinada Coast

Fluorescence line height (FLH) is a relative measure of the amount of radiance leaving the sea surface in the chlorophyll fluorescence emission band. Satellite-derived FLH images provide information about the surface chlorophyll distribution which in turn can be used for monitoring surface phytoplankton blooms in coastal waters. The present study aims at observing a phytoplankton bloom in the Kakinada coastal waters along the east coast of India using nFLH product of MODIS-A. This quasi-permanent bloom could not be precisely observed using chlorophyll concentration in the turbid coastal waters due to the presence of other optically active constituents; hence, chlorophyll fluoresce is used to study the bloom phenomenon from nFLH product of MODIS. The analysis has been carried out by analysing the spectral variability using both in situ and satellite Rrs spectra over the bloom and non-bloom waters. Comparative study between in situ chlorophyll with satellite-derived FLH showed a good correlation with an R2 of 0.43. The bloom phenomenon observed/assessed from the MODIS nFLH data showed > 0.25 W m−2 sr−1 μm in the bloom-dominated pixels. This proves that the satellite-derived nFLH can also be used as an indicator in detecting the phytoplankton blooms and also as one of the important parameters for the ocean colour applications in case-II waters.

Regional differences and inter-annual variability in the timing of surface phytoplankton blooms in the Labrador Sea

The annual phytoplankton bloom is an important marine event and its inter-annual variability can be used to monitor changes in the pelagic ecosystem. In this context, the use of a bio-regionalization analysis to objectively identify ocean areas that differ in their biological characteristics and environmental conditions may be useful to disentangle the functioning of the ecosystem. In this work, satellite surface chlorophyll-a concentration observations were used to obtain a bio-regionalization of the Labrador Sea (LS). Specifically, bioregions were defined using a K-means cluster analysis applied on the satellite time-series of chlorophyll-a concentration. Finally, the resulting biologically regionalization was used in conjunction with simulated data from a state-of-the-art ocean circulation model (NEMO) to investigate difference in spring bloom timing response across the Labrador Sea (LS). Chlorophyll-a seasonal cycle in the LS varies markedly between two neighboring but distinct bioregions: the north (>60 °N) and the south (<60 °N). The north LS blooms earlier (∼early-mid April) and more intensely, while the south LS blooms later (∼May) and its duration may persist all summer long. In the southern LS bioregion, the maximum phytoplankton growth rate (i.e., the initiation of the surface spring bloom) coincides closely with the timing of the first cooling-to-heating shift in air-sea heat fluxes. In this area, the bloom timing tends to precede the shoaling of the mixed-layer depth and the vernal development of the upper ocean stratification. In the southern bioregion, the cooling-to-heating shift in air-sea heat fluxes provided a more suitable indicator for the spring bloom initiation. Meanwhile, in the northern LS bioregion, the early onset of the spring bloom is related to the seasonal evolution (i.e., shoaling) of the mixed-layer depth and precedes the cessation of wintertime cooling. Over the northern bioregion, freshwater exchanges may suppress vertical mixing while the air-sea fluxes are still negative (i.e., ocean cooling). Given the heterogeneity of the LS basin, this study emphasizes the importance to consider the bioregion-specific differences in the upper ocean physical-biological interactions.

Determination of phytoplankton abundances (Chlorophyll-a) in the optically complex inland water - The Baltic Sea

A novel approach, termed Summed Positive Peaks (SPP), is proposed for determining phytoplankton abundances (Chlorophyll-a or Chl-a) and surface phytoplankton bloom extent in the optically complex Baltic Sea. The SPP approach is established on the basis of a baseline subtraction method using Rayleigh corrected top-of-atmosphere data from the Medium Resolution Imaging Spectrometer (MERIS) measurements. It calculates the reflectance differences between phytoplankton related signals observed in the MERIS red and near infrared (NIR) bands, such as sun-induced chlorophyll fluorescence (SICF) and the backscattering at 709nm, and considers the summation of the positive line heights for estimating Chl-a concentrations. The SPP algorithm is calibrated against near coincident in situ data collected from three types of phytoplankton dominant waters encountered in the Baltic Sea during 2010 (N=379). The validation results show that the algorithm is capable of retrieving Chl-a concentrations ranging from 0.5 to 3mgm−3, with an RMSE of 0.24mgm−3 (R2=0.69, N=264). Additionally, the comparison results with several Chl-a algorithms demonstrates the robustness of the SPP approach and its sensitivity to low to medium biomass waters. Based on the red and NIR reflectance features, a flagging method is also proposed to distinguish intensive surface phytoplankton blooms from the background water.

An advective mechanism for deep chlorophyll maxima formation in southern Drake Passage

AbstractWe observe surface and subsurface fluorescence‐derived chlorophyll maxima in southern Drake Passage during austral summer. Backscatter measurements indicate that the deep chlorophyll maxima (DCMs) are also deep biomass maxima, and euphotic depth estimates show that they lie below the euphotic layer. Subsurface, offshore and near‐surface, onshore features lie along the same isopycnal, suggesting advective generation of DCMs. Temperature measurements indicate a warming of surface waters throughout austral summer, capping the winter water (WW) layer and increasing off‐shelf stratification in this isopycnal layer. The outcrop position of the WW isopycnal layer shifts onshore, into a surface phytoplankton bloom. A lateral potential vorticity (PV) gradient develops, such that a down‐gradient PV flux is consistent with offshore, along‐isopycnal tracer transport. Model results are consistent with this mechanism. Subduction of chlorophyll and biomass along isopycnals represents a biological term not observed by surface satellite measurements which may contribute significantly to the strength of the biological pump in this region.