Subsurface Chlorophyll-a Maximum Research Articles

Fine-scale vertical relationships between environmental conditions and sound scattering layers in the Southwestern Tropical Atlantic.

Ocean dynamics initiate the structure of nutrient income driving primary producers, and these, in turn, shape the distribution of subsequent trophic levels until the whole pelagic community reflects the physicochemical structure of the ocean. Despite the importance of bottom-up structuring in pelagic ecosystems, fine-scale studies of biophysical interactions along depth are scarce and challenging. To improve our understanding of such relationships, we analyzed the vertical structure of key oceanographic variables along with the distribution of acoustic biomass from multi-frequency acoustic data (38, 70, and 120 kHz) as a reference for pelagic fauna. In addition, we took advantage of species distribution databases collected at the same time to provide further interpretation. The study was performed in the Southwestern Tropical Atlantic of northeast Brazil in spring 2015 and autumn 2017, periods representative of canonical spring and autumn conditions in terms of thermohaline structure and current dynamics. We show that chlorophyll-a, oxygen, current, and stratification are important drivers for the distribution of sound scattering biota but that their relative importance depends on the area, the depth range, and the diel cycle. Prominent sound scattering layers (SSLs) in the epipelagic layer were associated with strong stratification and subsurface chlorophyll-a maximum. In areas where chlorophyll-a maxima were deeper than the peak of stratifications, SSLs were more correlated with stratification than subsurface chlorophyll maxima. Dissolved oxygen seems to be a driver in locations where lower oxygen concentration occurs in the subsurface. Finally, our results suggest that organisms seem to avoid strong currents core. However, future works are needed to better understand the role of currents on the vertical distribution of organisms.

Estimation of the vertical phytoplankton distribution in the Philippine Sea: Influence of turbulence following passage of typhoons

The subsurface chlorophyll-a maximum (SCM) is a phenomenon that contributes significantly to the total primary production of the open ocean but it is not observable from remote sensing, thus primary production based on satellite information is highly underestimated. In a highly dynamic region such as the Philippine Sea, turbulence caused by tropical storms may exert significant impact on the SCM feature. In this study, we attempt to estimate the vertical phytoplankton profile in the Philippine Sea from remote sensing images by applying a generic quantitative approach. Generalized Additive Models (GAM) followed by Generalized Linear Models (GLMs) were used create predictive equations between response variables (Chl-a profile parameters) and predictor variables (RS parameters). GAM was able to predict integrated Chl-a biomass using photosynthetic active radiation (PAR), wind speed and wind stress, the depth of the Chl-a peak using surface Chl-a, wind speed and wind stress, and the baseline Chl-a concentration using sea surface temperature, sea surface salinity and PAR. GLM found wind stress and wind speed as significant predictors for integrated Chl-a biomass, while surface Chl-a, wind speed and wind stress were significant predictors for depth of the Chl-a peak. When the predictive equations were applied to 2020 monthly satellite images, they were seen to adequately estimate the offshore spatial distribution of the two Chl-a parameters. Increased turbulence due to high wind speed and wind stress during passage of tropical storms was seen to result in shallowing of the SCM and subsequent increase in Chl-a within the water column. These equations could be refined if long-term observational data was available. The capacity to estimate vertical distribution of primary productivity in the Philippines provides a means to better understand fisheries productivity and biogeochemical cycling in the region.

Effect of Anthropogenic Aerosol Addition on Phytoplankton Growth in Coastal Waters: Role of Enhanced Phosphorus Bioavailability.

Atmospheric deposition can supply nutrients to induce varying responses of phytoplankton of different sizes in the upper ocean. Here, we collected surface and subsurface chlorophyll a maximum (SCM) seawaters from the Yellow Sea and East China Sea to conduct a series of onboard incubation experiments, aiming to explore the impact of anthropogenic aerosol (AR, sampled in Qingdao, a coastal city in Northern China) addition on phytoplankton growth using schemes with (unfiltered seawater, UFS) and without (filtered seawater, FS) microsized (20–200 μm) cells. We found that AR addition stimulated phytoplankton growth obviously, as indicated by chlorophyll a (Chl a) in surface incubations, and had stimulatory or no effects in SCM incubations, which was related to nutrient statuses in seawater. The high ratio of nitrogen (N) to phosphorus (P) in the AR treatments demonstrated that P became the primary limiting nutrient. The alkaline phosphatase activity (APA), which can reflect the rate at which dissolved organic P (DOP) is converted into dissolved inorganic P, was 1.3–75.5 times higher in the AR treatments than in the control, suggesting that AR addition increased P bioavailability in the incubated seawater. Dinoflagellates with the capacity to utilize DOP showed the dominant growth in the AR treatments, corresponding to the shift in phytoplankton size structure toward larger cells. Surprisingly, we found that nanosized (2–20 μm) and picosized (0.2–2 μm) Chl a concentrations in UFS were generally higher than those in FS. The APA in UFS was at least 1.6 times higher than in FS and was proportional to the contribution of microsized cells to the total Chl a, suggesting that microsized cells play an important role in the increase in APA, which contributes to the growth of nanosized and picosized phytoplankton. Current work provides new insight into the increase of P bioavailability induced by atmospheric deposition and resultant ecological effect in coastal waters.

Subsurface phytoplankton vertical structure from lidar observation during SCS summer monsoon onset.

Subsurface phytoplankton vertical structure was observed for the first time by lidar during the onset of the SCS summer monsoon. Based on the lidar data that were obtained by continuous day-and-night measurements over a two-week period, a hybrid retrieval method to determine the vertical structure of the seawater chlorophyll-a concentrations using lidar data was proposed. We compared the data obtained from the lidar retrievals with the ocean color data and studied the spatial variations and hourly diurnal variations in the subsurface chlorophyll-a maximum layer (SCML). The significant changes in the depth of the SCML in the SCS may be due to the variations in light availability and nutrient supply during the onset of the SCS summer monsoon. The preliminary results indicated that lidar measurements allow the submesoscale oceanic dynamics mechanisms to be understood from a new perspective.

Spatial dynamics of active microeukaryotes along a latitudinal gradient: Diversity, assembly process, and co-occurrence relationships

Recent global warming is profoundly and increasingly influencing the Arctic ecosystem. Understanding how microeukaryote communities respond to changes in the Arctic Ocean is crucial for understanding their roles in the biogeochemical cycles of nutrients and elements. Between July 22 and August 19, 2016, during cruise ARA07, seawater samples were collected along a latitudinal transect extending from the East Sea of Korea to the central Arctic Ocean. Environmental RNA was extracted and the V4 hypervariable regions of the reverse transcribed SSU rRNA were amplified. The sequences generated by high throughput sequencing were clustered into zero-radius OTUs (ZOTUs), and the taxonomic identities of each ZOTU were assigned using SINTAX against the PR2 database. Thus, the diversity, community composition, and co-occurrence networks of size fractionated microeukaryotes were revealed. The present study found: 1) the alpha diversity of pico- and nano-sized microeukaryotes showed a latitudinal diversity gradient; 2) three distinct communities were identified, i.e., the Leg-A, Leg-B surface, and Leg-B subsurface chlorophyll a maximum (SCM) groups; 3) distinct network structure and composition were found in the three groups; and 4) water temperature was identified as the primary factor driving both the alpha and beta diversities of microeukaryotes. This study conducted a comprehensive and systematic survey of active microeukaryotes along a latitudinal gradient, elucidated the diversity, community composition, co-occurrence relationships, and community assembly processes among major microeukaryote assemblages, and will help shed more light on our understanding of the responses of microeukaryote communities to the changing Arctic Ocean.

High-Resolution Vertical Observations of Phytoplankton Groups Derived From an in-situ Fluorometer in the East China Sea and Tsushima Strait

Vertical distribution of phytoplankton composition in the East China Sea (ECS) and Tsushima Strait (TS) was highly variable in the region where the Changjiang River diluted water (CDW), Kuroshio water (KW), and Tsushima water (TW) intersected. An in-situ multiple excitation fluorometer was used to obtain the high-resolution phytoplankton groups data from every meter of the water column. Sharp differences were noted in the distribution of phytoplankton groups in the CDW, KW, and TW. In the CDW, brown algae were generally present ~60% of all depths with exception of subsurface chlorophyll-a maximum (SCM), whereas cyanobacteria (>40%) and green algae plus cryptophytes (>40%) were found above and below the SCM, respectively. In TW, where chlorophyll a (CHL) was lower than in the CDW, brown algae predominated the water column (>60%) and SCM (>80%), except the surface layer where cyanobacteria dominated. In KW, a high fraction of cyanobacteria (>40%) extended up to 40 m, while brown and green algae dominated (>60%) the deeper waters below 40 m at western and eastern stations, respectively. These results can be further related to water property and nutrient concentration of the water masses in each region. This new data show that the in-situ multiple excitation fluorometer can be a powerful tool to estimate high-resolution vertical profiles of phytoplankton groups on a large scale in marine environments.

Dominant physical-biogeochemical drivers for the seasonal variations in the surface chlorophyll-a and subsurface chlorophyll-a maximum in the Bay of Bengal

The seasonal variations and driving mechanisms of the surface and subsurface chlorophyll-a concentrations in the Bay of Bengal are far from resolved, as only a few local, short-term studies have been performed. Hence, this study investigates a comprehensive basin-wide framework of the seasonal variations in the chlorophyll-a concentration, its dominant external forcing, and the internal dynamics of the Bay of Bengal. Multivariate empirical orthogonal function decomposition and heterogeneous correlation analyses are applied to numerous observational, reanalysis, and satellite datasets, including chlorophyll-a, nutrients, temperature, salinity, turbidity, and wind stress curl datasets collected from various sources, including the Copernicus Marine Environment Monitoring Service, World Ocean Atlas, and ERA-Interim. This study suggests that the chlorophyll-a concentrations at both the surface and the subsurface chlorophyll-a maximum (SCM) are higher during summer and early autumn than during the other seasons, especially along the coastal regions and western part of the Bay of Bengal. During summer and early autumn, riverine nutrient inputs, the intrusion of nutritious water from the Arabian Sea, and coastal upwelling are the three dominant drivers controlling the chlorophyll-a concentrations at both the surface and the SCM. The positive wind stress curl-induced uplift of the thermocline increases the nutrient supply and thus significantly enhances the chlorophyll-a concentration at the SCM along the entire western side of the bay during the second half of the year. During spring, the deep euphotic depth plays a vital role in controlling the concentration and depth of the SCM. The depth of the 26 °C isotherm can be used as a proxy of the depth of the SCM. This study provides an improved understanding of the high chlorophyll-a concentrations and their drivers in five potential zones within the Bay of Bengal, which will help to identify the rich marine ecosystems therein.

Internal hydraulic jump in the Tsugaru Strait

Sea straits are known for important sites to modify water-mass properties and transport nutrients through vigorous vertical mixing. The first microstructure measurements ever taken in the Tsugaru Strait, between the Sea of Japan and the Northwest Pacific Ocean, showed elevated turbulent mixing. The turbulent energy dissipation rate was especially enhanced to O(10−6 W kg−1) via an internal hydraulic jump when the throughflow passed over an abrupt sill. Beyond the sill, the associated vertical eddy diffusivity reached O(10−2 m2 s−1) and the vertical turbulent nitrate flux below the subsurface chlorophyll-a maximum layer was estimated to reach 1 mmolN m−2 day−1, which could elevate the new production of phytoplankton within the layer. Large vertical velocities exceeding 0.1 m s−1 were also observed near the seafloor of the sill where the flow was hydraulically controlled. A simple advection–diffusion model suggested that the energy dissipation rate of O(10−5 W kg−1) was necessary to reproduce the observed modification of subsurface salinity maximum across the strait. Upwelling and/or vertical straining associated with the internal hydraulic jump is also believed to play an important role in the water-mass modification in the strait and could enhance new primary production for a wide area downstream.

Invertebrate larval distributions influenced by adult habitat distribution, larval behavior, and hydrodynamics in the retentive upwelling shadow of Monterey Bay, California, USA

Larval dispersal phases are a key determinant of population dynamics in recruitment-limited, coastal upwelling regions. Larvae were long considered to be highly susceptible to offshore transport, except in the lee of headlands where eddies form during upwelling conditions. We examined the spatial variation of benthic invertebrate larval assemblages in relation to the retentive upwelling shadow in northern Monterey Bay (California, USA) during strong upwelling (August 2013) and weak upwelling (October 2013). We characterized the spatial variation in physical characteristics of the water column, determined the cross-shore and depth distributions of invertebrate larvae in relation to the upwelling shadow, and examined how these physical and biological patterns change with upwelling strength. Larval abundances and environmental data (water temperature, salinity, chlorophyllaconcentration) were collected simultaneously using a plankton pump and profiling CTD at 3 depths: above, within, and below the chlorophyllamaximum layer. Larvae were primarily detected near the bottom. Larvae of most taxa were positively associated with a subsurface chlorophyllamaximum layer in August, but not in October when this layer was near the surface. Adult habitat distribution was related to the spatial distribution of larvae. Larvae of nearshore taxa occurred in the inner bay, while larvae of predominately offshore taxa occurred in the outer bay. Taxa with similar adult habitat (nearshore versus offshore) co-occurred in water samples. In addition, larvae of offshore taxa were commonly associated with offshore water types. Thus, the distribution of larvae within northern Monterey Bay appears to be strongly influenced by adult habitat distribution, vertical positioning of larvae in the water column, and upwelling strength.

Effects of the 2013–2016 Northeast Pacific warm anomaly on physical and biogeochemical variables off northwestern Baja California, derived from a numerical NPZD ocean model

Interannual climatic anomalies have important physical and biological consequences for the southern part of the California Current System (CCS). Two examples of these anomalies are “The Blob” and El Niño phenomena. The Blob was an anomalously warm water mass that formed in the Gulf of Alaska during fall 2013 and then affected most of the eastern North Pacific, including the waters off Baja California, Mexico. In this work we use a biogeochemical nutrient-phytoplankton-zooplankton-detritus (NPZD) model coupled to a three-dimensional hydrodynamic model, which allowed the simulation of some of the physical and biological consequences of the anomalous warm 2013–2016 period (The Blob was observed in 2014 and El Niño by mid-2015). A model-observation comparison showed that in some cases the model solution deviates substantially from the observed vertical thermal structure, but the temporal evolution of the spatially-averaged variables diagnosed by the model shows good agreement with the observations, except for the period from late 2015 to mid-2016, when the sea surface temperature (SST) is underestimated and the model failed to reproduce the correct sign of the surface chlorophyll-a (sCHL) anomalies. An EOF analysis of the 15-year (2003–2017) record of monthly mean modeled variables allowed the identification of two main contributions to the model variability: one mode mostly correlated with the Multivariate El Niño-Southern Oscillation (ENSO) Index (MEI), explaining ~50% of the variance of temperature (T) and dissolved nitrogen (N) and ~10% of the variance of chlorophyll-a (CHL), and another mode mostly correlated with the Coastal Upwelling Index (CUI) anomaly, explaining ~50-70% of variance of CHL and only ~10-20% of variance of T and N. In the 2013–2016 period, an El Niño signal diagnosed by the MEI induced a remarkable warming off northwestern Baja California. As a consequence, the thermocline and the nutricline deepened during this period and caused a deepening and weakening of the subsurface CHL maximum (SCM). Around the southern region of the study area (near Punta Baja) these effects were more intense at the surface over the shelf, but in the northern region (south of Southern California Bight) the effects were more intense subsurface seaward of the shelf break. This ENSO-modulated subsurface warming tends to weaken the Southern California Eddy during El Niño events, and intensify it during La Niña episodes. During the 2013–2015 period there was also a weakening of the upwelling-favorable winds, which caused a prolonged negative anomaly of the wind forcing, reducing the nutrient flux and causing negative CHL anomalies. Finally, in addition to the partial relation between the ENSO and the warming observed along the Baja California coast, the anomalies were observed in this area 4–6 months before the onset of the El Niño conditions (MEI>0.5) period, when the Blob was affecting the southern part of the CCS.

Spatial Distribution and Physical Controls of the Spring Algal Blooming Off the Changjiang River Estuary

The Changjiang River discharges huge amounts of nutrients, which cause frequent harmful algal blooms off its estuary. By analyzing historical and in situ observational data, this study aimed to determine the distribution and physical controls of these blooms. The results indicated that phytoplankton bloomed within the 30–50 m isobaths south of the Changjiang River mouth, with several high-value centers. South of the river mouth, the river plume front propagated along the coast as bottom-trapped river plume, which reached the bottom along the 25–30 m isobaths. The algae bloomed between the locations of the bottom and surface fronts. The surface front determined the seaward limit of the nutrient-rich river-influenced water and set the outer boundary of algal blooming; the latter separated the stratified (hence less turbid) offshore surface water from the well-mixed thus turbid nearshore surface water, forming the shoreward limit of the algal blooming due to high turbidity. The surface plume front was relatively unstable compared with the bottom. The surface plume front relaxes around the bathymetric irregularities, which enlarges the distance between the plume’s surface and bottom fronts, providing a larger area favorable to algal blooming. Consequently, several high chlorophyll-a centers developed around the bathymetric irregularities. East and northeast of the river mouth, the river plume floated above ambient seawater as a surface-trapped river plume. A subsurface chlorophyll-a maximum was observed above the pycnocline in the far-field plume area, which descended gradually to ~ 10 m in the offshore direction. The associated dynamical mechanism was also discussed.

Characterization of the synoptic‐scale diversity, biogeography, and size distribution of diatoms in the North Pacific

AbstractThe diversity, biogeography, and size distribution of diatoms in the North Pacific and underlying mechanisms shaping those patterns have little been characterized despite their importance in marine ecosystems. Here, we examined the community structure of diatoms in the surface and subsurface chlorophyll a maximum (SCM) layers of the North Pacific using light and scanning electron microscopy. Diatom carbon biomass in both the subarctic and temperate coastal regions was higher than that in the open subtropical and tropical waters. Species density was high in the temperate coastal region and certain open ocean stations where coastal water could be intruded. Diversity was generally higher in the SCM layer than that in the surface layer without a clear latitudinal trend. All diatom species in the open subtropical waters were observed in the temperate coastal waters, suggesting that the coastal regions act as a species bank. Certain indigenous species in the subarctic waters were considered as cryophilic species. We found that the general patterns of the size spectrum were nearly identical among different geographical regions of the North Pacific, irrespective of the species composition, environmental conditions, and seasonality. Our results provide mechanistic insights indicating that the diatom biodiversity in the species bank around the coastal region and the following current systems from the coastal to oceanic regions could principally determine biodiversity patterns in the North Pacific. The species‐specific ecophysiological traits of diatoms and environmental conditions may further modify the biodiversity patterns and size distribution of diatoms.

Vertical distribution of suspended particulate matter in the Caspian Sea in early summer

The features of the vertical distribution of chlorophyll a, particulate organic carbon and its isotopic composition, total suspended particulate matter (SPM), and the structure of the phytoplankton community in the Middle and South Caspian Sea in May–June 2012 are discussed. The subsurface chlorophyll a maximum (SCM) was found everywhere at depths of ~20 to 40–60 m. The position of this layer is confined to the depth of the seasonal thermocline, which is determined by the development of a cold-water (dark) phytocenosis. The genesis of this layer was studied. The increase in chlorophyll a concentration in this layer is caused by an abundance of phytoplankton or an increased concentration of this phytopigments per algal cell. The highest values of the studied organic compounds and phytoplankton biomass are revealed as close to the seasonal thermocline extending from the southern periphery of the Derbent Depression to the Apsheron Sill, which is determined by the bottom topography. The presence of chlorophyll a at depths exceeding 300 m (up to ≥1 mg/m3) was revealed. This was supported by findings of individual algal cells containing chlorophyll a and even their accumulations in the deep water layer. The most probable mechanisms responsible for the presence of these cells at the deep water level are discussed in the paper. The vertical distribution of the values of the organic carbon isotopic composition is primarily controlled by the vertical structure of phytoplankton and chlorophyll a in the water column up to ~500 m and by biogeochemical processes at the redox barrier (~600 m layer). The relative stability of chlorophyll a and the stability of pheophytin a in anaerobic environments were verified. A significant amount of weakly transformed chlorophyll a was found close the sea bottom.

Low particulate carbon to nitrogen ratios in marine surface waters of the Arctic

AbstractDuring the Canada's Three Oceans and Joint Ocean Ice Study projects in the summers of 2007 and 2008, we measured particulate organic carbon to nitrogen ratios (POC:PON) throughout the euphotic zone in Subarctic and Arctic waters. Average depth‐integrated values (2.65 ± 0.19) in the Beaufort Sea and Canada Basin (BS‐CB domain) were much lower than both the Redfield ratio (6.6) and the average ratios (3.9 to 5.6) measured across other Arctic‐Subarctic domains. Average uptake ratios of C and N (ρC:ρN) were also lower (0.87 ± 0.14) in BS‐CB than in the other four domains (2.10 to 3.51). Decreasing POC:PON ratios were associated with low concentrations of phytoplankton C, reduced abundance of biogenic silica (bSiO2), a smaller relative contribution of the >5 µm fraction to total chlorophyll a and a larger relative contribution of small flagellates (<8 µm) to total phytoplankton C. In the subsurface chlorophyll a maximum (SCM) within the BS‐CB domain, phytoplankton C represented only ~13% of POC; and therefore, the presence of heterotrophic microbes may have decreased POC:PON. These ratios are supported by data obtained during other Arctic programs in 2006, 2008, and 2009. Previous work has suggested a link between freshening of surface waters and increasing dominance of picophytoplankton and bacterioplankton in the Canada Basin, and the low POC:PON ratios measured during this study may be a consequence of this shift. Our results have ramifications for the conversion between C‐ and N‐based estimates of primary productivity, and for biogeochemical modeling of marine Arctic waters.

An assessment of phytoplankton primary productivity in the Arctic Ocean from satellite ocean color/in situ chlorophyll-a based models.

We investigated 32 net primary productivity (NPP) models by assessing skills to reproduce integrated NPP in the Arctic Ocean. The models were provided with two sources each of surface chlorophyll‐a concentration (chlorophyll), photosynthetically available radiation (PAR), sea surface temperature (SST), and mixed‐layer depth (MLD). The models were most sensitive to uncertainties in surface chlorophyll, generally performing better with in situ chlorophyll than with satellite‐derived values. They were much less sensitive to uncertainties in PAR, SST, and MLD, possibly due to relatively narrow ranges of input data and/or relatively little difference between input data sources. Regardless of type or complexity, most of the models were not able to fully reproduce the variability of in situ NPP, whereas some of them exhibited almost no bias (i.e., reproduced the mean of in situ NPP). The models performed relatively well in low‐productivity seasons as well as in sea ice‐covered/deep‐water regions. Depth‐resolved models correlated more with in situ NPP than other model types, but had a greater tendency to overestimate mean NPP whereas absorption‐based models exhibited the lowest bias associated with weaker correlation. The models performed better when a subsurface chlorophyll‐a maximum (SCM) was absent. As a group, the models overestimated mean NPP, however this was partly offset by some models underestimating NPP when a SCM was present. Our study suggests that NPP models need to be carefully tuned for the Arctic Ocean because most of the models performing relatively well were those that used Arctic‐relevant parameters.

Influence of the Changjiang River on the light absorption properties of phytoplankton from the East China Sea

Abstract. Phytoplankton light absorption properties were investigated at the surface and subsurface chlorophyll a maximum (SCM) layer in the East China Sea (ECS), a marginal sea which is strongly influenced by the Changjiang discharge in summer. Results from ECS were compared with those from the Tsushima Strait (TS) where the influence of Changjiang discharge is less. The probable controlling factors, packaging effect (cell size) and pigment composition of total chlorophyll a (Tchl a)-specific absorption coefficient (aph*(λ)) were examined by the corresponding measurements of pigments identified by high-performance liquid chromatography. We observed distinct phytoplankton size structure and thereby absorption properties between ECS and TS. At the surface, mixed populations of micro-, nano- and pico-phytoplankton were recorded in ECS while pico-phytoplankton dominated in TS, generating a lower average aph*(λ) in ECS than in TS. Within SCM, average aph*(λ) was higher in ECS than in TS because of the dominance of nano- and micro-phytoplankton in ECS and TS, respectively. By pooling surface and SCM samples, we found regular trends in phytoplankton size-fraction versus Tchl a; and correlations between aph*(λ) and Tchl a consistent with previous observations for the global ocean in TS but not in ECS. In ECS phytoplankton size-fraction was not correlated with Tchl a, which consequently caused poor relationships between aph*(λ) and Tchl a. The abnormal values mainly originated from the surface low-salinity waters and SCM waters beneath them. At high Tchl a, aph*(λ) of these samples was substantially higher compared to the values in TS and from the global regressions, which was attributable to the lower micro-phytoplankton fraction, and higher nano- and/or pico-phytoplankton fractions in ECS. These observations indicated that the distinct light absorption properties of phytoplankton in ECS were possibly influenced by the Changjiang discharge. Our findings imply that general bio-optical algorithms proposed based on the correlations between aph*(λ) and Tchl a or the patterns in size-fraction versus Tchl a are not applicable in ECS, and need to be carefully considered when using these general algorithms in river-influenced regions.

The timing and the magnitude of spring phytoplankton blooms and their relationship with physical forcing in the central Yellow Sea in 2009

The development of phytoplankton bloom and its association with physical forcing is examined through an interdisciplinary field-work conducted in the vicinity of the central trough of the southern Yellow Sea during March–April 2009, with the aid of a surface Lagrangian drifter deployed at the bloom site. Bloom patches were detected using an empirical value and two of them were traced by the drifter for a period of several days respectively. Both of them appears as thin-layer subsurface chlorophyll a maximum (SCM) throughout the tracing, although their dominant phytoplankton species are not identical at all. The magnitude as well as the onset of these two blooms is different from each other, but both found to be relevant to local oceanic and meteorological conditions. Both of them demonstrate that the changes in the stability of hydrographical structure, especially at layers around the SCM, take a substantial role in triggering or terminating the blooming processes. Those changes in meteorological conditions, like wind speed and directions, solar radiation, are short and cause daily or synoptic scale variations in phytoplankton concentrations, but the frequency of northerly wind events predating the bloom season has a positive effect on the occurrence of spring blooms. The horizontal advection is another contributing factor indicated by the drifter which accounts for the bloom extinction at the station B20. In addition, due to the weak orbital horizontal movement, the bloom above the central trough persists longer and larger.

동해 전역에 장기간 발달하는 아표층 엽록소 최대층과 수괴의 물리 화학적 특성과의 상관관계

하계 울릉분지에서 관측된 SCM층은 울릉분지에만 국한되어 나타나는 것이 아니라 동해 전역에서 관측되었다. 각기 다른 시기에 관측된 여러 문헌 자료와 하계 동해 전역에서 관측된 자료에 의하면 SCM층이 나타나는 시기는 5월부터 10월 하순까지 나타나는 것으로 추정된다. SCM층 바로 상층부에 표층보다 높은 용존산소가 관측되었고, SCM층이 영양염의 농도가 급격하게 증가하는 수심에 나타나는 것으로 보아 침강이나 낮은 광량에 대한 식물플랑크톤의 생리적인 적응보다는 식물플랑크톤이 활발하게 성장하고 있음 시사한다. SCM층에서 식물플랑크톤이 오랜기간 동안 지속적으로 성장하기 위한 영양염 공급과정으로 수직 확산, 소형 및 중형동물플랑크톤의 재생산등이 중요한 역할을 하는 것으로 알려져 있다. 이들 과정은 SCM층에서 식물플랑크톤 질소 요구량의 약 50% 정도만 설명할 수 있어 SCM층의 질소 요구량에 대한 추가적인 공급기작으로는 난수성 소용돌이가 제시되었다. 그러나 난수성 소용돌이가 발달하지 않은 해역과 시기에 관측된 SCM층에 영양염은 고온 저염의 특성을 가지면서 영양염이 고갈된 표층수 아래에서 대한해협을 통해서 유입되는 영양염이 비교적 풍부한 해수가 울릉분지내로 유입되어 북상하면서 유광층까지 직접으로 상승하거나 수평이동 응력의 차이로 발생하는 난류혼합등에 의해서 추가적으로 공급되는 것으로 사료된다(Fig. 17). 그리고 동해 전역에서 5월에서 10월까지 존재하는 SCM층은 표층에서 심층으로의 유기물 수송에 상당히 기여할 것으로 사료된다. To understand the scales of the spatial distribution and temporal duration of the subsurface chlorophyll-a maximum (SCM) observed in the Ulleung Basin of the East Sea, we analyzed physical and chemical data collected during the East Asian Seas Time-series-I (EAST-I) program. The SCM layer occurred at several observation lines from the Korea Strait to <TEX>$37.9^{\circ}N$</TEX> in the Ulleung Basin during August of 2008 and 2011. At each observation line, the SCM layer extended from the coast to about 200 km off the coast. The SCM layer was observed between 30 and 40 m depth in the Ulleung Basin as well as in the northwestern Japan Basin along <TEX>$132.3^{\circ}E$</TEX> from <TEX>$38^{\circ}N$</TEX> to <TEX>$42.3^{\circ}N$</TEX> during July 2009, and was observed around 50 m depth in the northeastern Japan Basin (<TEX>$135-140^{\circ}E$</TEX> and <TEX>$40-45^{\circ}N$</TEX>) during July 2010. From these observed features, we hypothesize that the SCM layer observed in the Ulleung Basin may exist in most of the East Sea and may last for at least half-year (from the early May to late October). The nutrient supply mechanism for prolonged the SCM layer in the East Sea was not known, but it may be closely related to the horizontal advection of the nutrient rich and low oxygen waters observed in the Korea Strait between a 50 m depth to near the bottom. The prolonged development of the SCM layer in the Ulleung Basin may result in high primary production and would also be responsible for the high organic carbon content observed in the surface sediment of the region.

Observed variability of chlorophyll-a using Argo profiling floats in the southeastern Arabian Sea

The time series of temperature, salinity, chlorophyll-a, particle scattering at 700nm, and dissolved oxygen obtained from Argo floats deployed in the southeastern Arabian Sea (AS), were used to investigate the variability of these quantities and the mechanisms that modulate them between March 2010 and March 2011. The observations show a persistent occurrence of a subsurface chlorophyll-a maximum (0.75–1mgm−3) near depths of 40–100m throughout the study period, just above the top of permanent thermocline and euphotic depth. The analysis shows that upward and downward movement of thermocline, which is primarily due to westward movement of low-mode baroclinic Rossby waves, significantly influences the depth of the subsurface chlorophyll maxima and its intensity. Further, the vertical movement of the thermocline significantly influences the depth of the oxycline in the AS. The mixed layer deepening associated with wind induced vertical mixing and convective overturning lead to near surface blooms during the summer and winter monsoons. The analysis clearly shows that, in addition to entrainment of nutrients from rich subsurface water in the near surface layer, vertical fluxes from the subsurface chlorophyll maximum also contribute significantly to mixed layer blooms. The availability of light also plays an active role in the mixed layer bloom, particularly during the summer monsoon season.

Latitudinal variation of phytoplankton communities in the western Arctic Ocean

Recent studies have shown that photosynthetic eukaryotes are an active and often dominant component of Arctic phytoplankton assemblages. In order to explore this notion at a large scale, samples were collected to investigate the community structure and biovolume of phytoplankton along a transect in the western Arctic Ocean. The transect included 37 stations at the surface and subsurface chlorophyll a maximum (SCM) depths in the Bering Sea, Chukchi Sea, and Canadian Basin from July 19 to September 5, 2008. Phytoplankton (>2μm) were identified and counted. A cluster analysis of abundance and biovolume data revealed different assemblages over the shelf, slope, and basin regions. Phytoplankton communities were composed of 71 taxa representing Dinophyceae, Cryptophyceae, Bacillariophyceae, Chrysophyceae, Dictyochophyceae, Prasinophyceae, and Prymnesiophyceae. The most abundant species were of pico- to nano-size at the surface and SCM depths at most stations. Nano- and pico-sized phytoplankton appeared to be dominant in the Bering Sea, whereas diatoms and nano-sized plankton provided the majority of taxon diversity in the Bering Strait and in the Chukchi Sea. From the western Bering Sea to the Bering Strait, the abundance, biovolume, and species diversity of phytoplankton provided a marked latitudinal gradient towards the central Arctic. Although pico- and nano-sized phytoplankton contributed most to cell abundance, their chlorophyll a contents and biovolumes were less than those of the larger micro-sized taxa. Micro-sized phytoplankton contributed most to the biovolume in the largely ice-free waters of the western Arctic Ocean during summer 2008.