Phytoplankton Absorption Research Articles

Variations in the light absorption coefficients of phytoplankton, non-algal particles and dissolved organic matter in reservoirs across China

Reservoirs were critical sources of drinking water for many large cities around the world, but progress in the development of large-scale monitoring protocols to obtain timely information about water quality had been hampered by the complex nature of inland waters and the various optical conditions exhibited by these aquatic ecosystems. In this study, we systematically investigated the absorption coefficient of different optically-active constituents (OACs) in 120 reservoirs of different trophic states across five eco-regions in China. The relationships were found between phytoplankton absorption coefficient at 675 nm (aph (675)) and Chlorophyll a (Chla) concentration in different regions (R2:0.60–0.82). The non-algal particle (NAP) absorption coefficient (aNAP) showed an increasing trend for reservoirs with trophic states. Significant correlation (p < 0.05) was observed between chromophoric dissolved organic matter (CDOM) absorption and water chemical parameters. The influencing factors for contributing the relative proportion of OACs absorption including the hydrological factors and water quality factors were analyzed. The non-water absorption budget from our data showed the variations of the dominant absorption types which underscored the need to develop and parameterize region-specific bio-optical models for large-scale assessment in water reservoirs.

High latitude Southern Ocean phytoplankton have distinctive bio-optical properties.

Studying the biogeochemistry of the Southern Ocean using remote sensing relies on accurate interpretation of ocean colour through bio-optical and biogeochemical relationships between quantities and properties of interest. During the Antarctic Circumnavigation Expedition of the 2016/2017 Austral Summer, we collected a spatially comprehensive dataset of phytoplankton pigment concentrations, particulate absorption and particle size distribution and compared simple bio-optical and particle property relationships as a function of chlorophyll a. Similar to previous studies we find that the chlorophyll-specific phytoplankton absorption coefficient is significantly lower than in other oceans at comparable chlorophyll concentrations. This appears to be driven in part by lower concentrations of accessory pigments per unit chlorophyll a as well as increased pigment packaging due to relatively larger sized phytoplankton at low chlorophyll a than is typically observed in other oceans. We find that the contribution of microphytoplankton (>20 µm size) to chlorophyll a estimates of phytoplankton biomass is significantly higher than expected for the given chlorophyll a concentration, especially in higher latitudes south of the Southern Antarctic Circumpolar Current Front. Phytoplankton pigments are more packaged in larger cells, which resulted in a flattening of phytoplankton spectra as measured in these samples when compared to other ocean regions with similar chlorophyll a concentration. Additionally, we find that at high latitude locations in the Southern Ocean, pheopigment concentrations can exceed mono-vinyl chlorophyll a concentrations. Finally, we observed very different relationships between particle volume and chlorophyll a concentrations in high and low latitude Southern Ocean waters, driven by differences in phytoplankton community composition and acclimation to environmental conditions and varying contribution of non-algal particles to the particulate matter. Our data confirm that, as previously suggested, the relationships between bio-optical properties and chlorophyll a in the Southern Ocean are different to other oceans. In addition, distinct bio-optical properties were evident between high and low latitude regions of the Southern Ocean basin. Here we provide a region-specific set of power law functions describing the phytoplankton absorption spectrum as a function of chlorophyll a.

Understanding optical absorption associated with phytoplanktonic groups in the marginal seas

Marine phytoplankton absorption plays an important role in oceanic biological productivity and ecological environmental dynamics. Understanding the optical absorption variability associated with phytoplanktonic groups still remains a challenge. In this study, samples (n = 206) were collected for the marginal seas of the northwest Pacific Ocean from six cruise surveys that covered different seasons. Using in situ parameters, including phytoplankton absorption coefficients and concentrations of the phytoplanktonic groups derived from phytoplankton pigments collected with high-performance liquid chromatography (HPLC), we developed a Gaussian model to characterize the specific absorption spectra of eight phytoplanktonic groups, including diatoms, chlorophytes, cryptophytes, cyanobacteria, prymnesiophytes, prasinophytes, dinoflagellates, and chrysophytes, without the package effect. The model was established by accurately identifying for the numbers and locations of the Gaussian peaks and their corresponding half-wave widths. The proposed model produced promising results, and a leave-one-out cross validation generated R2 values exceeding 0.7 for the whole visible light range and above 0.85 (correspondingly MAPE <40%) for the simulated wave bands, excluding the range of 550–650 nm. Meanwhile, a comparison with several spectra observed in the lab showed a high degree of similarity, indicative of the superior performance of our model. Applying the documented specific absorption spectra to the investigated water bodies (whether water surface or profiles) enabled us to quantify the absorption coefficients from different phytoplanktonic groups and characterize their relative contributions to the total. The findings of this study support our understanding of the dynamics of phytoplankton community structure with optical data.

Characterization of phytoplankton productivity and bio-optical variability in a polar marine ecosystem

Knowledge of Southern Ocean carbon cycling is limited by a paucity of phytoplankton primary productivity (PP) and spectral absorption data in this globally-important region. We measured 13C-based PP in the Indian sector of Southern Ocean (ISSO) during austral summer 2017, examining its link with spectral absorption coefficients and phytoplankton size structure derived from an absorption-based global model. Phytoplankton productivity was assessed at both coastal (60°S-69°S) and frontal stations (40°S-60°S), characterized by silicate- replete and -deplete water masses, respectively (indicated by measured nutrient ratios) to capture a range of phytoplankton growth conditions. Bio-optical relationships were used as indicators of phytoplankton community size structure and to assess the extent of cellular pigment packaging - a phenomenon reported previously for phytoplankton in this region. Blue-Red (B/R) ratios of phytoplankton absorption (aph) spectra indicated that microphytoplankton (more prone to “package effects”) were the dominant size class at most sites sampled. Overall, PP was better explained by aph (R2 = 0.85) than total chlorophyll-a (R2 = 0.64) in surface waters. The a*ph (675)-chlorophyll-a relationship explained package effects more effectively in frontal regions (R2 = 0.63) than stations further south (R2 = 0.30). The global absorption-based model captured smaller (pico, nano) phytoplankton size classes but failed to identify larger microphytoplankton, underscoring the need for region-specific algorithm modifications. Our findings improve existing understanding of spatio-temporal trends in PP and bio-optical variability within the Indian Sector of the Southern Ocean (ISSO) – knowledge that is essential to improve capacity to retrieve PP from satellite-based models in this region.

Dependence of the Photosynthetic Quantum Yield on Phytoplankton Light Absorption: Equations for Assessing Primary Production in the Black Sea

Purpose. Investigations were performed during a scientific cruise to characterize hydrophysical properties, chlorophyll a concentration, photosynthesis-irradiance curves, spectral light absorption coefficients by phytoplankton, and spectral quantum downwelling irradiance. From these results, the dependence of the photosynthetic quantum yield upon environmental factors was studied with the purpose of adapting an algorithm developed for the Baltic Sea so that it can be applied for the Black Sea. Methods and Results. Complex hydrophysical and biological studies were carried out at several depths within the euphotic zone. Spectral bio-optical parameters were measured in accordance with the latest NASA protocols (2018). Experiments to determine the photosynthesis-light relationship were performed under temperature and light conditions similar to those in situ. The quantum yield of photosynthesis was calculated from parameters of photosynthesis-light curves (photosynthesis efficiency, light saturation parameter) and the spectral light absorption coefficients by phytoplankton pigments. It was found out that the main photosynthetic characteristics of phytoplankton, including the maximum photosynthetic quantum yield and the fraction of phytoplankton absorption associated with photoprotective accessory pigments, varied with depth within the euphotic zone, due to phytoplankton acclimation to environment factors during the period of seasonal stratification. The relationship between the photosynthetic quantum yield and the quanta absorbed by photosynthetically active phytoplankton pigments was revealed. The results of this research made it possible to build on the approach developed for other regions and modify the equation for calculating the quantum yield to apply specifically for environmental conditions in the Black Sea. Conclusions. For the first time, comprehensive studies carried out in the Black Sea, including measurements of the photosynthesis-light dependence, spectral light absorption coefficients by phytoplankton and spectral downwelling irradiance as a function of optical depths within the euphotic zone, made it possible to reveal the equation for calculating photosynthetic quantum yield. This new equation can be applied for calculating primary production of the Black Sea using a spectral approach, based both on the results of in situ measurements and remote sensing data.

Effect of Phytoplankton Community Composition and Size Structure on Light Absorption Properties

In the fields of phytoplankton ecology, water optics, and water color remote sensing, phytoplankton absorption properties represent the light absorption capacity of phytoplankton, which affects photosynthesis efficiency and carbon fixation. Here, the biomass, community composition, and the absorption properties of phytoplankton were measured alongside other bio-optical parameters in Lake Tianmu are examined using data collected between January and November 2013 (except February). Based on the relationships between phytoplankton biomass, community composition, and absorption, the effects of abundance, biomass, and equivalent sphere diameter on phytoplankton absorption and specific absorption were revealed. The highest biomass and abundance of phytoplankton were recorded in the autumn and the lowest in the winter. Cryptomonas, Synedra, and Cyclotella were the dominant genera throughout the year. The dominant genera structure type was Bacillariophyta-Cryptophyta in the winter and spring, Bacillariophyta-Chlorophyta-Pyrroptata in the summer, and Cryptophyta-Bacillariophyta-Chlorophyta in the autumn. Phytoplankton diameter was ranked in the order summer>autumn>winter>spring, with mean values of 64.83 μm in summer and 29.54 μm in spring. Phytoplankton absorption coefficients of were ranked in the order autumn > spring > winter > summer, with mean values at 440 nm and 675 nm of (0.66±0.18) m-1 and (0.33±0.10) m-1 in autumn and (0.17±0.02) m-1 and (0.08±0.01) m-1 in summer, respectively. The specific absorption coefficients of the phytoplankton were ranked in the order spring > winter > autumn > summer, with mean values at 440 nm and 675 nm of (0.07±0.02) m2·mg-1 and (0.04±0.01) m2·mg-1 in spring and (0.03±0.004) m2·mg-1 and (0.01±0.002) m2·mg-1 in summer, respectively. Significant linear correlations were found between phytoplankton biomass, abundance, and absorption coefficients. Variations of Bacillariophyta and Cyanophyta biomass caused by temperature explained the seasonal variation in absorption coefficients. The specific absorption coefficient decreased with an increase in equivalent sphere diameter, and variations in phytoplankton community composition explained seasonal changes in the specific absorption coefficient.

Simultaneous inversion of concentrations of POC and its endmembers in lakes: A novel remote sensing strategy

Data on the concentration of particulate organic carbon (POC) and its endmembers provide a basis for the characterisation of lake biogeochemical cycles. Here, a novel remote sensing strategy (the SCPOC algorithm) was developed to determine total POC concentrations, as well as terrestrial and endogenous POC concentrations in lakes. This strategy provides a successful example for the combination of isotope tracer and remote sensing technology. First, we obtained the terrestrial and endogenous POC concentration at the sampling point based on isotope tracing technology. Afterwards, we established a relationship between the phytoplankton absorption coefficient and the endogenous POC concentration (Cend), and applied a semi-analytical algorithm to invert the Cend value. Finally, the POC source ratio model and Cend value were combined to obtain the POC concentration (CPOC) and terrestrial POC (Cter). The results of synchronisation verification based on ocean and land colour instrument (OLCI) images show that the SCPOC algorithm has high Cend, Cter, and CPOC inversion accuracy, with MAPE values of 26.07%, 30.43%, and 42.28%, respectively. In fact, the SCPOC algorithm not only improved the accuracy of lake POC mapping, but also fills the gap of optical retrieval of POC endmember concentrations. Additionally, data from the OLCI images indicated that the studied lakes were dominated by external POC. However, because of the greater contribution of algal blooms to POC, this dominant advantage weakens in summer, although the terrestrial organic carbon carried by rainfall runoff also affects lake POC composition. Different POC sources have different ecological roles in lakes, and the superior POC end-element estimation capability of the SCPOC algorithm can not only be used as a supplement to traditional tracing methods, but also provides accurate spatial data for lake management.

Linking phytoplankton absorption to community composition in Chinese marginal seas

Phytoplankton pigments significantly affect photosynthesis and play a crucial role in regulating marine ecological and biogeochemical processes. Assessment of phytoplankton pigments through optical means is desirable as it may be extended to satellite remote sensing. Here, using an extensive data set of high performance liquid chromatography (HPLC) phytoplankton pigment concentrations and phytoplankton absorption spectra (aph(λ)) collected through five cruise surveys of the Chinese marginal seas during 2016 and 2017, we explore the potentials of using aph(λ) to estimate twenty pigments. Specifically, the first and second derivatives of aph(λ) are used to construct an aph(λ) - pigment model. The validation of the aph(λ)-derived pigment classes, specific to individual phytoplankton community and size groups, shows a generally satisfactory model performance. Additionally, hierarchical cluster analysis also exhibits high similarity within the classification results based on the measured and modeled pigments, where only two pigments were decided into different clusters. Although still preliminary in nature, this proof-of-concept study for the Chinese marginal seas shows the potentials of using satellite remote sensing to assess phytoplankton pigment composition once hyperspectral satellite data are available and aph(λ) inversion algorithms are developed and validated.

Dissolved organic matter from tropical peatlands reduces shelf sea light availability in the Singapore Strait, Southeast Asia

Shelf seas provide valuable ecosystem services, but their productivity and ecological functioning depend critically on sunlight transmitted through the water column. Anthropogenic reductions in underwater light availability are thus a serious threat to coastal habitats. The flux of light-absorbing coloured dissolved organic matter (CDOM) from land to sea may have increased world-wide, but how this has altered the availability and spectral quality of light in shelf seas remains poorly known. We present time-series data from the Sunda Shelf in Southeast Asia, where the monsoon-driven reversal in ocean currents supplies water enriched in CDOM from tropical peatlands for part of the year, resulting in 5- to 10-fold seasonal variation in light absorption by CDOM. We show that this terrigenous CDOM can dominate underwater light absorption at wavelengths up to 500 nm, and shift the underwater irradiance spectrum towards longer wavelengths. The seasonal presence of terrigenous CDOM also reduces the 10% light penetration depth by 1-5 m, or 10-45%. We estimate that on average 0.6 m, or 25%, of this terrigenous CDOM-mediated shoaling might be attributable to the enhanced input of dissolved organic matter following peatland disturbance. The seasonal change in the light environment is correlated with changes in phytoplankton absorption spectra that suggest a photo-acclimation response, and we infer that terrigenous CDOM likely contributes to limiting the depth distribution of photosynthetic corals. The results reveal an ecologically important but largely overlooked impact of human modifications to carbon fluxes that is likely increasingly important in coastal seas.

Impact of Temperature on Absorption Coefficient of Pure Seawater in the Blue Wavelengths Inferred from Satellite and In Situ Measurements

There has been a long history of interest on how (if) the absorption coefficient of “pure” fresh water ( a fw λ ) and “pure” seawater ( a sw λ ) changes with temperature ( T ), yet the impact of T reported in the literature differs significantly in the blue domain. Unlike the previous studies based on laboratory measurements, we took an approach based on ~18 years (2002–2020) of MODIS ocean color and temperature measurements in the oligotrophic oceans, along with field measured chlorophyll concentration and phytoplankton absorption coefficient, to examine the relationship between T and the total absorption coefficient ( a λ ) at 412 and 443 nm. We found that the values of a 412 and a 443 in the summer are nearly flat (slightly decreasing) for the observed T range of ~19–27 °C. Since there are no detectable changes of chlorophyll during this period, the results suggest that T has a negligible impact on a sw 412 and a sw 443 in this T range. As a complement, the impact of salinity on a fw λ was also evaluated using three independent determinations of a sw λ and a fw λ , where good agreements were found from these observations.

Phytoplankton “Missing” Absorption in Marine Waters: A Novel Pigment Compensation Model for the Packaging Effect

AbstractThe packaging effect (Qa*) of phytoplankton pigments can account for over half of the variability in the phytoplankton absorption coefficient (aph) in oceanic waters. Given the significance of aph in many marine biogeochemical and environmental processes, exploring the packaging effect on absorption properties is a crucial task. In the present study, two pigment compensation models for quantifying the packaging effect are developed for Case I and Case II waters using high‐performance liquid chromatography (HPLC) derived pigments and aph data. The Case I model is based on data from the NOMAD data set from the Atlantic Ocean, and the Case II model is based on data taken in the marginal seas of China. Phytoplankton “missing” absorption is derived by subtracting the reconstructed aph without the packaging effect from the measured aph. Our proposed models use the established relationships between “missing” absorption and specific absorption coefficients of pigment groups without the packaging effect to quantify pigment group concentrations. Validation using independent in situ data sets demonstrates that significant improvements are achieved to quantify the packaging effect, especially for waters under abnormal packaging effect conditions (Qa* &gt; 1), where the mean ratio of normal Qa* increasing largest from 40% to 90% in 455 nm in Case I water and from 70% to 100% in 520 nm in Case II water. Applying the proposed models to satellite data displays the spatial distributions of the packaging effect in the Atlantic Ocean and the marginal seas of China, as delegates of Case I and II waters, respectively. The Qa* intensity negatively covaries with the chlorophyll‐a distribution. This study shows the spatial variability of the packaging effect from satellite observations for the first time.

Variability of phytoplankton light absorption in stratified waters of the NW Mediterranean Sea: The interplay between pigment composition and the packaging effect

The Variability of chlorophyll-specific phytoplankton light absorption [a*ph (λ)] was examined over depth and time in stratified offshore waters of the North-Western Mediterranean Sea. Coherent water patches were tracked with Lagrangian drifters during two oceanographic cruises in September (late summer) and May (post-spring bloom phase). By simultaneously analysing the phytoplankton absorption and pigment measurements, we explicitly separated the impact of pigment composition from that of pigment packaging on a*ph (λ). We further partitioned the packaging effect by comparing the variation of the packaging index [Qa*(440)], the phytoplankton community size structure (derived from diagnostic pigment analysis), and the chlorophyll-specific beam attenuation by particles [cp* (660)] as an optical index of phytoplankton photophysiology. In the ensemble of cruises, around 50% of the a*ph (440) vertical variation was explained by changes in the pigment composition (ruled by the decrease of photoprotective pigments with depth). The remaining vertical and inter-cruise variation of a*ph (440) was attributed to the packaging effect. We found that differences in the cp*(660) index (most likely indicating changes in the intracellular pigment concentration due to photoacclimation) mainly explained the observed variation in the packaging effect. Differences in cp* (660) were coincident with either the vertical gradient of light availability or the lower mean daily PAR irradiance in the euphotic layer of the September cruise. These explained the stronger packaging and lower a*ph (440) values observed with increasing depth in both cruises, and in September relative to May. On the other hand, differences in the phytoplankton community size structure did not explain the observed patterns in the packing effect. Our results highlight the importance of phytoplankton short-term acclimation to the prevailing light conditions, determining the vertical and temporal variability of a*ph (λ). A better understanding of the a*ph (λ) variability and its main drivers are key to improve different bio-optical applications.

Hyperspectral retrievals of phytoplankton absorption and chlorophyll-a in inland and nearshore coastal waters

Following more than two decades of research and developments made possible through various proof-of-concept hyperspectral remote sensing missions, it has been anticipated that hyperspectral imaging would enhance the accuracy of remotely sensed in-water products. This study investigates such expected improvements and demonstrates the utility of hyperspectral radiometric measurements for the retrieval of near-surface phytoplankton properties11Codes are accessible via https://github.com/STREAM-RS/STREAM-RS, i.e., phytoplankton absorption spectra (aph) and biomass evaluated through examining the concentration of chlorophyll-a (Chla). Using hyperspectral data (409–800 nm at ~5 nm resolution) and a class of neural networks known as Mixture Density Networks (MDN) (Pahlevan et al., 2020), we show that the median error in aph retrievals is reduced two-to-three times (N = 722) compared to that from heritage ocean color algorithms. The median error associated with our aph retrieval across all the visible bands varies between 20 and 30%. Similarly, Chla retrievals exhibit significant improvements (i.e., more than two times; N = 1902), with respect to existing algorithms that rely on select spectral bands. Using an independent matchup dataset acquired near-concurrently with the acquisition of the Hyperspectral Imager for the Coastal Ocean (HICO) images, the models are found to perform well, but at reduced levels due to uncertainties in the atmospheric correction. The mapped spatial distribution of Chla maps and aph spectra for selected HICO swaths further solidify MDNs as promising machine-learning models that have the potential to generate highly accurate aquatic remote sensing products in inland and coastal waters. For aph retrieval to improve further, two immediate research avenues are recommended: a) the network architecture requires additional optimization to enable a simultaneous retrieval of multiple in-water parameters (e.g., aph, Chla, absorption by colored dissolved organic matter), and b) the training dataset should be extended to enhance model generalizability. This feasibility analysis using MDNs provides strong evidence that high-quality, global hyperspectral data will open new pathways toward a better understanding of biodiversity in aquatic ecosystems.

Vertical Distribution of Bio-Optical Properties of the Azov – Black Sea Basin Waters in April – May, 2019

Purpose. The work is aimed at investigating spatial distribution of the chlorophyll a concentration and the spectral light absorption coefficients by all optically active components in the Azov and Black seas in spring, when the seawater hydrophysical structure changes. Methods and Results. The data collected in the 106th scientific cruise of R/V Professor Vodyanitsky in April 19 – May 10, 2019 were used. The chlorophyll a concentration was measured by the spectrophotometric method. The spectral light absorption coefficients were determined in accordance with the NASA protocol 2018. The optical measurements were performed using the dual-beam spectrophotometer Lambda 35 (PerkinElmer). It was shown that in the surface layer of the Black Sea, the chlorophyll a concentration varied from 0.21 to 1.2 mg/m3. At some stations in the deep-water region, the increased values of this parameter were observed in the lower part of the euphotic zone that was associated with the beginning of seasonal water stratification due to the surface water heating. At these stations, the phytoplankton absorption spectra were more smoothed in the lower part of the euphotic zone than those in the upper layer. In the deep-water region, the non-algal particles contribution to the total particulate light absorption at wavelength 438 nm changed with depth from 40 ± 15 % at the surface to 29 ± 12 % near the bottom of the euphotic zone; whereas in the coastal waters this parameter was almost unchangeable within the water column (54 ± 11 %). No significant change of the colored dissolved organic matter contribution to the total light absorption with depth was revealed (69% on average). In the Sea of Azov, vertical distribution both of the chlorophyll a concentration (6.2 mg/m3 on average) and the spectral light absorption coefficients by all the optically active components was uniform. The non-algal particles contribution to the particulate light absorption was 40 ± 14 %, and the colored dissolved organic matter contribution to the total light absorption constituted 52 ± 6 %. Conclusions. New data on spatial distribution of the chlorophyll a concentration and the spectral light absorption coefficients by the optically active components in the Black and Azov seas were obtained for the spring period when the seawater hydrophysical characteristics changed.

Modelling the impact of phytoplankton cell size and abundance on inherent optical properties (IOPs) and a remotely sensed chlorophyll-a product

Abstract Ocean colour data are commonly used to quantify primary production, study phytoplankton dynamics and calibrate marine models, thus understanding the origin of errors in the retrieved chlorophyll-a (Chl-a) product is critical. One source of uncertainty in retrieved Chl-a products can be related to large photosynthetic cells, characterised by lower mass-specific absorption coefficients due to increased packaging effect. Here, we explore the relationship between phytoplankton size structure and an ocean colour product using optical simulations and in situ observations. Specifically, we use an optical model to explore how phytoplankton cell size and abundance influence phytoplankton absorption and backscattering coefficients and the implication this has for water leaving radiance and the estimated Chl-a derived from satellite ocean colour. The optical model simulations show phytoplankton cell size has a significant impact on the remote-sensing reflectance, with Chl-a packaged in 5 to 10 μm cells resulting in about 54 to 76% the simulated ocean colour Chl-a compared to 1 μm cells, as determined by an algorithm that converts reflectances to Chl-a. To support optical simulations, size-fractionated Chl-a samples were collected from several water masses to investigate the phytoplankton size contribution (i.e., 10 μm) to the total Chl-a. We focused on the offshore eastern Australian ocean region, largely characterised by oligotrophic waters in which phytoplankton dominate the optical properties of the water column. Of the 22 stations sampled, a total of ten in situ size fractionated Chl-a measurements were matched-up with the corresponding clear-sky satellite Chl-a product. The matched-up points revealed a systematic underestimation of in situ Chl-a. With the low amount of data, it was not possible to statistically relate the satellite underestimation to a specific phytoplankton size class, but the observations showed that the largest satellite Chl-a underestimates were found when phytoplankton larger than 10 μm represented more than 50% of the phytoplankton community. Additional measurements that combine in situ optical measurements with phytoplankton size distribution and carbon/Chl-a ratio would help to clarify the relationship between phytoplankton size structures and remotely sensed Chl-a product.

Controls on Ocean Color Spectra Observed During the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES)

Satellite ocean color remote sensing is the primary method to retrieve synoptic measurements of the optical properties of the ocean on large spatial and regular time scales.Through bio-optical modelling, changes in ocean color spectra can be linked to changes in marine 2 ecosystem and biogeochemical properties. Bio-optical algorithms rely on assumptions about the covariance of marine constituents as well as the relationships among their inherent and apparent optical properties. Validation with in situ measurements of in-water constituents and their optical properties is required to extrapolate local knowledge about ocean color variations to global scales.Here, we evaluate seasonal and spatial relationships between optical constituents and their inherent and apparent optical properties throughout the annual cycle of the North Atlantic plankton bloom using bio-optical data from four cruises conducted as part of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). Our results show ocean color variability, quantified using field observations of the remote sensing reflectance spectrum at each NAAMES station, is driven by colored dissolved organic matter (CDOM) absorption in the ultraviolet wavelengths, phytoplankton absorption in the blue wavelengths, and total particulate backscattering in the green wavelengths. Results from a recently storm-mixed station at the height of the spring bloom demonstrate that significant changes in bio-optical properties can occur on daily scales. By testing the effects of variations in lighting conditions and solar geometries, we also demonstrate that, for this data set, remote sensing reflectance should be considered a quasi-inherent optical property.We find that the temporal and spatial chlorophyll concentrations and the magnitudes of inherent optical properties can be accurately assessed using previously published ocean color algorithms.However, changes in the spectral slopes of the inherent optical properties are often poorly retrieved, indicating the need for improvements in the retrieval of optical constituent composition.The characterization of such a dynamic environment provides beneficial insights for future biooptical algorithms.

Vertical structure in chlorophyll profiles: influence on primary production in the Arctic Ocean.

Subsurface chlorophyll maximum (SCM) layers are prevalent throughout the Arctic Ocean under stratified conditions and are observed both in the wake of retreating sea ice and in thermally stratified waters. The importance of these layers on the overall productivity of Arctic pelagic ecosystems has been a source of debate. In this study, we consider the three principal factors that govern productivity within SCMs: the shape of the chlorophyll profile, the photophysiological characteristics of phytoplankton and the availability of light in the layer. Using the information on the biological and optical parameters describing the vertical structure of chlorophyll, phytoplankton absorption and photosynthesis–irradiance response curves, a spectrally resolved model of primary production is used to identify the set of conditions under which SCMs are important contributors to water-column productivity. Sensitivity analysis revealed systematic errors in the estimation of primary production when the vertical distribution of chlorophyll was not taken into account, with estimates of water-column production using a non-uniform profile being up to 97% higher than those computed using a uniform one. The relative errors were shown to be functions of the parameters describing the shape of the biomass profile and the light available at the SCM to support photosynthesis. Given that SCM productivity is believed to be largely supported by new nutrients, it is likely that the relative contribution of SCMs to new production would be significantly higher than that to gross primary production. We discuss the biogeochemical and ecological implications of these findings and the potential role of new ocean sensors and autonomous underwater vehicles in furthering the study of SCMs in such highly heterogeneous and remote marine ecosystems.This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Variability in primary productivity and bio-optical properties in the Indian sector of the Southern Ocean during an austral summer

The Southern Ocean (SO), in spite of its major contribution to global primary productivity (PP), remains underexplored in this aspect. Light being the most limiting parameter affecting primary production, it is crucial to study the ambient light field to understand PP and associated processes. The current study makes a dual effort to present PP estimates as well as understand the bio-optical variability in the Indian sector of the Southern Ocean (ISSO). Results suggest that PP was highest at Sub-Tropical Front (STF) and lowest at Polar Front-2 (PF2). Most PP profiles were characterized by subsurface maxima, indicating probable photoinhibition or micronutrient limitation at surface layer. Strong correlation between measured and satellite-based integrated PP (R2 = 0.94, RMSE = 77.48, p < 0.01) indicated the efficacy of global models in their original formulation in bio-optically complex SO waters. The maximum photochemical efficiency of phytoplankton (Fv/Fm) measured by fast repetition rate fluorometry varied from 0.1–0.4, implying reduced phytoplankton photosynthetic efficiency in ISSO. The ratio between remote sensing reflectance (Rrs)-derived phytoplankton absorption (aph) at blue-red band (B/R ratio) indicated dominance of smaller phytoplankton in surface and larger phytoplankton at subsurface. Higher Chl-a specific phytoplankton absorption (aph* ) than phytoplankton absorption (aph) suggested an adaptation of dominant phytoplankton species to low light, yet a better light harvest efficiency. However, low contribution of aph suggested a strong influence of non-phytoplankton materials to the total absorption budget. We therefore infer that, the surrounding physical environment in terms of nutrients and bio-optical variability modulated phytoplankton size class and thereby productivity more critically in the surface than in the deeper layers of ISSO.

Seasonal evolution of particulate and dissolved absorption coefficients in a subtropical estuary

The seasonal evolution of colored dissolved organic matter (CDOM), phytoplankton, and non-algal particulate absorption coefficients were measured for the first time along a salinity gradient within the Paranaguá Estuarine Complex (PEC), in southern Brazil. CDOM (mostly of autochthonous origin) inside the estuary exhibited a conservative behavior that dominates the absorption budget at lower wavelengths (~380 nm). On average, absorption by non-algal particles was more important than absorption by phytoplankton throughout the year and across all wavelengths, except for the maximum absorption band of chlorophyll-a (665–675 nm), where the algal component was predominant year-round. Phytoplankton optical properties were generally consistent with variability in phytoplankton cell counts, particularly during bloom periods. Simultaneous condition of high suspended matter concentrations and low turbidity conditions associated with sampling of mucilaginous materials released by centric diatoms exemplified the complex relationships between biogeochemical and optical properties in this region. The relationship between chlorophyll-a concentration and phytoplankton absorption exhibited seasonal oscillations related to the variation of algal assemblages, ultimately determining the magnitude of the packaging effect. Finally, separation of the signals of these three components in the visible domain is possible only in the near-ultraviolet range (380 nm), associated with CDOM dominance, and at 665 nm where the phytoplankton signature can be isolated.

Dynamic Satellite Seascapes as a Biogeographic Framework for Understanding Phytoplankton Assemblages in the Florida Keys National Marine Sanctuary, United States

Physical, chemical, geological, and biological factors interact in marine environments to shape complex but recurrent patterns of organization of life on multiple spatial and temporal scales. These factors define biogeographic regions in surface waters that we refer to as seascapes. We characterize seascapes for the Florida Keys National Marine Sanctuary (FKNMS) and southwest Florida shelf nearshore environment using multivariate satellite and in situ measurements of Essential Ocean Variables (EOVs) and Essential Biodiversity Variables (EBVs). The study focuses on three periods that cover separate oceanographic expeditions (March 11-18, May 9-13, and September 12-19, 2016). We collected observations on bio-optical parameters (particulate and dissolved spectral absorption coefficients), phytoplankton community composition, and hydrography from a ship. Phytoplankton community composition was evaluated using 1) chemotaxonomic analysis (CHEMTAX) based on high-performance liquid chromatography (HPLC) pigment measurements, and 2) analysis of spectral phytoplankton absorption coefficients (aphy). Dynamic seascapes were derived by combining satellite time series of sea surface temperature, chlorophyll-a concentration, and normalized fluorescent line height (nFLH) using a supervised thematic classification. The seascapes identified areas of different salinity and nutrient concentrations where different phytoplankton communities were present as determined by hierarchical cluster analyses of HPLC pigments and aphy spectra. Oligotrophic, Mesotrophic and Transition seascape classes of deeper offshore waters were dominated by small phytoplankton ( 60 %). Spectral analysis of aphy indicated higher absorption levels at 492 and 550 nm wavelengths in seascapes carrying predominantly small phytoplankton than in classes dominated by larger taxa. Seascapes carrying large phytoplankton showed absorption peaks at the 673 nm wavelength. The seascape framework promises to be a tool to detect different biogeographic domains quickly, providing information about the changing environmental conditions experienced by coral reef organisms including coral, sponges, fish and higher trophic levels. The effort illustrates best practices developed under the Marine Biodiversity Observation Network (MBON) demonstration project, in collaboration with the South Florida Ecosystem Restoration Research (SFER) project managed by the Atlantic Oceanographic and Meteorological Laboratory of NOAA (AOML-NOAA).