Phytoplankton Pigment Concentrations Research Articles

Diversity, Distribution and Abundance of Phytoplankton from Palghar Coast of Maharashtra

Diversity of phytoplankton and water quality of Dandi creek was studied during September 2009 to September 2010.Physicochemical parameters indicated variations in depth, pH, and temperature. DO, BOD, salinity, nitrite, nitrate, ammonium nitrate, phosphate etc. A total of 59 genera of phytoplankton were observed from Dandi creek system of which 50 genera were diatoms, 7 were dinoflagellates and 2 were other algae. Coscinodisccus was the most abundant genus in Dandi creek. Followed by Ditylum, Biddulphia, Bacillaria, Certatium, Nitzschia, Naviculla, Plurosigma, Rhizosolenia, Thalasiossira and Thalassiothrix. Variations showed in phytoplankton pigments at Dandi creek waters phytoplankton community. Variation of chlorophyll a ranged between 0.47 and 6.87 mg/m3 (av.3.45 mg/m3) and phaeophytin ranged between 0.18 and 2.8 mg/m3 (av.1.04 mg/m3). Concentration of phytoplankton pigments in the creek was fairly high. The chlorophyll a was higher in the outer zone than the interior zone of the creek. In general, phaeophytin level was lower than the chlorophyll a suggesting that the phytoplankton were in good physiological state. Seasonal variation in phytoplankton pigments was marginal. Phytoplankton pigments showed variations in concentration during pre-monsoon, monsoon and post monsoon seasons at 5 different stations of creek.

Toward a synthesis of phytoplankton community composition methods for global‐scale application

AbstractThe composition of the marine phytoplankton community has been shown to impact many biogeochemical processes and marine ecosystem services. A variety of methods exist to characterize phytoplankton community composition (PCC), with varying degrees of taxonomic resolution. Accordingly, the resulting PCC determinations are dependent on the method used. Here, we use surface ocean samples collected in the North Atlantic and North Pacific Oceans to compare high‐performance liquid chromatography pigment‐based PCC to four other methods: quantitative cell imaging, flow cytometry, and 16S and 18S rRNA amplicon sequencing. These methods allow characterization of both prokaryotic and eukaryotic PCC across a wide range of size classes. PCC estimates of many taxa resolved at the class level (e.g., diatoms) show strong positive correlations across methods, while other groups (e.g., dinoflagellates) are not well captured by one or more methods. Since variations in phytoplankton pigment concentrations are related to changes in optical properties, this combined dataset expands the potential scope of ocean color remote sensing by associating PCC at the genus‐ and species‐level with group‐ or class‐level PCC from pigments. Quantifying the strengths and limitations of pigment‐based PCC methods compared to PCC assessments from amplicon sequencing, imaging, and cytometry methods is the first step toward the robust validation of remote sensing approaches to quantify PCC from space.

Monthly average remote sensing dataset of phytoplankton pigment concentrations in the Bohai sea， Yellow sea and east China sea during 1998-2020

Monthly average remote sensing dataset of phytoplankton pigment concentrations in the Bohai sea， Yellow sea and east China sea during 1998-2020

Global estimation of phytoplankton pigment concentrations from satellite data using a deep-learning-based model

Based on a global matchup between satellite observations and high performance liquid chromatography (HPLC) measurements, we developed a deep-learning-based model (DL-PPCE model) for globally estimating concentrations of 17 different phytoplankton pigments. The model adopted a fusion architecture of residual and pyramid networks to achieve robust estimation performance. The model inputs include three different data types: essential ocean color parameters, satellite-derived environmental parameters, and the slope of above-surface remote-sensing reflectance (Rrs). We compared the model performances with various input parameters to determine the most effective inputs. The results showed that Rrs in the essential ocean color parameters and sea surface temperature (SST) in the environmental parameters were the most critical input parameters. The estimation of phytoplankton pigment concentrations was validated against HPLC data using the leave-one-out cross-validation method. Except for three pigments, 19′-butanoyloxy-fucoxanthin, prasinoxanthin, and lutein, the estimated pigment concentrations and in-situ observations were strongly correlated for all other pigments (an average relative root-mean-square error of 0.59, R2≥0.60, and regression slopes close to 1). In addition, a time series analysis was performed on the MODIS retrieved global pigment concentrations during 2003–2021 using the established DL-PPCE model to explore the relationship between the distribution of phytoplankton groups and El Niño in the western equatorial Pacific. Our findings revealed that the prokaryotes-dominated area extended eastward from180°E to 150°W during the 2015/2016 El Niño event. From 2003 to 2021, prokaryotic abundance was positively correlated with El Niño intensity (R=0.65,P≪0.01) but negatively correlated with the abundance of the entire phytoplankton community (R=−0.53,P≪0.01). These results demonstrate that the DL-PPCE model presents a novel approach for estimating the concentration of 17 pigments worldwide, and the estimated pigment concentrations are advantageous for analyzing the phytoplankton community dynamics on a large spatiotemporal scale.

Daily Spectral Ocean Surface Albedo due to Small Chlorophyll Concentrations and Cloudy Conditions for 440 nm Wavelength in Coastal Waters

The spectral daily Ocean Surface Albedo (OSA) is a sensitive parameter dependent on sea surface bio-optical properties and solar radiation distribution due to the transmissivity of the atmosphere. We analyzed the OSA sensitivity to the small concentrations of sea surface phytoplankton due to cloudy conditions, based on measured half-hourly global radiation data, and phytoplankton variability represented by chlorophyll-a concentrations at the upper layers over the Malacca Strait. The influence of the sea surface phytoplankton was examined by using the chlorophyll-a maximum absorption wavelength (440 nm) for a detailed examination of the real phytoplankton impact presented by small concentrations (0.39 - 0.69 mg/m3). The atmosphere transmittance determination was made over the period from January 2016 to March 2016 and January 2017 to March 2017 using hourly clearness index (KTh) estimation. The aim of current study was to examine the influence of sea surface phytoplankton concentrations on the radiation budget and to account the role that the phytoplankton plays in detail short-scale OSA parameterization. Daily timescale spectral OSA includes the bio-optical modelling part, which allowed us to investigate the patterns of diurnal variability of the main reflectance parameters, using Jin et al. scheme for albedo components (direct and diffuse) estimation, was computed. The OSA variability analysis confirmed the bio-optical feedback presented by apparent properties of the coastal waters for the observed conditions. The proposed calculation scheme accounted for the sea surface optical behavior with low concentrations of chlorophyll-a and suggests that albedo variability depends not only on the angle factors, even with the low phytoplankton influence (less than 1 mg/m3). It was found that the phytoplankton pigment absorption properties have less effect in albedo parameterization while the chlorophyll-a concentrations are less than 0.5 mg/m3.

Seasonal variability of satellite-derived primary production in the South China Sea from an absorption-based model

Seasonal patterns of marine net primary production (NPP) are crucial for understanding the marine ecosystem and biogeochemical cycles. Uncoupling of seasonal variations between NPP and phytoplankton chlorophyll-a over different areas has attracted much attention. In this study, following a review of previous studies, monthly climatological NPP data from 2003 to 2020, estimated using the Size-fractioned Phytoplankton Pigment Absorption (aph)-based NPP Model (SABPM), were selected to study the seasonal variability of NPP in the South China Sea (SCS). Results showed the spatial differences of NPP seasonality and its departures from climatology in extreme El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) years. Cluster analysis for climatological monthly data identified significant differences of NPP seasonality in five typical regions. In coastal regions along the northern SCS and off eastern Vietnam, NPP exhibited the most obvious seasonal cycle with maximum (minimum) values in summer (winter), attributable mainly to river discharge and summer upwelling. In regions off northwestern Luzon and coast of southern SCS, NPP showed peaks in winter, which were related to strong mixing and upwelling. In northwestern SCS, NPP was high during May–September in phase with sea surface temperature and the primary controlling factors were found to be shallow nutricline depth and wind-driven mixing. Owing to the deep nutricline depth in the central basin, NPP exhibited little seasonal variability; only a weak signal was observed in spring in phase with photosynthetically active radiation. Local dynamics on regulating the nutrient supply and light availability contribute to these regional differences in NPP seasonality, which could also be affected by extreme climate events. The largest anomalies of the NPP seasonal cycle coincide with 2015/2016 ENSO and super IOD in 2020. During these events, enhanced (weakened) westerly winds caused fall (rise) of SLA and increase (decrease) of NPP in coastal regions along the northern SCS and that off eastern Vietnam. Overall, theaph-based model shows a new perspective to study the spatiotemporal variations of NPP in the SCS.

Integrating phytoplankton pigment and DNA meta‐barcoding observations to determine phytoplankton composition in the coastal ocean

AbstractQuantifying phytoplankton composition is critical to predicting marine ecosystem structure and function. DNA meta‐barcoding and high‐performance liquid chromatography (HPLC) pigment analysis are two widely used methods for assessing phytoplankton composition; however, comparing their performance has been done only rarely. Here, we integrate DNA meta‐barcoding and HPLC pigment observations to determine eukaryotic phytoplankton composition in the Santa Barbara Channel, California. We find that both methods identify the same four dominant eukaryotic phytoplankton taxa (diatoms, dinoflagellates, chlorophytes, and prymnesiophytes), but inter‐ and intra‐lineage variability in biomarker pigmentation (associated with both a lack of taxonomic specificity of biomarker pigments and intrinsic differences in accessory pigmentation) drives substantial disagreement between the methods. Covariation network analysis circumvents this disagreement and reveals that diverse assemblages of phytoplankton and other protists covary with distinct suites of biomarker pigments. Our results highlight the strengths and weaknesses of each method in characterizing phytoplankton composition and reveal novel insights into phytoplankton physiology that could only be gained by integrating the two methods. Finally, we suggest a path to monitor eukaryotic plankton communities on unprecedented spatiotemporal scales based on the covariation of unique phytoplankton and protistan assemblages with remotely sensible phytoplankton pigment concentrations.

Retrieving Pigment Concentrations Based on Hyperspectral Measurements of the Phytoplankton Absorption Coefficient in Global Oceans

Phytoplankton communities, which can be easily observed by optical sensors deployed on various types of platforms over diverse temporal and spatial scales, are crucial to marine ecosystems and biogeochemical cycles, and accurate pigment concentrations make it possible to effectively derive information from them. To date, there is no practical approach, however, to retrieving concentrations of detailed pigments from phytoplankton absorption coefficients (aph) with acceptable accuracy and robustness in global oceans. In this study, a novel method, which is a stepwise regression method improved by early stopping (the ES-SR method) based on the derivative of hyperspectral aph, was proposed to retrieve pigment concentrations. This method was developed from an extensive global dataset collected from layers at different depths and contains phytoplankton pigment concentrations and aph. In the case of the logarithm, strong correlations were found between phytoplankton pigment concentrations and the absolute values of the second derivative (aph″)/the fourth derivative (aph4) of aph. According to these correlations, the ES-SR method is effective in obtaining the characteristic wavelengths of phytoplankton pigments for pigment concentration inversion. Compared with the Gaussian decomposition method and principal component regression method, which are based on the derivatives, the ES-SR method implemented on aph″ is the optimum approach with the greatest accuracy for each phytoplankton pigment. More than half of the determination coefficient values (R2log) for all pigments, which were retrieved by performing the ES-SR method on aph″, exceeded 0.7. The values retrieved for all pigments fit well to the one-to-one line with acceptable root mean square error (RMSElog: 0.146–0.508) and median absolute percentage error (MPElog: 8.2–28.5%) values. Furthermore, the poor correlations between the deviations from the values retrieved by the ES-SR method and impact factors related to pigment composition and cell size class show that this method has advantageous robustness. Therefore, the ES-SR method has the potential to effectively monitor phytoplankton community information from hyperspectral optical data in global oceans.

Inherent optical properties of suspended particulate matter in the southern Baltic Sea in relation to the concentration, composition and characteristics of the particle size distribution; new forms of multicomponent parameterizations of optical properties

This paper reports on detailed studies of the relationship between the inherent optical properties (IOPs) of suspended particulate matter populations occurring in southern Baltic Sea waters, and the basic biogeochemical and morphological quantities enabling these populations to be generally characterized. Specifically, these IOPs are the spectral absorption coefficients by all particulate matter, phytoplankton pigments and detritus, the spectral scattering and backscattering coefficients by particles, and the backscattering ratio. The biogeochemical properties of suspended matter populations are characterized by measuring concentrations of suspended particulate matter (SPM), particulate organic matter (POM) and particulate inorganic matter (PIM), as well as concentrations of various phytoplankton pigments, including chlorophyll a (Chla). We also take particle size distributions (PSDs) into account. This diverse empirical material is used to describe how the typical relationships between the IOPs of suspended matter and the concentrations of the main water constituents (SPM or Chla) can be influenced by differences in the general composition and size distributions of the suspended matter. We statistically describe the possible impact on these relationships of the variability of composition ratios, such as POM/SPM and Chla/SPM, as well as the impact of the variability of certain characteristics calculated on the basis of PSDs: the average particle diameter weighted by the particle's projected area (DA), the average apparent density of particles (ρa), the average intracellular concentration of chlorophyll a (ci), and the corresponding products of these quantities. We also demonstrate the significant variability of the specific IOPs (SIOPs) by attempting to determine them in the classical way for our southern Baltic data, and we give a possible interpretation of this variability. On the basis of these analyses, we derive new, multicomponent IOP parameterizations that are functions of several variables. These new parameterizations enable the IOPs of suspended matter to be characterized and predicted with better accuracy than with the often-used standard one-component relationships.

Two-decadal estimation of sixteen phytoplankton pigments from satellite observations in coastal waters

Knowledge of the marine phytoplankton communities is vital for understanding the carbon cycle, ecological and biogeochemical processes, and climate change. Phytoplankton assemblages contain multiple pigments that play a significant role in describing the phytoplankton communities. Thus, detecting the concentrations of phytoplankton pigments is of great significance. Here, an extensive in situ observation dataset, including the phytoplankton absorption coefficient, concentrations of phytoplankton pigments, and remote sensing reflectance, was collected during seven cruise surveys in the Bohai Sea, Yellow Sea, and East China Sea. This study developed a customized absorption-based remote sensing model using the Gaussian decomposition technique, which closely links phytoplankton absorption characteristics with various pigments. This model was repeatedly assessed to guarantee its stable performance, and was evaluated using the in situ observation dataset and the leave-one-out cross-validation method, yielding results of competitive and acceptable errors (e.g., mean absolute percentage errors (MAPEs) below ∼60% for most pigments). Satellite match-up validation also generated promising predictive errors, with the derived MAPEs in a ∼10%–40% range for most pigments. Another contribution of our study is the application of the developed model to a long time series of satellite data (1998–2019), and the 22-year data record that documents the spatiotemporal variation of 16 pigment concentrations in the study areas. Accordingly, the long-term spatial and temporal evolution rules of 16 pigments are reported for the first time. This work concludes that the pigments (as the second-order delegates of phytoplankton pigments, compared to chlorophyll-a) can be retrieved from satellite data, despite the presence of optically complex turbid coastal waters, thus providing refined characteristics on phytoplankton assemblages for future marine research.

Long-term changes in phytoplankton pigment contents in the Baltic Sea: Trends and spatial variability during 20 years of investigations

The changes in phytoplankton pigment compositions and concentrations during a 20-year period of exploration of the Baltic Sea have been analysed. Both chlorophylls and carotenoids are characterized in the context of their monthly variations in the Gulf of Gdańsk and the open waters of southern Baltic Sea. It is assumed that pigment compositions are good markers of environmental changes, especially in the context of phytoplankton composition and biomass growth. Long-term trends and changes for both regions are established on the basis of marker pigment analyses for a period of two decades (1999–2018). Chlorophyll a, a marker of phytoplankton biomass, is clearly decreasing in both regions: −2.11% yr−1 (slope of trend curve −0.21) in the Gulf of Gdańsk and −2.41% yr−1 (slope −0.08) in the southern Baltic. Decreasing trends for several marker pigments have also been noted in both regions for the period in question, namely, fucoxanthin (diatom marker): −0.10% yr−1 in the Gulf of Gdańsk and the southern Baltic (respective slopes −0.0012 and −0.0003); alloxanthin (cryptophyte marker): −2.28% yr−1 (slope −0.012) in the Gulf of Gdańsk and −0.54% yr−1 (slope −0.001) in the southern Baltic; peridinin (dinoflagellate marker): −4.35% yr−1 (slope −0.05) in the Gulf of Gdańsk and −5.55% yr−1 (slope −0.02) in the southern Baltic. The decreases in the average concentrations of chlorophyll b (−1.30% yr−1, slope −0.008) and neoxanthin (−1.37% yr−1, slope −0.0014) have been slower in the Gulf of Gdańsk than in the southern Baltic (−3.31% yr−1, slope −0.013, and −3.38% yr−1, slope −0.0019 – respectively). In contrast, the yearly mean concentrations of zeaxanthin (the main cyanobacteria carotenoid) have increased significantly, to 7.16% yr−1 (slope 0.016) in the Gulf of Gdańsk and 1.45% yr−1 (slope 0.0022) in the southern Baltic. This may be the consequence of the reduced riverine supply of nutrients to these regions in recent years. The results indicate that the ecosystem of the Gulf of Gdańsk is responding more dynamically to eutrophication-related changes than the open waters of the southern Baltic Sea.

Modeling surface ocean phytoplankton pigments from hyperspectral remote sensing reflectance on global scales

Phytoplankton community composition impacts food webs, climate, and fisheries on regional and global scales, and can be assessed at coarse taxonomic resolution from biomarker pigments measured using high-performance liquid chromatography (HPLC). Presently, satellite ocean color provides unprecedented coverage of the global surface ocean and offers reliable estimates of bulk biological properties; however, existing multispectral sensors have limited ability to provide information about phytoplankton community composition. Satellite ocean color at hyperspectral resolution (e.g., NASA's upcoming Plankton, Aerosol, Cloud, and ocean Ecosystem sensor, PACE) is expected to improve estimates of phytoplankton community composition from space. Phytoplankton impact ocean color via contributions to absorption and fluorescence (through phytoplankton pigments) and scattering, especially on narrow spectral scales (5–100 nm). Here, a global open ocean dataset of concurrent HPLC pigments and hyperspectral remote sensing reflectance (Rrs(λ)) observations is used to model phytoplankton pigment composition from optical data. Phytoplankton pigments are reconstructed from Rrs(λ) using optimized principal components regression modeling. This work demonstrates that thirteen phytoplankton pigments, representing five phytoplankton pigment groups (e.g., diatoms, dinoflagellates, haptophytes, green algae, and cyanobacteria), can be modeled from hyperspectral Rrs(λ). Spectral information needed to model each phytoplankton pigment concentration is found throughout the entire visible spectrum and the model results are best at high spectral resolution (≤5 nm). The resulting model recreates observed relationships among pigment concentrations, providing support for the designation of five pigment-based phytoplankton groups for the global open ocean. This work represents a step toward developing robust, global spectral models for phytoplankton pigment composition. However, more high-quality data from a wide range of ecosystems and environments are still needed to achieve this goal.

XGBoost-Based Inversion of Phytoplankton Pigment Concentrations from Field Measured Fluorescence Excitation Spectra

XGBoost-Based Inversion of Phytoplankton Pigment Concentrations from Field Measured Fluorescence Excitation Spectra

Resilience of phytoplankton dynamics to trophic cascades and nutrient enrichment

AbstractResilience was compared for alternate states of phytoplankton pigment concentration in two multiyear whole‐lake experiments designed to shift the manipulated ecosystem between alternate states. Mean exit time, the average time between threshold crossings, was calculated from automated measurements every 5 min during summer stratification. Alternate states were clearly identified, and equilibria showed narrow variation in bootstrap analysis of uncertainty. Mean exit times ranged from 13 to 290 h. In the reference ecosystem, Paul Lake, mean exit time of the low‐pigment state was about 100 h longer than mean exit time of the high‐pigment state. In the manipulated ecosystem, Peter Lake, mean exit time of the high‐pigment state exceeded that of the low‐pigment state by 30 h in the cascade experiment. In the enrichment experiment mean exit time of the low‐pigment state was longer than that of the high‐pigment state by about 100 h. Mean exit time is a useful measure of resilience for stochastic ecosystems where high‐frequency measurements are made by consistent methods over the full range of ecosystem states.

Diagnosing seasonal to multi-decadal phytoplankton group dynamics in a highly productive coastal ecosystem

The Santa Barbara Channel, CA (SBC) is a biodiverse marine ecosystem fueled largely by phytoplankton productivity, and the composition of the phytoplankton community influences the magnitude and fates of this productivity. Here, we create a 22-year monthly time series of phytoplankton biomarker pigment concentrations in the SBC by combining 12 years of high performance liquid chromatography phytoplankton pigment concentrations with bio-optical models and 10 additional years of bio-optical observations. The bio-optical models skillfully predict biomarker pigment concentrations representative of five distinct phytoplankton groups (PGs; diatoms, dinoflagellates, chlorophytes, prymnesiophytes, and picophytoplankton) and resolve seasonal responses to the annual upwelling-relaxation cycle for all PGs except the dinoflagellates. Our observations indicate that nanophytoplankton groups respond most rapidly to seasonal upwelling, followed by diatoms, and then by picophytoplankton as the water column stratifies in the summer. A Regional Ocean Modeling System (ROMS) solution is used to relate advection of different source waters to the observed PG dynamics. The ROMS simulation results suggest that, on seasonal time scales, pronounced cross-SBC differences in PG seasonality are related to cross-SBC differences in source waters. El Niño Southern Oscillation events drive interannual variability in the upwelling response of most PGs. On decadal time scales, dinoflagellate blooms are associated with the warm phase of the North Pacific Gyre Oscillation and anomalous advection of Southern California Bight source waters into the SBC. Taken together, our results provide a novel view of phytoplankton community succession in response to seasonal upwelling by considering the dynamics of pico- and nano-phytoplankton and suggest that regional surface ocean advection plays a substantial role in driving phytoplankton composition in the SBC.

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.

A comparative study of phytoplankton epi- and metalimnetic communities under different light and thermal regimes

Our study explores the variability of chlorophyll a and phycocyanin as well as the taxonomic and functional structure of phytoplankton in epilimnetic and metalimnetic strata regarding the main environmental features of the water column. The maximum phytoplankton pigment concentrations are located differently depending on the type of thermal stratification and light availability. In general, there were three situations observed during the summer stratification: the euphotic zone did not extend to the thermocline and the highest chlorophyll concentration developed above the thermocline level, at the bottom of the euphotic zone (type I); the euphotic zone reached beyond the thermocline, and the maximum chlorophyll concentration occurred above the thermocline (type II); or the euphotic zone was much greater than the thermocline depth and the peak phytoplankton density was located below the thermocline (type III). In metalimnetic samples, the most dominant cyanobacteria belonged to the Reynolds Functional Groups (RFG) H1, S1 , and Lo ; their occurrence differed depending on the types of light and thermal regimes. It was assumed that vertical heterogeneity affects the ecological status assessment based on phytoplankton. Calculating the two applied indices (TI index and Q index) based on metalimnetic and epilimnetic data (as opposed to only epilimnetic data) yielded worse ecological status in > 70% of cases.

Exploration of an ice-cliff grounding zone in Antarctica reveals frozen-on meltwater and high productivity

Ice fluxes across the grounding zone affect global ice-sheet mass loss and sea level rise. Although recent changes in ice fluxes are well constrained by remote sensing, future projections remain uncertain, because key environments affecting ice-sheet dynamics – the ice-sheet bed and grounding zone – are largely unknown. Here, we used a remotely operated vehicle to explore the grounding zone of a Weddell Sea tidewater ice cliff. At 148 m we found a 0.3–0.5 m gap between the ice and the seafloor and a 0.4 m clear facies of debris- and bubble-free basal ice, suggesting freeze-on of meltwater in the distal marine portion of the ice sheet over the last 400 yr. Ploughmarks and low epifauna cover reveal recent grounding line retreat, as corroborated by satellite remote sensing. We found dense algal tufts on the ice cliff and high phytoplankton pigment concentrations, suggesting high productivity fuelled by nutrients from ice melt. As grounded tidewater ice cliffs rim 38% of the Antarctic continent, sinking and downwelling of organic matter along with low benthic turnover may contribute to enhanced carbon sequestration, providing a potentially important feedback to climate.

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.

Long-term dynamics of a cladoceran community from an early stage of lake formation in Lake Fukami-ike, Japan.

An increase in nutrient levels due to eutrophication has considerable effects on lake ecosystems. Cladocerans are intermediate consumers in lake ecosystems; thus, they are influenced by both the bottom‐up and top‐down effects that occur as eutrophication progresses. The long‐term community succession of cladocerans and the effects cladocerans experience through the various eutrophication stages have rarely been investigated from the perspective of the early‐stage cladoceran community assemblage during lake formation. In our research, long‐term cladoceran community succession was examined via paleolimnological analysis in the currently eutrophic Lake Fukami‐ike, Japan. We measured the concentration of total phosphorus and phytoplankton pigments and counted cladoceran and other invertebrate subfossils in all layers of collected sediment cores, and then assessed changes in the factors controlling the cladoceran community over a 354‐year period from lake formation to the present. The cladoceran community consisted only of benthic taxa at the time of lake formation. When rapid eutrophication occurred and phytoplankton increased, the benthic community was replaced by a pelagic community. After further eutrophication, large Daphnia and high‐order consumers became established. The statistical analysis suggested that bottom‐up effects mainly controlled the cladoceran community in the lake's early stages, and the importance of top‐down effects increased after eutrophication occurred. Total phosphorus and phytoplankton pigments had positive effects on pelagic Bosmina, leading to the replacement of the benthic cladoceran community by the pelagic one. In contrast, the taxa established posteutrophication were affected more by predators than by nutrient levels. A decrease in planktivorous fish possibly allowed large Daphnia to establish, and the subsequent increase in planktivorous fish reduced the body size of the cladoceran community.