Phenology Of Phytoplankton Blooms Research Articles

Extreme mismatch between phytoplankton and grazers during Arctic spring blooms and consequences for the pelagic food-web

Food-web structure determines the cycling pathways and fate of new production in marine ecosystems. Herbivorous zooplankton populations are usually seasonally coupled with pelagic primary producers. Synchrony of phytoplankton blooms with reproduction, recruitment and seasonal ascent of their main grazers ensures efficient transfer of organic carbon to higher trophic levels, including commercially harvested species, especially in high-latitude systems. Changes in light, nutrient, and sea-ice dynamics due to accelerating climate change in the Arctic, however, create large uncertainties in how these systems will function in the future. To address such knowledge gaps, we surveyed the pelagic ecosystem of the Barents Sea Polar Front in May of two consecutive years (2021 and 2022) to investigate the pelagic food-web from primary producers to planktivorous fish. In both years we observed unprecedentedly high phytoplankton chlorophyll a values in open as well as ice-covered waters, much of which was invisible to satellite remote sensing. We also measured very low densities of grazing zooplankton across a wide area and extending for at least one month. This extreme mismatch resulted in low feeding by capelin, and further suggests a high potential for vertical export of carbon to the benthos rather than efficient assimilation into the pelagic food web. As the Arctic continues to warm and is characterized by thinner and more mobile sea ice, we may expect higher variability in phytoplankton bloom phenology and more frequent mismatches with grazer life-histories. This could have significant impacts on ecosystem functioning by re-directing the flow of energy through the system towards seafloor rather than to the production of commercially valuable pelagic marine resources.

The phenology of the spring phytoplankton bloom in the North Atlantic does not trend with temperature

Climate change is anticipated to alter the phenology of phytoplankton blooms in the ocean, making their recent dynamics of interest to inform models of future ocean states. We characterized temperature change in the North Atlantic using metrics that track the patterns of sea surface water temperature (SST) defined by quantiles. To complement these thermal indicators, we estimated a thermal phenology index in the form of the date of the spring transition, taken as the date that temperature achieved the long-term mean at a specific location. We then used ocean color data (1998–2022) and characterized spring bloom phenology using change point methods to derive bloom initiation, duration, magnitude, and intensity. The North Atlantic has warmed over recent decades, averaging a rate of increase of 0.27°C decade−1, yet throughout most of the basin, spring transition timing has remained constant, with the exception of small areas with either delayed or advanced transitions. There were no clear trends in bloom start or duration in the North Atlantic, indicating that spring bloom phenology was independent of climate-driven temperature change. Bloom magnitude and intensity trended downward in some North Atlantic continental shelf seas, indicating that increased temperatures may have had negative effects on overall bloom productivity. However, exclusive of the areas where the bloom parameters were trending, there was a decrease in magnitude and intensity with warmer winter temperatures, suggesting that the inter-annual variability of these parameters may be affected by thermal conditions at the onset of the bloom. While temperature has increased in the North Atlantic, vernal light availability has remained unchanged, which may explain why spring bloom phenology has remained resistant to climate change. Consequently, it seems plausible that future climate change may have limited effects on spring bloom phenology, but could have substantial effects on overall phytoplankton production.

Phytoplankton spring bloom in the NW Mediterranean Sea under climate change

The spring phytoplankton bloom is the main event influencing ecosystem richness in the pelagic realm of the Northwestern Mediterranean Sea (NW Med Sea). The Marine Strategy Framework Directive requires the achievement of a good ecological status for the pelagic habitat, and phytoplankton bloom phenology has been used as an indicator of the status of offshore waters. In this work we investigate interannual changes in the timing and magnitude of the phytoplankton bloom in the NW Med Sea, using phenological metrics. Daily maps of Chl-a concentration from 1998 to 2022 obtained by CMEMS were used to analyse bloom phenological metrics in 5 representative sites in the area. Chlorophyll-a data from 1998 to 2007 were used for determining the climatological behaviour, while 2008–2022 was identified as the study period. For this latter period, yearly spring bloom were identified and interannual variability and overall trends were analysed for each of the phenological metrics considered. Winter oceanographic and meteorological data were analysed to investigate possible correlations with the subsequent spring bloom. The frequency of anomalous years is increasing, both for bloom intensity and sea temperature. Bloom analysis revealed a negative trend only in some areas, but a steep decrease in the last 7 years was noticeable for all sites considered. Correlations of the Chl-a concentration during bloom with oceanographic variables revealed the importance of temperature, both marine and atmospheric, while Mixed Layer Depth played a lesser role. This work contributes to a better understanding of the dynamics of an area already under severe threat from human activities.

Spring phytoplankton bloom phenology during recent climate warming on the Bering Sea shelf

High-latitude ecosystems commonly experience large phytoplankton blooms in spring, which provide basal resources for a range of grazers including zooplankton, benthic consumers and fishes. Variation of the timing and intensity of the spring phytoplankton bloom influences the degree of spatial and temporal overlap with consuming organisms. In the Bering Sea, blooms occur in association with ice retreat or as pelagic open-water blooms. In the last few years, the Bering Sea shelf experienced unprecedented and widespread warming. Understanding how those climatic changes subsequently influenced phytoplankton bloom dynamics is critical for evaluating Bering Sea food web responses. We estimate spring bloom timing and type (ice-associated, open-water) across the Bering Sea shelf using a combination of data from the long-running oceanographic moorings on the eastern shelf (M2, M4, M5, M8) and satellite ocean color data from 1998 to 2022. We assess 1) if the Bering Sea shelf experienced noticeable changes in spring bloom timing or type in the last two decades, 2) whether bloom phenology was accentuated by the recent warm period (2018–2019), and 3) what influences do winds and sea surface temperatures have on spring bloom timing and where are these variables influential? Our spatial analyses reinforce the conclusion that ice retreat is the dominant forcing factor of bloom timing for the Bering Sea shelf with some influence of wind for open-water blooms. Overall, bloom timing has not shifted seasonally with climate warming in the last two decades for most of the Bering Sea shelf, except for nearshore areas and mainly in the northern Bering Sea. In warm years when ice retreats early prior to the last week of March, blooms form in open waters and bloom timing on the middle and outer shelf is delayed when wind mixing is prevalent in ice-free springs. In recent years, open-water blooms were more widespread than previously experienced, and even occurred in the northern Bering Sea during 2018–2019. A progression to more open water blooms in a future warmer climate will influence the availability of basal resources for pelagic and benthic consumers.

Mapping Algal Blooms in Aquatic Ecosystems Using Long-Term Landsat Data: A Case Study of Yuqiao Reservoir from 1984–2022

Water eutrophication poses a dual threat to ecological and human well-being. Gaining insight into the intricate dynamics of phytoplankton bloom phenology holds paramount importance in comprehending the complexities of aquatic ecosystems. Remote sensing technologies have gained attention for mapping algal blooms (ABs) effectively, but distinguishing them from aquatic vegetation (AV) remains challenging due to their similar spectral characteristics. To address this issue, we propose a meticulous three-step methodology for AB mapping employing long-term Landsat imagery. Initially, a multi-index decision tree model (DTM) is deployed to identify the vegetation signal (VS) encompassing both AV and ABs. Subsequently, the annual maximum growth range of AV is precisely delineated using vegetation presence frequency (VPF) in conjunction with normal and low water level imagery. Lastly, ABs are accurately extracted by inversely intersecting VS and AV. The performance of our approach is thoroughly validated using the interclass correlation coefficient (ICC) based on a Gaofen-2 Panchromatic Multi-spectral (GF-2 PMS) image, demonstrating strong consistency with notable values of 0.822 longitudinally, 0.771 latitudinally, and 0.797 overall. The method is applied to Landsat images from 1984 to 2022 to quantify the spatial distribution and temporal variations of ABs in Yuqiao Reservoir—a significant national water body spanning a vast area of 135 km2 in China. Our findings reveal a pervasive and uneven dispersion of ABs, predominantly concentrated in the northern sector. Notably, the intensity of ABs experienced an initial surge from 1984 to 2008, followed by a subsequent decline from 2014 to 2022. Importantly, anthropogenic activities, such as fish cage culture, alongside pollution stemming from nearby industrial and agricultural sources, exert a profound influence on the dynamics of water eutrophication. Fortunately, governmental initiatives focused on water purification exhibit commendable efficacy in mitigating the ecological burden on reservoirs and upholding water quality. The methodological framework presented in this study boasts remarkable precision in AB extraction and exhibits considerable potential in addressing the needs of aquatic ecosystems.

Recent Changes of Phytoplankton Bloom Phenology in the Northern High‐Latitude Oceans (2003–2020)

AbstractEighteen years (2003–2020) of data on remotely‐sensed ocean color in high northern latitudes show a significant increase (0.56% yr−1) in double blooms of phytoplankton. Starting in 2015 the rate of increase rose to 1.10% yr−1. Double blooms have become especially prevalent in regions north of 50°N in the North Atlantic, Gulf of Alaska and southern Chukchi Sea. Our correlation analysis suggests that double bloom occurrences are dependent on latitude and seasonal changes in both sea surface temperature (SST) and mixed layer depth (MLD), particularly in the subpolar North Pacific (SNP) and polar oceans (65°–75°N), although we cannot attribute the increased double blooms to specific causes. Single blooms indicated by Chlorophyll a (Chl a) dominate in subpolar‐polar waters and occur close to the timing of the Particulate Inorganic Carbon (PIC) peak (i.e., June–August). In the SNP single bloom, Chl a peaks occur earlier than PIC peaks. This interval was shorter during 2016–2020, which is associated with warming anomalies. In the southern part of the Chukchi Sea, however, we found that PIC peaks occur earlier than Chl a peaks, suggesting a potentially different succession pattern between calcifiers and other functional groups. We speculate that in the near future double blooms of phytoplankton will become a prevalent feature in high‐latitude oceans with concurrent changes in the timing of blooms and seasonal phytoplankton succession in the surface ocean due to climate change.

Trophic level decoupling drives future changes in phytoplankton bloom phenology

Climate change can drive shifts in the seasonality of marine productivity, with consequences for the marine food web. However, these alterations in phytoplankton bloom phenology (initiation and peak timing), and the underlying drivers, are not well understood. Here, using a 30-member Large Ensemble of climate change projections, we show earlier bloom initiation in most ocean regions, yet changes in bloom peak timing vary widely by region. Shifts in both initiation and peak timing are induced by a subtle decoupling between altered phytoplankton growth and zooplankton predation, with increased zooplankton predation (top-down control) playing an important role in altered bloom peak timing over much of the global ocean. Only in limited regions is light limitation a primary control for bloom initiation changes. In the extratropics, climate-change-induced phenological shifts will exceed background natural variability by the end of the twenty-first century, which may impact energy flow in the marine food webs. The authors show earlier future phytoplankton bloom initiation timing in most oceans, while shifts in bloom peak timing will vary widely by region. In the extratropics, these phenological changes will exceed background natural variability by the end of the twenty-first century.

Mesoscale Eddies Regulate Seasonal Iron Supply and Carbon Drawdown in the Drake Passage

AbstractThe Southern Ocean is a major region for uptake of anthropogenic carbon. The transport of carbon and nutrients are dominated by the Antarctic Circumpolar Current and its rich mesoscale eddy field; however, the current generation of Earth System Models (ESMs) lack the resolution to resolve eddies. Here we show that a computational model that explicitly represents mesoscale eddies can reproduce observed phases and amplitudes of seasonal biological productivity and partial pressure of carbon dioxide. Our sensitivity study demonstrates that when eddies are suppressed or parameterized, the model cannot reproduce these seasonal cycles. Experiments with suppressed eddy activity show that the lack of eddies significantly changes the iron supply and phenology of phytoplankton blooms. The mismatch in the timing and intensity of the bloom causes significant biases in the seasonal carbon cycle of the region with implications to the enigmatic biases in partial pressure of carbon dioxide among the state‐of‐the‐art ESMs.

Twenty-One Years of Phytoplankton Bloom Phenology in the Barents, Norwegian, and North Seas

Phytoplankton blooms provide biomass to the marine trophic web, contribute to the carbon removal from the atmosphere and can be deadly when associated with harmful species. This points to the need to understand the phenology of the blooms in the Barents, Norwegian, and North seas. We use satellite chlorophyll-a from 2000 to 2020 to assess robust climatological and the interannual trends of spring and summer blooms onset, peak day, duration and intensity. Further, we also correlate the interannual variability of the blooms with mixed layer depth (MLD), sea surface temperature (SST), wind speed and suspended particulate matter (SPM) retrieved from models and remote sensing. The climatological spring blooms start on March 10th and end on June 19th. The climatological summer blooms begin on July 13th and end on September 17th. In the Barents Sea, years of shallower mixed layer (ML) driven by both calm waters and higher freshwaters input keeps the phytoplankton in the euphotic zone, causing the spring bloom to start earlier and reach higher biomass but end sooner due to the lack of nutrients upwelling from the deep. In the Norwegian Sea, a correlation between SST and the spring blooms is found. Here, warmer waters are correlated to earlier and stronger blooms in most regions but with later and weaker blooms in the eastern Norwegian Sea. In the North Sea, years of shallower ML reduces the phytoplankton sinking below the euphotic zone and limits the SPM increase from the bed shear stress, creating an ideal environment of stratified and clear waters to develop stronger spring blooms. Last, the summer blooms onset, peak day and duration have been rapidly delaying at a rate of 1.25-day year–1, but with inconclusive causes based on the parameters assessed in this study.

Phenology and Environmental Control of Phytoplankton Blooms in the Kong Håkon VII Hav in the Southern Ocean

Knowing the magnitude and timing of pelagic primary production is important for ecosystem and carbon sequestration studies, in addition to providing basic understanding of phytoplankton functioning. In this study we use data from an ecosystem cruise to Kong Håkon VII Hav, in the Atlantic sector of the Southern Ocean, in March 2019 and more than two decades of satellite-derived ocean color to study phytoplankton bloom phenology. During the cruise we observed phytoplankton blooms in different bloom phases. By correlating bloom phenology indices (i.e., bloom initiation and end) based on satellite remote sensing to the timing of changes in environmental conditions (i.e., sea ice, light, and mixed layer depth) we studied the environmental factors that seemingly drive phytoplankton blooms in the area. Our results show that blooms mainly take place in January and February, consistent with previous studies that include the area. Sea ice retreat controls the bloom initiation in particular along the coast and the western part of the study area, whereas bloom end is not primarily connected to sea ice advance. Light availability in general is not appearing to control the bloom termination, neither is nutrient availability based on the autumn cruise where we observed non-depleted macronutrient reservoirs in the surface. Instead, we surmise that zooplankton grazing plays a potentially large role to end the bloom, and thus controls its duration. The spatial correlation of the highest bloom magnitude with marked topographic features indicate that the interaction of ocean currents with sea floor topography enhances primary productivity in this area, probably by natural fertilization. Based on the bloom timing and magnitude patterns, we identified five different bloom regimes in the area. A more detailed understanding of the region will help to highlight areas with the highest relevance for the carbon cycle, the marine ecosystem and spatial management. With this gained understanding of bloom phenology, it will also be possible to study potential shifts in bloom timing and associated trophic mismatch caused by environmental changes.

Assessing Phytoplankton Bloom Phenology in Upwelling-Influenced Regions Using Ocean Color Remote Sensing

Phytoplankton bloom phenology studies are fundamental for the understanding of marine ecosystems. Mismatches between fish spawning and plankton peak biomass will become more frequent with climate change, highlighting the need for thorough phenology studies in coastal areas. This study was the first to assess phytoplankton bloom phenology in the Western Iberian Coast (WIC), a complex coastal region in SW Europe, using a multisensor long-term ocean color remote sensing dataset with daily resolution. Using surface chlorophyll a (chl-a) and biogeophysical datasets, five phenoregions (i.e., areas with coherent phenology patterns) were defined. Oceanic phytoplankton communities were seen to form long, low-biomass spring blooms, mainly influenced by atmospheric phenomena and water column conditions. Blooms in northern waters are more akin to the classical spring bloom, while blooms in southern waters typically initiate in late autumn and terminate in late spring. Coastal phytoplankton are characterized by short, high-biomass, highly heterogeneous blooms, as nutrients, sea surface height, and horizontal water transport are essential in shaping phenology. Wind-driven upwelling and riverine input were major factors influencing bloom phenology in the coastal areas. This work is expected to contribute to the management of the WIC and other upwelling systems, particularly under the threat of climate change.

Calving Event Led to Changes in Phytoplankton Bloom Phenology in the Mertz Polynya, Antarctica

AbstractPolynyas are subject to variability in winds and ocean circulation and are important sites of ecological productivity. In February 2010, the B09B iceberg collided with the Mertz Glacier Tongue (MGT), calving a 78 × 40‐km giant iceberg which modified the icescape and primary productivity of the Mertz polynya. In this study, we use satellite ocean color and sea ice concentration to investigate the variability, trends, and drivers of phytoplankton blooms in the Mertz polynya since 1997. During the bloom, over 21 years, we found (i) a later ice retreat time, (ii) an increase in sea ice concentration, (iii) a decrease in open‐water period, (iv) a later bloom start, and (v) a decrease in bloom duration. Our results suggest that major postcalving changes in the physical characteristics of the polynya, mainly its icescape, are the primary drivers of phytoplankton phenology. More specifically, the MGT calving event resulted in significant seasonal and regional changes, with higher eastern chl‐a and mean summer chl‐a postcalving. While satellite data are useful to study long‐term variability in these inhospitable areas, they only focus on the ocean surface and are obscured by ice and clouds. Additional subsurface parameters from seal tags, gliders and moorings in the southernmost polar regions would strengthen our comprehension of phytoplankton and physical changes in ocean dynamics that may have far‐reaching consequences, from global circulation to carbon export.

Spatiotemporal Variability in Phytoplankton Bloom Phenology in Eastern Canadian Lakes Related to Physiographic, Morphologic, and Climatic Drivers

Phytoplankton bloom monitoring in freshwaters is a challenging task, particularly when biomass is dominated by buoyant cyanobacterial communities that present complex spatiotemporal patterns. Increases in bloom frequency or intensity and their earlier onset in spring were shown to be linked to multiple anthropogenic disturbances, including climate change. The aim of the present study was to describe the phenology of phytoplankton blooms and its potential link with morphological, physiographic, anthropogenic, and climatic characteristics of the lakes and their watershed. The spatiotemporal dynamics of near-surface blooms were studied on 580 lakes in southern Quebec (Eastern Canada) over a 17-year period by analyzing chlorophyll-a concentrations gathered from MODIS (Moderate Resolution Imaging Spectroradiometer) satellite images. Results show a significant increase by 23% in bloom frequency across all studied lakes between 2000 and 2016. The first blooms of the year appeared increasingly early over this period but only by 3 days (median date changing from 6 June to 3 June). Results also indicate that high biomass values are often reached, but the problem is seldom extended to the entire lake surface. The canonical correlation analysis between phenological variables and environmental variables shows that higher frequency and intensity of phytoplankton blooms and earlier onset date occurred for smaller watersheds and higher degree-days, lake surface area, and proportion of urban zones. This study provides a regional picture of lake trophic state over a wide variety of lacustrine environments in Quebec, a detailed phenology allowing to go beyond local biomass assessments, and the first steps on the development of an approach exploiting regional trends for local pattern assessments.

Stirring, Mixing, Growing: Microscale Processes Change Larger Scale Phytoplankton Dynamics

The quantitative description of marine systems is constrained by a major issue of scale separation: phytoplankton production processes occur at sub-centimeter scales, while the contribution to the Earth's biogeochemical cycles is expressed at much larger scales, up to the planetary one. In spite of vastly improved computing power and observational capabilities, the modeling approach has remained anchored to an old view that sees the microscales as unable to substantially affect larger ones. The lack of a widespread theoretical appreciation of the interactions between vastly different scales has led to the proliferation of numerical models with uncertain predictive capabilities. In this paper, we use the phenology of phytoplankton blooms as one example of a macroscopic ecosystem feature affected by microscale interactions. We describe two distinct mechanisms that produce patchiness within a productive water column: turbulent entrainment of less-productive water at the base of the mixed layer, and stirring by slow turbulence of a vertical phytoplankton gradient sustained by depth–dependent light availability. In current eddy–diffusive models, patchiness produced in this way is wiped out very rapidly, because the time scales of irreversible mixing largely overlap those of mechanical stirring. We propose a novel Lagrangian modeling framework that allows for the existence of microscale patchiness, even when that is not fully resolved. We show, with a mixture of theoretical arguments and numerical simulations of increasing realism, how the presence of patchiness, in turn, affects larger-scale properties, demonstrating that the timing of phytoplankton blooms and vertical variability of chlorophyll in the oceanic upper layers is determined by the mutual interplay between the stirring, mixing and growing processes.

Interplay of regional oceanography and biogeochemistry on phytoplankton bloom development in an Arctic fjord

Ongoing sea-ice melting and associated environmental changes influence the bloom phenology and biogeochemistry of the Arctic Ocean. Kongsfjorden, a fjord in Svalbard, has already undergone the transition of being sea-ice covered in winter to sea-ice free, and now stands vulnerable to Atlantic water intrusion and glacier melting. We monitored physical and biogeochemical variables in the fjord between July 2015 and July 2016 using Indian Arctic subsurface mooring in Kongsfjorden. The year-round records of nitrate represent the first reported sensor-based observation in the region. Here, we used the high-resolution time-series of chlorophyll a, nitrate, dissolved oxygen, photosynthetically active radiation, turbidity, temperature, and salinity to study their interactions and assess how do they relate to the development of phytoplankton bloom. Results indicated a high nitrate concentration (up to 13.3 μM) and near-zero chlorophyll a in winter. The spring bloom was spotted from mid-April to the first week of June (up to 7.2 μg/L chlorophyll a at 25 m), however, with discontinuities due to high winds events. The chlorophyll a was negatively correlated with nitrate (r = −0.7, p < 0.001). Further, we observed temperature, salinity, and density characteristic of Atlantic water at 25 m and 35 m in autumn and early-winter, which coincided with nitrate peaks. This indicated nitrate supply by Atlantic inflow during the non-bloom period. We say that the features of the bloom start evolving from rather quiescent autumn and winter before it reaches an active state in spring-summer. Year-round high-resolution observations are crucial to study phytoplankton bloom phenology as any shift or change in the phenology will affect higher trophic levels and biogeochemical cycles.

Environmental Forcings on the Remotely Sensed Phytoplankton Bloom Phenology in the Central Ross Sea Polynya

AbstractWe investigated the interannual variability of the phytoplankton bloom in the central Ross Sea Polynya derived from the annual phenology metrics of the bloom based on ocean color satellite measurements obtained between 2002 and 2017. The phenology metrics determined by the adjusted Gaussian fitting method include the bloom amplitude (BA), bloom initiation timing (BIT), and bloom peak timing (BPT). We found the following results for three phenology metrics. The BA tended to increase since 2002, probably related to the formation of open water area by the atmospheric circulation changes on the synoptic scale over the Ross Sea. The significant sea ice loss trend due to the changing winds over the entire southern coast of the Ross Sea was found. Continuous winds in widened open water can move surface water masses more easily along the wind direction, transferring water masses or chlorophyll pigments themselves accordingly. This process has led to the recent intense bloom in the central Ross Sea Polynya. The interannual variability of the BIT is a function of the sea surface temperature in November and wind speed in October, implying that there is a strong association between ice drift and melting, which can be primarily related to the onset of the polynya expansion. Although there was no direct factor to the BPT, it was somewhat related to the BIT and BA. In other words, the environmental factors forming the BIT and BA might have indirectly influenced the BPT, suggesting that early polynya and large biomass could lead to promoting the bloom decay.

Phenology of Phytoplankton Blooms in a Trophic Lake Observed from Long-Term MODIS Data

Phytoplankton phenology critically affects elements biogeochemical cycles, ecosystem structure, and productivity. However, our understanding about the phenological process and driving mechanism is still very limited due to the shortage of long-term observation data. We used all available daily MODIS-Aqua data from 2003 to 2017 to determine bloom start dates (BSDs) in a typical trophic lake (Lake Taihu) and investigate how phytoplankton BSDs respond to climate change and environmental factors. The results indicate that BSDs have advanced 29.9 days for the entire Lake Taihu from 2003 to 2017. Spatially, an earlier phytoplankton bloom was recorded in the northern bays and the littoral regions than in the center of open water. Air temperature, wind speed, and N/P ratio (N, total nitrogen; P, total phosphorus) were three important factors affecting phytoplankton phenology. Multiple linear correlation showed that air temperature, wind speed, and N/P ratio in Spring could explain 59.9% variability of BSDs for Lake Taihu. This study provides a quantitative assessment of phytoplankton phenological shifts and elucidates the inter-relationship between phenology parameters and environmental factors, thus improving our understanding on the potential impact of climate change and eutrophication on lake ecosystems. The starting earlier and lasting longer of phytoplankton are consistent with the expected effects of climate warming on aquatic ecosystem in recent decades, which will bring new challenges for algal bloom management in eutrophic Lake Taihu.

Applications of DINEOF to Satellite-Derived Chlorophyll-a from a Productive Coastal Region

A major limitation for remote sensing analyses of oceanographic variables is loss of spatial data. The Data INterpolating Empirical Orthogonal Functions (DINEOF) method has demonstrated effectiveness for filling spatial gaps in remote sensing datasets, making them more easily implemented in further applications. However, the spatial and temporal coverage of the input image dataset can heavily impact the outcomes of using this method and, thus, further metrics derived from these datasets, such as phytoplankton bloom phenology. In this study, we used a three-year time series of MODIS-Aqua chlorophyll-a to evaluate the DINEOF reconstruction output accuracy corresponding to variation in the form of the input data used (i.e., daily or week composite scenes) and time series length (annual or three consecutive years) for a dynamic region, the Salish Sea, Canada. The accuracy of the output data was assessed considering the original chla pixels. Daily input time series produced higher accuracy reconstructing chla (95.08–97.08% explained variance, RMSExval 1.49–1.65 mg m−3) than did all week composite counterparts (68.99–76.88% explained variance, RMSExval 1.87–2.07 mg m−3), with longer time series producing better relationships to original chla pixel concentrations. Daily images were assessed relative to extracted in situ chla measurements, with original satellite chla achieving a better relationship to in situ matchups than DINEOF gap-filled chla, and with annual DINEOF-processed data performing better than the multiyear. These results contribute to the ongoing body of work encouraging production of ocean color datasets with consistent processing for global purposes such as climate change studies.

Variability in the mechanisms controlling Southern Ocean phytoplankton bloom phenology in an ocean model and satellite observations

AbstractA coupled global numerical simulation (conducted with the Community Earth System Model) is used in conjunction with satellite remote sensing observations to examine the role of top‐down (grazing pressure) and bottom‐up (light, nutrients) controls on marine phytoplankton bloom dynamics in the Southern Ocean. Phytoplankton seasonal phenology is evaluated in the context of the recently proposed “disturbance‐recovery” hypothesis relative to more traditional, exclusively “bottom‐up” frameworks. All blooms occur when phytoplankton division rates exceed loss rates to permit sustained net population growth; however, the nature of this decoupling period varies regionally in Community Earth System Model. Regional case studies illustrate how unique pathways allow blooms to emerge despite very poor division rates or very strong grazing rates. In the Subantarctic, southeast Pacific small spring blooms initiate early cooccurring with deep mixing and low division rates, consistent with the disturbance‐recovery hypothesis. Similar systematics are present in the Subantarctic, southwest Atlantic during the spring but are eclipsed by a subsequent, larger summer bloom that is coincident with shallow mixing and the annual maximum in division rates, consistent with a bottom‐up, light limited framework. In the model simulation, increased iron stress prevents a similar summer bloom in the southeast Pacific. In the simulated Antarctic zone (70°S–65°S) seasonal sea ice acts as a dominant phytoplankton‐zooplankton decoupling agent, triggering a delayed but substantial bloom as ice recedes. Satellite ocean color remote sensing and ocean physical reanalysis products do not precisely match model‐predicted phenology, but observed patterns do indicate regional variability in mechanism across the Atlantic and Pacific.

Changes in phytoplankton bloom phenology over the North Water (NOW) polynya: a response to changing environmental conditions

Marine ecological indicators can be used to assess the condition of the pelagic ecosystems. The bloom onset provides a warning bell for possible changes in trophic interactions and biogeochemical processes. However, depicting the phenology of phytoplankton blooms at high latitudes, where long-term observations are sparse or unavailable, is not a straightforward task. A data-interpolating empirical orthogonal function algorithm was applied to daily satellite-retrieved chlorophyll-a images to produce a long-term (1998–2014) and cloud-free data set over the North Water (NOW) polynya. The seasonal bloom was modeled using a multi-Gaussian approach from which a baseline of phenological characteristics was extracted. The correlation analysis highlights the influence of environmental factors, such as sea surface temperature, cloud fraction, wind stress, and sea-ice concentration, in modulating the bloom start date, its duration, and amplitude. The year-to-year variability in bloom onset appears to be controlled by a delicate balance between oceanographic and meteorological conditions. Blooms last longer during years characterized by a longer open-water period and are shorter during those characterized by greater sea-ice coverage. Noteworthy is the decrease in phytoplankton bloom amplitude over the 17 years examined. Collectively, these outcomes depict the NOW as a climate-sensitive region in which the pelagic marine ecosystem seems to be going toward a decline in chlorophyll-a concentrations. Satellite time series are still too short to differentiate between inter-annual variability, inter-decadal variability, and climate change signal. Should these changes persist; however, the NOW may no longer act as a productive regional oasis supporting thriving populations of zooplankton and top predators.