Seasonal Cycles Of Phytoplankton Research Articles

On the Seasonal Cycle of Phytoplankton Bio‐Optical Properties Inside a Warm Core Ring in the Gulf of Mexico

AbstractFour underwater glider missions were carried out to sample the physical and bio‐optical properties inside a Loop Current Eddy (LCE) in the Gulf of Mexico, to investigate whether the winter deepening of the mixed‐layer and erosion of the nitracline stimulates phytoplankton growth. Recent coupled physical‐biogeochemical numerical models support this mechanism, but observations using profiling floats suggest that there is no seasonal cycle on integrated phytoplankton biomass. Here, data collected by underwater gliders during a full seasonal cycle and inside the LCE Poseidon support the idea of an increase in phytoplankton biomass during winter, consistent with nutrient entrainment into the euphotic zone. The changes in fluorescence emission per chlorophyll‐a unit and their implications for interpreting bio‐optical variability were also assessed. Linear regressions between in vivo chlorophyll‐a fluorescence and satellite chlorophyll‐a concentration show the largest (smallest) slopes during winter (summer), suggesting a shift in the phytoplankton community along the year or photoacclimation. Although the glider data set is convolved by temporal and spatial variability, and chlorophyll‐a fluorescence is affected by multiple factors, the concomitant enhancement of particle backscattering coefficient and chlorophyll‐a observed during winter supports the occurrence of a seasonal cycle in phytoplankton biomass. Deep vertical mixing in winter inside the core of the LCE, can promote fertilization through vertical diffusion of nutrients. Poseidon was an extraordinary, large, and strong, LCE that prompted phytoplankton blooms in winter highlighting their relevance for primary production and in general for biogeochemical processes.

Southern Ocean phytoplankton dynamics and carbon export: insights from a seasonal cycle approach.

Quantifying the strength and efficiency of the Southern Ocean biological carbon pump (BCP) and its response to predicted changes in the Earth's climate is fundamental to our ability to predict long-term changes in the global carbon cycle and, by extension, the impact of continued anthropogenic perturbation of atmospheric CO2. There is little agreement, however, in climate model projections of the sensitivity of the Southern Ocean BCP to climate change, with a lack of consensus in even the direction of predicted change, highlighting a gap in our understanding of a major planetary carbon flux. In this review, we summarize relevant research that highlights the important role of fine-scale dynamics (both temporal and spatial) that link physical forcing mechanisms to biogeochemical responses that impact the characteristics of the seasonal cycle of phytoplankton and by extension the BCP. This approach highlights the potential for integrating autonomous and remote sensing observations of fine scale dynamics to derive regionally optimized biogeochemical parameterizations for Southern Ocean models. Ongoing development in both the observational and modelling fields will generate new insights into Southern Ocean ecosystem function for improved predictions of the sensitivity of the Southern Ocean BCP to climate change. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Seasonal cycles of phytoplankton and net primary production from Biogeochemical Argo float data in the south-west Pacific Ocean

We present annual cycles of chlorophyll a, phytoplankton carbon, nitrate and oxygen for Subtropical (STW), Subantarctic (SAW), and Subantarctic Mode (SAMW) waters near Aotearoa New Zealand from data collected by two Biogeochemical (BGC) Argo floats. We develop two simple models of depth-integrated net primary production (NPP), tuned against 14C-uptake measurements, to compare with Vertically-Generalised Production Model (VGPM) satellite-based estimates of NPP. One model is the simplest possible, and assumes production is proportional to light multiplied by chlorophyll a concentration. The second model modifies the light response profile to account for photoacclimation. In STW at 30–35°S, enhanced production is initiated in austral autumn when the mixed layer deepens to entrain nutrients into the photic zone. For about half the year, there is substantial production within a deep chlorophyll maximum that sits below the mixed layer. Consequently, depth-integrated NPP is only loosely related to surface biomass as imaged from satellite remote-sensing, and BGC Argo-based model estimates of depth-integrated NPP are about double VGPM estimates. In SAW at 45–55°S, production is initiated when vertical mixing decreases in austral spring. Production is largely within the mixed layer, and depth-integrated phytoplankton biomass and depth-integrated NPP follow surface phytoplankton biomass. Model estimates of depth-integrated NPP based on BGC Argo float profiles are comparable with VGPM estimates for the southern water masses.

Phytoplankton Biomass and the Hydrodynamic Regime in NEOM, Red Sea

NEOM (short for Neo-Mustaqbal) is a $500 billion coastal city megaproject, currently under construction in the northwestern part of the Red Sea, off the coast of Tabuk province in Saudi Arabia, and its success will rely on the preservation of biodiverse marine ecosystems. Monitoring the variability of ecological indicators, such as phytoplankton, in relation to regional environmental conditions, is the foundation for such a goal. We provide a detailed description of the phytoplankton seasonal cycle of surface waters surrounding NEOM using satellite-derived chlorophyll-a (Chl-a) observations, based on a regionally-tuned product of the European Space Agency’s Ocean Colour Climate Change Initiative, at 1 km resolution, from 1997 to 2018. The analysis is also supported with in situ cruise datasets and outputs of a state-of-the-art high-resolution hydrodynamic model. The open waters of NEOM follow the oligotrophic character of the Northern Red Sea (NRS), with a peak during late winter and a minimum during late summer. Coral reef-bound regions, such as Sindala and Sharma, are characterised by higher Chl-a concentrations that peak during late summer. Most of the open waters around NEOM are influenced by the general cyclonic circulation of the NRS and local circulation features, while shallow reef-bound regions are more isolated. Our analysis provides the first description of the phytoplankton seasonality and the oceanographic conditions in NEOM, which may support the development of a regional marine conservation strategy.

Long-term dynamics of photosynthetic pigments in plankton of a large plains reservoir

Long-term observations are necessary to analyze and predict changes occurring in biological communities under global and regional climatic changes. The data on seasonal and long-term dynamics of chlorophyll in plankton of the Rybinsk Reservoir (Upper Volga, Russia) obtained in May – October 2009–2019 at six standard stations using the fluorescence method are presented. In the years with contrasting hydroclimatic conditions and water regime, the chlorophyll content varied from 1–3 to >100 μg/L. The significant variability of mean seasonal values (7.9 ± 0.5 μg/L in 2009 to 27.6 ± 1.7 μg/L in 2013 with variation coefficients of 52–134%) indicates the low resistance of the community. The total chlorophyll content is associated with the development of three main phytoplankton divisions i.e., diatoms, cyanoprokaryots, and green algae. The trophic status of the reservoir was characterized as mesotrophic in 2009 and 2017, eutrophic in 2011–2014, and moderately eutrophic in other years. In the long-term seasonal cycle of phytoplankton, there are five periods with stable temperature conditions and transparency, but variable chlorophyll content. A moderate positive relationship was found between the seasonal dynamics of chlorophyll and water temperature, but a moderate negative relationship with transparency. The priority factors regulating the long-term dynamics of chlorophyll include the NAO indices, Wolf numbers, temperature, and underwater light conditions, as well as the inflow volume and water level. Water regime limits the development of phytoplankton.

Modeling the seasonal variability of phytoplankton in the subarctic northeast Pacific Ocean

The subarctic northeast Pacific Ocean (SNEP) is a high-nitrate, low-chlorophyll region where, due to iron (Fe) limitation, chlorophyll a (chl a) concentration is persistently low throughout the year despite abundant nitrate. Observations from a Biogeochemical-Argo float (BGC-Argo) deployed in the SNEP in 2012 show prominent variations in chl a in the upper ocean (0-150 m). Chl a peaks in late summer and autumn at a value >3�times that in winter; these observations at a fine resolution can help us disentangle the mechanisms driving the full seasonal cycle of phytoplankton. A one-dimensional physical-biogeochemical coupled model was applied at OSP to investigate the factors driving phytoplankton dynamics in the SNEP. Seasonal variability of chl a in the upper layer is reproduced by the model; results are reasonably comparable to the BGC-Argo observations in 2012 near OSP. Phytoplankton growth is limited in winter by the deep mixed layer, which reduces light availability; light gradually increases later in the year due to shoaling of the mixed layer and increasing solar radiation. In spring and early summer, Fe limitation restricts phytoplankton growth, especially diatoms, and grazing pressure on small phytoplankton from overwintering microzooplankton inhibits accumulation of phytoplankton biomass. Chl a accumulates throughout the summer, peaking in late summer and autumn as mesozooplankton consume microzooplankton, which reduces microzooplankton grazing on phytoplankton. Although the model study was restricted to the seasonal pattern of BGC-Argo observations in 2012, it nevertheless fully describes seasonal variability in chl a in the SNEP and is crucial to understanding mechanisms underlying phytoplankton dynamics in high-nitrate, low-chlorophyll regions.

Seasonal Cycles of Phytoplankton Expressed by Sine Equations Using the Daily Climatology from Satellite-Retrieved Chlorophyll-a Concentration (1997–2019) Over Global Ocean

The global coverage of Chlorophyll-a concentration (Chl-a) has been continuously available from ocean color satellite sensors since September 1997 and the Chl-a data (1997–2019) were used to produce a climatological dataset by averaging Chl-a values at same locations and same day of year. The constructed climatology can remarkably reduce the variability of satellite data and clearly exhibit the seasonal cycles, demonstrating that the growth and decay of phytoplankton recurs with similarly seasonal cycles year after year. As the shapes of time series of the climatology exhibit strong periodical change, we wonder whether the seasonality of Chl-a can be expressed by a mathematic equation. Our results show that sinusoid functions are suitable to describe cyclical variations of data in time series and patterns of the daily climatology can be matched by sine equations with parameters of mean, amplitude, phase, and frequency. Three types of sine equations were used to match the climatological Chl-a with Mean Relative Differences (MRD) of 7.1%, 4.5%, and 3.3%, respectively. The sine equation with four sinusoids can modulate the shapes of the fitted values to match various patterns of climatology with small MRD values (less than 5%) in about 90% of global oceans. The fitted values can reflect an overall pattern of seasonal cycles of Chl-a which can be taken as a time series of biomass baseline for describing the state of seasonal variations of phytoplankton. The amplitude images, the spatial patterns of seasonal variations of phytoplankton, can be used to identify the transition zone chlorophyll fronts. The timing of phytoplankton blooms is identified by the biggest peak of the fitted values and used to classify oceans as different bloom seasons, indicating that blooms occur in all four seasons with regional features. In global oceans within latitude domains (48°N–48°S), blooms occupy approximately half of the ocean (50.6%) during boreal winter (December–February) in the northern hemisphere and more than half (58.0%) during austral winter (June–August) in the southern hemisphere. Therefore, the sine equation can be used to match the daily Chl-a climatology and the fitted values can reflect the seasonal cycles of phytoplankton, which can be used to investigate the underlying phenological characteristics.

Improved Representation of Underwater Light Field and Its Impact on Ecosystem Dynamics: A Study in the North Sea

AbstractUnderstanding ecosystem state on the North‐West European (NWE) Shelf is of major importance for both economy and climate research. The purpose of this work is to advance our modeling of in‐water optics on the NWE Shelf, with important implications for how we model primary productivity, as well as for assimilation of water‐leaving radiances. We implement a stand‐alone bio‐optical module into the existing coupled physical‐biogeochemical model configuration. The advantage of the bio‐optical module, when compared to the preexisting light scheme is that it resolves the underwater light spectrally and distinguishes between direct and diffuse downwelling streams. The changed underwater light compares better with both satellite and in situ observations. The module lowered the underwater photosynthetically active radiation, decreasing the simulated primary productivity, but overall, the improved underwater light had relatively limited impact on the phytoplankton seasonal dynamics. We showed that the model skill in representing phytoplankton seasonal cycle (e.g., phytoplankton bloom) can be substantially improved either by assimilation of satellite phytoplankton functional type (PFT) chlorophyll, or by assimilating a novel PFT absorption product. Assimilation of the two PFT products yields similar results, with an important difference in the PFT community structure. Both assimilative runs lead to lower plankton biomass and increase the nutrient concentrations. We discuss some future directions on how to improve our model skill in biogeochemistry without using assimilation, for example, by improving nutrient forcing, retuning the model parameters, and using the bio‐optical module to provide a two‐way physical‐biogeochemical coupling, improving the consistency between model physical and biogeochemical components.

Annual phytoplankton cycle in a meromictic anoxic basin of a Rhode Island (USA) estuarine river

Estuarine Pettaquamscutt River is a unique habitat 10 km in length with physical and biogeochemical characteristics analogous to a miniature fjord. Its meromictic upper basins are characterized by a permanent oxic–anoxic vertical gradient in which a well-oxygenated upper layer overlies a deeper, anoxic reservoir, with persistent blooms of phototrophic anoxygenic bacteria (Chromatium, Chlorobium) at the oxic–anoxic transition layer. A diverse assemblage of nanoplanktonic, centric diatoms (Cyclotella caspia, Thalassiosira pseudonana, cf. Cyclotella cryptica) dominated the seasonal phytoplankton cycle in the aerobic layer, similar to comparable meromictic habitats elsewhere. This assemblage of nano-centric diatoms appears to be trait-separated from other species clusters and is potentially useful as a functional group flora with ecophysiology diagnostic of marine estuarine rivers and meromictic habitat niche structure. The most conspicuous phytoplankton feature, however, was the year-round occurrence of the photoautotrophic euglenid Euglenaformis (Euglena) proxima, restricted in its upper water column distribution to the O2–H2S boundary layer where it formed a consortium with the photosynthetic green and purple sulfur bacteria community. The association of E. proxima with meromixis, H2S, and phototrophic anoxygenic bacteria is similar to that reported previously in a brackish Norwegian oyster poll and a brackish loch in Scotland.

Comparison of Seasonal Cycles of Phytoplankton Chlorophyll, Aerosols, Winds and Sea-Surface Temperature off Somalia

In climate research, an important task is to characterise the relationships between Essential Climate Variables (ECVs). Here, satellite-derived data sets have been used to examine the seasonal cycle of phytoplankton (chlorophyll concentration) in the waters off Somalia, and its relationship to aerosols, winds and Sea Surface Temperature (SST). Chlorophyll-a (Chl-a) concentration, Aerosol Optical Thickness (AOT), A° ngstro¨m Exponent (AE), Dust Optical Thickness (DOT), Sea Surface Temperature (SST) and sea-surface wind data for a 16-year period were assembled from various sources. The data were used to explore whether there is evidence in the data to show that dust aerosols enhance Chl-a concentration in the study area. The Cross 11 Correlation Function (CCF) showed highest positive correlation (r2=0.3) in the western Arabian Sea when AOT led Chl-a by 1 to 2 time steps (here, 1 time step is 8 days). A 2o□ 2obox off Somalia was selected for further investigations. The correlations of alongshore wind speed, Ekman Mass Transport (EMT) and SST with Chl-a were higher than that of AOT, for a lag of 8 days. When all four variables were considered together in a multiple linear regression, the increase in r2 associated with the AOT is only about 0.02, a consequence of covariance among AOT, SST, EMT and alongshore wind speed. The AOT data show presence of dust aerosols most frequently during the summer monsoon season (June- September). When the analyses were repeated for the dust aerosol events, the correlations were generally lower, but still significant. Again, the inclusion of DOT in the multiple linear regression increased the correlation coefficient by only 2%, indicating minor enhancement in Chl-a concentration. Interestingly, during summer monsoon season, there is a higher probability of finding more instances of positive changes in Chl-a after one time step, regardless of whether there is dust aerosol or not. On the other hand, during the winter monsoon season (November-December) and rest of the year, the probability of Chl-a enhancement is higher when dust aerosol is present than when dust aerosol is present than when it is absent. The phase relationship in the annual climatologies of Chl-a and AOT

Diversity, temporal distribution and physiology of the centric diatom Leptocylindrus Cleve (Bacillariophyta) from a southern hemisphere upwelling system

The marine diatom Leptocylindrus is a major component of phytoplankton blooms in coastal ecosystems and upwelling regions worldwide, however, little is known about this genus in the southern hemisphere. Whilst Leptocylindrus danicus has been reported from south-eastern (SE) Australia since the 1930s, there has been neither unequivocal species identification nor focused examination of the temporal abundance of Leptocylindrus in this region. Such investigations are crucial in the context of climate change and the strengthening of the East Australian Current, which is expected to result in alterations to the seasonal abundance and distribution of Leptocylindrus along the east Australian coast. Thus we also describe the temporal distribution of Leptocylindrus based on 50 years of records, revealing that this diatom is a key component of the seasonal phytoplankton cycle, with greatest abundance in the austral spring and summer. Using light and transmission electron microscopy and molecular phylogenetics based on the nuclear-encoded ITS1–5.8S–ITS2 rDNA region, our study unambiguously revealed three species, L. danicus, Leptocylindrus convexus and Leptocylindrus aporus from 34 clonal isolates from SE Australia, with the majority (82%) of strains identified as L. danicus. Furthermore, we investigated the growth, auxospore and resting spore formation of the most commonly occurring species, L. danicus, under four temperature and irradiance scenarios. The diatom reached maximum growth rates (µMax, 1.71 divisions day−1) under relatively high temperatures (25°C) and light conditions (100 µmol photons m−2 s−1) between days 2 and 7 of the experiment. When temperature and light regimes were reduced (18°C, 50 µmol photons m−2 s−1) auxospores and resting spores were formed. The rapid growth rate and potential of L. danicus to form auxospores are important survival mechanisms in coastal upwelling systems and likely to result in the continued success of this species in Eastern Australia. The ecological, physiological and evolutionary response of this significant diatom group to further ocean warming should be the focus of future investigations.

Optimal PAR intensity for spring bloom in the Northwest Pacific marginal seas

Using ten years (2003–2012) of satellite Chlorophyll-a data, we report that annual phytoplankton bloom climax in the Northwest Pacific marginal seas (17°–58°N) delays northward at a rate of 22.98±2.86kmday−1. The spring bloom is a dominant feature of the phytoplankton seasonal cycle in this region, except for the northern South China Sea, which features a winter bloom. The sea surface hourly Photosynthetically Available Radiation (PAR) intensity averaged over the bloom peak duration is nearly uniform (1.04±0.10Wm−2h−1) among the four sub-regions (i.e. the northern South China Sea, the Kuroshio waters, the Sea of Japan and the Sea of Okhotsk), although different algal species in these four distinct ecological provinces could adapt to a much larger change in other environmental parameters (including total daily PAR, day length, sea surface temperature, net surface heat flux, mixed layer depth, wind speed and euphotic depth). The differences of the hourly PAR intensity between the four provinces during their bloom periods are smaller than those during non-bloom seasons. In contrast, an increasing total daily PAR (Wm−2day−1), due to the longer day length at higher latitudes, may balance decreasing sea surface temperature and induce algal flowering. Our results point to an optimal hourly light intensity for the annual phytoplankton bloom peak timing in this entire region, which could potentially become an indicator for the requirement of these annual bloom peaks.

Influence of photoacclimation on the phytoplankton seasonal cycle in the Mediterranean Sea as seen by satellite

Abstract Photoacclimation is a cellular process that allows phytoplankton to change the intracellular chlorophyll- a concentration ( Chl ) in relation to environmental factors such as light and nutrients. This process is currently overlooked by standard operational ocean colour algorithms used to retrieve information about both the phytoplankton standing stock and production. Here, we describe the photoacclimation effect on the phytoplankton seasonal cycle through the chlorophyll to carbon ratio ( Chl : C ) over the Mediterranean Sea, using SeaWiFS (1998–2007) monthly data. Chlorophyll was calculated through a regionalized empirical algorithm and Carbon was estimated from the particle backscattering, after subtraction of the non-algal contribution (b bp NAP ). Historically, b bp NAP has been neglected or assumed constant, and only recently its space-time variability started to be taken into account. Here, bootstrapping simulations are used to compute the monthly b bp NAP s over a former bio-regionalization map of the Mediterranean Sea, all highlighting significant space-time variability. Resulting Chl : C varies within a factor of three, demonstrating the important role that photoacclimation plays at seasonal and basin scales. Lowest Chl : C values (~ 0.0013) are observed during summer; this is due to the combined effect of high irradiation, shallow mixing and low nutrient concentration. Maximum values (~ 0.031) are observed in winter and spring dominated by both low solar radiation, deeper mixing and high nutrient availability. It emerges that a better view of the phytoplankton space and time variability can be achieved through the joint use of Chl and C .

Examining features of enhanced phytoplankton biomass in the Bay of Bengal using a coupled physical‐biological model

AbstractA coupled bio‐physical ocean model is used to describe areas of enhanced phytoplankton biomass, seen in remotely sensed observations, in the otherwise oligotrophic environment of the Bay of Bengal. The model is based on the Naval Coastal Ocean Model (NCOM), which is one‐way coupled to the 13‐component Carbon, Silicate, and Nitrogen Ecosystem (CoSiNE) model and configured for the Indian Ocean. Model results are compared and evaluated against a set of in situ shipboard observations as well as ocean color data acquired from several remote sensing platforms. The model is shown to successfully simulate the seasonal cycle of phytoplankton, the markedly contrasting scenarios of phytoplankton distribution in the north versus the south Bay of Bengal, and the biological impact from the 1997/1998 Indian Ocean Dipole (IOD) event. The model simulation provides us with vertical cross sections of phytoplankton biomass from summer and winter blooms in the southwest of the bay, information not found in remotely sensed data. It also successfully reproduces the timing of the onset of the blooms and their spatial extent, thereby providing a measure of its potential for augmenting in situ and remotely sensed observations to improve understanding of the dynamics of primary producers and carbon cycling in one of the most poorly sampled regions of the world's oceans.

Onset, intensification, and decline of phytoplankton blooms in the Southern Ocean

AbstractThe seasonal cycle of phytoplankton biomass in the Southern Ocean (SO) is characterized by a period of rapid accumulation, known as bloom, that is typical of high-latitude regions. Recent studies have illustrated how spatial and temporal dynamics of blooms in the SO are more complex than in other oceans. This complexity is likely related to differences in vertical mixing and the iron availability. In this work, we examine the sensitivity of bloom dynamics to changes in vertical mixing and iron availability using a biogeochemical model. Under idealized physical forcing, we produce seasonal cycles of phytoplankton for an ensemble of SO scenarios and we describe the bloom dynamics in terms of the net biomass accumulation rate. Based on this metric, we define three crucial bloom phases: the onset, the climax, and the apex. For the ensemble of modelled blooms, onsets always occur in winter and can be either bottom-up (increase in productivity) or top-down (decrease in grazing) controlled. Climaxes are mostly found in spring and their magnitudes are bottom-up controlled. Apexes are always found in late spring and strongly top-down controlled. Our results show that while a “strict” onset definition is consistent with a winter onset, the surface spring bloom is associated with the climax of the integrated bloom. Furthermore, we demonstrate that onset phase can be distinguished from climax phase using appropriate bloom detection methods based on surface satellite-based products. The ensemble of these results suggests that Sverdrup's blooming conditions are not indicative of the bloom onset but of the climax. We conclude that the recent bloom onset debate may partly be due to a confusion between what is defined here as the bloom onset and the climax, and that the SO observed complexity is due to the factors that control the climax.

Long-term changes of the phytoplankton community and biomass in the subtropical shallow Patos Lagoon Estuary, Brazil

Seasonal and interannual changes (1993–2012) of water temperature and transparency, river discharge, salinity, water quality properties, chlorophyll a (chl-a) and the carbon biomass of the main taxonomical phytoplankton groups were evaluated at a shallow station (∼2 m) in the subtropical Patos Lagoon Estuary (PLE), Brazil. Large variations in salinity (0–35), due to a complex balance between Patos Lagoon outflow and oceanic inflows, affected significantly other water quality variables and phytoplankton dynamics, masking seasonal and interannual variability. Therefore, salinity effect was filtered out by means of a Generalized Additive Model (GAM). River discharge and salinity had a significant negative relation, with river discharge being highest and salinity lowest during July to October. Diatoms comprised the dominant phytoplankton group, contributing substantially to the seasonal cycle of chl-a showing higher values in austral spring/summer (September to April) and lowest in autumn/winter (May to August). PLE is a nutrient-rich estuary and the phytoplankton seasonal cycle was largely driven by light availability, with few exceptions in winter. Most variables exhibited large interannual variability. When varying salinity effect was accounted for, chl-a concentration and diatom biomass showed less irregularity over time, and significant increasing trends emerged for dinoflagellates and cyanobacteria. Long-term changes in phytoplankton and water quality were strongly related to variations in salinity, largely driven by freshwater discharge influenced by climatic variability, most pronounced for ENSO events. However, the significant increasing trend of the N:P ratio indicates that important environmental changes related to anthropogenic effects are undergoing, in addition to the hydrology in the PLE.

Modeling the biogeochemical seasonal cycle in the Strait of Gibraltar

A physical-biological coupled model was used to estimate the effect of the physical processes at the Strait of Gibraltar over the biogeochemical features of the Atlantic Inflow (AI) towards the Mediterranean Sea. This work was focused on the seasonal variation of the biogeochemical patterns in the AI and the role of the Strait; including primary production and phytoplankton features. As the physical model is 1D (horizontal) and two-layer, different integration methods for the primary production in the Biogeochemical Fluxes Model (BFM) have been evaluated. An approach based on the integration of a production-irradiance function was the chosen method. Using this Plankton Functional Type model (BFM), a simplified phytoplankton seasonal cycle in the AI was simulated. Main results included a principal bloom in spring dominated by nanoflagellates, whereas minimum biomass (mostly picophytoplankton) was simulated during summer. Physical processes occurring in the Strait could trigger primary production and raise phytoplankton biomass (during spring and autumn), mainly due to two combined effects. First, in the Strait a strong interfacial mixing (causing nutrient supply to the upper layer) is produced, and, second, a shoaling of the surface Atlantic layer occurs eastward. Our results show that these phenomena caused an integrated production of 105g C m−2year−1 in the eastern side of the Strait, and would also modify the proportion of the different phytoplankton groups. Nanoflagellates were favored during spring/autumn while picophytoplankton is more abundant in summer. Finally, AI could represent a relevant source of nutrients and biomass to Alboran Sea, fertilizing the upper layer of this area with 4.95 megatons nitrate year−1 (79.83 gigamol year−1) and 0.44 megatons C year−1. A main advantage of this coupled model is the capability of solving relevant high-resolution processes as the tidal forcing without expensive computing requirements, allowing to assess the effect of these phenomena on the biogeochemical patterns at longer time scales.

Diagnosis of physical and biological control over phytoplankton in the Gulf of Maine-Georges Bank region using an adjoint data assimilation approach

The linkage between physical and biological processes, particularly the effect of the circulation field on the distribution of phytoplankton, is studied by applying a two-dimensional model and an adjoint data assimilation approach to the Gulf of Maine-Georges Bank region. The model results, comparing well with observation data, reveal seasonal and geographic variations of phytoplankton concentration and verify that the seasonal cycles of phytoplankton are controlled by both biological sources and advection processes which are functions of space and time and counterbalance each other. Although advective flux divergences have greater magnitudes on Georges Bank than in the coastal region of the western Gulf of Maine, advection control over phytoplankton concentration is more significant in the coastal region of the western Gulf of Maine. The model results also suggest that the two separated populations in the coastal regions of the western Gulf of Maine and on Georges Bank are self-sustaining.

Recent Arctic Climate Change and Its Remote Forcing of Northwest Atlantic Shelf Ecosystems

During recent decades, historically unprecedented changes have been observed in the Arctic as climate warming has increased precipitation, river discharge, and glacial as well as sea-ice melting. Additionally, shifts in the Arctic's atmospheric pressure field have altered surface winds, ocean circulation, and freshwater storage in the Beaufort Gyre. These processes have resulted in variable patterns of freshwater export from the Arctic Ocean, including the emergence of great salinity anomalies propagating throughout the North Atlantic. Here, we link these variable patterns of freshwater export from the Arctic Ocean to the regime shifts observed in Northwest Atlantic shelf ecosystems. Specifically, we hypothesize that the corresponding salinity anomalies, both negative and positive, alter the timing and extent of water-column stratification, thereby impacting the production and seasonal cycles of phytoplankton, zooplankton, and higher-trophic-level consumers. Should this hypothesis hold up to critical evaluation, it has the potential to fundamentally alter our current understanding of the processes forcing the dynamics of Northwest Atlantic shelf ecosystems.

Hydrographical forcing and phytoplankton variability in two semi-enclosed estuarine bays

Alfacs and Fangar (North East of the Iberian Peninsula) are two embayments of the Ebre Delta complex with typical Mediterranean characteristics. Both are subject to the same meteorological forcing and receive similar freshwater inputs from irrigation drainage channels. However the basin volume in Alfacs is about ten times larger than in Fangar. We studied the temporal patterns of series of chlorophyll a and phytoplankton counts sampled between 1990 and 2003 from two depths of a fixed station in each bay, and related them to the variability of environmental variables (water, temperature, salinity and stratification). A principal component analysis performed on the correlation matrix among the (log-transformed) abundance data of the most frequent taxa revealed three main trends of variability. The first principal component (PC1) indicated a gradient of marine (more important in Alfacs) versus freshwater (particularly in Fangar) influence. PC2 reflected the seasonal cycle of phytoplankton in Alfacs, characterized by the dominance of a diatom assemblage typical of Mediterranean coastal waters in autumn and a group of dinoflagellates, including toxic taxa, in winter–early spring. PC3 expressed mainly the seasonal changes in Fangar and opposed a mixed phytoplankton group, including mostly dinoflagellates, with population maxima between May and October, to dinoflagellates of the winter group. Empirical Mode Decomposition was applied to the environmental variables and to the principal components in order to analyze the temporal structure of the data. All the series presented strong seasonal modes; an index based on phase shift between pairs of series revealed correlations between some of the principal components and environmental variables (temperature and salinity in Alfacs and temperature, salinity and stratification in Fangar). Water temperature showed a slight increasing trend along the sampling period. Between 1997 and 2003, some phytoplankton taxa also presented a weak increasing trend, particularly in the bottom samples of Fangar. This finding does not indicate a direct relationship between phytoplankton variability and the actual magnitudes of temperature or salinity. Rather, these environmental variables should be considered here as proxies of the seasonal behavior of a complex of environmental and biotic factors. Differences among the seasonal patterns of phytoplankton variability in Alfacs and Fangar could be attributed to the lower residence times of the water in Fangar, which resulted in a stronger hydrological control of phytoplankton abundance and composition.