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While primary productivity is more stable in oceanic regions, it may vary to a great extent with the proximity to coasts, where mesoscale processes may intertwine and shape phytoplankton community composition and biomass. Sometimes, this may lead to the development of anomalous phytoplankton blooms (i.e., episodic blooms that exceed several times the average phytoplankton biomass). A massive bloom observed off the Western Iberian Coast (SW Europe) during March 2009 prompted a full investigation on its spatial and temporal extent, its causes, and its potential impact on the ecosystem. Results revealed that the March 2009 bloom was both novel in terms of biomass in a regional context and one of the largest anomalous blooms until now described in terms of relative magnitude. Its causes were due to a concurrence of long-term (deep winter MLD) and short-term factors (coastal upwelling, sudden changes in the water column, consistent offshore water transport). Its impact on the regional ecosystem is difficult to gauge, although the high concentrations of particulate organic carbon at surface during the bloom period suggests that it may have had a significant local impact. Since climate change is expected to increase the frequency and intensity of extreme weather events, it is possible that anomalous blooms will also become more frequent, expanding their role in shaping carbon export and food webs. These results are crucial for the monitoring of the Western Iberian Coast and are applicable to other complex coastal upwelling regions where phytoplankton biomass and variability have a crucial link to fisheries.

To establish effective water quality monitoring strategies in estuaries, it is imperative to identify and understand the main drivers for the variation of water quality parameters. The tidal effect is an important factor of the daily and fortnightly variability in several estuaries. However, the extent of that influence on the different physicochemical and biological parameters is still overlooked in some estuarine systems, such as the Sado Estuary, a mesotidal estuary located on the west coast of Portugal. The main objective of this study was to determine how the water quality parameters of the Sado Estuary varied with the fortnightly and the semidiurnal tidal variation. To achieve this goal, sampling campaigns were conducted in May/18, Nov/18 and Jun/19, under neap and spring tidal conditions, with data collection over the tidal cycle. Results were observed to be significantly influenced by the tidal variation, in a large area of the estuary. Flood seemed to mitigate possible effects of nutrient enrichment in the water column. Additionally, significant differences were also observed when considering the different sampling stations. Temperature, Suspended Particulate Matter (SPM) and nutrients showed the highest values at low water. Lastly, the implications of the tidal variability in the evaluation of the water quality according to Water Framework Directive were also discussed, highlighting the importance of studying short-time scale variations and the worst-case scenario to ensure water quality is maintained. These findings are relevant for the implementation of regional management plans and to promote sustainable development.

Coccolithophores are among the most important calcifying pelagic organisms. To assess how coccolithophore species with different coccolith-carbonate mass and distinct ecological resilience to ocean warming will influence the “rain ratio” and the “biological carbon pump”, 1 yr of species-specific coccolith-carbonate export fluxes were quantified using sediment traps moored at four sites between NW Africa and the Caribbean (i.e., CB-20°N/21°W, at 1214 m; M1-12°N/23°W, at 1150 m; M2-14°N/37°W, at 1235 m; M4-12°N/49°W, at 1130 m). Highest coccolith-CaCO3 fluxes at the westernmost site M4, where the nutricline is deepest along the tropical North Atlantic, were dominated by deep-dwelling small-sized coccolithspecies Florisphaera profunda and Gladiolithus flabellatus. Total coccolith-CaCO3 fluxesof 371 mg m−2 yr−1 at M4 were followed by 165 mg m−2 yr−1 at the north-easternmost CB, 130 mg m−2 yr−1 at M1, and 114 mg m−2 yr −1 at M2 in between. Coccoliths accounted for nearly half of the total carbonate flux at M4 (45%), much higher compared to 23% at M2 and 15% at M1 and CB. At site M4, highest ratios of coccolith-CaCO3 to particulate organic carbon fluxes and weak correlations between the carbonate of deep-dwelling species and particulate organic carbon suggest that increasing productivity in the lower photic zone in response to ocean warming might enhance the rain ratio and reduce the coccolith-ballasting efficiency. The resulting weakened biological carbon pump could, however, be counterbalanced by increasing frequency of Saharan dust outbreaks across the tropical Atlantic, providing mineral ballast as well as nutrients to fuel fast-blooming and ballast-efficient coccolithophore species.

Hyperspectral satellite data are a source of the top of the atmosphere radiance signal which can be used for novel algorithms aimed for observations of marine ecosystems and the light-lit ocean. Atmospheric sensors such as SCIAMACHY, GOME-2 and OMI have proven in the past to yield valuable information on phytoplankton diversity, sun-induced marine fluorescence, and the underwater light field, however at low coverage and spatial resolution. Within the ESA Sentinel-5p+ Innovation themes, we explore TROPOMI's potential for deriving the diffuse attenuation coefficient and the quantification of different phytoplankton groups. As commonly used for the retrieval of atmospheric trace gases, we apply the differential optical absorption spectroscopy combined with radiative transfer modeling (RTM) to infer these oceanic parameters. We present results on a measure describing the diminishing of incoming radiation in the ocean with depth, the diffuse attenuation coefficient KD. KD is derived by the retrieval of the vibrational Raman scattering signal in backscattered radiances measured by TROPOMI in the UV and spectral range which then is further converted to the associated KD using RTM. The final TROMPOMI KD data sets resolved for three spectral regions (UV-B+short wave UV-A, UV-A and short blue) agree well with in situ data sampled during an expedition with RV Polarstern in 2018 in the Atlantic Ocean. Further, KD-blue compared to wavelength-converted KD(490nm) products (OLCI-A and the merged OC-CCI) from common, multispectral, ocean color sensors, show that differences between the three data sets are within uncertainties given for the OC-CCI product. Our study shows for the first time KD products for the UV spectral range retrieved from space based data. TROPOMI KD-blue results have higher quality and much higher spatial coverage and resolution than previous ones from SCIAMACHY, GOME-2 and OMI. Additionally, first results on TROPOMI’s potential for retrieving three phytoplankton groups will be shown and compared to similar multispectral phytoplankton group data for the same time period and ocean region as shown for TROPOMI KD.

Monitoring water quality parameters and their ecological effects in transitional waters is usually performed through in situ sampling programs. These are expensive and time-consuming, and often do not represent the total area of interest. Remote sensing techniques offer enormous advantages by providing cost-effective systematic observations of a large water system. This study evaluates the potential of water quality monitoring using Sentinel-2 observations for the period 2018-2020 for the Sado estuary (Portugal), through an algorithm intercomparison exercise and time-series analysis of different water quality parameters (i.e., colored dissolved organic matter (CDOM), chlorophyll-a (Chl-a), suspended particulate matter (SPM), and turbidity). Results suggest that Sentinel-2 is useful for monitoring these parameters in a highly dynamic system, however, with challenges in retrieving accurate data for some of the variables, such as Chl-a. Spatio-temporal variability results were consistent with historical data, presenting the highest values of CDOM, Chl-a, SPM and turbidity during Spring and Summer. This work is the first study providing annual and seasonal coverage with high spatial resolution (10 m) for the Sado estuary, being a key contribution for the definition of effective monitoring programs. Moreover, the potential of remote sensing methodologies for continuous water quality monitoring in transitional systems under the scope of the European Water Framework Directive is briefly discussed.

Changes in temperature and CO2 are typically associated with climate change, but they also act on shorter time scales, leading to alterations in phytoplankton physiology and community structure. Interactions among stressors may cause synergistic or antagonistic effects on phytoplankton dynamics. Therefore, the main goal of this work is to understand the short-term isolated and interactive effects of warming and high CO2 on phytoplankton nutrient consumption, growth, production, and community structure in the Ria Formosa coastal lagoon (southern Portugal). We performed microcosm experiments with temperature and CO2 manipulation, and dilution experiments under temperature increase, using winter phytoplankton assemblages. Phytoplankton responses were evaluated using inverted and epifluorescence microscopy. Overall, phytoplankton growth and microzooplankton grazing on phytoplankton decreased with warming. Negative antagonist interactions with CO2 alleviated the negative effect of temperature on phytoplankton and cryptophytes. In contrast, higher temperature benefited smaller-sized phytoplankton, namely cyanobacteria and eukaryotic picophytoplankton. Diatom growth was not affected by temperature, probably due to nutrient limitation, but high CO2 had a positive effect on diatoms, alleviating the effect of nutrient limitation. Results suggest that this winter phytoplankton assemblage is well acclimated to ambient conditions, and short-term increases in temperature are detrimental, but can be alleviated by high CO2. DL57/2016/CP1361/CT0017 info:eu-repo/semantics/publishedVersion

Early detection of dense harmful algal blooms (HABs) is possible using ocean colour remote sensing. Some algorithms require a training dataset, usually constructed from satellite images with a priori knowledge of the existence of the bloom. This approach can be limited if there is a lack of in situ observations, coincident with satellite images. A laboratory experiment collected biological and bio-optical data from a culture of Karenia mikimotoi, a harmful phytoplankton dinoflagellate. These data showed characteristic signals in chlorophyll-specific absorption and backscattering coefficients. The bio-optical data from the culture and a bio-optical model were used to construct a training dataset for an existing statistical classifier. MERIS imagery over the European continental shelf were processed with the classifier using different training datasets. The differences in positive rates of detection of K. mikimotoi between using an algorithm trained with purely manually selected areas on satellite images and using laboratory data as training was overall in situ observations of non-harmful high chlorophyll blooms in the area would improve testing of the ability to distinguish harmful from non-harmful high chlorophyll blooms. This approach can be expanded to use additional wavelengths, different satellite sensors and different phytoplankton genera.

The Northern Antarctic Peninsula (NAP), located in West Antarctica, is amongst the most impacted regions worldwide by recent warming events. Its vulnerability to climate change has already led to an accumulation of severe changes along its ecosystems. This work reviews the current findings on impacts observed in phytoplankton communities occurring in the NAP, with a focus on its causes, consequences, and the potential research priorities for an integrated comprehension of the physical-biological coupling and climate perspective. Evident changes in phytoplankton biomass, community composition and size structure, as well as potential bottom-up impacts to the ecosystem are discussed. Surface wind, sea ice and meltwater dynamics, as the main drivers of the upper layer structure, are identified as the leading factors shaping phytoplankton. Short- and long-term scenarios are suggested for phytoplankton communities in the NAP, both indicating a future increase of the importance of small flagellates at the expense of diatoms, with potential devastating impacts for the ecosystem. Five main research gaps in the current understanding of the phytoplankton response to climate change in the region are identified: i) anthropogenic signal has yet to be disentangled from natural climate variability; ii) the influence of small-scale ocean circulation processes on phytoplankton is poorly understood; iii) the potential consequences to regional food webs must be clarified; iv) the magnitude and risk of potential changes in phytoplankton composition is relatively unknown; and v) a better understanding of phytoplankton physiological responses to changes in the environmental conditions is required. Future research directions, along with specific suggestions on how to follow them, are equally suggested. Overall, while the current knowledge has shed light on the response of phytoplankton to climate change, in order to truly comprehend and predict changes in phytoplankton communities there must be a robust collaboration effort integrating both Antarctic research programs and the whole scientific community under a common research framework.

Aquaculture has become the fastest-growing sector of the food industry worldwide. The increase of intensive aquaculture practices, however, has been raising global concern about economic and social impacts, but mostly due to the associated potential environmental impacts. The aim of this report is to make a preliminary assessment of the impact of an intensive sea bass aquaculture (Dicentrarchus labrax, L. 1758) on surrounding coastal waters. The aquaculture site is located at the SW Iberian coast (Sines, Portugal), having 16 cages, each holding approximately 150,000 specimens at different stages of growth. We present a spatial and temporal description of environmental physical, chemical, and biological parameters taken in the course of four monitoring campaigns conducted between June 2018 and April 2019. All monitored parameters, except phosphate concentration in October only at one sampling station, showed values within the desirable ranges for marine finfish production and the natural range of Portuguese coastal waters. So far, results do not reveal any detrimental impact of the production units on local water quality, although more research is needed. The preliminary findings suggest that the lack of stress on the receiving waters may be attributed to the hydrodynamic regime in the production area, the feeding strategy, and the dimension of the production.

Spectrally resolved water-leaving radiances (ocean colour) and inferred chlorophyll concentration are key to studying phytoplankton dynamics at seasonal and inter-annual scales, for a better understanding of the role of phytoplankton in marine biogeochemistry; the global carbon cycle; and the response of marine ecosystems to climate variability, change and feedback processes. Ocean colour data also have a critical role in operational observation systems monitoring coastal eutrophication, harmful algal blooms, and sediment plumes. The contiguous ocean-colour record reached 21 years in 2018; however, it is comprised of a number of one-off missions such that creating a consistent time-series of ocean-colour data requires merging of the individual sensors (including MERIS, Aqua-MODIS, SeaWiFS, VIIRS, and OLCI) with differing sensor characteristics, without introducing artefacts. By contrast, the next decade will see consistent observations from operational ocean colour series with sensors of similar design and with a replacement strategy. Also, by 2029 the record will start to be of sufficient duration to discriminate climate change impacts from natural variability, at least in some regions. This paper describes the current status and future prospects in the field of ocean colour focusing on large to medium resolution observations of oceans and coastal seas. It reviews the user requirements in terms of products and uncertainty characteristics and then describes features of current and future satellite ocean-colour sensors, both operational and innovative. The key role of in situ validation and calibration is highlighted as are ground segments that process the data received from the ocean-colour sensors and deliver analysis-ready products to end-users. Example applications of the ocean-colour data are presented, focusing on the climate data record and operational applications including water quality and assimilation into numerical models. Current capacity building and training activities pertinent to ocean colour are described and finally a summary of future perspectives is provided.

Understanding the natural variability of coastal ecosystems, and in particular distinguishing between the natural fluctuations and the ones that are caused by anthropogenic interventions and long-term climatic variability, is a major concern for establishing adequate management and adaptation strategies. The Aveiro lagoon, a shallow coastal lagoon (Portugal), holds one of the largest saltmarshes and saltpans in Europe and is a very important ecosystem from both economic and ecological viewpoints, making the protection of its water masses a requirement. To better understand the variability of its ecosystem, the factors controlling seasonal, inter-annual and long-term variability of the water quality in the Aveiro lagoon were thus analyzed. The statistical analysis was based on a set of climatic, hydrological and water quality observations undertaken between 1985 and 2010. Seasonal variations were mostly related with the seasonal variation of the main climatic and hydrological drivers, while long-term shifts were typically driven by the anthropogenic interventions in the lagoon. After the adoption of secondary treatment for industrial effluents on 1992, a recovery from hypoxia conditions occurred in the upstream area of the lagoon. After 2000 lower concentrations of silicates occurred downstream, and may also derive from some anthropogenic modifications (e.g., shunting of river water to the sewage system, deepening of the inlet) that may have affected the physical dynamics. In the downstream area of the lagoon, chlorophyll a presented a downward trend between 1985 and 2010 and lower concentrations after 2000, which were probably associated with the lower concentrations of silicates. Results from the data analysis showed that the seasonal, inter-annual and long-term trends observed in the Aveiro lagoon depend on the influence of both anthropogenic and climate drivers, putting in evidence the need to combine these different drivers when evaluating and developing management strategies for estuarine ecosystems.

Monitoring temporal changes in phytoplankton dynamics in high latitude lakes is particularly timely for understanding the impacts of warming on aquatic ecosystems. In this study, we analyzed 33-years of high resolution (30 m) Landsat (LT) data for reconstructing seasonal patterns of chlorophyll a (chl a) concentration in four lakes across Finland, between 60°N and 64°N. Chl a models based on LT spectral bands were calibrated using 17-years (2000–2016) of field measurements collected across the four lakes. These models were then applied for estimating chl a using the entire LT-5 and 7 archives. Approximately 630 images, from 1984 to 2017, were analyzed for each lake. The chl a seasonal patterns were characterized using phenology metrics, and the time-series of LT-based chl a estimates were used for identifying temporal shifts in the seasonal patterns of chl a concentration. Our results showed an increase in the length of phytoplankton growth season in three of the lakes. The highest increase was observed in Lake Köyliönjärvi, where the length of growth season has increased by 28 days from the baseline period of 1984–1994 to 2007–2017. The increase in the length of season was mainly attributed to an earlier start of phytoplankton blooms. We further analyzed surface temperature (Ts) and precipitation data to verify if climatic factors could explain the shifts in the seasonal patterns of chl a. We found no direct relationship between Ts and chl a seasonal patterns. Similarly, the phenological metrics of Ts, in particular length of season, did not show significant temporal trends. On the other hand, we identify potential links between changes in precipitation patterns and the increase in the phytoplankton season length. We verified a significant increase in the rainfall contribution to the total precipitation during the autumn and winter, accompanied by a decline in snowfall volumes. This could indicate an increasing runoff volume during the beginning of spring, contributing to an earlier onset of the phytoplankton blooms, although further assessments are needed to analyze historical streamflow values and nearby land cover data. Likewise, additional studies are needed to better understand why chl a patterns in some lakes seem to be more resilient than in others. peerReviewed

Surface sediments were sampled at 18 stations from 7 to 64 m depth off the SE coast of Algarve to characterize the organic matter contents in the inner and middle shelf. For each sediment sample, grain size, mineralogy and total organic carbon analyses were carried out, as well as pigment analyses (19’ but-fucoxanthin, fucoxanthin, 19’ hex-fucoxanthin, chlorophyll-a, chlorophyll-b, pheophytin-a, pheophorbide-a, alpha-carotene, beta-carotene, anteraxanthin, diadinoxanthin, diatoxanthin, lutein and zeoxanthin). In addition, the ratios between each pigment and the total organic carbon (TOC) percentage, and between the chlorophyll-a concentration and the pheophytin-a and the pheophorbide-a concentrations were calculated to establish pigment indices. A Multidimensional scaling (MDS) analysis highlights three regions in the SE coast of Algarve according to depth, percentages of TOC, sediment grain size and mineralogy, concentration of pigments highly susceptible to degradation, concentration of refractory pigments (pigments not susceptible to degradation) and different preservation conditions (based on the pigment index values). The results highlight a correlation between pigments concentration and environmental variables, showing the eastern limit of West coastal upwelling plumes influence along the Algarve continental shelf.

This document concerns the collection, analysis and on-line experimentation of ocean color data off the Western Iberian Peninsula. Field measurements have been acquired during the BIOMETORE field campaign in summer 2016 to evaluate and enhance Earth observation capabilities of the Copernicus program. Deliverables of the Ocean and Land Colour Instrument on board of the Sentinel-3 satellite of the European Space Agency are of specific interest. Preliminary evaluations confirm the quality of the in situ measurements to address the match-up future analysis of radiometric values and derived data products. On-line experimentation undertaken with the WebEnhanced Service To Ocean Color demonstrates the feasibility of enabling in a transparent way the user's access to complex functionalities such as neural network applications. FCT [IF/00541/2013/CP1181] info:eu-repo/semantics/publishedVersion

Ocean colour is recognised as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS), and spectrally-resolved water-leaving radiances (or remote-sensing reflectances) in the visible domain, and chlorophyll-a concentration are identified as required ECV products. Time series of the products at the global scale and at high spatial resolution, derived from ocean-colour data, are key to studying the dynamics of phytoplankton at seasonal and inter-annual scales, their role in marine biogeochemistry, the global carbon cycle, the modulation of how phytoplankton distribute solar-induced heat in the upper layers of the ocean, and the response of the marine ecosystem to climate variability and change. However, generating a long time series of these products from ocean-colour data is not a trivial task: algorithms that are best suited for climate studies have to be selected from a number that are available for atmospheric correction of the satellite signal and for retrieval of chlorophyll-a concentration, since satellites have a finite life span, data from multiple sensors have to be merged to create a single time series, and any uncorrected inter-sensor biases could introduce artefacts in the series, e.g., different sensors monitor radiances at different wavebands such that producing a consistent time series of reflectances is not straightforward. Another requirement is that the products have to be validated against in situ observations. Furthermore, the uncertainties in the products have to be quantified, ideally on a pixel-by-pixel basis, to facilitate applications and interpretations that are consistent with the quality of the data. This paper outlines an approach that was adopted for generating an ocean-colour time series for climate studies, using data from the MERIS (MEdium spectral Resolution Imaging Spectrometer) sensor of the European Space Agency, the SeaWiFS (Sea-viewing Wide-Field-of-view Sensor) and MODIS-Aqua (Moderate-resolution Imaging Spectroradiometer-Aqua) sensors from the National Aeronautics and Space Administration (USA), and VIIRS (Visible and Infrared Imaging Radiometer Suite) from the National Oceanic and Atmospheric Administration (USA). The time series now covers the period from late 1997 to end of 2018. To ensure that the products meet, as well as possible, the requirements of the user community, marine-ecosystem modellers, and remote-sensing scientists were consulted at the outset on their immediate and longer-term requirements as well as on their expectations of ocean-colour data for use in climate research. Taking the user requirements into account, a series of objective criteria were established, against which available algorithms for processing ocean-colour data were evaluated and ranked. The algorithms that performed best with respect to the climate user requirements were selected to process data from the satellite sensors. Remote-sensing reflectance data from MODIS-Aqua, MERIS, and VIIRS were band-shifted to match the wavebands of SeaWiFS. Overlapping data were used to correct for mean biases between sensors at every pixel. The remote-sensing reflectance data derived from the sensors were merged, and the selected in-water algorithm was applied to the merged data to generate maps of chlorophyll concentration, inherent optical properties at SeaWiFS wavelengths, and the diffuse attenuation coefficient at 490 nm. The merged products were validated against in situ observations. The uncertainties established on the basis of comparisons with in situ data were combined with an optical classification of the remote-sensing reflectance data using a fuzzy-logic approach, and were used to generate uncertainties (root mean square difference and bias) for each product at each pixel.

The spatio-temporal variation of microphytobenthos (MPB) at the scale of a large estuary (Tagus estuary, Portugal) was studied using a combination of field and satellite remote sensing data during 2003. This is the first attempt to use remote sensing to study MPB in an ecosystem with a Mediterranean-like climate. Satellite pour l'Observation de la Terre (SPOT) and Medium Resolution Imaging Spectrometer (MERIS) images were used to map benthic microalgae through the application of a Normalized Difference Vegetation index (NDVI). A significant relationship between in-situ benthic chlorophyll a measurements and SPOT NDVI values was used to derive a map for biomass spatial distribution. At the scale of the whole intertidal area, NDVI time-series from 2003 revealed that MPB showed clear temporal variations, with lower values observed in summer compared to winter. This seasonal trend was found both in the SPOT and MERIS images and may be the result of extreme high temperatures that inhibit MPB growth. The main MPB biofilms were spatially stable through time at a large scale. Maximum NDVI values during the winter were found in the high shore with decreasing NDVI values towards the low shore. MPB light limitation at the lowest bathymetries is likely to occur in winter due to the high turbidity of Tagus estuary. The biomass spatial distribution map, obtained for January 2003, indicated low values ranging from 0 to 20 mg Chl a m

Tagus estuary is one of the largest estuaries of Western Europe. With the aim of unravelling the drivers of primary production in this shallow and turbid nutrient replete estuary, we tested the hypothesis that light availability is a major factor controlling phytoplankton production. Environmental parameters, phytoplankton biomass, community composition, and photosynthetic parameters were monitored at two sites in the estuary during a complete annual cycle. Despite the fact that nutrient concentrations were always above growth-limiting values, Chl a concentrations were relatively low throughout the study period. High water column turbidity, due to riverine inputs, promoted a rapid attenuation of light and created a compressed profile with optimal photosynthetic conditions. Therefore, the phytoplankton community, dominated by small cells, such as diatoms and cryptophycean flagellates, displayed highly photosynthetic efficiency and low light-saturated photosynthetic rates as a photo-acclimation response to low light conditions year-round. Primary production rate was unimodal, peaking in the summer months. In such estuarine system, gross primary production could thus be predicted by an existing robust empirical model based on pigment standing crop (Chl a), surface irradiance (E 0) and optical depth (Z eup). Compared to other shallow estuaries, the Tagus can be classified as a low- to moderately productive estuary, being the turbidity-induced low light conditions the principal factor limiting phytoplankton growth.

Time series of satellite-derived surface chlorophyll-a concentration (Chl) in 1997-2009 were used to examine for trends in the timing of the annual phytoplankton bloom maximum. Significant trends towards earlier phytoplankton blooms were detected in about 11 % of the area of the Arctic Ocean with valid Chl data, e.g. in the Hudson Bay, Foxe Basin, Baffin Sea, off the coasts of Greenland, in the Kara Sea and around Novaya Zemlya. These areas roughly coincide with areas where ice concentration has decreased in early summer (June), thus making the earlier blooms possible. In the selected areas, the annual phytoplankton bloom maximum has advanced by up to 50 days which may have consequences for the Arctic food chain and carbon cycling. Outside the Arctic, the annual Chl maximum has become earlier in boreal North Pacific but later in the North Atlantic.

Coastal ecosystem processes are largely influenced by the interaction of different factors operating at various temporal and spatial scales, specifically those responsible for primary production patterns that modulate zooplankton and subsequent trophic levels. Hydrological processes, such as tidal cycles and coastal currents, nutrients availability, phytoplankton groups (studied through algal pigment signatures analysed by HPLC), and zooplankton abundance and distribution were investigated at the Sofala Bank (Mozambique), with special emphasis on their horizontal distribution and vertical dynamics (48 h). Horizontal distribution has shown inshore–offshore gradients in all analysed parameters, as well as inshore waters intrusion probably related to Zambezi River delta runoff. Tidal currents were responsible for major hydrological vertical variations and for horizontal and vertical advection of phytoplankton biomass in the surface and deepest layers, respectively. Nutrient concentrations were typical from oligotrophic regions, and nutrient ratios were strongly influenced by depleted nitrate + nitrite concentrations, indicating low estuarine discharges typical from the dry season. The very low N:P ratio obtained suggests strong nitrogen limitation to phytoplankton communities, supporting the low phytoplankton abundance observed. Both phytoplankton pigments and zooplankton were found mainly near the bottom (40 m depth), despite the latter displayed vertical migrations triggered by light variations. Phytoplankton community was dominated by microflagellates, specifically prymnesiophyceans, and behaved as a whole, except Cyanobacteria that displayed vertical distribution movements different from other phytoplankton groups, being mainly concentrated at mid-water column depths (10–20 m). This investigation enhances physico-chemical phenomena and their importance determining the planktonic communities vertical dynamics at Sofala Bank, a tropical coastal ecosystem of the Western Indian Ocean where planktonic dynamics are still poorly described and understood.

Phytoplankton biomass and composition were monitored for four coastal lagoons of the Algarve, Portugal, from February to November 2006. The phytoplankton community in the freshwater lagoons - Dunas Douradas (FW1) and Garrao (FW2) - as determined by biomarker pigment concentration using high-performance liquid chromatography and the pro- gram CHEMTAX, was dominated by diatoms and chlorophytes. For the brackish lagoons - Salgados (BW1) and Almargem (BW2) - cyanophytes and diatoms were the most important groups respectively. The trophic state indices, based on algal biomass, classify FW1, FW2 and BW1 lagoons as hypertrophic, whereas BW2 is classified as mesotrophic. Signs of eutro- phication in the former lagoons include large phytoplankton biomass (chlorophyll-a) fluctuations, the presence of phyto- plankton taxa that usually thrive under nutrient-enriched conditions and high concentrations of chlorophyll-a-degradation products. Chemotaxonomic analyses derived from high-performance liquid chromatography data might constitute a valu- able tool for achieving Water Framework Directive requirements for an evaluation system based on phytoplankton taxo- nomic composition.

Data were collected in the Tagus estuary from 1999–2007 on a monthly basis and combined with published results and for several previous years between 1980 and 1995, so that a comprehensive analysis could be performed over a non-continuous 27-year period. Sampling conditions and methods were similar for all datasets. Extreme wet and dry years were observed. River flow was strongly linked to phytoplankton abundance, with the highest biomass attained in dry years. The observed range of annual median Chl a was 1.8–7.6 µg L−1 and the overall median was 3.5 µg L−1. Dissolved inorganic nitrogen (DIN) and silicate showed a clear seasonal pattern, with a maximum in winter−spring, indicating a freshwater origin. Although wastewater treatment started in 1990, no difference was detected from 1980 to the present in terms of DIN and phosphorus. The recorded seasonal pattern for biomass with highest values in late spring–summer period is comparable to other temperate tidally influenced ecosystems. In spite of interannual differences in terms of Chl a concentration or the time of the maximum Chl a occurrence, a repeatable pattern could be identified. The mean growth development time for phytoplankton was 163 days (June 12) ranging 129–206 days (May 9–July 26) during the sampling period. No obvious changes in phytoplankton community structure were observed over time: diatoms were always the dominant group, and cryptophytes were relatively abundant throughout autumn–winter. The dominant species have remained essentially the same since 1969. River inflow, light availability, and temperature were the major factors shaping phytoplankton variability patterns. The strong influence of tidal mixing on the estuarine waters appears to lower the risk of potential eutrophication in the Tagus estuary. The lack of change in nutrients and phytoplankton biomass and composition observed in this study is an important contribution towards the assessment of natural variability versus responses to man-induced inputs in this severely anthropogenically disturbed estuary.

Ocean colour is the only essential climate variable that targets a biological variable (chlorophyll-a concentration (chl-a)) and is also amenable to remote sensing at the global scale. However, the finite lifetime of individual ocean-colour sensors, and the differences in their characteristics increase the difficulty of creating a long-term, consistent, ocean-colour time series that meets the requirements of climate studies. The Ocean Colour Climate Change Initiative (OC-CCI), a European Space Agency programme, has recently produced a time series of satellite-based ocean-colour products at the global scale, merging data from three sensors: Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer on the Aqua Earth Observing System (MODIS-Aqua), and Medium Resolution Imaging Spectrometer (MERIS), while attempting to reduce inter-sensor biases.In this work we present a comparison between the OC-CCI chlorophyll-a product and precursor satellite-derived data sets, from both single missions (SeaWiFS, MODIS-Aqua, and MERIS) and multi-mission products (global ocean colour (GlobColour) and Making Earth Science Data Records for Use in Research Environments (MEaSUREs)). To this end, OC-CCI global monthly composites are compared to the similar products offered by single-mission and multi-mission records. Our results indicate that the OC-CCI product provides a higher number of observations. Comparing the observations that match with precursors, the OC-CCI product was generally most similar to the single-mission products. Relationships between OC-CCI and other precursors did not change significantly during a common and continuous period, and, on average the root-mean-square differences between log-transformed chlorophyll-a concentration are below or equal to 0.11. Further, when considering variability that could arise when merging data from different sources, it is shown that the OC-CCI product is a longer term constant than those from other multi-mission initiatives studied here.

A 7-year (March 1999eNovember 2005) monitoring program was developed in the Tagus estuary to study phytoplankton dynamics and several key controlling factors, namely nutrient content, light availability, atmospheric and hydrodynamic conditions (temperature, wind, rainfall, river flow, and salinity). Water was collected at four sampling sites on a monthly basis. Phytoplankton biomass, analyzed as Chl a, was moderate to low, when compared to other mesotidal estuaries: interannual average Chl a values ranged from 1.4 in winter to 8.0 mg L � 1 in summer. A consistent seasonal pattern was observed, with a unimodal peak extending from late spring to summer. The phytoplankton community, as determined by biomarker pigment concentration using HPLC and CHEMTAX, was dominated by diatoms (57%), and included cryptophytes (23%), dinoflagellates (6.8%), chlorophytes (5.4%), euglenophytes (4.9%), and prasinophytes (2.6%). The method was capable of detecting phytoplankton taxa generally underestimated or overlooked when using standard microscopic techniques. Diatoms were the main bloom-formers in the summer Chl a maximum. A stepwise regression analysis showed that air temperature, river flow and irradiance explained 47% of the observed Chl a variance, illustrating the importance of climatic factors as driving forces for seasonal and interannual variability of phytoplankton.

The role of hydrological droughts in shaping meiofauna abundance through alterations in biofilm biomass and composition was investigated. In January 2005, continental Portugal was under a moderate to severe drought resulting from a 40% to 60% decrease in rainfall during the previous 12 months relative to the long-term average (1961–1990). Reservoir capacity was reduced by 30–50% relative to average values and the width of streams was reduced by 20–80% in the Zezere River Basin (central Portugal). Algal biomass and algal class composition of biofilms was assessed through quantification of algal pigments in three reservoir and six river locations. During drought, habitat alterations are expected to be sharp in rivers while, in the absence of water quality deterioration, the habitat characteristics of reservoirs are expected to remain fairly unaffected. Chlorophylls and carotenoid pigments were extracted from biofilm samples and analysed using high performance liquid chromatography (HPLC). In the winter of 2003, during the period of average rainfall, biofilm biomass did not exceed 5 μg chlorophyll a cm−2 at any location. River biofilm biomass was roughly half of that measured in the reservoirs. In the winter of 2005 (drought), biofilm biomass increased by more than 5-fold in river locations and remained low or decreased in the reservoirs. Algal biofilms were either dominated by Bacillariophyceae or by Chlorophyceae regardless of the existence of drought. The relative contribution of Bacillariophyceae to total biofilm biomass was higher during the drought than under average hydrological conditions. The abundance of harpacticoids, cladocerans and ostracods was favoured by the drought only in the reservoirs where an increase in diatom proportion in biofilms was observed. The increase in the abundance of cyclopoid copepods, turbellarians, nematodes and chironomids in rivers during the drought could be explained by algal class composition and biomass of biofilms and environmental variables (organic matter sediment content, phosphorus availability content and sediment granulometry). The hydrological drought appears to regulate meiofauna abundance only in river locations, possibly through the promotion of the growth of biofilms and the availability of organic matter deposited in rivers during the drought.

The distribution of microphytobenthos on intertidal flats is heterogeneous and described as “patchy” over spatial scales of centimetres to kilometres. The spatial structure and the dynamics of microphytobenthic biomass during tidal emersion were investigated by creating high resolution fluorescence distribution maps of biomass ( F o ) at the sediment surface (0.5 cm pixels, 20 cm×20 cm). The PSII maximum quantum efficiency was also calculated for correlation with biomass structure. Intertidal areas of the Biezelingsche Ham (Westerschelde) and Zandkreek (Oosterschelde), The Netherlands, were sampled during June 2000. The space–time variability of the discrete fields obtained was analysed by studying their spatial variogram and their time autocorrelation. Sites showed significant differences in absolute variation of microphytobenthic biomass and in the dynamics of the development of spatial structure throughout the emersion period. The variograms indicated little spatial correlations in the biomass fields above the 2 cm scale, while the time autocorrelation showed a strong persistence of biomass structure at time scales between 1 h and 5 h. The Biezelingse Ham biofilm was shown to be migratory in nature while the Zandkreek system was more stable. At the former site, biomass distribution became more homogenous with time, as upward migration of cells attenuated horizontal patchiness. At the Biezelingsche Ham site, maximum PSII quantum efficiencies varied inversely with biomass areas; reasons for this may include nutrient depletion and CO 2 limitation within dense biofilms.

The spatial and temporal variability of phytoplankton abundance (in terms of chlorophyll a and cell number), inorganic nitrogen, suspended particulate matter (SPM), and light availability was determined throughout one year in the Tagus estuary, Portugal. Chlorophyll a concentrations showed a strong seasonal variation with values ranging from 1 to 32 μg l−1(average 5.4 μg l−1). Chlorophyll patterns were unimodal for sites 1, 2, and 3 and bimodal for site 4. Diatoms and cryptophytes were, throughout the year, the dominant groups in this shallow and unstratified estuarine system. Nitrate concentrations were seasonally related to river flow and ammonium concentrations spatially related to sources of sewage input. Lower river inputs and long water residence times during summer initially promoted the accumulation of phytoplankton, but the resulting low dissolved inorganic nitrogen (DIN) concentrations lead to limitation of phytoplankton growth. Chlorophyll a and DIN values obtained in the present study were comparable to those reported 20 years ago for the Tagus estuary.

Estuaries are among the most productive ecosystems on Earth and provide multiple ecosystem services. They harbor ecologically important habitats for fish, shellfish, and birds and support diverse human activities (e.g., marine transportation, fishing, and tourism). However, climate change, together with the predicted increase of human activities, may increase the hazards in these systems and alter estuarine ecosystems dynamics. Coupled hydrodynamics and biogeochemical numerical models, which jointly simulate the physical, chemical, and biological processes at the relevant spatial and temporal scales, are useful tools to support climate change impact studies on estuarine ecosystems dynamics. A general overview of some well-established coupled hydrodynamic–biogeochemical models is presented and their use to support the study of climate change impacts on estuarine ecosystems is discussed. The use of coupled hydrodynamic–biogeochemical models to support the long-term, climate-adapt management of estuarine ecosystems and the definition of mitigation and adaptation strategies within a climate change context is demonstrated with a case study: the evaluation of climate change impacts in the lower trophic levels dynamics in the Aveiro lagoon using the model ECO-SELFE. Existing coupled hydrodynamic–biogeochemical models already have numerous benefits in climate change impact studies. Future research should improve these models’ capabilities to reduce limitations and uncertainties, in particular those related to the coupled representation of the physical and biological processes and their feedbacks over long time scales.

Chlorophyll a concentration (Chl) product validation off theWestern Iberian coast is here undertaken by directly comparing remote sensing data with in situ surface reference values. Both standard and recently developed alternative algorithms are considered for match-up data analysis. The investigated standard products are those produced by the MERIS (algal 1 and algal 2) and MODIS (OC3M) algorithms. The alternative data products include those generatedwithin the CoastColour Project and Ocean Color Climate Change Initiative (OC-CCI) funded by ESA, as well as a neural net model trained with field measurements collected in the Atlantic off Portugal (MLPATLP). Statistical analyses showed that satellite Chl estimates tend to be larger than in situ reference values. The study also revealed that a non-uniform Chl distribution in the water column can be a concurring factor to the documented overestimation tendency when considering larger optical depth match-up stations. Among standard remote sensing products, MODIS OC3M and MERIS algal 2 yield the best agreement with in situ data. The performance of MLPATLP highlights the capability of regional solutions to further improve Chl retrieval by accounting for environmental specificities. Results also demonstrate the relevance of oceanographic regions such as the Nazaré area to evaluate how complex hydrodynamic conditions can influence the quality of Chl products. This studywas performed in the framework of HabSpot FCT Project, PTDC/MAR/100348/2008 and European Space Agency projects DUE CoastColour (ESRIN/AO/1-6141/09/l-EC) and Climate Change Iniciative — Ocean Color (AO-1/6207/09/I-LG). The work has been also partially supported by the European Space Agency within the framework of the MERIS Validation Activities under contract n. 12595/09/I-OL, and sampling activities benefited from European projects HERMES (GOCE-CT-2005-511234) and Hermione (EC contract 226354) support. We would like to thank NASA OBPG for the MODIS data and ESA Project AOPT-2423 for providing MERIS full resolution images. Ana C. Brito was funded by a Portuguese Post-doc grant from FCT (BPD/63017/2009) and by the Investigador FCT Program (IF/00331/2013). Davide D'Alimonte was funded by Investigador FCT Program (IF/00541/2013). info:eu-repo/semantics/publishedVersion

Increases in ultraviolet radiation (UVR) and CO2 affect phytoplankton growth and mortality in a variety of different ways. However, in situ responses of natural phytoplankton communities to climate change, as well as its effects on phytoplankton annual cycles, are still largely unknown. Although temperature and UVR have been increasing in temperate latitudes during winter, this season is still particularly neglected in climate change studies, being considered a non-active season regarding phytoplankton growth and production. Additionally, coastal lagoons are highly productive ecosystems and very vulnerable to climate change. This study aims, therefore, to evaluate the short-term effects of increased UVR and CO2 on the composition and growth of winter phytoplankton assemblages in a temperate coastal lagoon. During winter 2012, microcosm experiments were used to evaluate the isolated and combined effects of UVR and CO2, under ambient and high CO2 treatments, exposed to ambient UV levels and photosynthetically active radiation (PAR), or to PAR only. Phytoplankton composition, abundance, biomass and photosynthetic parameters were evaluated during the experiments. Significant changes were observed in the growth of specific phytoplankton groups, leading to changes in community composition. The cyanobacterium Synechococcus was dominant at the beginning of the experiment, but it was negatively affected by UVR and CO2. Diatoms clearly benefited from high CO2 and UVR, particularly Thalassiosira. Despite the changes observed in specific phytoplankton groups, growth and production of the whole phytoplankton community did not show significant responses to UVR and/or CO2.

In recent years, the global distribution of phytoplankton functional types (PFT) and phytoplankton size classes (PSC) has been determined by remote sensing. Many of these methods rely on interpretation of phytoplankton size or type from pigment data, but independent validation has been difficult due to lack of appropriate in situ data on cell size. This work uses in situ data (photosynthetic pigments concentration and cell abundances) from the north-east Atlantic, along a trophic gradient, sampled from 2005 to 2010, as well as Atlantic Meridional Transect (AMT) data for the same region, to test a previously developed conceptual model, which calculates the fractional contributions of pico-, nano- and micro-plankton to total phytoplankton chlorophyll biomass ( Brewin et al., 2010 ). The application of the model proved to be successful, as shown by low mean absolute error between data and model fit. However, regional values obtained for the model parameters had some effect on the relative distribution of size classes as a function of chlorophyll-a, compared with the results according to the original model. The regional parameterisation yielded a dominance of micro-plankton contribution for chlorophyll-a concentrations greater than 0.5 mg m − 3 , rather than from 1.3 mg m − 3 in the original model. Intracellular chlorophyll-a (Chla) per cell, for each size class, was computed from the cell enumeration results (microscope counts and flow cytometry) and the chlorophyll-a concentration for that size class given by the model. The median intracellular chlorophyll-a values computed were 0.004, 0.224 and 26.78 pg Chla cell − 1 for pico-, nano-, and micro-plankton respectively. This is generally consistent with the literature, thereby providing an indirect validation of the method based on pigments to assign size classes. Using a satellite-derived composite image of chlorophyll-a for the study area, a map of cell abundance was generated based on the computed intracellular chlorophyll-a for each size-class, thus extending the remote-sensing method for mapping size classes of phytoplankton from chlorophyll-a concentration to mapping cell numbers in each class. The map reveals the ubiquitous presence of pico-plankton, and shows that all size classes are more abundant in more productive areas.

The purpose of this study was to measure primary productivity of microphytobenthos of intertidal mudflats of Tagus estuary. Sampling occurred from 1991 to 1992, during low tide of spring waters on 2 stations, each one representative of a typical habitat. Net photosynthesis rates (NP) were measured in undisturbed sediment cores incubated in the laboratory andin situ. The results obtained indicated that NP in the laboratory was similar for the two stations, (mean values of 1.1 and 1.3 mmol O2 m−2 h−1, within a range of 5-fold). A stepwise regression analysis combining biotic and abiotic factors was performed to explain temporal variability, indicating that NP in each site was influenced by different parameters, with the exception of precipitation, which might play an important role in the disruption of sediment-water interface. Photosynthesis-Irradiance curves constructed with the results obtained insitu showed similar photophysiological responses of the two communities. Based on the values of photosynthetic efficiency (α), and of optimal light intensity (lk) measured and on the combination of the photoperiod pattern and the amount of light available for each sampling site, an estimation of total annual productivity is given: 47–178 g C m−2.

The Water Framework Directive (WFD) requires the assessment of ecological quality of water bodies based on physico-chemical and biological elements. This paper is focused on the use of phytoplankton as a biological indicator of ecological quality in Portuguese transitional waters (TWs). Phytoplankton is considered to be a sensitive indicator to environmental changes, especially nutrient enrichment, and is therefore useful for evaluating eutrophication processes. According to the WFD, three metrics should be generally considered to assess ecological status: phytoplankton biomass (chlorophyll a), phytoplankton composition and abundance, and phytoplankton bloom intensity and frequency. This work presents an integrated methodology that comprises the first and the last metrics indicated above, analyzed during the growing season (February–October). Significant (p

Microelectrode oxygen profiles were measured in intertidal sediments from Ria Formosa (S. Portugal), a very productive shallow coastal lagoon. Four intertidal sampling sites were selected according to different sediment characteristics. Individual profiles revealed a high degree of lateral variability on a centimeter spatial scale. Nevertheless, consistent differences were observed between oxygen profiles measured in atmosphere-exposed and inundated intertidal sediments: in organically poor sand oxygen-penetration depth varied from 3 mm in inundated cores to more than 7 mm in exposed ones, while in organically rich muddy sand and mud it remained between 0.5–2.0 mm. The oxygen input from inundated to exposed conditions was estimated for each sampling site. Semi-diurnal tidal fluctuation, leading to periodical atmospheric exposure of sediments plays a major role in the oxygenation process of intertidal zones of Ria Formosa.

The Tagus Estuary is one of the largest estuaries in Europe and merges large urban and industrial areas. Understanding phytoplankton community variability is key for an appropriate assessment of the estuarine ecological status. The objective of the present study was to assess the importance of the tidal influence over the phytoplankton community and to evaluate its main drivers of variation. Weekly sampling was performed at two stations on the Tagus Estuary with different anthropogenic pressures (Alcântara and Barreiro). The sampling covered periods with different tidal amplitude. Alcântara presented both the lowest and highest concentrations of dissolved inorganic nitrogen (DIN) and orthophosphate concentration (DIP), depending on the tidal height. Such high variability in this sampling station is probably due to its proximity to a sewage treatment station outfall and to the estuary mouth. In the present study, both seasonal and tidal variations influenced the chlorophyll a concentration of which the tidal cycle explained up to 50% of the chlorophyll a variations. Chlorophyll a displayed a seasonal trend with two peaks of phytoplankton biomass between spring and mid-summer. The main drivers of chlorophyll a variation were radiation, water temperature, tidal amplitude, salinity, river discharge, and the inorganic nutrients DIN and DSi. The estuarine phytoplankton community was mainly dominated by Bacillariophyceae, especially at Alcântara. Bacillariophyceae were less important at Barreiro, where communities had a higher representation from other phytoplankton groups, such as Cryptophyceae and Prasinophyceae. The drivers of variability in the community composition were similar to those influencing the total biomass. In conclusion, the spring-neap tidal cycle strongly influenced the phytoplankton community, both in terms of biomass and community composition. Of the several tidal conditions, spring tides were the tidal condition that presented both higher biomass and higher Bacillariophyceae representativity in the community.

Dissolved nitrogen fluxes and denitrification were studied during 1 yr in intertidal sediments of the Tagus estuary (Portugal). This study focused on the factors regulating both nitrogen fluxes across the sediment-water interface and denitrification, and on the effect of microphytobenthos activity in controlling nitrogen cycling in these areas. Sampling was performed monthly at 2 stations located in inner and outer intertidal areas. Fluxes of O 2 , NO 3 - , NO 2 - , NH 4 + and N 2 O, and denitrification (determined by the nitrogen-isotope pairing technique) were measured simultaneously in closed chambers incubated in the laboratory under simulated in situ temperature and light conditions, as well as in the dark. At the sediment-water interface, higher DIN fluxes and lower denitrification rates were registered at higher temperatures and lower NO 3 - concentration in the water column. Oxygen uptake by the sediment was generally higher than release, particularly in summer. Primary productivity displayed a seasonal cycle, positively influenced by temperature. Denitrification rates were closely related to NO 3 - river-input, Temperature, NO 3 - concentration in the water column, microphytobenthos, infauna and tidal height were the key parameters involved in controlling nitrogen cycling at the sediment-water interface in the Tagus estuary. A comparison of annual nitrogen fluxes and denitrification rates between sites was made, taking into account tidal immersion periods. Hence, N-removal by denitrification accounted for 156 mmol m -2 yr -1 in the inner station and 482 mmol m -2 yr -1 in the outer station. These rates represent respectively ca 3 and 9% of total DIN available in the estuarine water column at those 2 stations, respectively. N 2 O production was comparatively very low (0.3 to 0.6 mmol m -2 yr -1 ). The estimated nitrogen assimilation rates by microphytobenthos were 707 mmol m -2 yr -1 in the inner station and 333 mmol m -2 yr -1 in the outer station, indicating that a considerable amount of nitrogen was retained within benthic microalgae. The assimilation/denitrification ratio, with a mean value of 2, shows the relative importance of the denitrification role as a N-sink. Apart from denitrification, it is suggested that other processes must be involved in the removal of nitrogen from the estuary.

Monitoring lakes in high-latitude areas can provide a better understanding of freshwater systems sensitivity and accrete knowledge on climate change impacts. Phytoplankton are sensitive to various conditions: warmer temperatures, earlier ice-melt and changing nutrient sources. While satellite imagery can monitor phytoplankton biomass using chlorophyll a (Chl) as a proxy over large areas, detection of Chl in small lakes is hindered by the low spatial resolution of conventional ocean color satellites. The short time-series of the newest generation of space-borne sensors (e.g., Sentinel-2) is a bottleneck for assessing long-term trends. Although previous studies have evaluated the use of high-resolution sensors for assessing lakes&rsquo, Chl, it is still unclear how the spatial and temporal variability of Chl concentration affect the performance of satellite estimates. We discuss the suitability of Landsat (LT) 30 m resolution imagery to assess lakes&rsquo, Chl concentrations under varying trophic conditions, across extensive high-latitude areas in Finland. We use in situ data obtained from field campaigns in 19 lakes and generate remote sensing estimates of Chl, taking advantage of the long-time span of the LT-5 and LT-7 archives, from 1984 to 2017. Our results show that linear models based on LT data can explain approximately 50% of the Chl interannual variability. However, we demonstrate that the accuracy of the estimates is dependent on the lake&rsquo, s trophic state, with models performing in average twice as better in lakes with higher Chl concentration (&gt, 20 &micro, g/L) in comparison with less eutrophic lakes. Finally, we demonstrate that linear models based on LT data can achieve high accuracy (R2 = 0.9, p-value &lt, 0.05) in determining lakes&rsquo, mean Chl concentration, allowing the mapping of the trophic state of lakes across large regions. Given the long time-series and high spatial resolution, LT-based estimates of Chl provide a tool for assessing the impacts of environmental change.

Reliable satellite estimates of chlorophyll-a concentration (Chl-a) are needed in coastal waters for applications such as eutrophication monitoring. However, because of the optical complexity of coastal waters, retrieving accurate Chl-a is still challenging. Many algorithms exist and give quite different performance for different optical conditions but there is no clear definition of the limits of applicability of each algorithm and no clear basis for deciding which algorithm to apply to any given image pixel (reflectance spectrum). Poor quality satellite Chl-a data can easily reach end-users. To remedy this and provide a clear decision on when a specific Chl-a algorithm can be used, we propose simple quality control tests, based on MERIS water leaving reflectance (ρw) bands, to determine on a pixel-by-pixel basis if any of three popular and complementary algorithms can be used. The algorithms being tested are: 1. the OC4 blue-green band ratio algorithm which was designed for open ocean waters; 2. the OC5 algorithm which is based on look-up tables and corrects OC4 overestimation in moderately turbid waters and 3. a near infrared-red (NIR-red) band ratio algorithm designed for eutrophic waters. Using a dataset of 348 in situ Chl-a / MERIS matchups, the conditions for reliable performance of each of the selected algorithms are determined. The approach proposed here looks for the best compromise between the minimization of the relative difference between In situ measurements and satellite estimations and the number of pixels processed. Conditions for a reliable application of OC4 and OC5 depend on ρw412/ρw443 and ρw560, used as proxies of coloured dissolved organic matter and suspended particulate matter (SPM), as compared to ρw560/ρw490, used as a proxy for Chl-a. Conditions for reliable application of the NIR-red band ratio algorithm depend on Chl-a and SPM. These conditions are translated into pixel-based quality control (QC) tests with appropriately chosen thresholds. Results show that by removing data which do not pass QC, the performance of the three selected algorithms is significantly improved. After combining these algorithms, 70% of the dataset could be processed with a median absolute percent difference of 30.5%. The QC tests and algorithm merging methodology were then tested on four MERIS images of European waters. The OC5 algorithm was found to be suitable for most pixels, except in very turbid and eutrophic waters along the coasts where the NIR-red band ratio algorithm helps to fill the gap. Finally, a test was performed on an OLCI-S3A image. Although some validations of water reflectance are still needed for the OLCI sensors, results show similar behavior to the MERIS applications which suggests that when applied to OLCI data the present methodology will help to accurately estimate Chl-a in coastal waters for the next decade.