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The utility of global and regional algorithms for retrieving surface chlorophyll-a values from satellite images of MODIS (Moderate-Resolution Imaging Spectroradiometer) onboard Aqua and Terra Satellites, SeaWiFS (Sea-Viewing Wide Field-of-View Sensor), MERIS (MEdium Resolution Imaging Spectrometer) and VIIRS (Visible Infrared Imaging Radiometer Suite) is tested in a wide region of the Western Mediterranean Sea, the Spanish Mediterranean. The comparison between chlorophyll-a concentration obtained from samples collected and satellite concentrations calculated with global and regional algorithms for this area demonstrates that satellites overestimate the surface chlorophyll-a. In this work, a set of new algorithms, namely SMED (Spanish MEDiterranean algorithms), are proposed to improve the estimations of surface chlorophyll-a in our study area. The SMED set of algorithms employs the linear function to fit the standard Maximum Band Ratios (MBR) to the in situ surface chlorophyll-a concentration (in logarithmic scale). The implementation of the SMED algorithms is simple, and the accuracy is as good as more complex algorithms like OC5 (5 band algorithms tuned for European Atlantic and tested successfully in the Mediterranean Sea). The improvement of SMED algorithms is important respect to MedOC (built based in data of the Mediterranean Sea) for all sensors, ranging 17%-37% in terms of the coefficient of determination (R2). In the case of OC algorithms (standard global algorithms currently operative for most ocean colour sensors), SMED also improves the estimates in 11%-22%, except in 4% for MERIS. Additionally, when comparing satellite estimates with in situ chlorophyll-a in log-log scale, SMED achieved the best correspondence, even with respect to OC5. During the calibration procedure MERIS remote sensing reflectances (RRS) showed a higher sensitivity to the presence of mineral aerosols, and therefore it is strongly recommended to include the value of Aerosol Optical Thickness (AOT) in the algorithms for chlorophyll in the region under study. A regional algorithm for VIIRS ocean colour sensor is also proposed based on a matchup database built from spatially and temporary distributed SMED chlorophyll-a values of MODIS-Aqua and VIIRS RRS. Our analysis suggests that SeaWiFS, MODIS-Aqua, and VIIRS based SMED algorithms can support generating a multiplatform time series in the region. [ABSTRACT FROM AUTHOR]

The utility of three different algorithms for retrieving surface chlorophyll-avalues from satellite images of MODIS-Aqua is tested in the northern Alboran Sea. The available global algorithm to calculate chlorophyll-afrom reflectance of MODIS-Aqua (OC3M) overestimates the surface chlorophyll-ain the study area. Another regional algorithm specifically developed for the Mediterranean Sea (MedOC3) improves the estimates although the best outcome is obtained with OC5, which was developed for Atlantic coastal waters. The three tested algorithms perform worse atin situchlorophyll-aconcentrations higher than 1 mg m−3and exhibit uncertainty levels higher than 35% for this range of concentrations. A new algorithm (ALBOC3) is proposed which produces a good estimation of thein situchlorophyll-afor the whole range of concentrations normally registered in the study area (0.1–3.5 mg m−3). We hypothesize that the particular bio-optical features of the northern Alboran Sea phytoplankton explain the poor functioning of the published algorithms that have been tested in this work. [ABSTRACT FROM AUTHOR]

Some of the most important effects of global change on coastal marine systems include increasing nutrient inputs and higher levels of ultraviolet radiation (UVR, 280–400 nm), which could affect primary producers, a key trophic link to the functioning of marine food webs. However, interactive effects of both factors on the phytoplankton community have not been assessed for the Mediterranean Sea. An in situ factorial experiment, with two levels of ultraviolet solar radiation (UVR+PAR vs. PAR) and nutrients (control vs. P-enriched), was performed to evaluate single and UVR×P effects on metabolic, enzymatic, stoichiometric and structural phytoplanktonic variables. While most phytoplankton variables were not affected by UVR, dissolved phosphatase (APAEX) and algal P content increased in the presence of UVR, which was interpreted as an acclimation mechanism of algae to oligotrophic marine waters. Synergistic UVR×P interactive effects were positive on photosynthetic variables (i.e., maximal electron transport rate, ETRmax), but negative on primary production and phytoplankton biomass because the pulse of P unmasked the inhibitory effect of UVR. This unmasking effect might be related to greater photodamage caused by an excess of electron flux after a P pulse (higher ETRmax) without an efficient release of carbon as the mechanism to dissipate the reducing power of photosynthetic electron transport. [ABSTRACT FROM AUTHOR]

In the Bay of Algeciras (BA), intensive urban and industrial activityis underway, which is potentially responsible for the release of significant quantities of nutrients. However, the assessment of the impact of these discharges is complex. Nutrient concentration in the surface layer is per se strongly variable due to the variability associated with the upwelling of nutrient-enriched deep Mediterranean water (MW), which in turn is regulated by atmospheric forcing. The aim of this study is to determine the effects of changes in the upwelling intensity on the load of nitrate and phosphate in the BA and to appraise their impact on chlorophyll a variability. Based on this analysis, the possible influence of the nutrients released from land-based sources is indirectly inferred. Data and samples collected during nine research cruises carried out in different seasonal cycle periods between 2010 and 2015 in the BA were analysed. The vertical variation of temperature and salinity indicates that the MW upwelling was favoured in spring, as occurred in other coastal areas of the northern Alboran Sea. However, principal component analysis conducted on physical and chemical data reveals that shifts in nutrients and chlorophyll a in the euphotic layer are poorly explained by changes in the upwelling intensity. Furthermore, during some of these research surveys (particularly in summer), chlorophyll a concentrations were higher in the BA as compared to a nearby coastal area also affected by MW upwelling. Scarce information about land-based pollution sources precludes quantitative analysis of the impact of nutrient loads on water quality; however, the available data suggest that the main source of allochthanous inorganic nitrogen over the period 2010–2015 in the BA was nitrate. Therefore, it is reasonable to hypothesize that the high concentrations of nitrate and chlorophyll a in BA in summer are a consequence of those discharges. Our study highlights the need of more exhaustive inventories of sewage and river discharges to adequately rate their impact in the BA. [ABSTRACT FROM AUTHOR]

Thermal stratification of the water column promotes contrasting conditions with respect to irradiance level and nutrient concentration for phytoplankton growth, demanding more research on which environmental factor has more importance in determining the photosynthetic performance of the communities. For this purpose, two research surveys were performed in several coastal stations located in the Alboran Sea (Western Mediterranean Sea) at the end of 2008 and 2009 summers. Primary productivity was estimated by determining photosynthetic electron transport rates (ETRB) and inorganic carbon assimilation rates (PP C B) at two different depths within the water column, surface and chlorophyll a vertical maximum depth (CM). At the surface, communities presented similar photo-acclimation features during the two surveys, which were different from those obtained for the CM samples that grew at more favorable nutrient and light conditions. The acclimation response to surface conditions consisted in the reduction in the chlorophyll a (Chl a) cell content, and the increase in the Chl a specific light absorption coefficient [a*(λ)], initial slope of ETRB vs. Irradiance curves (αB) and light saturation parameter (E k), indicating that the communities were acclimated to stress conditions (i.e. high PAR and UVR dose and low nutrient availability). However, the in situ primary productivity was unaffected; on the contrary, both PP C B and ETRB in situ increased with respect to the values obtained for communities growing at CM depth. Furthermore, Φ ETR,C (i.e. the molar ratio of transported e− to assimilated carbon) in surface samples was similar to CM samples (8.0 ± 3.5 compared with 7.5 ± 5.3 mol e− [mol−1 C], respectively). The analysed phytoplankton communities were mainly dominated by diatom. This corroborates that diatoms possess a greater capacity to cope with high irradiance levels, even at low nutrient concentrations registered during this study at surface waters. • Variability in phytoplankton photosynthetic parameters is controlled by irradiance. • Diatoms possess high capacity to cope with high irradiance and low nutrients. • Primary production modeling based on chlorophyll shall be done with caution. [ABSTRACT FROM AUTHOR]