Photoacclimation of picophytoplankton in the central Cantabrian Sea

Photoacclimation of picophytoplankton was studied in the mixed layer of 3 stations in the central Cantabrian Sea (southern Bay of Biscay). Picophytoplankton chl a:carbon ratios (θ) presented minimum values during summer, when irradiance, temperature, and biomass of prokaryotes reached maximum values and inorganic nutrient concentrations were low. Conversely, the maximum θ were reached during winter, coincident with lowest annual irradiance but maximum concentration of inorganic nutrients and higher relative biomass of eukaryotes. Changes in θ were modeled using irradiance as an independent variable. Exponentially decreasing functions of θ with irradiance were significant only when the mean temperatures in the mixed layer were above 14°C. These functions presented light-saturated minimum ratios (θmin) that decreased linearly with temperature and low-light maximum ratios (θmax) that increased exponentially with temperature. Such relationships were used to establish an empirical model that reproduced the seasonality of picophytoplankton θ in the mixed layer, with minima in summer and maxima in winter. A maximum potential θ, θN,T-max, was determined to estimate picophytoplankton growth rates in the central Cantabrian Sea. Combinations of picophytoplankton growth rates and biomass in the mixed layer were used to estimate areal picophytoplankton primary production rates in the euphotic zone that presented a bimodal seasonal cycle, with maxima in late winter (ca. 100 mg C m-2 d-1) and in late autumn (>200 mg C m-2 d-1) and mean annual values around 120 mg C m-2 d-1.En prens

Seasonality of picophytoplankton chlorophyll a and biomass in the central Cantabrian Sea, southern Bay of Biscay

Seasonal changes in the abundance and biomass of cyanobacteria (Synechococcus and Prochlorococcus) and picoeukaryotes were studied by flow cytometry in the upper layers of the central Cantabrian Sea continental shelf, from April 2002 to April 2006. The study area displayed the typical hydrographic conditions of temperate coastal zones. A marked seasonality of the relative contribution of prokaryotes and eukaryotes was found. While cyanobacteria were generally more abundant for most of the year (up to 2.4 105 cells mL− 1), picoeukaryotes dominated the community (up to 104 cells mL− 1) from February to May. The disappearance of Prochlorococcus from spring through summer is likely related to shifts in the prevailing current regime. The maximum total abundance of picophytoplankton was consistently found in late summer–early autumn. Mean photic-layer picoplanktonic chlorophyll a ranged from 0.06 to 0.53 µg L− 1 with a relatively high mean contribution to total values (33 ± 2% SE), showing maxima around autumn and minima in spring. Biomass (range 0.58–40.16 mg C m− 3) was generally dominated by picoeukaryotes (mean ± SE, 4.28 ± 0.27 mg C m− 3) with an average contribution of cyanobacteria of 30 ± 2%. Different seasonality of pigment and biomass values resulted in a clear temporal pattern of picophytoplanktonic carbon to chlorophyll a ratio, which ranged from 10 (winter) to 140 (summer). This study highlights the important contribution of picoplanktonic chlorophyll a and carbon biomass in this coastal ecosystem.Versión del edito

Picoplankton diel variability and estimated growth rates in epipelagic and mesopelagic waters of the central Red Sea

The diel variability of the abundance and cell size of picoplanktonic groups in the central Red Sea was monitored every 2 h in situ on 4 occasions (once per season) from 2015 to 2016. We distinguished Prochlorococcus, low (LF-Syn) and high (HF-Syn) fluorescence Synechococcus, small (Speuk) and large (Lpeuk) picoeukaryotes and two groups of heterotrophic prokaryotes of low (LNA) and high (HNA) nucleic acid content. The diel variability in abundance was less marked than in cell size and more apparent in autotrophs than heterotrophs. Specific growth rates were estimated by an empirical relationship from measurements obtained in bottle incubations of surface and deep samples collected in the winter compared with in situ variations in cell size over 24 h. Autotrophic picoplankton groups generally grew faster (0.23–0.77 d–1) than heterotrophic prokaryotes (0.12–0.50 d–1). Surface to 100 m depth-weighted specific growth rates displayed a clear seasonal pattern for Prochlorococcus, with maxima in winter (0.77 ± 0.07 d–1) and minima in fall (0.52 ± 0.07 d–1). The two groups of Synechococcus peaked in spring, with slightly higher growth rates of LF-Syn (0.57 ± 0.04 d–1) than HF-Syn (0.43 ± 0.04 d–1). Speuk and Lpeuk showed different seasonal patterns, with lower values of the former (0.27 ± 0.02 and 0.37 ± 0.04 d–1, respectively). HNA consistently outgrew LNA heterotrophic prokaryotes, with a higher growth in the epipelagic (0–200 m, 0.36 ± 0.03 d–1) than in the mesopelagic (200–700 m, 0.26 ± 0.03 d–1), while no differences were found for LNA cells (0.19 ± 0.03 d–1 and 0.17 ± 0.02 d–1, respectively). With all data pooled, the mean diel abundances of autotrophic picoplankton in the upper epipelagic and of HNA cells in the epipelagic and mesopelagic layers were significantly correlated with the specific growth rates estimated from cell size variations. Our high-resolution sampling dataset suggests that changes in growth rates underlie the noticeable seasonality of picoplankton recently described in these tropical waters

Localized effects of offshore aquaculture on water quality in a tropical sea

Aquaculture production has increased steadily in many tropical countries over the past few decades, although impact assessments have been frequently neglected. We investigated the impacts of an offshore barramundi fish farm on water quality in the southern-central Red Sea, a traditionally understudied tropical, oligotrophic, and semi-enclosed basin. Inorganic nutrients, particulate matter, chlorophyll-a, and heterotrophic bacteria were measured periodically over 8 months around the farm. Water down-current from the farm had, on average, more heterotrophic bacteria and chlorophyll-a than up-current (11% and 34% higher, respectively). Ratios of dissolved inorganic nitrogen:phosphorus down-current from the farm were lower than ratios up-current (mean 9.8 vs 16.0, respectively). Phosphate, inorganic nitrogen, and particulate matter showed patterns of enrichment associated with the farm after a fish feeding event. Strategies such as feed optimization and considering hydrodynamics in site selection may improve water quality for future fish farms in Saudi Arabia and other tropical countries.En prens

Temperature enhances the functional diversity of dissolved organic matter utilization by coastal marine bacteria

Although bulk bacterial metabolism in response to temperature has been determined for different oceanic regions, the impact of temperature on the functional diversity of dissolved organic matter (DOM) utilization has been largely unexplored. Here, we hypothesized that besides modifying the rates of carbon utilization, temperature can also alter the diversity of substrates utilized. The patterns of utilization of 31 model DOM compounds (as represented in Biolog EcoPlates™) by bacterioplankton were assessed using inocula from surface waters of the southern Bay of Biscay continental shelf over 1 year. Bacteria utilized more polymers and carbohydrates in late spring and summer than in winter, likely reflecting changes in substrate availability linked to the release and accumulation of DOM in phytoplankton post-bloom conditions. Seawater temperature correlated positively with the number of substrates utilized (i.e. functional richness) and this relationship was maintained in monthly experimental incubations spanning 3°C below and above in situ values. The enhancement of functional richness with experimental warming displayed a unimodal response to ambient temperature, peaking at 16°C. This temperature acted as a threshold separating nutrient-sufficient from nutrient-deficient conditions at the study site, suggesting that trophic conditions will be critical in the response of microbial DOM utilization to future warming

Size-dependent photoacclimation of the phytoplankton community in temperate shelf waters (southern Bay of Biscay)

Shelf waters of the Cantabrian Sea (southern Bay of Biscay) are productive ecosystems with a marked seasonality. We present the results from 1 yr of monthly monitoring of the phytoplankton community together with an intensive sampling carried out in 2 contrasting scenarios during the summer and autumn in a mid-shelf area. Stratification was apparent on the shelf in summer, while the water column was comparatively well mixed in autumn. The size structure of the photoautotrophic community, from pico-to micro-phytoplankton, was tightly coupled with the meteo-climatic and hydrographical conditions. Over the short term, variations in the size structure and chlorophyll content of phytoplankton cells were related to changes in the physico-chemical environment, through changes in the availability of nutrients and light. Uncoupling between the dynamics of carbon biomass and chlorophyll resulted in chlorophyll to carbon ratios dependent on body size. The slope of the size dependence of chlorophyll content increased with increasing irradiance, reflecting different photoacclimation plasticity from pico-to micro-phytoplankton. The results have important implications for the productivity and the fate of biogenic carbon in this region, since the size dependence of photosynthetic rates is directly related to the size scaling of chlorophyll content.Versión del edito

Chlorophyll and primary production in the North Atlantic.

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Cytometric Diversity of Marine Bacterioplankton: A 10 Years Interannual study In the Southern Bay of Biscay.

The application of molecular methods to marine ecology in the last decades has completely changed our view of the patterns of diversity and distribution of microorganisms in the ocean (Giovannoni et al. 1990, Zinger et al. 2012). However, these methods are expensive and time-consuming when applied on a large number of samples. Flow-cytometry, on the other hand, allows an efficient and rapid processing of a large number of samples. In this sense, the use of single-cell measurements by flow-cytometry for diversity purposes would be a great advance. In marine ecosystems, this concept has been introduced by Li 1997 as `cytometric diversity'. OBJECTIVES: In this study we evaluated the power of cytometric diversity to detect changes in the composition of bacterioplankton communities: Cant 1) By comparing changes in bacterial composition of 3.5 years surface samples obtained by cytometric diversity and molecular approaches. 2) Analysing the cytometric diversity patterns of a set of 10-years monthly bacterioplankton flow-cytometry samples for 3 coastal stations

Hydrological structures along the N y NW Iberian Shelf during the winter-spring transition

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