63,445 research outputs found
High numbers of Trichodesmium and diazotrophic diatoms in the southwest Indian Ocean
No full textWe observed high cell numbers of large (>50?m) nitrogen-fixing phytoplankton to the south and to the east of Madagascar in February 2005. The distribution of the diazotrophic taxa we found was markedly different: Trichodesmium was most abundant (500-1000 trichomes L-1) in the waters at the southern tip of Madagascar, while diazotrophic diatoms (125-500 cells L-1) were found to be abundant to the southeast of Madagascar. From these cell numbers, using literature values of nitrogen-fixation per cell, we estimate potential rates of nitrogen-fixation for Trichodesmium (south of Madagascar) of 1-5 mmol N m-2 d-1 and for diazotrophic diatoms (east of Madagascar) of 0.4-2.4 mmol N m-2 d-1. These cell-based estimates highlight the potential for significant nitrogen-fixation in the southwest Indian Ocean. High numbers of diazotrophic diatoms in subtropical waters to the east of Madagascar may have important implications for the biogeochemistry of the austral phytoplankton blooms that occur in this area
New production and the f ratio around the Crozet Plateau in austral summer 2004–2005 diagnosed from seasonal changes in inorganic nutrient levels
No full textRecent mesoscale iron-fertilisation experiments suggest that iron may be an important micronutrient in HNLC regions but estimates of carbon export from such experiments are inconclusive. An alternative strategy to estimate export from such environments is to observe naturally productive ecosystems associated with topography. One such system is the Crozet islands and associated plateau (Crozet), at 46°S, 52°E. Each year a large bloom of phytoplankton occurs to the north of Crozet with a reduced bloom occurring to the south. We use nitrate data from the Crozet region collected during austral summer 2004–2005 to estimate new production (NP) via the Redfield ratio. Peak integrated values of up to 50 g C m?2 to the north of the plateau and up to 15 g C m?2 to the south are inferred. We estimate total integrated primary production (TP) using satellite techniques and calculate f for each station. Overall NP is linearly related to TP. However, f declines at very high levels of TP because nitrate usage ceases despite continuing PP and because nitrate levels increased from their postbloom low. This results either from a resupply of nitrate from beneath the thermocline due to mixing processes or to the mixed-layer ammonification and nitrification of accumulated organic nitrogen. We discount the first possibility because our estimates of the mixing flux of nitrate appear to be inadequate to cause the entire recovery in nitrate levels, and because any mixing flux of nitrate would likely be accompanied by a resupply of iron, which would induce NP to occur and erode the resupply of nitrate. Instead we consider the recycling of accumulated organic nitrogen to be a more likely explanation based on our observation of high organic nitrogen levels in the mixed layer north of Crozet during the cruise. The implications of this conclusion are that euphotic zone nitrification is a significant process, and that in this system new and export production are not equivalent. This recycling is sufficiently large that it reduces our estimate of NP north of the plateau to a level where it is equivalent to NP in the south. Whether a similar refertilisation of the mixed layer occurred in the south of the study region, which would be consistent with a meridional gradient in carbon export, is unknown due to the limited duration of the shipboard programme
Biogeographical patterns and environmental controls of phytoplankton communities from contrasting hydrographical zones of the Labrador Sea
No full textThe Labrador Sea is an important oceanic sink for atmospheric CO2 because of intensive convective mixing during winter and extensive phytoplankton blooms that occur during spring and summer. Therefore, a broad-scale investigation of the responses of phytoplankton community composition to environmental forcing is essential for understanding planktonic food-web organisation and biogeochemical functioning in the Labrador Sea. Here, we investigated the phytoplankton community structure (>4 ?m) from near surface blooms (<50 m) from spring and early summer (2011–2014) in detail, including species composition and environmental controls. Spring blooms (>1.2 mg chla m?3) occurred on and near the shelves in May and in offshore waters of the central Labrador Sea in June due to haline- and thermal-stratification, respectively. Sea ice-related (Fragilariopsis cylindrus and F. oceanica) and Arctic diatoms (Fossula arctica, Bacterosira bathyomphala and Thalassiosira hyalina) dominated the relatively cold (<0 °C) and fresh (salinity < 33) waters over the Labrador shelf (e.g., on the southwestern side of the Labrador Sea), where sea-ice melt and Arctic outflow predominates. On the northeastern side of the Labrador Sea, intense blooms of the colonial prymnesiophyte Phaeocystis pouchetii and diatoms, such as Thalassiosira nordenskioeldii, Pseudo-nitzschia granii and Chaetoceros socialis, occurred in the lower nutrient waters (nitrate < 3.6 ?M) of the West Greenland Current. The central Labrador Sea bloom occurred later in the season (June) and was dominated by Atlantic diatoms, such as Ephemera planamembranacea and Fragilariopsis atlantica. The data presented here demonstrate that the Labrador Sea spring and early summer blooms are composed of contrasting phytoplankton communities, for which taxonomic segregation appears to be controlled by the physical and biogeochemical characteristics of the dominant water masses
Environmental drivers of coccolithophore abundance and calcification across Drake Passage (Southern Ocean)
No full textAlthough coccolithophores are not as common in the Southern Ocean as they are in sub-polar waters of the North Atlantic, a few species, such as Emiliania huxleyi, are found during the summer months. Little is actually known about the calcite production (CP) of these communities, or how their distribution and physiology relates to environmental variables in this region. In February 2009, we made observations across Drake Passage (between South America and the Antarctic Peninsula) of coccolithophore distribution, CP, primary production, chlorophyll-a and macronutrient concentrations, irradiance and carbonate chemistry. Although CP represented less than 1 % of total carbon fixation, coccolithophores were widespread across Drake Passage. The B/C morphotype of E. huxleyi was the dominant coccolithophore, with low estimates of coccolith calcite (~ 0.01 pmol C coccolith−1) from biometric measurements. Both cell-normalised calcification (0.01–0.16 pmol C cell−1 d−1) and total CP (< 20 μmol C m−3 d−1) were much lower than those observed in the sub-polar North Atlantic where E. huxleyi morphotype A is dominant. However, estimates of coccolith production rates were similar (0.1–1.2 coccoliths cell−1 h−1) to previous measurements made in the sub-polar North Atlantic. A multivariate statistical approach found that temperature and irradiance together were best able to explain the observed variation in species distribution and abundance (Spearman's rank correlation ρ = 0.4, p < 0.01). Rates of calcification per cell and coccolith production, as well as community CP and E. huxleyi abundance, were all positively correlated (p < 0.05) to the strong latitudinal gradient in temperature, irradiance and calcite saturation states across Drake Passage. Broadly, our results lend support to recent suggestions that coccolithophores, especially E. huxleyi, are advancing pole-wards. However, our in situ observations indicate that this may owe more to sea-surface warming and increasing irradiance rather than increasing CO2 concentrations
Nitrogen uptake responses to a naturally Fe-fertilised phytoplankton bloom during the 2004/2005 CROZEX study
No full textAnnual phytoplankton blooms are observed around most sub-Antarctic islands during austral spring and summer, but are absent in the surrounding high-nutrient low-chlorophyll (HNLC) ocean. The CROZEX study (2004/2005) tested the hypothesis that annual blooms occurring immediately north of the Crozet islands in the Polar Frontal Zone (PFZ) develop because of natural iron fertilisation, while to their south in HNLC waters, there is insufficient iron (Fe) to promote blooms. Size-fractionated nitrogen uptake (?N) experiments using 15N-NO3?, NH4+ and urea addressed three major goals. Firstly, measurements of NO3? uptake (?NO3?) aimed to establish whether ?NO3? responds to natural Fe fertilisation. Secondly, we compared regional ?NO3? rates, hypothesising that in Fe-fertilised regions, ?NO3? should exceed that in HNLC regions. Thirdly, by using satellite imagery, we extrapolated ?NO3? measurements made during a declining bloom to reconstruct seasonal ?NO3? by the spring bloom. Finally, we estimated the ‘new’ Fe demand required to support ?NO3?, comparing this with estimated Fe fluxes.Diatoms and colonial Phaeocystis dominated phytoplankton communities north of the islands, while to the south, Phaeocystis was absent. Total ?N was elevated north of the islands (400 ?mol m?2 d?1) relative to south of the islands (250 ?mol m?2 d?1). Nitrate uptake showed a clear response to Fe fertilisation, exhibiting a strong north (198 ?mol m?2 d?1) to south (74 ?mol m?2 d?1) gradient, while neither ?NH4+ nor ?urea showed such significant latitudinal gradients. The N–S integrated f-ratio gradient was 0.47–0.28 while specific N uptake (VNO3 d?1) rates were significantly higher in the Fe-fertilised region relative to those in the southern HNLC region. The potential for NH4+ inhibition of ?NO3? did not appear to be significant. High PON:chl-a ratios combined with relatively low 14C:15N uptake ratios suggested that most phytoplankton were relatively chlorotic and carbon stressed, with the exception of those growing actively within a cyclonic eddy where neither Fe nor light appeared to be limiting. Size-fractionated ?NO3? and f-ratios exhibited a complex response to NH4+ and Fe availability, with f-ratios in the >20-?m fraction being low (0.3) in the HNLC region, but significantly higher (0.7) in a localised diatom-dominated bloom in the northern Fe-fertilised region. In contrast, f-ratios in the <2-?m size class were similar everywhere (0.44), indicative of Fe-limitation for large-celled diatoms in the southern HNLC region. As a result of Fe-regulated ?NO3?, new production showed a N–S gradient of 24 to 15 mmol C m?2 d?1, very similar to carbon export determined from NO3? ‘draw-down’ and from 234Th measurements. The estimated DFe demand required to support seasonal ?NO3? in the northern region, based on conservative cellular Fe:N quotas, required surface (to 100 m) pre spring-bloom DFe concentrations of 0.75 nmol l?1. Our results support the hypothesis that phytoplankton blooms north of the islands are stimulated by natural Fe fertilisation, with a direct impact on ?NO3?, particularly for larger cells, resulting in higher new production rates relative to those from the Fe-limited HNLC region south of the Crozet islands
Erratum to: Effect of moderate red wine intake on cardiac prognosis after recent acute myocardial infarction of subjects with Type 2 diabetes mellitus (Diabetic Medicine, (2006), 23, 9, (974-981), 10.1111/j.1464-5491.2006.01886.x)
No full textIn an article by Marfella et al, the author name C. Saron is incorrect and should be listed as C. Sardu. Therefore the correct author list is: R. Marfella, F. Cacciapuoti, M. Siniscalchi, F. C. Sasso, F. Marchese, F. Cinone, E. Musacchio, M. A. Marfella, L. Ruggiero, G. Chiorazzo, D. Liberti, G. Chiorazzo, G. F. Nicoletti, C. Sardu, F. D'Andrea, C. Ammendola, M. Verza and L. Coppola.In an article by Marfella et al, the author name C. Saron is incorrect and should be listed as C. Sardu. Therefore the correct author list is: R. Marfella, F. Cacciapuoti, M. Siniscalchi, F. C. Sasso, F. Marchese, F. Cinone, E. Musacchio, M. A. Marfella, L. Ruggiero, G. Chiorazzo, D. Liberti, G. Chiorazzo, G. F. Nicoletti, C. Sardu, F. D'Andrea, C. Ammendola, M. Verza and L. Coppola
The supply of nutrients due to vertical turbulent mixing: A study at the Porcupine Abyssal Plain study site in the northeast Atlantic
No full textAs part of a multidisciplinary cruise to the Porcupine Abyssal Plain (PAP) study site (49°00?N 16°30?W), in June and July 2006, observations were made of the vertical nitrate flux due to turbulent mixing. Daily profiles of nitrate and turbulent mixing, at the central PAP site, give a mean nitrate flux into the euphotic zone of 0.09 (95% confidence intervals: 0.05–0.16) mmol N m?2 d?1. This is a factor of 50 lower than the mean observed rate of nitrate uptake within the euphotic zone (5.1±1.3 mmol N m?2 d?1). By using our direct observations to ‘validate’ a previously published parameterisation for turbulent mixing, we further quantify the variability in the vertical turbulent flux across a roughly 100×100 km region centred on the PAP site, using hydrographic data. The flux is uniformly low (0.08±0.26 mmol N m?2 d?1, the large standard deviation being due to a strongly non-Gaussian distribution) and is consistent with direct measurements at the central site. It is demonstrated that on an annual basis convective mixing supplies at least 40-fold more nitrate to the euphotic zone than turbulent mixing at this location. Other processes, such as those related with mesoscale phenomena, may also contribute significantly
Phytoplankton carbon fixation, chlorophyll-biomass and diagnostic pigments in the Atlantic Ocean
No full textWe have made daily measurements of phytoplankton pigments, size-fractionated (<2 and >2-?m) carbon fixation and chlorophyll-a concentration during four Atlantic Meridional Transect (AMT) cruises in 2003–04. Surface rates of carbon fixation ranged from <0.2-mmol C m?3 d?1 in the subtropical gyres to 0.2–0.5-mmol C m?3 d?1 in the tropical equatorial Atlantic. Significant intercruise variability was restricted to the subtropical gyres, with higher chlorophyll-a concentrations and carbon fixation in the subsurface chlorophyll maximum during spring in either hemisphere. In surface waters, although picoplankton (<2-?m) represented the dominant fraction in terms of both carbon fixation (50–70%) and chlorophyll-a (80–90%), nanoplankton (>2-?m) contributions to total carbon fixation (30–50%) were higher than to total chlorophyll-a (10–20%). However, in the subsurface chlorophyll maximum picoplankton dominated both carbon fixation (70–90%) and chlorophyll-a (70–90%). Thus, in surface waters chlorophyll-normalised carbon fixation was 2–3 times higher for nanoplankton and differences in picoplankton and nanoplankton carbon to chlorophyll-a ratios may lead to either higher or similar growth rates. These low chlorophyll-normalised carbon fixation rates for picoplankton may also reflect losses of fixed carbon (cell leakage or respiration), decreases in photosynthetic efficiency, grazing losses during the incubations, or some combination of all these. Comparison of nitrate concentrations in the subsurface chlorophyll maximum with estimates of those required to support the observed rates of carbon fixation (assuming Redfield stoichiometry) indicate that primary production in the chlorophyll maximum may be light rather than nutrient limited. <br/
Iron cycling during the decline of a south Georgia diatom bloom
No full textThe Southern Ocean is the largest high nutrient low chlorophyll (HNLC) oceanic region, where iron limits phytoplankton growth and productivity and ultimately influences the Biological Carbon Pump (BCP). Natural exceptions to the HNLC regime occur where island wakes cause iron to be mixed into surface waters from sediments, enabling large, prolonged phytoplankton blooms and increased carbon drawdown. Interactions between iron and phytoplankton are reciprocal in blooms: with plankton regulating the (re)cycling of iron through cellular uptake and remineralisation. The depth of iron remineralisation then influences either re-supply to the surface mixed layer biota or sequestration into deeper waters. Water column trace metal observations and shipboard experiments, using bioassays and radioisotope (55Fe, 32Si, 14C) cycling, were undertaken to investigate surface mixed layer phytoplankton iron limitation, iron uptake, and mesopelagic iron remineralisation relative to carbon and silica within the November 2017 bloom downstream of South Georgia. Surface phytoplankton residing in the iron depleted mixed layer were iron limited throughout the four-week sampling period. Experiments designed to investigate particulate water column (re)cycling revealed limited iron remineralisation from freshly produced upper ocean particles. The main pathway of iron transfer from particulates into the dissolved phase was through rapid (<2 d) release of extra-cellular adsorbed iron, which, if occurring in situ, could contribute to observed higher sub-surface dissolved Fe concentrations. This was accompanied by a small loss of cellular carbon, likely through respiration of the fixed 14C, and limited dissolution of particulate 32Si to dissolved 32Si. Decoupling of the remineralisation length scales for Fe, C and Si, with Fe having the fastest turnover, is thus likely in the upper mesopelagic zone beneath the bloom.</p
Determination of nitrate and phosphate in seawater at nanomolar concentrations
No full textOver much of the world’s surface oceans, nitrate and phosphate concentrations are below the limit of detection (LOD) of conventional techniques of analysis. However, these nutrients play a controlling role in primary productivity and carbon sequestration in these waters. In recent years, techniques have been developed to address this challenge, and methods are now available for the shipboard analysis of nanomolar (nM) nitrate and phosphate concentrations with a high sample throughput.This article provides an overview of the methods for nM nitrate and phosphate analysis in seawater. We outline in detail a system comprising liquid waveguide capillary cells connected to a conventional segmented-flow autoanalyser and using miniaturised spectrophotometers. This approach is suitable for routine field measurements of nitrate and phosphate and achieves LODs of 0.8 nM phosphate and 1.5 nM nitrate
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