1,721,000 research outputs found
On the significance of nitrification within the euphotic zone of the subpolar North Atlantic (Iceland basin) during summer 2007
No full textThe oxidation of ammonium to nitrite was investigated within surface waters (< 125 m) of the sub-polar North Atlantic (~ 60°N, 20°W) during late summer 2007. Sampling occurred within a mesoscale eddy dipole system and a definite bias towards waters beneath the euphotic zone as the focus for nitrification was evident. The patchy occurrence of significant nitrification rates within the euphotic zone is interpreted as providing minimal indication for widespread nitrification within surface waters of the sub-polar gyre. However, isolated occurrences of significant nitrification rates within the euphotic zone above the cyclonic eddy were sufficient to account for observed in-situ NO3?concentrations. It is proposed that the deeper mixed layer associated with the cyclonic eddy enhanced the likelihood of nitrification within the euphotic zone through the vertical displacement of sub-euphotic zone bacterial communities
A mesoscale eddy driving spatial and temporal heterogeneity in the productivity of the euphotic zone of the northeast Atlantic
No full textIn this paper we show how different water masses from a similar geographic region provide an explanation for perturbations in the signal of declining productivity at the Porcupine Abyssal Plain (PAP) study site in the Northeast Atlantic. Furthermore we show that the passage of these different water masses is affected by the filamentary instabilities of a cyclonic eddy just southwest of the PAP site. We describe a high-resolution spatial hydrographic survey conducted with a towed instrument package, complemented by biogeochemical sampling. Maximum rates of primary production of 110 mmol C m-2 d-1 seen at the centre of the survey area were associated with the passage of an eddy filament and were enhanced 3 fold relative to far-field conditions (not, vert, similar36 mmol C m-2 d-1). The rotation and stirring influence of the eddy resulted in the sequential passage of 3 distinct water masses past the observation point. This understanding of the lateral stirring around the site enabled us to explain the sharp changes observed in daily primary production rates and other biogeochemical parameters. The spatial survey also revealed a fluorescence maxima associated with the cyclonic eddy that was laterally displaced northwards away from the core, an observation supportive of recent modelling studies
Seasonality, phytoplankton succession and the biogeochemical impacts of an autumn storm in the northeast Atlantic Ocean
Full text linkPhytoplankton chemotaxonomic distributions are examined in conjunction with taxon specific particulate biomass concentrations and phytoplankton abundances to investigate the biogeochemical consequences of the passage of an autumn storm in the northeast Atlantic Ocean. Chemotaxonomy indicated that the phytoplankton community was dominated by nanoplankton (2-20 ?), which on average represented 75±8% of the community. Microplankton (20-200 ?) and picoplankton (<2 ?) represented 21±7% and 4±3% respectively with the microplankton group composed of almost equal proportions of diatoms (53±17%) and dinoflagellates (47±17%). Total chlorophyll-a (TCHLa = CHLa + Divinyl CHLa) concentrations ranged from 22 to 677 ng L-1, with DvCHLa making minor contributions of between <1% and 13% to TCHLa. Higher DvCHLa contributions were seen during the storm, which deepened the surface mixed layer, increased mixed layer nutrient concentrations and vertically mixed the phytoplankton community leading to a post-storm increase in surface chlorophyll concentrations. Picoplankton were rapid initial respondents to the changing conditions with pigment markers showing an abrupt 4-fold increase in proportion but this increase was not sustained post-storm. 19’-HEX, a chemotaxonomic marker for prymnesiophytes, was the dominant accessory pigment pre- and post-storm with concentrations of 48-435 ng L-1, and represented 44% of total carotenoid concentrations. Accompanying scanning electron microscopy results support the pigment-based analysis but also provide detailed insight into the nano- and microplankton communities, which proved to be highly variable between pre-storm and post-storm sampling periods. Nanoplankton remained the dominant size class pre- and post-storm but the microplankton proportion peaked during the period of maximum nutrient and chlorophyll concentrations. Classic descriptions of autumn blooms resulting from storm driven eutrophication events promoting phytoplankton growth in surface waters should be tempered with greater understanding of the role of storm driven vertical reorganization of the water column and of resident phytoplankton communities. Crucially, in this case we observed no change in integrated chlorophyll, particulate organic carbon or biogenic silica concentrations despite also observing a ?50% increase in surface chlorophyll concentrations which indicated that the surface enhancement in chlorophyll concentrations was most likely fed from below rather than resulting from in situ growth. Though not measured directly there was no evidence of enhanced export fluxes associated with this storm. These observations have implications for the growing practice of using chlorophyll fluorescence from remote platforms to determine ocean productivity late in the annual productivity period and in response to storm mixing
The spatial variability of vertical velocity in an Iceland basin eddy dipole
No full textThis paper quantitatively assesses the mesoscale spatial variability in vertical velocity associated with an open ocean eddy dipole. High-resolution, in situ data were collected during a research cruise aboard the NERC research ship RRS Discovery to the Iceland Basin in July/August 2007. A quasi-synoptic SeaSoar spatial survey revealed a southeastward flowing jet with counter-rotating eddies on either side. The anti-cyclonic component was identified as a mode water eddy, characterised by a homogenous core (?35.5 psu and 12 °C) centred at a depth of ?600 m. Vertical velocities were calculated by inverting the quasi-geostrophic (QG) Omega equation at each point in a three-dimensional grid encompassing the dipole. The strongest vertical velocities (up to 5 m day?1) were found primarily in the central jet between the eddies, as fast flowing water was forced over raised isopycnals associated with the large potential vorticity anomaly of the mode water eddy. Weaker upward (downward) vertical velocity was diagnosed ahead of the cyclonic (mode water) eddy in the direction of propagation, reaching 0.5 m day?1 (2.5 m day?1) at the depth of maximum potential vorticity (PV) anomaly. The results demonstrate that the mesoscale velocity field cannot be accurately reconstructed from analysis of individual isolated eddy features and that detailed three-dimensional maps of potential vorticity are required to quantify the cumulative effects of their interactions. An examination of potential sources of error associated with the vertical velocity diagnosis is presented, including sampling strategy, quasi-synopticity, sensitivity to interpolation length scale and the unquantified effect of lower boundary conditions. The first three of these errors are quantified as potentially reaching 50%, ?20% and ?25% of the calculated vertical velocity, respectively, indicating a potential margin of error in the vertical velocity diagnosis of order one
Urea distribution and uptake in the Atlantic Ocean between 50° N and 50°S.
No full textWe investigate the distribution of urea and its uptake by phytoplankton during 3 meridional
transects of the Atlantic Ocean between 50°N and 50° S. Significant relationships were identified
between urea uptake and Prochlorococcus abundance (p < 0.01) in the northern subtropical
Atlantic, where Prochlorococcus appears likely to dominate urea uptake, and between urea concentration
and the <200 µm microplankton biomass fraction (p < 0.005) in the South Atlantic, which may
be associated with the production of urea. These results suggest that the distribution of urea in the
subtropical ocean may be controlled by regional imbalances between urea consumption and urea
production. In parallel with these simple relationships significant spring-autumn seasonal changes in
the distribution of urea were identified in southern subtropical and tropical latitudes. Urea was twice
as abundant during local spring than during local autumn in the subtropical South Atlantic but 2.5
times more abundant in equatorial waters during the boreal autumn period. Euphotic zone integrated
urea uptake rates also varied seasonally, being considerably higher in the North Atlantic temperate
and subtropical latitudes during the boreal spring whilst in the subtropical South Atlantic urea
uptake peaked in local autumn
Unusual subpolar North Atlantic phytoplankton bloom in 2010: Volcanic fertilisation or North Atlantic Oscillation?
Full text linkIn summer and autumn 2010, a highly anomalous phytoplankton bloom, with chlorophyll concentration more than double that of previous years, was observed in the Irminger Basin, southwest of Iceland. Two unusual events occurred during 2010 which had the potential to promote the unusual bloom. First, in spring 2010, the Eyjafjallajökull volcano in Iceland erupted, depositing large quantities of tephra into the subpolar North Atlantic. Second, during the winter of 2009/2010 the North Atlantic Oscillation (NAO) became extremely negative, developing into the second strongest negative NAO on record. Hydrographic conditions were highly anomalous in the region, with an influx of freshwater spreading through the basin, and unusual nutrient and mixed layer depth conditions. Here we use a combination of satellite, modeled and in situ data to investigate whether the input of iron from the volcanic eruption or change in hydrographic conditions due to the extreme negative NAO were responsible for the anomalous phytoplankton bloom. We conclude that changes in physical forcing driven by the NAO, and not the volcanic eruption, stimulated the unusual bloom
Summertime trends in pelagic biogeochemistry at the Porcupine Abyssal Plain study site in the northeast Atlantic
No full textMeasurements of nitrate and carbon uptake made in July 2006 in the Northeast Atlantic Ocean are evaluated with reference to the photophysiology of the attendant phytoplankton population. Over the 11-day observation period integrated chlorophyll concentrations and carbon fixation rates decreased by 76% and 60%, respectively. Integrated nitrate uptake decreased by 50% from initial to final rates but was generally less variable than carbon fixation and chlorophyll in the intervening period. Satellite derived estimates of surface chlorophyll concentrations reveal the uptake observations to be coincident with, and subsequent to, a peak in summer time production. Large reductions in diatom and dinoflagellate abundance were also seen at this time, with indications that increased grazing, due to an increase in ciliate abundance, was an important mechanism terminating summertime production in the NE Atlantic. Meanwhile, the presence of consistently low values of Fv/Fm (<0.3), particularly in surface waters, suggests that production occurs, or is inhibited, with suboptimal photochemical efficiency widespread amongst the phytoplankton population. Furthermore, the low values of Fv/Fm were not alleviated by day-to-day variability in macronutrient concentration. The timing of our observations places them within the seasonal period recognised for the widespread phenomena of carbon overconsumption, and we estimate C:N uptake ratios at this time could be as high as 13:1
A review of the measurement and modelling of dinoflagellate bioluminescence
No full textBioluminescence is a striking phenomenon that is ubiquitous throughout the world’s oceans. Here we bring together the findings of in situ observations of bioluminescence in the upper ocean (<300 m depth) taken over several decades. We describe the distribution and diel variability of mechanically stimulated bioluminescence within the upper ocean, as well as its relationships with other environmental parameters. As dinoflagellates are often the dominant source of stimulated bioluminescence in the upper ocean we review current knowledge regarding the bioluminescence of these organisms including its potential ecological function. Modelling and prediction of the bioluminescent field has previously had only limited success, especially over timescales greater than a few days. We suggest that the potential exists to improve the forecasting of upper ocean bioluminescence potential on longer, seasonal, timescales by utilising and improving methods to model dinoflagellates
Phenological characteristics of global coccolithophore blooms
Full text linkCoccolithophores are recognized as having a significant influence on the global carbon cycle through the production and export of calcium carbonate (often referred to as particulate inorganic carbon or PIC). Using remotely sensed PIC and chlorophyll data, we investigate the seasonal dynamics of coccolithophores relative to a mixed phytoplankton community. Seasonal variability in PIC, here considered to indicate changes in coccolithophore biomass, is identified across much of the global ocean. Blooms, which typically start in February–March in the low-latitude (~30°) Northern Hemisphere and last for ~6–7?months, get progressively later (April–May) and shorter (3–4?months) moving poleward. A similar pattern is observed in the Southern Hemisphere, where blooms that generally begin around August–September in the lower latitudes and which last for ~8?months get later and shorter with increasing latitude. It has previously been considered that phytoplankton blooms consist of a sequential succession of blooms of individual phytoplankton types. Comparison of PIC and chlorophyll peak dates suggests instead that in many open ocean regions, blooms of coccolithophores and other phytoplankton can co-occur, conflicting with the traditional view of species succession that is thought to take place in temperate regions such as the North Atlantic
Plankton patchiness investigated using simultaneous nitrate and chlorophyll observations
No full textThe complex patterns observed in marine phytoplankton distributions arise from the interplay of biological and physical processes, but the nature of the balance remains uncertain centuries after the first observations. Previous observations have shown a consistent trend of decreasing variability with decreasing length-scale. Influenced by similar scaling found for the properties of the water that the phytoplankton inhabit, ‘universal' theories have been proposed that simultaneously explain the variability seen from meters to hundreds of kilometers. However, data on the distribution of phytoplankton alone has proved insufficient to differentiate between the many causal mechanisms that have been suggested. Here we present novel observations from a cruise in the North Atlantic in which fluorescence (proxy for phytoplankton), nitrate and temperature were measured simultaneously at scales from 10 m to 100 km for the first time in the open ocean. These show a change in spectra between the small scale (10–100 m) and the mesoscale (10–100 km) which is different for the three tracers. We discuss these observations in relation to the current theories for phytoplankton patchiness
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