1,720,983 research outputs found
The role of mesozooplankton in the biological carbon pump of the North Atlantic
No full textThesis embargoed until 21 June 201
Unusual subpolar North Atlantic phytoplankton bloom in 2010: Volcanic fertilisation or North Atlantic Oscillation?
Get PDFIn 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
Elevated iron to nitrogen recycling by mesozooplankton in the Northeast Atlantic Ocean
No full textLow dissolved iron (DFe) concentrations limit primary production in most high-nutrient low-chlorophyll (HNLC) regions. Increased recycling of iron (Fe) relative to nitrogen (N) by zooplankton may help to sustain phytoplankton production in these conditions. We concurrently determined rates of DFe and ammonium (NH4+) recycling by natural mesozooplankton communities in HNLC conditions of the Northeast Atlantic. NH4+ excretion remained constant and ranged between 14.2–54.1 nmol NH4+ mg dry weight−1 h−1. Fe recycling ranged between 6–138 pmol DFe mg dry weight−1 h−1 during the first hour and decreased thereafter, reflecting the transition from the loss of phytoplankton-derived Fe to basal DFe excretion. Mesozooplankton-driven nutrient recycling was estimated to support 6–59% and <1–13% of the respective phytoplankton requirements for DFe and N; DFe:N regeneration ratios were 5–26 times larger than those required by phytoplankton. Our data suggest that Fe recycling by grazing organisms has the potential to reduce the intensity of HNLC conditions
Microbial gardening in the ocean's twilight zone : Detritivorous metazoans benefit from fragmenting, rather than ingesting, sinking detritus
Get PDFFunded by NERC NE/K001833/1 ‘Oceans 2025’Peer reviewe
Slow Sinking Particulate Organic Carbon in the Atlantic Ocean: magnitude, flux and potential controls
No full textThe remineralization depth of particulate organic carbon (POC) fluxes exported from the surface ocean exert a major control over atmospheric CO₂ levels. According to a long held paradigm most of the POC exported to depth is associated with large particles. However, recent lines of evidence suggest that slow sinking POC (SSPOC) may be an important contributor to this flux. Here we assess the circumstances under which this occurs. Our study uses samples collected using the Marine Snow Catcher throughout the Atlantic Ocean, from high latitudes to mid latitudes. We find median SSPOC concentrations of 5.5 μg L-1, 13 times smaller than suspended POC concentrations and 75 times higher than median fast sinking POC (FSPOC) concentrations (0.07 μg L-1). Export fluxes of SSPOC generally exceed FSPOC flux, with the exception being during a spring bloom sampled in the Southern Ocean. In the Southern Ocean SSPOC fluxes often increase with depth relative to FSPOC flux, likely due to midwater fragmentation of FSPOC, a process which may contribute to shallow mineralization of POC and hence to reduced carbon storage. Biogeochemical models do not generally reproduce this behaviour, meaning that they likely overestimate long term ocean carbon storage
Seasonal variation of zooplankton community structure and trophic position in the Celtic Sea: a stable isotope and biovolume spectrum approach
No full textZooplankton on continental shelves represent an important intermediary in the transfer of energy and matter from phytoplankton to the wider ecosystem. Their taxonomic composition and trophic interactions with phytoplankton vary in space and time, and interpreting the implications of this constantly evolving landscape remains a major challenge. Here we combine plankton taxonomic data with the analysis of biovolume spectra and stable isotopes to provide insights into the trophic interactions that occur in a shelf sea ecosystem (Celtic Sea) across the spring-summer-autumn transition. Biovolume spectra captured the seasonal development of the zooplankton community well, both in terms of total biomass and trophic positioning, and matched trophic positions estimated by stable isotope analysis. In early April, large microplankton (63-200 µm) occupied higher trophic positions than mesozooplankton (>200 µm), likely reflecting the predominance of nanoplankton (2-20 µm) that were not readily available to mesozooplankton grazers. Biomass and number of trophic levels increased during the spring bloom as elevated primary production allowed for a higher abundance of predatory species. During July, the plankton assemblage occupied relatively high trophic positions, indicating important links to the microbial loop and the recycling of organic matter. The strong correlation between biomass and community trophic level across the study suggests that the Celtic Sea is a relatively enclosed and predominantly energy-limited ecosystem. The progression of the zooplankton biomass and community structure within the central shelf region was different to that at the shelf-break, potentially reflecting increased predatory control of copepods by macrozooplankton and pelagic fishes at the shelf break. We suggest that the combination of size spectra and stable isotope techniques are highly complementary and useful for interpreting the seasonal progression of trophic interactions in the plankton
Particle flux in the oceans: Challenging the steady state assumption
Get PDFAtmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption; that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (<40?m d-1). We were able to reproduce this behaviour using a simple model using satellite-derived export rates and literature-derived remineralization rates. We further provide a simple equation to estimate ‘additional’ (or ‘excess’) POC supply to the dark ocean during non-steady state conditions, which is not captured by traditional sediment trap deployments. In seasonal systems, mesopelagic net organic carbon supply could be wrong by as much as 25% when assuming steady state. We conclude that the steady state assumption leads to misinterpretation of particle flux profiles when input fluxes from the upper ocean vary on the order of weeks, such as in temperate and polar regions with strong seasonal cycles in export
Evidence of nitrification associated with globally distributed pelagic jellyfish
Get PDFBioavailable nitrogen is a scarce resource in most of the surface oceanand often limits primary productivity. Although Pelagic jellyfish excretesubstantial amounts of ammonia (the preferred form of nitrogen formost phytoplankton), they are overlooked players in marine nitrogencycling. Here, we observed high rates of nitrification (NH4+ → NO3-, 5.7– 40.8 nM gWW-1 (wet weight) h-1) associated with the scyphomedusaeAurelia aurita, Chrysaora hysoscella and Chrysaora pacifica and low ratesof incomplete nitrification (NH4+ → NO2-, 1-2.7 nM gWW-1 h-1)associated with Chrysaora fulgida, Chrysaora hysoscella and Chrysaorapacifica. These observations indicate that microbes living in associationwith the jellyfish thrive by oxidizing the readily available ammonia tonitrite and nitrate. The four studied species are abundant over a largegeographic distribution and exhibit frequent population outbreaks. Weshow that, during such outbreaks, jellyfish-associated release of nitrogencan provide more than 100% of the nitrogen required for primaryproduction. These findings reveal a so far overlooked pathway whenassessing pelagic nitrification rates that might be of particular relevancein nitrogen depleted surface waters and at high jellyfish populationdensities
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