Mesoscale Eddies Are Oases for Higher Trophic Marine Life

Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific western subarctic gyre (WSG) revealed seasonal changes in δ15N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting traps (DST; 100-200 m) and moored traps (MST; 200 and 500 m). All particles showed higher δ15N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ15N(SUS) of 0.4-3.1 ‰ in the euphotic zone (EZ). The δ15N(SUS) signature was reflected by δ15 N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ15 N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ15 N(DST) variations of 2.4-7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ15 N(DST) vs. PP regression to δ15 N(MST) of 1.9-8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. Moreover, the monthly export ratio at 500 m was calculated using both estimated PP and measured organic carbon fluxes. Results suggest a 1.6 to 1.8 times more efficient transport of photosynthetically-fixed carbon to the intermediate layers occurs in summer/autumn rather than winter/spring

Effects of physical fragmentation on remineralization of marine snow

The vast majority of carbon lost from ocean surface waters sinks as large, relatively rare, marine snow and fecal pellets. Fragmentation of these particles into more slowly sinking daughter particles has been proposed previously to partly explain the rapid loss of sinking carbon below the mixed layer. In this study we investigated 2 other less obvious potential consequences of aggregate disruption, namely the release of dissolved interstitial compounds upon fragmentation and accelerated degradation due to increased particle surface area. We found that upon fragmentation natural marine aggregates, ranging in size from 3 to 6 mm diameter, released dissolved organic carbon (DOC) (mean 0.12 mu mol aggregate(-1)) and nitrate (mean of 0.013 mu mol aggregate(-1)) into surrounding seawater, making these nutrients available to free-living biota. Filtration of whole aggregates failed to result in an equivalent release, suggesting that marine snow may not be as leaky as expected based on high aggregate porosity. Decomposition of aggregate particulate organic carbon (POC) to DOC was similar for whole and fragmented aggregates ranging from 0.6 to 0.9 mu mol POC aggregate(-1) d(-1), resulting in calculated aggregate POC turnover times of 2 to 11 d. Remineralization of DOC was also similar for both aggregate treatments and suggested a tight coupling between solubilization and uptake by attached bacteria. Our results indicate that the longer residence times predicted for smaller aggregates in the mixed layer, rather than changes in decomposition rate, may be the most influential impact of aggregate fragmentation on reduction of particle flux to depth

Comparison of mesozooplankton assemblages across quasi-synoptic oceanographic features on the north-western iberian shelf break

The mesozooplankton community at the north-western Iberian shelf break was studied among adjacent oceanographic regimes (including upwelling, stratification and anticyclonic eddies) during 17 days in autumn 2009. Zooplankton sampling locations were determined in situ, after identifying the oceanographic regimes from CTD profiles performed over the continental shelf and upper slope. Zooplankton samples were sorted indentifying taxonomically the main zooplankton groups, from phylum to subclass. Copepods were the most abundant group (ind m−3) in all stations, followed by appendicularians, doliolids and siphonophores. The mesozooplankton community was significantly different amongst oceanographic conditions. Meroplankton abundance was higher in upwelling stations; particularly lamellibranchia, polychaeta and bryozoan larvae abundance, and decreased from early to late upwelling conditions. Medusae and chaetognata were found exclusively under the latter oceanographic regime. However, dissimilarity between the oceanographic conditions was mostly based on the varying contribution of the four most common groups.Versión del editor1,784