One of the main reasons behind our current lack of understanding of iron cycling in the oceans is our inability to characterize the ligands that control iron solubility, photosensitivity, reactivity, and bioavailability. We currently lack consensus about the nature and origin of these ligands. Here, we present the first field application of a new methodological development that allows the selective quantification of the fraction of Fe complexed to humic substances (HS). In the HS-rich surface Arctic waters, including the Fe-rich Transpolar Drift (TPD), we found that HS iron binding groups were largely occupied by iron (49%). The overall contribution of Fe−HS complexes to DFe concentrations was substantial at 80% without significant differences between TPD and non-TPD waters. Stabilization and transport of large concentrations of DFe across the surface of the Arctic Ocean are due to the formation of high concentrations of Fe−HS complexes. Competition of Arctic Fe−HS complexes with desferrioxamine and EDTA indicated that their stability constants are considerably higher than the stability constants previously found for riverine HS in temperate estuaries and HS standard material. This is the first case of identification of the ligand-dominating iron speciation over a specific region of the global ocean.

Tuna are among the most ubiquitous oceanic predators, and range globally from the equator to temperate regions (0 to 55° latitude). While the distribution of adult fish has been mapped from fishing records, the extent of tuna spawning and larval habitats is less well understood. We compiled and analyzed data on the global distributions of larval occurrence for 7 major oceanic tuna species to investigate environmental predictors of larval habitat. Our results showed that tuna larvae occur within the adults' distributional range, but were restricted to lower latitudes and higher water temperatures than adults, largely consistent with Schaefer's 'temperature hypothesis'. Temperature requirements explained much of the variation in larval occurrence, though temperature by itself tended to over-predict the extent of larval habitats. We also demonstrate that tuna larvae have an elevated probability of occurrence at intermediate values of eddy kinetic energy, generally supporting Bakun's 'ocean triad hypothesis', which relates tuna larval habitats to mesoscale oceanographic activity. However, some deviations in this pattern were also observed, such as for albacore. Regions of suitable larval habitats were most commonly found in western boundary currents, where warm water masses coincide with intermediate eddy kinetic energy. Bluefin tuna species are exceptional though, in that their spawning habitats tended to be much more confined than predicted from oceanographic conditions. Our results provide support for a combination of the 2 hypotheses to explain global environmental requirements for tuna larvae. We have identified oceanographic parameters that can easily be measured by remote sensing and features that should be considered when determining areas of critical habitat for tuna larvae.KEY WORDS: Spawning habitat · Global · Spatial distribution · Large predators · Tuna · Larvae · Thunnus · Temperature hypothesis · Ocean triad hypothesis Resale or republication not permitted without written consent of the publisher Editorial responsibility: Nicholas Tolimieri,

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Seascape ecology is an emerging discipline focused on understanding how features of the marine habitat influence the spatial distribution of marine species. However, there is still a gap in the development of concepts and techniques for its application in the marine pelagic realm, where there are no clear boundaries delimitating habitats. Here we demonstrate that pelagic seascape metrics defined as a combination of hydrographic variables and their spatial gradients calculated at an appropriate spatial scale, improve our ability to model pelagic fish distribution. We apply the analysis to study the spawning locations of two tuna species: Atlantic bluefin and bullet tuna. These two species represent a gradient in life history strategies. Bluefin tuna has a large body size and is a long-distant migrant, while bullet tuna has a small body size and lives year-round in coastal waters within the Mediterranean Sea. The results show that the models performance incorporating the proposed seascape metrics increases significantly when compared with models that do not consider these metrics. This improvement is more important for Atlantic bluefin, whose spawning ecology is dependent on the local oceanographic scenario, than it is for bullet tuna, which is less influenced by the hydrographic conditions. Our study advances our understanding of how species perceive their habitat and confirms that the spatial scale at which the seascape metrics provide information is related to the spawning ecology and life history strategy of each species.

The ecology of highly migratory marine species is tightly linked to dynamic oceanographic processes occurring in the pelagic environment. Developing and applying techniques to characterize the spatio-temporal variability of these processes using operational oceanographic data is a challenge for management and conservation. Here we evaluate the possibility of modelling and predicting spawning habitats of Atlantic bluefin tuna in the Western Mediterranean, using pelagic seascape metrics specifically designed to capture the dynamic processes affecting the spawning ecology this species. The different seascape metrics applied were processed from operational oceanographic data products providing information about the temporal and spatial variability of sea surface temperature, kinetic energy and chlorophyll a. Spawning locations were identified using larval abundances sampled in the Balearic Sea, one of the main reproductive areas for this species in the Mediterranean Sea. Results confirm the high dependence of bluefin tuna spawning ecology on mesoscale oceanographic processes while providing spawning habitat maps as a tool for bluefin tuna assessment and management, based on operational oceanographic data. Finally, we discuss the coming challenges that operational fisheries oceanography and pelagic seascape ecology face to become fully implemented as predictive tools.

Abstract. In the western Mediterranean Sea, the RADMED monitoring programme is already conducting several of the evaluations required under the Marine Strategy Framework Directive (MFSD) along the Spanish Mediterranean coast. The different aspects of the ecosystem that are regularly sampled under this monitoring programme are the physical environment and the chemical and biological variables of the water column, together with the planktonic communities, biomass and structure. Moreover, determinations of some anthropogenic stressors on the marine environment, such as contaminants and microplastics, are under development.Data are managed and stored at the Instituto Español de Oceanografía (IEO) Data Centre that works under the SeaDataNet infrastructure, and are also stored in the IBAMar database. In combination with remote sensing data, they are used to address open questions on the ecosystems in the western Mediterranean Sea.

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In oligotrophic regions, picophytoplankton can play a key role in total carbon production and energy transfer. Since the mesoscale hydrographic variability can influence the resource availability and therefore the biological communities, here we studied the linkage between hydrography, resource supply, abundance, and biomass contribution of prokaryotic picoplankton in the south Balearic Sea during the stratified season. The sampling area covered the confluence of two different water masses, the fresher new Atlantic water, and the saltier resident Atlantic water. Our results showed higher Synechococcus abundances in the more oligotrophic new Atlantic water mass and suggest that the spatial patterns of prokaryotic picophytoplankton are dictated by the mesoscale processes in this region. The summer stratification condition separated clearly the surface mixed layer (ML) from the deep layer (DL); our results support different limiting factors for picophytoplankton in the two layers: nutrient and light availability in the ML and DL, respectively. We also obtained no significant difference in the Synechococcus biomass contribution to total autotrophic biomass within the water column, but higher contribution in the new Atlantic water mass. These results demonstrate the general importance of picophytoplankton as carbon producers in oligotrophic waters and particularly their variability as biomass source at the mesoscale.