The North Atlantic is characterized by diatom-dominated spring blooms that results in significant transfer of carbon to higher trophic levels and the deep ocean. These blooms are terminated by limiting silicate concentrations in summer. Numerous regional studies have demonstrated phytoplankton community shifts to lightly-silicified diatoms and non-silicifying plankton at the onset of silicate limitation. However, to understand basin-scale patterns in ecosystem and climate dynamics, nutrient inventories must be examined over sufficient temporal and spatial scales. Here we show, from a new comprehensive compilation of data from the subpolar Atlantic Ocean, clear evidence of a marked pre-bloom silicate decline of 1.5–2 µM throughout the winter mixed layer during the last 25 years. This silicate decrease is primarily attributed to natural multi-decadal variability through decreased winter convection depths since the mid-1990s, a weakening and retraction of the subpolar gyre and an associated increased influence of nutrient-poor water of subtropical origin. Reduced Arctic silicate import and the projected hemispheric-scale climate change-induced weakening of vertical mixing may have acted to amplify the recent decline. These marked fluctuations in pre-bloom silicate inventories will likely have important consequences for the spatial and temporal extent of diatom blooms, thus impacting ecosystem productivity and ocean-atmosphere climate dynamics.

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The distribution of the leptocephalus larvae of European (Anguilla anguilla) and American (Anguilla rostrata) eels collected during recent Sargasso Sea surveys was used to model larval drift. The drift trajectories of individual larva were back-calculated to the estimated time of spawning, using current data from two global oceanographic assimilation models. The results of both models give the same overall result; widespread spawning extended in time from December to March. The drift was also calculated forwards for approximately 1 year. The forward drift modelling showed that most leptocephali remained in the area south of the Subtropical Frontal Zone. One conclusion is that the majority of leptocephali remain trapped and possibly die in the retention area. A small proportion of leptocephali are entrained into the Gulf Stream system. An implication is that the spawning success may be highly sensitive to oceanographic and climatic factors that alter the dispersion of leptocephali out from the retention area. An alternative interpretation is that the surveys were made too late after the peak spawning period and that the core spawning area was missed.

Since the open-ocean subpolar Atlantic is amongst the most predictable regions in the world, our results hold promise for predicting the general production to seabird populations over a large geographical region adjacent to the northern North Atlantic and the Arctic Mediterranean. Colonies of black-legged kittiwakes Rissa tridactyla in the North Atlantic have declined markedly since the mid-1990s, partly due to repeatedly failing breeding seasons. We show a close link between the breeding success of a kittiwake colony in the Faroe Islands and the subpolar gyre index. Successful breeding follows winters with an expanded subpolar gyre and, by inference, increased zooplankton abundances southwest of Iceland. The environmental conditions in the northwestern Atlantic during the non-breeding and pre-breeding seasons might therefore be important. Furthermore, the subpolar gyre dynamics might influence the local food abundance on the Faroe shelf during the breeding season.

One hypothesis to explain the large decline in the recruitment of the European eel (Anguilla anguilla) from the 1980s onwards is that a change in ocean circulation has influenced the drift of eel larvae, resulting in lower recruitment. To test this hypothesis, a simple Lagrangian model was constructed to simulate a passive drift from the spawning area in the Sargasso Sea to the European shelf. The simulation utilised the velocity data from a reanalysis of ocean climate, the Simple Ocean Data Assimilation (SODA 2.1.6), for the period 1958-2008 covering the time of regime shift in eel recruitment. The average drift time and latitudinal distribution of the arrival of eel larvae were explored for a range of constant depth levels and instantaneous mortalities. The model showed that the proportion of eel larvae carried by the North-East Atlantic Current to northern latitudes of arrival was greater before 1970, whereas there was an increase in amount of larvae being entrained into the southbound current branches after this time. The overall success of drift from the spawning area to the East Atlantic was not affected and clearly contradicts that this could explain the observed recruitment decline.

The Northeast Atlantic mackerel (Scomber scombrus) stock has increased and expanded its summer feeding migration west-and northwards since 2006, entailing large geopolitical challenges for the countries harvesting this species. A common perspective is that climatic warming opens up new regions for biota in the north. It has also been suggested that the presently large pelagic fish stocks deplete prey resources in the eastern North Atlantic during their summer feeding phase, forcing the stocks west towards the Irminger Sea in their search for food. Here, we suggest that the declining nutrient (silicate) concentrations observed along the northern European continental slope reduce primary and thus secondary production, exacerbating food scarceness in the east and adding to the incentive to migrate westward. The new westward feeding route requires that the fish cross the Iceland Basin, which during the summer season quickly becomes nutrient-depleted and thus might act as a barrier to migration after the spring bloom. Using mackerel and zooplankton abundance data from the International Ecosystem Summer Surveys in the Nordic Seas, we suggest that the oligotrophic waters in the central Iceland Basin force the fish to migrate through a narrow 'corridor' along the south Iceland shelf, where nutrients are replenished and both primary and secondary production are higher.

Pacariz, S., Björk, G., Jonsson, P., Börjesson, P., and Svedäng, H. 2014. A model study of the large-scale transport of fish eggs in the Kattegat in relation to egg density. – ICES Journal of Marine Science, 71: 345–355. A process-oriented model, with high vertical resolution, has been used to investigate variation in the transport of fish eggs and early larvae in relation to egg density. The main focus is on gadoid eggs from the spawning grounds in the southern Kattegat. Additionally, transport from the neighbouring areas, the central Kattegat and Öresund, is presented. The model results clearly indicate that transport is dependent on the egg density; lighter eggs are transported northwards whereas heavier eggs are to a larger extent retained or transported southwards. This study suggests that optimum densities in order to promote retention in the southern Kattegat are in the range of 1023–1026 kg m−3. Observations from 2005 and 2006 of the vertical distribution of gadoid eggs combined with hydrographical data indicated high concentrations of eggs at the upper part of the pycnocline at densities of 1017–1022 kg m−3. Combining the observations and modelling results on amount of dispersal and retention, suggests that gadoid eggs are mainly retained in the southern Kattegat (although shifted from maximum retention density) and simultaneously dispersed northwards. Even though the results of the study are described in the context of gadoid eggs, the results are applicable for other marine species with pelagic stages and buoyant particles within the tested density range.

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