Mesoscale physical processes and zooplankton transport-retention in the northern Norwegian shelf region

Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100 m on the shelf in the survey region is approximately 6.4×10³ tonnes wet weight day⁻¹ northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100 m over Malangsgrunnen was approximately 0.08 day⁻¹, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort.     

Microzooplankton and mesozooplankton in an upwelling filament off Galicia: modelling and sensitivity analysis of the linkages and their impact on the carbon dynamics

This paper reports estimates of trophic flows of carbon off the Galician coast from a 1D ecological model, which are compared with field data from a two week Lagrangian drift experiment. The model consists of 9 biological components: nitrate, ammonium, >5μm phytoplankton, <5μm phytoplankton, heterotrophic nanoflagellates/dinoflagellates (5–20 μm), heterotrophic dinoflagellates (>20 μm), ciliates, fast sinking detritus and slow sinking detritus. Calculations were made for the fluxes of carbon between biological components within the upper 45m of the water column. The temporal development of primary production during the simulation period of two weeks was in good agreement with field estimates, which varied between 248 and 436mgC.m⁻².d⁻¹. Heterotrophic nanoflagellates had the greatest impact on carbon flux, with a grazing rate of 168mgC.m⁻².d⁻¹. Herbivorous grazing by microzooplankton amounted to 215mgC.m⁻².d⁻¹, whereas grazing by copepods on phytoplankton was 35mgC.m⁻² d⁻¹. Copepods grazing on microzooplankton was minor (0.47mgC.m⁻².d⁻¹) and the export flux from the upper 45m was 302mgC.m⁻².d⁻¹. Sensitivity analyses, in which the grazing parameters (i.e the functional relationship between ingestion and food concentration) were changed, were carried out on the heterotrophic dinoflagellate, ciliate and heterotrophic nanoflagellates/dinoflagellate components of the model. These changes did not alter the temporal development of heterotrophic nanoflagellates/dinoflagellates biomass significantly, but ciliates and heterotrophic dinoflagellates were more sensitive to variations in the grazing parameters. The overall conclusion from this modelling study is that the coupling between small phytoplankton and heterotrophic nanoflagellates was the quantitatively most important process controlling carbon flow in this region.     

Mesoscale distribution and advection of overwintering Calanus finmarchicus off the shelf of northern Norway

Data collected on a cruise in January 2008, using a laser optical plankton counter, conductivity–temperature–depth sensors, and a lowered acoustic Doppler current profiler, was used to study the mesoscale distribution and advection of overwintering Calanus finmarchicus in its deep water winter habitat off the shelf of northern Norway. The overwintering animals were generally located immediately below the Atlantic Water (AW) in Arctic Intermediate Water (AIW), in the 600–1200 m depth range. The depth of the interface between AW and AIW varied considerably in the area and this was clearly reflected in the C. finmarchicus distribution. Maximum abundance varied from about 80 ind m^?3 to more than 200 ind m^?3 at the different stations. Current measurements showed the richness of mesoscazle eddies, with speeds exceeding 70 cm s^?1 at the surface and rapidly decreasing with depth. In the main overwintering layer the eddy features were also clearly seen, but with speeds generally below 20 cm s^?1. C. finmarchicus were found in the whole survey area, but they were not homogeneously distributed. Advection of the copepods resulted in relatively high local rates of change in overwintering C. finmarchicus abundance with mean value of 8% per day in the area. It is clear that mesoscale physical processes greatly contribute to the variability in the abundance of overwintering C. finmarchicus off the shelf of northern Norway. The collected data are also a valuable addition to the generally sparse datasets on the C. finmarchicus winter distribution and the role of the Lofoten basin in the large scale system is also discussed.     

Productivity, trophic levels and size spectra of zooplankton in northern Norwegian shelf regions

Many studies have been conducted in northern Norwegian shelf regions to assess distributions and abundances of zooplankton in the last decade using towed Scanfish-conductivity, temperature and depth sensors (CTD)–optical plankton counter (OPC), and plankton nets. Significant progresses have been made in understanding dominant species, life histories and behavior, and in using size-structured data to identify dominant species in a certain size range. Using these Scanfish–CTD–OPC data, the analysis of zooplankton community size structures, compositions and their relationships with water types is made along the shelf region from Lofoten, North Cape to Varangerfjorden. From the relationships between the water types and zooplankton communities, the transports and exchanges of zooplankton communities between the Norwegian Coastal and Norwegian Atlantic Waters in regions near Malangsgrunnen and Nordvestbanken are examined. The biovolume (biomass) spectra are further analyzed for the productivity, trophic levels and seasonality of communities in these regions, indicating a steeper slope of the biovolume spectrum for a community dominated by herbivorous species in spring and a flatter slope for a community dominated by carnivorous–omnivorous species in winter. The comparison with the zooplankton biovolume spectra obtained in areas west of Antarctic Peninsula is made to examine and understand the differences in the zooplankton biovolume spectra, their trophic dynamics and potential human impacts between different regions.