Sünnje Linnéa Basedow

Biovolume spectrum theories applied: spatial patterns of trophic levels within a mesozooplankton community at the polar front

Three-dimensional data on the mesoscale distribution of hydrography and mesozooplankton were collected at the Polar Front, northwestern Barents Sea, in spring 2008 (29 April–15 May) using a combination of multinet and towed instrument platform equipped with Laser Optical Plankton Counter, fluorometer and CTD. Trophic levels (TLs) within the zooplankton community (whole community and size-separated) were analysed for three consecutive periods using biovolume spectrum theory, which proved to be a powerful tool in the physically and biologically variable frontal system. Trophic structure was highly variable in time and across the Polar Front, but was mostly related to the phytoplankton bloom (as determined by fluorescence). High TLs of 5.5 within the zooplankton community were observed outside bloom situations (mostly in Atlantic Water) and were likely due to increased omnivory of Calanus spp., which dominated the large zooplankton size group that had a lower TL (2.2) during the bloom than outside blooms (max. TL 5.6). A strong input of herbivorous barnacle nauplii (Cirripedia) into the upper layer (35 000 ind. m−3 in net samples) substantially decreased mean TL in the marginal ice zone. Differences in TL estimates based on biovolume spectrum theory and other methods (stable isotopes, lipid markers, dietary analyses) are discussed.

A major Calanus finmarchicus overwintering population inside a deep fjord in northern Norway: implications for cod larvae recruitment success

High Calanus finmarchicus abundances were recorded in wintertime in Vestfjorden, close to the main cod breeding grounds off Lofoten and Vesterålen, northern Norway. The mean abundance for locations with water depth >500 m was ∼37000 ind. m−2 (range: 26700–49000 ind. m−2). To our knowledge, this is the first report of massive overwintering of C. finmarchicus on the Norwegian shelf. Because of the observed size and location of this population, we argue that local overwintering on the northern Norwegian shelf can contribute significantly to sustain a C. finmarchicus population on the shelf during the period of first feeding for cod larvae. This is supported by a particle tracking model.

Seasonal Variation in Transport of Zooplankton Into the Arctic Basin Through the Atlantic Gateway, Fram Strait

The largest contribution of oceanic heat to the Arctic Ocean is the warm Atlantic Water (AW) inflow through the deep Fram Strait. The AW current also carries Atlantic plankton into the Arctic Basin and this inflow of zooplankton biomass through the Atlantic-Arctic gateway far exceeds the inflow through the shallow Pacific-Arctic gateway. However, because this transport has not yet been adequately quantified based on observational data, so that the present contribution is poorly defined, and future changes in Arctic zooplankton communities are difficult to project and observe. Our objective was to quantify the inflow of zooplankton biomass through the Fram Strait during different seasons, including winter. We collected data with high spatial resolution covering hydrography (CTD), currents (ADCP and LADCP) and zooplankton distributions (LOPC and MultiNet) from surface to 1000 m depth along two transects crossing the AW inflow during three cruises in January, May and August 2014. Long-term variations (1997-2016) in the AW inflow were analyzed based on moored current meters. Water transport across the inflow region was of the same order of magnitude during all months (January 2.2 Sv, May 1.9 Sv, August 1.7 Sv). We found a higher variability in zooplankton transport between the months (January 51 kg C s-1, May 34 kg C s-1, August 50 kg C s-1), related to seasonal changes in the vertical distribution of zooplankton. However, high abundances of carbon-rich copepods were observed in the AW inflow during all months. Surface patches with high abundances of C. finmarchicus, Microcalanus spp., Pseudocalanus spp. and Oithona similis clearly contributed to the advected biomass, also in winter. The data reveal that the phenology of species is important for the amount of advected biomass, and that the advective input of zooplankton carbon into the Arctic Basin is important during all seasons. The advective zooplankton input might be especially important for mesopelagic planktivorous predators that were recently observed in the region, particularly during winter. The inflow of C. finmarchicus with AW was estimated to be in the order of 500 000 metric tons C y-1, which compares well to modeled estimates.