Dynamical Controls of the Eastward Transport of Overwintering Calanus finmarchicus From the Lofoten Basin to the Continental Slope

Diapausing populations of Calanus finmarchicus at depth in the Lofoten Basin (LB) return to the continental shelf and slope off the Lofoten-Vesterålen Islands during the phytoplankton spring bloom to feed and spawn, forming surface swarms with a great abundance. To study how overwintering populations of C. finmarchicus move with the deep currents and return to the shelf, Lagrangian transport characteristics of particles in deep water between 2008 and 2019 were analyzed using Global Ocean Reanalysis and Simulation re-analysis data and Lagrangian Coherent Structures (LCSs). Our analyses revealed that persistent eastward transport of diapausing C. finmarchicus between LB and continental slope occurred mainly between 600 and 1,100 m in the Arctic Intermediate Water. The consistency of the vertical distributions of C. finmarchicus abundance and salinity further suggests that physical factors control the horizontal distribution of the species. Hovmöller diagrams of kinetic energy indicate that there is an eastward advection of mean current at depth. The co-occurrence between the eastward transport of LCSs and the eastward advection of the mean current provides direct evidence that the life history of C. finmarchicus is subjected to physical control in the Norwegian Sea

On the use of the HUGIN 1000 HUS Autonomous Underwater Vehicle for high resolution zooplankton measurements

This paper evaluates and tests the HUGIN 1000 HUS AUV as a carrier platform for the Laser Optical Plankton Counter (LOPC). The LOPC was mounted outside on top of the carrier platform, and the tests confirm that this concept did not create turbulence and zooplankton avoidance. A negligible velocity difference between the flow passing through the sampling tube and the AUV speed was detected. The abundance recorded by the LOPC onboard HUGIN 1000 HUS was within the same order of magnitude compared to the zooplankton abundance from traditional net-based measurements. Our results showed zooplankton abundance in the range of 70 to 180 individuals m-3, while other, traditional net-based measurements have indicated 27 - 332 individuals m-3 from the same area and same depth layer. Note that these numbers are minimum and maximum values observed from a vast number of samples. The mean is approximately 200 individuals m-3. The results provide new and unique high-resolution biological data of deep-water copepod communities. The application of AUVs in marine ecological research introduces advanced methodology with potential to address new scientific questions in deep water habitats

Jellyfish summer distribution, diversity and impact on fish farms in a Nordic fjord

Jellyfish can cause high mortality of farmed fish and hence significant economic losses for the aquaculture industry. Despite their socio-economic importance, distribution and diversity data on gelatinous plankton are scarce from northern Norwegian fjords and other Nordic systems. Intense blooms of jellyfish have repeatedly been observed in Ryggefjord, Finnmark (Norway), sometimes concurrent with severe health problems of salmon. In the present study, the jellyfish community of this fjord was studied in summer 2015. In July, at least 13 species were identified using a combination of morphological and molecular techniques. High densities of small Beroe spp. and ctenophore larvae in cydippid stage dominated the surface waters. Adult Beroe cucumis were also present. Molecular identification revealed the presence of juvenile Euphysa tentaculata, as well as 2 species each of Clytia and Obelia. Obelia longissima was identified from both its pelagic (medusa) and benthic (polyp) stages, indicating that some local populations can complete their entire life cycle in the fjord. Abundances were significantly different between inner and outer parts of the fjord, and in relation to the prevailing wind direction. A dense bloom of the hydrozoan Dipleurosoma typicum in September coincided with high mortalities of farmed fish, suggesting a causal relationship. We conclude that the jellyfish assemblage in Ryggefjord is dynamic on short time scales and structured by both oceanographic conditions and local reproduction. A better understanding of seasonal population development and the relationships between hydrography, abundance and species composition is required to develop mitigation strategies for aquaculture operation

Jellyfish summer distribution, diversity and impact on fish farms in a Nordic fjord

Jellyfish can cause high mortality of farmed fish and hence significant economic losses for the aquaculture industry. Despite their socio-economic importance, distribution and diversity data on gelatinous plankton are scarce from northern Norwegian fjords and other Nordic systems. Intense blooms of jellyfish have repeatedly been observed in Ryggefjord, Finnmark (Norway), sometimes concurrent with severe health problems of salmon. In the present study, the jellyfish community of this fjord was studied in summer 2015. In July, at least 13 species were identified using a combination of morphological and molecular techniques. High densities of small Beroe spp. and ctenophore larvae in cydippid stage dominated the surface waters. Adult Beroe cucumis were also present. Molecular identification revealed the presence of juvenile Euphysa tentaculata, as well as 2 species each of Clytia and Obelia. Obelia longissima was identified from both its pelagic (medusa) and benthic (polyp) stages, indicating that some local populations can complete their entire life cycle in the fjord. Abundances were significantly different between inner and outer parts of the fjord, and in relation to the prevailing wind direction. A dense bloom of the hydrozoan Dipleurosoma typicum in September coincided with high mortalities of farmed fish, suggesting a causal relationship. We conclude that the jellyfish assemblage in Ryggefjord is dynamic on short time scales and structured by both oceanographic conditions and local reproduction. A better understanding of seasonal population development and the relationships between hydrography, abundance and species composition is required to develop mitigation strategies for aquaculture operation