Bi-modal vertical distribution of the carnivorous copepod Paraeuchaeta norvegica

Bi-modal vertical distribution of Paraeuchaeta norvegica duringlate autumn was observed in a Norwegian fjord in three differentyears, and hence seems to be a recurring phenomenon. An uppermode performed diel vertical migration, foraging in shallowwater at night. A deep mode did not migrate vertically, butsome feeding took place at depth.     

Reply to Horizons Article 'Some ideas about the role of lipids in the life cycle of Calanus finmarchicus' Irigoien (2004): II

Recently, Irigoien (Irigoien, 2004) suggested a conceptual modelof the role of lipids in the life cycle of the marine copepodCalanus finmarchicus. As he pointed out, lipids accumulatedbefore     

The feeding strategies of two large marine copepods

We compared the feeding behaviour of the two copepods Paraeuchaetanorvegica and Chiridius armatus, allowing them to prey on othercalanoids in small-scale laboratory experiments. Several differenceswere found. When fed either live, free-swimming or dead, non-movingprey, P.norvegica seemed unable to locate the dead prey itemswhile C.armatus foraged heavily on them. When starved, P.norvegicaincreased its feeding rate while C.armatus reduced its feedingafter an initial increase. None of the predators changed theirfeeding rates when exposed to light. Nighttime versus daytimefeeding was tested only with P.norvegcia, which seemed to possessan endogenous feeding rhythm with increased rates at night.Chiridius armatus infected with epizooic ciliates appeared tohave increased feeding rates.     

Spatially structured interactions between a migratory pelagic predator, the Norwegian spring-spawning herring Clupea harengus L., and its zooplankton prey

Spring-spawning herring Clupea harengus was patchily distributed over large parts of the Norwegian Sea in May 1995–2005, during the early phase of the annual feeding migration. Overall, herring tended to be found in areas with intermediate biomasses of zooplankton prey, intermediate water temperatures and relatively high salinities. Herring had more food in their stomachs in areas of relatively low water temperature and high herring abundance. Hydrographical conditions revealed that herring was feeding mainly within Atlantic water masses, and more intensely in western and northern regions of the Norwegian Sea. Zooplankton biomass was patchily distributed, and was generally higher towards the western parts of the Norwegian Sea. Here, zooplankton biomass in year _{i +1} was also negatively associated with herring spawning stock biomass in year _i , while there was no evidence for such an association in the eastern region; indicating that herring may have a geographically structured 'top–down' effect on the recruitment of its zooplankton prey. The fact that herring was not typically associated with the areas containing the greatest zooplankton biomasses may reflect that the fish had not yet reached the most profitable feeding grounds or alternatively that herring was depleting zooplankton biomass.     

The Submarine Volcano Eruption off El Hierro Island: Effects on the Scattering Migrant Biota and the Evolution of the Pelagic Communities

The submarine volcano eruption off El Hierro Island (Canary Islands) on 10 October 2011 promoted dramatic perturbation of the water column leading to changes in the distribution of pelagic fauna. To study the response of the scattering biota, we combined acoustic data with hydrographic profiles and concurrent sea surface turbidity indexes from satellite imagery. We also monitored changes in the plankton and nekton communities through the eruptive and post-eruptive phases. Decrease of oxygen, acidification, rising temperature and deposition of chemicals in shallow waters resulted in a reduction of epipelagic stocks and a disruption of diel vertical migration (nocturnal ascent) of mesopelagic organisms. Furthermore, decreased light levels at depth caused by extinction in the volcanic plume resulted in a significant shallowing of the deep acoustic scattering layer. Once the eruption ceased, the distribution and abundances of the pelagic biota returned to baseline levels. There was no evidence of a volcano-induced bloom in the plankton community.     

Mesoscale eddies are oases for higher trophic marine life

Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.     

Population genetics of drifting (Calanus spp.) and resident (Acartia clausi) plankton in Norwegian fjords

Kaartvedt distinguished between drifting and resident planktonand hypothesized that the latter were distinguished by theirability to maintain their horizontal position in desired habitats(Kaartvedt, 1993). In this study, we examined the populationgenetic consequences of these two lifestyles for copepods infour fjords of western Norway (Lurefjorden, Masfjorden, Sognefjordenand Sørfjorden) and one fjord in eastern Norway (Oslofjorden).Based on DNA sequence variation of a region of mitochondrial16S rRNA, we contrasted population genetic diversity and structurein drifting populations of Calanus spp. with that of residentpopulations of Acartia clausi. With the exception of Sørfjorden(where Calanus spp. were rare), two or three species of Calanusco-occurred in significantly different proportions in the fjords.Based on a 350 base-pair region of mitochondrial 16S rRNA, Calanusspp. varied in molecular genetic diversity, with the highestvalues for C.helgolandicus. There was no evidence of significantgenetic structure of fjord populations for either C.finmarchicusor C.helgolandicus; the population structure of C.glacialiscould not be evaluated as the species was only abundant in Lurefjorden.Acartia clausi was abundant in all five fjords sampled for thisstudy. Molecular genetic diversity of A.clausi, based on a 220bp region of mt 16S rRNA, was within the range of Calanus spp.values. Populations of A.clausi showed significant genetic structure(i.e. haplotype frequencies differed markedly) among the fjords.The results of this study indicated that little exchange (geneflow) occurs between populations of A.clausi in different fjords,and suggested that the populations are long-term residents ofa fjord. In contrast, most Calanus spp. fjord populations maybe replaced periodically, as they drift with currents flowingto and from coastal and fjord environments.     

Advection of euphausiids in a Norwegian fjord system subject to altered freshwater input by hydro-electric power production

Due to hydro-electric power production, the freshwater inputto the Sandsfjord system has increased during winter, whilethe freshwater input to Jøsenfjorden has been removed.In studies conducted during autumn and winter, both Meganyctiphanesnorvegica and Thysanoessa raschii were most common in Jøsenfjorden,and T.raschii was almost wanting in the Sandsfjord system. Theeuphausiids generally swam to the surface layer at night, butavoided low salinity water in the uppermost metres of the Sandsfjordsystem. Still, in most locations 50% of M.norvegica was foundin the upper 10 m. Their vertical distribution overlapped withfreshwater-driven currents, and estuarine circulation was apparentlyprominent in governing the horizontal distribution within theSandsfjord system. At the entrance to the fjord, M. norvegicawas distributed deeper than within the fjord, reducing the importanceof transport with estuarine circulation relative to currentsdriven by wind at the coast. Estimates of advective rates ofM.norvegica suggest that exchange with coastal waters was moreimportant for the standing stock than local growth within thefjords. In Jtfsenfjorden, maxima of T.raschii were always foundin a zone of shallowing bottom topography (bottom depth 130m), where this species possibly maintained its horizontal positionby means of vertical migrations.     

State-dependent vertical distribution of the carnivore copepod Pareuchaeta norvegica

We explored the impact of body condition on vertical distributionof the carnivore copepod Pareuchaeta norvegica. Individualssampled in the upper part of the water column had lower dryweight and slightly shorter prosome than individuals sampleddeeper. Non-ovigerous females occurred higher in the water columnthan ovigerous females. Our results suggest that ovigerous femalesand individuals with large energy reserves to a large extentprioritize predator avoidance in deep water versus feeding inthe upper part of the water column compared to non-ovigerousfemales and individuals with low energy reserves. Adult femalescollected closest to the surface were less pigmented than thosefound deeper. This, however, proved to be a quickly adjustabletrait depending on prevailing light conditions.     

Effect of freshwater discharge, intrusions of coastal water, and bathymetry on zooplankton distribution in a Norwegian fjord system

Freshwater discharge from a hydroelectric power plant had astrong impact on plankton transport and distribution withinthe fjord system. Most zooplankters submerged beneath the outgoingbrackish current, and subsequent advection by the compensationcurrent caused high concentrations near the power plant outlet.Currents driven by wind-induced changes in the density fieldin the adjacent coastal waters dominated transport between thefjord system and Outer regions. Normal plankton advection ratesappeared to be on the same order of magnitude as local zooplanktongrowth within the fairly enclosed system. However, during strong,short-term intrusions of coastal water, plankton compositionand distribution changed markedly in the course of a few days.The bathymetry seemed to be decisive for the distribution ofAcartia, which was recurrently found in highest numbers at theshallowest sampling location.