Ketil Eiane

Diel vertical migration of Arctic zooplankton during the polar night

High-latitude environments show extreme seasonal variation in physical and biological variables. The classic paradigm of Arctic marine ecosystems holds that most biological processes slow down or cease during the polar night. One key process that is generally assumed to cease during winter is diel vertical migration (DVM) of zooplankton. DVM constitutes the largest synchronized movement of biomass on the planet, and is of paramount importance for marine ecosystem function and carbon cycling. Here we present acoustic data that demonstrate a synchronized DVM behaviour of zooplankton that continues throughout the Arctic winter, in both open and ice-covered waters. We argue that even during the polar night, DVM is regulated by diel variations in solar and lunar illumination, which are at intensities far below the threshold of human perception. We also demonstrate that winter DVM is stronger in open waters compared with ice-covered waters. This suggests that the biologically mediated vertical flux of carbon will increase if there is a continued retreat of the Arctic winter sea ice cover.

Vertical distribution of Calanus spp. and Metridia longa at four Arctic locations

Abstract We investigated the vertical distribution of Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa at four locations around the archipelago of Svalbard in autumn. The older and larger copepodites of Calanus spp. were generally located deeper in the water column. Differences in vertical distribution between stations partly reflected a south–north gradient in developmental progress with higher abundance of older stages in the southern locations. The C. finmarchicus and M. longa observations were consistent with the hypothesis that the developmental stages distributed according to certain preferences for light intensity, and different optical properties at the four locations are likely to have affected the vertical distributions. Diel vertical migration was only observed for older developmental stages of M. longa while young stages of M. longa remained in deep waters both day and night. A mortality index indicated that non-migrating Calanus spp. suffered higher mortality than migrating M. longa.

ADVECTION AND ZOOPLANKTON FITNESS

Abstract Successful adaptation to the environment by zooplankton is constrained by the agents of mortality (starvation, predation) and losses due to advection. A fitness measure which explicitly includes risk of advective loss is presented. The authors show that as horizontal current speeds and vertical shear increase, the behavioral strategies that maximize fitness of zooplankton possessing different life history strategies are affected. For a Calanus finmarchicus-type life history, fitness is maximized by diel vertical migrations when advection risk in surface waters is low and by spending less time in the surface layers as flow rate increases. For a Paracalanus parvus-type life history, vertical migration is postulated to occur as advective loss increases. The geographic length scale of the habitat of residence also affects the optimal mode of habitat selection. In the Calanus case an abrupt change in the optimal vertical migration pattern is postulated as a function of length scale of habitat and rate...

The significance of optical properties in competition among visual and tactile planktivores: a theoretical study

Abstract A mathematical model describing competition for a common food resource among visual and tactile planktivores is developed. Sensitivity of the competitive regime to environmental changes are studied by analysing their effects on foraging rates and niche availability of both planktivore types. Depth, spatial, daily and seasonal variations in factors of biotic and abiotic origin may result in great changes in the competitive regime. Optical properties of the water column is of greatest importance for competition. In addition the abundance and size of prey, small scale turbulence, and the seasonal and daily variations in sun irradiance are of importance. Inherent contrast of prey organisms is likely to modify the effect from the above factors under certain circumstances only, but may together with prey size, also enhance niche differentiation among the competitors. By studying the zero growth rate isoclines for populations of tactile and visual planktivores at different depths, we show that the water column is a habitat divided into different competitive regimes. These can be thought of as relatively stable niches separated by an ecotone of rather unpredictable and ever changing competitive regime, due to changes in environmental factors. In most cases it is optimal for visual planktivores to stay at shallow depths, while tactile planktivores compete better at greater depths. However, by simulations we show that high predation pressure can make near surface waters hostile to many visual planktivores such as fish, thereby making this highly productive niche available to tactile planktivores such as jellyfish. Results are discussed in relation to naturally occurring environmental variations and the impact of anthropogenic factors. Changes in optical properties due to eutrophication might have drastic effects on the outcome of competition, and thus on the structure of marine food chains. While too little is known about the physiology, life history and behaviour of most pelagic organisms to make specific predictions, this work indicates a potential for strong competitive interactions among different functional types of planktivores.

Variability in Calanus spp. abundance on fine- to mesoscales in an Arctic fjord : implications for little auk feeding

Abstract We studied the abundance of calanoid copepods of the genus Calanus in a high-resolution sampling grid located in the vicinity of a breeding colony of an avian Calanus predator, the little auk (Alle alle) in an Arctic fjord (Isfjorden, Svalbard) in July 2007. Within diving reach of little auks, all copepodite stages of C. finmarchicus, as well as stages CIII–CV of C. glacialis, were recorded in all 26 stations sampled. Spatial autocorrelation analysis was used to detect spatial heterogeneity (patches) in the distribution of Calanus spp. Positive spatial autocorrelation was detected on scales up to 1.2 km for C. finmarchicus stages CIII–CV and for C. glacialis CIV and CV, but was rarely detected for the younger stages. This suggests that the tendency to form patches varies with ontogeny in Calanus spp. At an adjacent little auk colony, the diet prey composition from 30 gular pouches of little auks returning from feeding trips was investigated. Calanus glacialis CIV and CV were numerically dominant in the prey samples, while C. finmarchicus CIV which was dominant in the net samples did not contribute to the little auk diet. This could suggest selective feeding, a detection of patches beyond the scale of our survey, feeding beyond the sampling area or a combination of these factors. Large Calanus abundance differences within the sampling grid underline the necessity of a proper choice of grid size, in this case 2 km between stations.

Towards a robust concept for modelling zooplankton migration

Abstract We develop a model framework for handling some aspects of zooplankton behaviour in biophysical simulation models. The vertical behaviour of zooplankton (their diel vertical migration pattern, or DVM) is modelled as a stimuli-response process where inputs from the environment (i.e. presence of light, food, and predators) are used as decision parameters. By use of simple artificial neural networks to control behaviour and optimising these by genetic algorithms we obtain close to optimal sets of DVM strategies. We show how adding increasing levels of environmental complexity (number of stimuli, stochasticity, and seasonality) to the model affects predicted behaviour. Within 50 generations of adaptation the model strategies perform reasonable well under the levels of environmental complexity used in simulations. The performance of strategies tended to increase with the complexity level of inputs used to control behaviour. By adapting strategies to environments with a stochastic variability we show that the strategies become more plastic to unpredictable changes in environmental forcing. Also, we discuss how plastic strategies with high degree of robustness to environmental fluctuations of the magnitude encountered in nature can be derived from this method. Results are discussed in relation to the behavioural patterns observed in the field.

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.