- PEP -

Project Abstract (MAS3-CT95-0013)

 Within its extraordinarily wide range of occurrence from the Arctic to the Mediterranean, the Northern krill, Meganyctiphanes norvegica are exposed to a variety of environmental conditions. With respect to the thermal regime they are able to cope with low as well as high temperatures. This property makes krill a suitable organism to study the potential for ecophysiological optimisation in different climatic as well as trophic environments. The project has brought together a multidisciplinary closely-knit group of European physiologists, ecologists, acousticians, and geneticists to study the ways the Northern krill has adapted itself to a wide range of contrasting environmental conditions as a pelagic model in zooplankton.

1. The project considered abiotic and biotic conditions at three selected study sites, the cool Clyde sea, the warmer Mediterranean in the Ligurian sea, and the thermally variable Danish Kattegat. As a first conclusion, krill populations are found in areas were seasonal feeding is facilitated by environmental conditions, particularly linked to the existence of frontal systems or generally, areas of enhanced advection. Krill can hold its position at these favourable locations, withstanding tidal and larger scale currents. The well developed vertical and horizontal swimming capacity in krill appears to be an underlying principle.

 

2. The adaptive capacity of M. norvegica in terms of the rates, ranges and limits of physiological performance in relation to the differing climatic conditions was compared. Nutrition: The food ingested in krill was extremely wide ranging from detritus over phytoplankton to other euphausiids, highlighting the opportunistic nature of krill in its choice of prey items. Krill showed a diel-rhythm in feeding activity that is believed to be an adaptation to minimising predation risk. Lipid metabolism: a strong relationship between latitude and lipid accumulation as well as lipid class composition was apparent. Reproductive strategies allow for a flexible response to widely diverse environmental conditions. A conceptual model of coupled processes of moulting and spawning was established, valid for all three sites of the climatic triangle. Fecundity is a function of factors like: the biomass and size structure of the population, the rate of growth and body-size attained at maturity. The number of the successive cycles of egg production which are possible during the reproductive period, are controlled by temperature and trophic conditions at the three sites. Overall metabolism: in krill from different climatic regions metabolic rates, as measured by respiration and exemplified by the kinetic properties of a metabolic enzyme, are adjusted to maintain a constant metabolic level at the local prevailing ambient temperature. However, in contrast to the Northern locations, in the Mediterranean, trophic adaptation supersedes that to thermal influences in krill.

 

3. The extraordinarily wide North Atlantic distribution of Meganyctiphanes norvegica suggested the possible existence of separate, geographically isolated stocks. Accordingly, a molecular genetic analysis of the populations at the mitochondrial and nuclear DNA level was fundamental. Three distinct genetic pools were identified: the first was represented by a population collected in the Atlantic Cadiz bay, the second obtained from the Mediterranean Ligurian sea, and the third included all the North East Atlantic samples. The first molecular genetic analysis in European Atlantic waters indicates that M. norvegica, although endowed with a high dispersal capacity because of its pelagic habit, can evolve separate breeding units that exhibit different adaptations to local conditions.

 

4. Meganyctiphanes norvegica is certainly a successful species having adapted to a wide range of climatic conditions, and thus can play a substantial role in different ecosystems. Obviously, a "natural experiment" of hydroclimatic adaptation of the species can equally be considered successful. The combination of physiological and genetic approaches of the PEP-project laid the basis to establish a "functional biodiversity" in zooplankton as exemplified in the Northern krill. Particularly, the plasticity found in most physiological processes lead to the conclusion that the species is characterised by a pronounced eurythermic capacity. One unexpected finding of the genetic analysis was that the Cadiz-population differed substantially from all other populations investigated. The swarm sampled may have been advected from warmer Atlantic regions, possibly from the area of the Canary islands. This finding indicates that the species bears the potential of having adapted to warmer regimes, warmer even as Mediterranean conditions. Although the physiological properties of this Southern population are not known so far, the functional thermal plasticity (eurythermy) in the Northern krill may be underlined, and thus lead to the conclusion, that the species M. norvegica can in fact cope with an extremely wide range of environmental temperatures. This extraordinary adaptive capacity may suggest that Northern krill may only be little impacted by the expected global warming.

 

5. One of the major outputs of the project is a thorough analysis of causes and effects of the typical diel vertical migration (DVM) behaviour in krill. The range of these migrations is at more than 500m in the Mediterranean. The light regime governs regular DVM, where sunlight acts a the phase-setter ("Zeitgeber") of endogenous rhythms. Moon light can be perceived, and equally modifies migratory behaviour. However, although DVM patterns depend on underlying endogenous cues, external and also further internal factors override these frequently. Among modifying internal factors, directly influencing levels of ascent, are moult, reproductive processes and spawning. Apparently, DVM, and physiology, i.e. the reproductive cycle as well as moult processes are intimately interlinked. The use of an ADCP (Acoustic Doppler Current Profiler), and its development to document shifts of biomass and speeds of moving krill, over its normal use as a current metre, was an achievement of the PEP-project.

 

6. Multi-frequency sonar, ADCP, Optical Plankton Counter and discrete net sampling, used in conjunction with each other, have given a comprehensive assessment of biomass, distribution and (nutritive) behaviour of krill populations in the Kattegat, the Clyde Sea and the Ligurian Sea. The combined data comprise the first quantitative assessment of local krill stocks as compared at major areas of occurrence. The revised interannual population dynamics can be used to estimate krill production and can be combined with the new assessments of krill biomass in the areas. The high local standing stock of the omnivorous krill should have a major impact on both phytoplankton and zooplankton stocks. Although outside of the direct scope of the PEP-project, a basis has been laid to compute energy transfer at the three locations, and derive the importance of this transfer into krill stocks as potential food source for top predators as fish (cod, herring) or whales in the Ligurian Sea. The PEP-data in this respect may be used in future modelling exercises in food-web analysis in fishery biology.

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