Population dynamic of high latitude copepods - with emphasis on Metridia longa

High latitude ecosystems are shaped by seasonality in light, ranging from complete darkness in winter to midnight sun in summer, influencing both temperature and primary production. Copepods are important grazers on phytoplankton in marine systems and
occupy a central role in the marine food-web, linking smaller and larger organisms. Research of copepod communities in the Arctic has traditionally focused on larger taxa present in the surface layers. Consequently, little is known about reproduction strategies and population dynamics of smaller copepods located deeper in the water column. The aim of this thesis was to identify the dominant copepod species in an open ocean system covering different water masses in Baffin Bay and Davis Strait, as well as a sub-Arctic fjord during spring and summer. Both investigations were conducted using fine meshed nets, sampling of deeper water masses and including diurnal and nocturnal sampling. The seasonal fjord study further identified different life strategies with respect to seasonal adaptations in reproduction and migration patterns.
In contrast to the general assumption that large calanoid copepods dominate high latitude ecosystems, we find that smaller species numerically dominated the copepod community in the fjord. The contribution of the smaller species was less pronounced in Baffin Bay. However, even in this region, smaller species dominated in the part of the study area when the large Calanus species had left the surface layer to overwinter at
depth. The large calanoid Metridia longa dominated the biomass in the fjord during spring and represented a significant part of the biomass in Baffin Bay and Davis Strait. Metridia longa exhibited a pronounced diel vertical migration, avoiding the upper 150 meters of the water column during day. Thus, sampling only the upper water column during the day-as is a usual procedure-would underestimate this potential key species.
Reproduction patterns of the large calanoids suggested lifecycles adapted to the seasonal and episodic food availability, and consequently had a pulsed reproduction. In contrast, small copepod species were less dependent on the spring phytoplankton bloom, and their reproduction and population dynamics were less pulsed. Likewise, a large proportion of Oithona similis was ovigerous from March to August. Reproduction
of Microsetella norvegica, another of the small key species in the fjord, was more restricted and indicated a life-history strategy that combines the advantages of eggcarrying with inter-clutch duration independent of hatching time. Our findings stress the need for sampling with small meshed nets, sampling deeper in the water column, and sampling both diurnally and nocturnally to target the entire copepod community. We speculate that the complexity of the copepod community will increase in a warmer climate, as smaller species will profit more than the larger species
from increased water temperatures