Seasonal occurrence of Oithona similis (cyclopoida), Microsetella norvegica (harpacticoida) and Microcalanus spp. (calanoida), and productivity of O. similis, in three high-latitude Norwegian fjords.

Abstract

The Seasonal population dynamics of the small copepod species Oithona similis (cyclopoida), Microsotella norvegica (harpacticoida) and Microcalanus spp. were investigated in the northern Norwegian fjords Balsfjord, Altafjord and Porsangerfjord. In this study a WP-2 net with 64 µm mesh size was used to sample all the stages of O. similis, M. norvegica and Microcalanus spp. Copepodite stages CI – CVI were identified for each species to determine seasonal distribution population structure. To assess the relative importance of small copepod species in the marine ecosystem, the seasonal and annual secondary production of O. similis in the three study fjords was estimated. Production was estimated with specific egg production rates, that is based on experimentally determined egg hatching rates, and a temperature-dependent method.

The species were present year-round in the fjords but differed from each other in both geographical and seasonal distribution, but all copepodite stages for each specie was present in all sample months. In December, copepod abundance and biomass were low with the exception of inner Porsangerfjord and Balsfjord. In March, peak in total copepod abundance and biomass was observed in inner Porsangerfjord and Balsfjord while in the other areas this remained low. In April, March and October, abundance and biomass were comparatively less than the peak found in March. Population abundance for O. similis remained relatively stable during the months and seasons, where all life stages were observed during the study that indicated year-round reproduction. M. norvegica accounted for the high abundance found in December and March at inner Porsangerfjord and Balsfjord, that mainly consisted of overwintering stages (females without eggs and CIV – CV copepodites). Egg carrying M. norvegica females was first observed in small numbers in April and in greater numbers in August. All stages of Microcalanus spp. was found during this study but was less abundant than the other two species. Reproduction for Microcalanus spp. had likely taken place between December and March.

The egg hatching experiments shows there is a strong correlation between O. similis egg hatching rate (HR) and egg hatching time (HT), where HR increased with higher temperature and HT decreased. Estimated SEPR showed clear seasonal trend as it remained low during December, March and April (< 0,001 mg C m-1 d-1) in all the fjords which increased in August and October when sea surface temperature increased, most notably in Porsangerfjord and Balsfjord. The annual secondary production reveals that O. similis were most productive in Balsfjord and the outer area of Porsangerfjord, yielding an annual estimation of > 1 g C m-2 y-1 in these fjords. This is comparatively less than other copepod species found in sub-Arctic/Arctic waters such as those belonging to the Calanus genus, but the fact that productivity remained continuous even during winter is of great importance nevertheless. Temperature is the main regulating factor for O. similis productivity as overall low temperatures in the fjords inhibited maximum egg production large portions of the year. A future scenario where sea surface temperature increases in the sub-Arctic/Arctic ecosystem will much likely promote higher seasonal and annual O. similis secondary production.