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northern Skagerrak Associated Investigators: Related Web Sites: The Arendal sampling site (northern Skagerrak) is located at 58 23 N 8 49 E, approximately one nautical mile offshore from the Flodevigen Research Station (Institute of Marine Research, IMR) off southern Norway. The water depth at the site is 105 m. Sampling for hydrographic parameters and abundance of phytoplankton and zooplankton (biomass and species) has been carried out twice a month since January 1994. Zooplankton is sampled fortnightly with a WP-2 net (180 µm mesh) towed vertically from a depth of 50 m to the surface. Each sample is split in half, providing data on both species composition/abundance and biomass. The seasonal maximum in zooplankton biomass generally occurs in April/May, with a secondary, smaller peak in the zooplankton occurring in July/August. Large differences can be seen between years in the observed biomass of zooplankton, with maximum values in 2003 and minimum values in 1998. A general increase in biomass and abundance was observed from 1998 to 2003, but a lower abundance overall was observed in 2004-2007. The observed lower abundance in recent years is especially pronounced in the autumn peak (July/August). This is mainly caused by the reduced abundances of the copepods Oithona spp and Pseudocalanus spp in the period 2004-2007. The seasonal maximum in zooplankton biomass (April/May) is dominated by Calanus finmarchicus, whereas the secondary peak (July/August) is dominated by smaller copepods (Pseudocalanus, Oithona, Acartia, Temora). The important common copepod genus, Calanus, is represented by three species at the Arendal sampling site: C. finmarchicus, C. helgolandicus, and C. hyperboreus. C. finmarchicus is the most abundant species during spring. This species overwinters in the Norwegian Deep (20 nautical miles farther offshore from this station), and interannual variability in overwinter survival is likely to affect the population dynamics. C. helgolandicus generally occurs in smaller numbers than C. finmarchicus, although the proportion of C. helgolandicus increases from spring (<10%) to autumn (>80%). C. hyperboreus is rarely observed in spring (March/April) and is associated with the influx of Atlantic water from the Norwegian Sea. The Arendal sampling site is influenced by relatively fresh coastal waters (25-32 psu) in the upper 30 m and by saltier Skagerrak water (32-35 psu) in the greater depths. Water movement is generally westerly and is caused by the coastal current bringing low-salinity water from the Baltic Sea and Kattegat. The site is also influenced by Atlantic water (>35 psu) advected from the Norwegian Sea into the Skagerrak Deep during winter. Together, these influxes create a relatively large seasonal cycle in salinity. The seasonal minimum temperature in the surface layer generally occurs in February (2 C) and the maximum in August (>20 C). At 75 m, the variation is less pronounced (minimum 4 C in February/March to maximum 14 C in August/September). Although the water column is mixed throughout the winter, increased freshwater run-off causes a strong halocline to appear from February/March to June. A spring bloom usually occurs in April/March, dominated by Diatoms. The chlorophyll values are generally low during summer (May-August), followed by an autumn bloom of Dinoflagellates in August/September. In summer, the water remains stratified because of surface heating. During the last 20 years, a trend towards higher temperatures has been observed in Skagerrak, both in surface and deeper layers. Since 2001, water temperatures in the region have been higher than previously seen in the last 100 years. |