Abstract
The copepods Calanus finmarchicus and Calanus helgolandicus co-exist in the North Sea, but their spatial distribution and phenology are very different. Long-term changes in their distributions seem to occur due to climate change resulting in a northward extension of C. helgolandicus and a decline of C. finmarchicus in this region. The aim of this study is to use life-stage structured models of the two Calanus species embedded in a 3D coupled hydrodynamic–biogeochemical model to investigate how the biogeography of C. finmarchicus and C. helgolandicus is modified by changes in ±2°C sea water temperatures, overwintering and oceanic inflow in the North Sea. Life-stage structured models are validated against CPR data and vertical distributions north of the Dogger Bank in the North Sea for the reference year 2005. The model shows that (1) ±2°C changes from the current level mainly influence the seasonal patterns and not the relative occurrence of the two species, (2) changes due to oceanic inflow mainly appeared in the northern and southern part of the North Sea connected to the NE Atlantic and not in the central part and (3) the abundance of Calanus species were very sensitive to the degree of overwintering within the North Sea because it allows them to utilise the spring bloom more efficiently and independently of the timing and amount of oceanic inflow. The combination of lower temperatures, higher overwintering and oceanic inflow simulating the situation in the 1960s largely favoured C. finmarchicus and their relative contribution to Calanus spp. increased from 40% in the reference year to 72%. The +2°C scenario suggest that in a warmer future, C. finmarchicus is likely to decline and C. helgolandicus abundance will probably continue to increase in some areas.
Highlights
The North Sea is a continental shelf system located in the North-East Atlantic Ocean and sustains a large fishery (Mackinson and Daskalov, 2007)
The two copepods Calanus finmarchicus and C. helgolandicus are considered key species in the ecosystem because they serve as prey for commercially important fish species (Gaard and Reinert, 2002; Gislason and Astthorsson, 2002; Heath, 2007; Munk and Nielsen, 1994; Ringuette et al, 2002)
Since 1980, the spring phenology of temperature is advanced by 2.08 days per decade in the Northern Hemisphere (Burrows et al, 2011) and sea surface temperatures in the North Sea have been observed to increase about 1.5°C (Philippart et al, 2007)
Summary
The North Sea is a continental shelf system located in the North-East Atlantic Ocean and sustains a large fishery (Mackinson and Daskalov, 2007). Since 1980, the spring phenology of temperature is advanced by 2.08 days per decade in the Northern Hemisphere (Burrows et al, 2011) and sea surface temperatures in the North Sea have been observed to increase about 1.5°C (Philippart et al, 2007) This is believed to have implications for the geographical distribution and seasonality of zooplankton (Mackas et al, 2012). The two Calanus species are difficult to distinguish Their life cycle includes eggs, six naupliar stages followed by five copepodite stages until maturation to the 6th copepodite stage, the adult stage. Despite these similarities, the two species differ in the spatial distribution and phenology (Beaugrand et al, 2002; Bonnet et al, 2005).
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