Climate fluctuations and the spring invasion of the North Sea by Calanus finmarchicus

Abstract

This version is available at https://strathprints.strath.ac.uk/18569/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the ABSTRACTThe population of Calanus finmarchicus in the North Sea is replenished each spring by invasion from an overwintering stock located beyond the shelf edge. A combination of field observations, statistical analysis of Continuous Plankton Recorder (CPR) data, and particle tracking model simulations, was used to investigate the processes involved in the cross-shelf invasion. The results showed that the main source of overwintering animals entering the North Sea in the spring is at depths of greater than 600 m in the Faroe Shetland Channel, where concentrations of up to 620 m 73 are found in association with the overflow of Norwegian Sea Deep Water (NSDW) across the Iceland Scotland Ridge. The input of this water mass to the Faroe Shetland Channel, and hence the supply of overwintering C. finmarchicus, has declined since the late 1960s due to changes in convective processes in the Greenland Sea. Beginning in February, animals start to emerge from the overwintering state and migrate to the surface waters, where their transport into the North Sea is mainly determined by the incidence of north-westerly winds that have declined since the 1960s. Together, these two factors explain a high proportion of the 30-year trends in spring abundance in the North Sea as measured by the CPR survey. Both the regional winds and the NSDW overflow are connected to the North Atlantic Oscillation Index (NAO), which is an atmospheric climate index, but with different time scales of response. Thus, interannual fluctuations in the NAO can cause immediate changes in the incidence of northwesterly winds without leading to corresponding changes in C. finmarchicus abundance in the North Sea, because the NSDW overflow responds over longer (decadal) time scales.

Update on the relationship between the North Atlantic Oscillation and Calanus finmarchicus

“…It shows that the surface has warmed throughout the region. Heath et al (1999) proposed that the decline of C. finmarchicus abundance was a result of a reduction in overwintering habitat available due to a decrease in the volume of Norwegian Sea Deepwater. Warm-water zooplankton species have become prevalent in this region (Beaugrand et al 2002, Beaugrand 2003.…”

Unanticipated biological changes and global warming

“…They also showed that the warm temperate species C. helgolandicus was posi-tively related to the NAO although the relationship was weaker and with a 1 yr lag. Many hypotheses were proposed to explain the link between the NAO and these species: wind-induced turbulence and prey-predator interactions, the influence of temperature on competition between both species, bottom-up control, changes in biogeographical boundaries between both species, the volume of Norwegian Sea Deep Water in which C. finmarchicus is known to overwinter and the flow of the European shelf edge current (Heath et al 1999, Beaugrand 2009). …”

Temporal patterns in the surface abundance of Calanus finmarchicus and C. helgolandicus in the northern North Sea (1958-1996) inferred from Continuous Plankton Recorder data

“…One explanation for the temporal Patterns Seen in the 2 Calanus species is that Atlantic influence and inflow (Corten 1990, Stephens et al 1998) into the North Sea were greater in the 1960s and more Calanus were imported: C. finmarchicus from overwintenng 1.ocations beyond the contmental shelf and C. helgolandicus from the north and West of Scotland via the Fair lsle Current (Backhaus et al 1994, Heath et al 1999. The waning of relatively warm Atlantic inflow left a signature of falling temperatures (See Fig.…”