Chapter 1.

Bottom-up introduction

1. Pulses
   Periods with an intensified atmospheric jet stream, often proxied by a high North Atlantic Oscillation (NAO) index (Hurrell, 1995), increases heat losses from the SPNA oceans and this induces deep winter convection, which is especially strong in the Labrador and Irminger Seas (Yashayaev, 2007). Strong convection increases the volume/size of the SPG and invigorates nutrient upwelling and thus primary production. Increased SPG volume, as well as the action of winds (through the so-called wind stress curl) brings the nutrient and zooplankton rich SPG water closer to the south Iceland and Faroes shelves, and can in this way “blow life” to these shelf ecosystems (Hátún et al., 2016). Such pulses have been documented to a) increase the nutrient contents all the way from the Labrador Sea, across the Irminger Sea and the Iceland Basin and into the southern Norwegian Sea (Hátún, et al., 2017), b) increase the abundance of the subarctic zooplankton species C. finmarchicus in the central Irminger Sea, the south Iceland shelf (Hátún et al., 2016) and in the subartic water masses in the Norwegian Sea (Kristiansen et al., 2021), c) increase the breeding success of the seabird black-legged kittiwake (Hátún, et al., 2017) and the total abundance of juvenile fish on the Faroe shelf (Jacobsen et al., 2019). During the last half century, such pulses occurred during the following years: 1976, 1984–1985, 1987, 1993–1995, 2000–2001, 2009, and 2017.
   
2. Major shifts
   Associated with the same atmospheric drivers, but with additional profound shifts in gyre circulation and major ocean currents, we have also witnessed longer lasting shifts in the northeastern Atlantic. After a period with generally high NAO index values during the late 1980s and early 1990s, an abrupt weakening in the atmospheric forcing during winter 1995–1996 led to much weakened winter convection, which again initialized a major decline of the SPG size and circulation (Hátún et al., 2005; Häkkinen & Rhines, 2004). Less subarctic water reached the mixing region west of the British Isles, and the AW therefore became both warmer and more saline, but poorer in nutrients and the nutritious C. finmarchicus (Hátún, et al., 2009). Likely as a result, seabird populations like the large puffin colony in the Westman Islands started to decline (Lilliendahl et al., 2013). The strong subarctic pulse in 2000-2001 gave calanus-dependent species on the south Iceland and the Faroe shelf a much-needed injection, but after this pulse had passed these shelf ecosystems went into a serious recession (Hátún et al., 2022) . This event also had detrimental impacts along the European margin, were e.g. the sandeel abundance (ICES, 2019) and the prosperity of several seabird species strongly decline (Coulson, 2011). The mid-1990s change had, however, positive impact on the large pelagic blue whiting stock (Hátún et al., 2009), which is less sensitive to food availability early in the season (before May), and which benefits from a warmer and more stratified ocean. Many new marine species with warmer water affinity entered Faroese and Icelandic waters during this event (Valdimarsson et al., 2012), and it is therefore justifiable to refer to this as the mid-1990s regime shift (Hátún et al., 2009a).
   Since the above discussed pulses and longer-lasting shifts are related to similar physical processes, it is not trivial to determine whether a present or recently observed change represents a pulse, or if it marks a shift to a new state. Although the pulses around 2000–2001 and 2008–2009 were evident in hydrographic records around the northeastern Atlantic (clearest in salinity) and in biological production on the Faroe shelf (Jacobsen et al., 2019), they were not strong enough to flip the SPNA back into a subarctic state – like in the early 1990s. A major pulse initiated by increased convection during the winters 2014–2016 (Yashayaev & Loder, 2016) managed, however, to re-invigorate the SPG. This was observed as a drop in sea surface heights south of Iceland (Hátún & Chafik, 2018) and in the most rapid salinity drop in historical records in the same region (Holliday et al., 2020). Observations during the following years have demonstrated that the system shifted back to a state resembling the early 1990s. It is therefore timely to review possible ecological changes in the northeastern Atlantic for the years after 2015. And for those species exhibiting a change, to distinguish if this was a post-2015 pulse or maybe a more lasting shift back to a subarctic/more productive state.
   Discussions on the Norwegian Sea must include the additional influence by the variable EIC. The properties and transport in the EIC are, as mentioned, related to the SPG story due to similar atmospheric forcing, and since the AW properties in the clockwise circulation constitutes one of the sources for the EIC (Valdimarsson et al., 2012). The SPG and EIC signals are, however, not the same. The EIC influx was strong during the 1990s, and this brought large amounts of large and lipid rich copepods (Calanus hyperboreus and large stages of C. finmarchicus) into the Norwegian Sea. This influx declined sharply in 2003–2005, and with it decline the abundance of the nutritious zooplankton (Kristiansen et al., 2019). This relatively warm and saline Atlantic period prevailed until around 2016, when the EIC influx increased yet again, which immediately brought more of the subarctic copepods to the waters north of the Faroes, and even east to the Norwegian slope (Kristiansen et al., 2021; Skagseth et al., 2021). Possible implications of these shifts in the Norwegian Sea have already been discussed for zooplankton and the fish species, herring (Clupea harengus) (Kristiansen et al., 2021; Eliasen et al., 2022), mackerel (Scomber scombrus) (Homrum et al., 2022) and salmon (Salmo salar) (Utne et al., 2022)). Updates on the further development of these processes and their potential impacts on adjacent shelves and seabirds is warranted.
   
3. Long-term trends
   Evidence for longer-term physical and ecological trends must also be discussed, especially now anthropogenic climate change is projected to fundamentally alter the functioning of marine ecosystem during the next 50–100 years (Pörtner et al., 2019). Only few hydrographical and biological records are longer than 50 years, and since the North Atlantic Ocean is characterized by a natural cycle of 50–60 years (the Atlantic Multidecadal Oscillation, AMO, Goldenberg et al., 2001), it is not trivial to distinguish between uni-directional trends and natural variability. Although we regularly read about warming trends and biota migrating polewards, fact is that increasing temperature trends are difficult to establish from hydrographic records in the northeastern Atlantic (González-Pola et al., 2023). The first order signal in these hydrographic records is a mid-1990s warming, followed by about twenty years with anomalously high temperatures and a subsequent temperature decrease down to the early 1990s level, i.e. the above mentioned shift. The pre-bloom (winter) silicate levels throughout the entire SPNA have, on the other hand, been declining since the 1980s in a more linear fashion than what can be ascribed to SPG-related natural variability alone (Hátún et al., 2017). Silicate is the limiting nutrient in the SPNA, so if this trend persists through the coming decades, the working of North Atlantic subarctic marine ecosystems is bound to change – fundamentally. It is conceivable that the mentioned bio-physical linkages, established during a period with higher nutrient concentrations already are altered.
   Based on a review of recent ecosystem changes on the Iceland, Faroe and Norwegian shelves, we should be able to test and maybe adjust/improve previously proposed hypotheses on bio-physical linkages.

References