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Marine ecosystem dynamics can vary on timescales ranging from months to centuries, but many observational data are limited to just a few decades. The bivalve Arctica islandica may live up to five centuries depositing annual growth increments in its shells which can serve as an indicator for ecosystem productivity. In the present study, 154 specimens of A. islandica were collected on the Faroe Shelf and standardised annual growth increments for 143 of them – 44 from coastal stations and 99 from shelf stations – were compared with climatic, oceanographic and biological variables. A. islandica growth from coastal and shelf stations was not correlated with basin-scale climate indices (the AMO index, the NAO index, the AO index or the subpolar gyre index) or, more locally, with windspeed or sea surface temperature on the Faroe Shelf. For the shelf stations there was a significant negative correlation between A. islandica growth and the volume transport of the Faroe Current flowing just north of Faroe Islands (r = -0.62). There was a weak nonsignificant positive correlation with an index of primary production on the Faroe Shelf (r = 0.31) and a strong negative correlation with a zooplankton biomass index in mid-summer (r = -0.76). There was also a strong positive correlation between A. islandica growth and the biomass of the bottom-feeding fish species Melanogrammus aeglefinus two years later (r = 0.62). These results seem to suggest that A. islandica growth may represent the amount of fresh phytoplankton that reaches the near-bottom water layers and could probably be regarded as a proxy for the strength of pelagic-benthic coupling that is modulated through phytoplankton-zooplankton interactions in the overlying water. Our results highlight the potential for A. islandica to serve as a long-term proxy for linking variability in pelagic ecosystem dynamics to demersal fish stocks.

Zooplankton availability during spring and summer determines the growth and survival of first-feeding fish larvae, and thus impacts the recruitment to both fish prey species and commercial fish stocks. On the Faroe shelf, however, the relative importance of oceanic versus neritic zooplankton species has hitherto not been well understood. In this study, spatio-temporal variability in zooplankton community structure and size spectra on the Faroe shelf is investigated using observations from late April during the period 1997–2016. The main objective was to explore which environmental variables influence the zooplankton community structure in early spring. The zooplankton community in the permanently well mixed central shelf inside the tidal front consists of a mixture of neritic, cosmopolitan and oceanic species. In this region, redundancy analyses showed that chlorophyll concentration had a positive effect on abundance of neritic copepods and meroplankton as well as all zooplankton Calanus finmarchicus , is however observed off-shore from the tidal front, especially on the western side of the Faroe Plateau. A shift in C. finmarchicus phenology occurred around 2007, resulting in earlier reproduction of this species, and this variability could not be explained by the employed regional environmental parameters. Our results indicate that the Faroe shelf biological production is more dependent on the local primary production and neritic zooplankton species than on the large oceanic C. finmarchicus stock.

The highly variable primary production on the spatially confined Faroe shelf has been comprehensively monitored since the early 1990s. Utilizing these data together with output from a realistic high-resolution numerical model, we here give a unique description of an interplay between bloom phenology in a productive temperate well mixed Central Shelf and the mixed layer dynamics in a surrounding seasonally stratified Outer Shelf. There is a shift to a persistently stratified Outer Shelf during late May – early June, which thereafter holds the highest phytoplankton biomass and becomes nutrient limited in the upper layer. The Central Shelf chlorophyll accumulation becomes impeded around this transition, which is coincident with strong silicate drawdown. Subsequently, a succession of blooms can occur, and these coincide with nutrient enriching erosion of the Outer Shelf stratification and become terminated by re-establishments of the stratification. We hypothesize that the mixed layer dynamics of the Outer Shelf regulates the lateral nutrient fluxes to the Central Shelf.

Marked inter-annual fluctuations in the primary production on the Faroe shelf propagate to higher trophic levels and influence commercial fish stocks. This has previously been demonstrated based on weekly chlorophyll samples from a coastal station, dating back to 1997. However, the spatial extent, for which the coastal samples are representative, has not been well defined, and potential bio-geographical segregations of the shelf have not been considered. By integrating 18 years of chlorophyll satellite data, supplemented by in-situ , model, and meteorological reanalysis data, we identify three regions with unique characteristics with regards to surface chlorophyll and vertical structure - the Central Shelf, the Outer Shelf and the Eastern Banks. The observed difference in timing of the spring bloom in these regions helps explain different spawning patterns of important fish stocks, and the spatial division of the Faroe Shelf should be considered when studying biology and hydrography in these waters. A positive correlation between annual means on the outer Faroe Shelf and parts of the outer northwest Scottish Shelf indicates similarities between these neighbouring regions. We suggest that this similarity arises from the commonality in nutrient composition of the water masses shared by these neighbouring regions.

The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope – at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel., The authors show that overflow waters flowing toward the Faroe Bank Channel can take a previously unidentified path to the Faroe-Shetland Channel where it joins an unrecognized deep-reaching jet located along its eastern rather than its western boundary.

Warm water of subtropical origin flows northward in the Atlantic Ocean and transports heat to high latitudes. This poleward heat transport has been implicated as one possible cause of the declining sea-ice extent and increasing ocean temperatures across the Nordic Seas and the Arctic Ocean, but robust estimates are still lacking. Here, we use a box inverse model and more than 20 years of volume transport measurements to show that the mean ocean heat transport was 305 ± 26 TW for 1993–2016. A significant increase of 21 TW occurred after 2001, which is sufficient to account for the recent accumulation of heat in the northern seas. Ocean heat transport may therefore have been a major contributor to climate change since the late 1990s. This increased heat transport contrasts with the Atlantic Meridional Overturning Circulation (AMOC) slowdown at mid-latitudes and indicates a discontinuity of the overturning circulation measured at different latitudes in the Atlantic Ocean. An increase in ocean transport from the North Atlantic into the Nordic Seas and Arctic Ocean is warming the region. Observations from 1993 to 2016 show a significant increase in heat transport after 2001, with the heat being transported over the Greenland–Scotland Ridge.

The variable annual primary production on the Faroe Shelf drives changes in higher trophic levels. However, the dynamics triggering the onset of the on-shelf bloom have previously not been well understood. Here we investigate how solar radiation and stratification on the outer periphery of the shelf affect the primary production on the central part of the shelf in spring, and we introduce a Critical Volume Hypothesis (CVH) to explain the initial onset of the bloom. The water masses on the entire shelf are vertically well mixed in winter but horizontally, the Central Shelf is separated from the deeper Outer Shelf by a tidal front. While the Central Shelf is always vertically mixed, the Outer Shelf becomes stratified during the spring which causes the front to change character and find a new position nearer shore. Simultaneously, the increasing radiation induces net phytoplankton accumulation in increasingly deeper water extending out from the shelf – i.e. an increased critical volume. In the CVH the onset of the spring bloom occurs when the net primary production within the Central Shelf becomes positive, which in an idealized circular symmetric version of the CVH translates to an average distance out to the front (Central Shelf width) becoming narrower than the critical width – analogous to a mixed layer becoming shallower than the critical depth in Sverdrup's vertical critical depth hypothesis. Utilizing comprehensive observations and simulations from 1997 to present, we show that more often than not, the CVH helps explain the onset time of the Faroe shelf spring bloom.

The interannual variation of the spring bloom and its effect on the marine ecosystem on the Faroe Shelf has been observed for a couple of decades. However, the mechanism controlling the spring bloom has so far not been known and attempts to explain the mechanism have mostly ruled out possibilities. The Faroe Shelf is to a variable degree isolated from the surrounding waters by a tidal front. It has previously been suggested that variations in the density difference across the front and how water masses are transferred across it affect the spring primary production, which is thought to be a driver of the shelf ecosystem. Using air–sea heat flux data and sea temperature observations on the shelf and off the shelf, we estimate the cross-frontal volume exchange in January–April and find that it increases with the tidal current speed and decreases with the cross-frontal temperature difference. Using the observed exchange rates, we show that the phytoplankton growth rate may be reduced by more than 0.05 day − 1 when the exchange is intense and off-shelf production is still low. Based on frontal dynamics theory, we suggest that the cross-frontal exchange rate in the above mentioned period is determined by the rate of vertical turbulent diffusion through the front. A simple theoretical model is found to support this hypothesis qualitatively as well as quantitatively. This supports that variations in horizontal exchange are an important controlling factor of the initial spring bloom and that the horizontal exchange during the winter can be determined by vertical turbulent diffusion. Our results will be relevant for the primary production in other similar systems of small geographical extent and also for other problems involving cross-shelf exchange, such as oil spill dispersal.

The flow of warm and saline water from the Atlantic Ocean, across the Greenland–Scotland Ridge, into the Nordic Seas – the Atlantic inflow – is split into three separate branches. The most intense of these branches is the inflow between Iceland and the Faroe Islands (Faroes), which is focused into the Faroe Current, north of the Faroes. The Atlantic inflow is an integral part of the North Atlantic thermohaline circulation (THC), which is projected to weaken during the 21st century and might conceivably reduce the oceanic heat and salt transports towards the Arctic. Since the mid-1990s, hydrographic properties and current velocities of the Faroe Current have been monitored along a section extending north from the Faroe shelf. From these in situ observations, time series of volume, heat, and salt transport have previously been reported, but the high variability of the transport has made it difficult to establish whether there are trends. Here, we present results from a new analysis of the Faroe Current where the in situ observations have been combined with satellite altimetry. For the period 1993 to 2013, we find the average volume transport of Atlantic water in the Faroe Current to be 3.8 ± 0.5 Sv (1 Sv = 106 m3 s−1) with a heat transport relative to 0 °C of 124 ± 15 TW (1 TW = 1012 W). Consistent with other results for the Northeast Atlantic component of the THC, we find no indication of weakening. The transports of the Faroe Current, on the contrary, increased. The overall increase over the 2 decades of observation was 9 ± 8 % for volume transport and 18 ± 9 % for heat transport (95 % confidence intervals). During the same period, the salt transport relative to the salinity of the deep Faroe Bank Channel overflow (34.93) more than doubled, potentially strengthening the feedback on thermohaline intensity. The increased heat and salt transports are partly caused by the increased volume transport and partly by increased temperatures and salinities of the Atlantic inflow, which have been claimed mainly to be caused by the weakened subpolar gyre.

Workshop on Currents and Transports Across the Iceland-Faroe-Scotland Ridge; Tórshavn, Faroe Islands, 9–10 January 2017

The ecosystem on the Faroe shelf has been shown to be tightly controlled by the primary production. It has been suggested that the primary production is governed by the physical processes controlling this water mass. The objective of this study is to identify the physical control mechanisms that control this water mass, link these to the interannual variability of the chlorophyll content on the Faroe shelf and through this discuss the influence on the primary production. In order to achieve this, a 10 year hindcast (2000–2009) with a regional ocean circulation model has been set up for the focus area. Results are compared with measurements on the Faroe shelf. The model reproduces the clockwise residual circulation around the Faroe Islands. The vertical velocity profile is validated using observations at a location west of the Islands. Observations show a logarithmic profile in the entire water column indicating a fully developed boundary layer. The modeled profile matches the observations in the bottom part of the water column, however the thickness of the bottom boundary layer is underestimated, which results in a constant profile in the upper part of the water column. As a consequence, the modeled velocity in the upper part of the water column is up to 20% lower than the observed velocity. The direction of the modeled velocity profile compares well with observations. The model realistically forms the partly isolated unique shelf water mass. Years with anomalously early and persistent modeled spring stratification correspond with years with a high on-shelf chlorophyll concentration. An integration of the exchange across the 120 m isobath shows intense water mass exchange across this depth contour. The major part of this includes tidal shifting of the front between on-shelf and off-shelf waters and is associated with little effective water mass exchange. The result is a shelf water mass that is relatively isolated. The modeled net exchange is constituted by an on-shelf flow near the surface and an off-shelf flow near the bottom associated with the frictional boundary layer. This is confirmed by the tracer experiment. Both the tracer experiment and the exchange across the 120 m isobath indicate that there is spatial variability in the exchange of the shelf water. The renewal rate of the shelf water mass during the spring bloom period is assessed by a set of passive tracer simulations. Using passive tracers, the time scale for the half-life is found to be between 30 and 40 days. This compares favorably with the time-scale based on the net, advected water mass exchange that yields 60–70 days, considering that this estimate neglects turbulent and diffusive exchange.

From 1994 to 2011, instruments measuring ocean currents (Acoustic Doppler Current Profilers; ADCPs) have been moored on a section crossing the Faroe–Shetland Channel. Together with CTD (Conductivity Temperature Depth) measurements from regular research vessel occupations, they describe the flow field and water mass structure in the channel. Here, we use these data to calculate the average volume transport and properties of the flow of warm water through the channel from the Atlantic towards the Arctic, termed the Atlantic inflow. We find the average volume transport of this flow to be 2.7 ± 0.5 Sv (1 Sv = 106 m3 s–1) between the shelf edge on the Faroe side and the 150 m isobath on the Shetland side. The average heat transport (relative to 0 °C) was estimated to be 107 ± 21 TW (1 TW = 1012 W) and the average salt import to be 98 ± 20 × 106 kg s−1. Transport values for individual months, based on the ADCP data, include a large level of variability, but can be used to calibrate sea level height data from satellite altimetry. In this way, a time series of volume transport has been generated back to the beginning of satellite altimetry in December 1992. The Atlantic inflow has a seasonal variation in volume transport that peaks around the turn of the year and has an amplitude of 0.7 Sv. The Atlantic inflow has become warmer and more saline since 1994, but no equivalent trend in volume transport was observed.

Faroe Shelf Water (FSW) is the water mass that occupies the shallow parts of the Faroe Shelf, surrounding the Faroe Islands (62°N, 7°W). Intensive tidal mixing induces a high degree of homogeneity and the circulation system allows a partial isolation from surrounding waters. This water mass, therefore, supports a unique ecosystem of great importance for commercial fish stocks and studies have shown a clear dependence of the ecosystem on the physical processes that maintain this system and control the exchange between the FSW and the off-shelf waters. In order to identify and quantify these processes, a large observational dataset has been analysed and related to alternative theories. From this analysis, the extent and properties of the FSW have been quantified and the degree of stratification explained in terms of the Simpson–Hunter theory. The residual clockwise circulation system, which is responsible for the partial isolation from off-shelf waters, is shown to be mainly generated by tidal rectification. The typical exchange rate of water between the FSW and the off-shelf regimes has been determined by the use of simple models based on the heat and the salt budgets but the actual exchange rate is found to vary considerably in time and space. These results support earlier suggestions that this exchange is the main limiting factor for the phytoplankton spring bloom on the Faroe Shelf and that variations in exchange rate are responsible for the large inter-annual variation in spring bloom timing and intensity. The observations indicate that the on-shelf/off-shelf exchange intensity is not symmetrically distributed around the shelf, but rather concentrated around the narrow southern tip of the Faroe Shelf, where off-shelf waters during intensive exchange events may be imported all the way to the shore.

The flow of warm and saline water from the Atlantic Ocean, across the Greenland–Scotland Ridge, into the Nordic Seas – the Atlantic inflow – is split into three separate branches. The most intensive of these branches is the inflow between Iceland and the Faroe Islands (Faroes), which is focused into the Faroe Current, north of the Faroes. The Atlantic inflow is an integral part of the North Atlantic thermohaline circulation (THC), which is projected to weaken during the 21 century and might conceivably reduce the oceanic heat and salt transports towards the Arctic. Since the mid-1990s, hydrographic properties and current velocities of the Faroe Current have been monitored along a section extending north from the Faroe shelf. From these in situ observations, time series of volume, heat, and salt transport have previously been reported, but the high variability of the transport series has made it difficult to identify trends. Here, we present results from a new analysis of the Faroe Current where the in situ observations have been combined with satellite altimetry. For the period 1993 to 2013, we find the average volume transport of Atlantic water in the Faroe Current to be 3.8 ± 0.5 Sv (1 Sv =106 m3 s−1) with a heat transport relative to 0 °C of 124 ± 15 TW (1 TW =1012 W). Consistent with other results for the Northeast Atlantic component of the THC, we find no indication of weakening. The transports of the Faroe Current, on the contrary, increased. The overall trend over the two decades of observation was 9 ± 8% for volume transport and 18 ± 9% for heat transport (95% confidence intervals). During the same period, the salt transport relative to the salinity of the deep Faroe Bank Channel overflow (34.93) more than doubled, potentially strengthening the feedback on thermohaline intensity. The increased heat and salt transports are partly caused by the increased volume transport and partly by increased temperatures and salinities of the Atlantic inflow, attributed mainly to the weakened subpolar gyre.

One of the most basic paradigms in marine ecology is the “Sverdrup mechanism,” where the spring bloom is retarded if the surface mixed layer is too deep, due to the algae being mixed vertically out of the euphotic layer. A similar mechanism may operate in vertically homogeneous water over a small shallow area, surrounded by deep waters, if the horizontal exchange is sufficiently intense. In systems with inter-annual variations in the horizontal exchange rate, this may induce inter-annual variations in the timing and intensity of the spring bloom. A numerical primary production model with circular symmetry and prescribed horizontal exchange rate is developed. Using the Faroe Shelf as an example, the model and observations show that the timing and intensity of the spring bloom in the shallow parts of the system may be critically dependent upon the rate of the horizontal exchange. Numerical experiments confirm that the effect of horizontal exchange on the spring bloom is less pronounced for banks than for shelf systems around islands of similar scales, and that the effect increases in importance as the horizontal scale is reduced.

Previous investigations have shown that new primary production on the Faroe Shelf during the spring bloom varies considerably from one year to another, both with regard to timing and intensity. It has also been found that variations in new primary production are transmitted up the food chain to top predators such as cod and haddock. An observed inverse relationship between new primary production and zooplankton biomass, especially Calanus finmarchicus, might be due to grazing, but could also reflect a dependence of both new primary production and C. finmarchicus import on the horizontal exchange between waters on the shelf and off it. Here, we investigate this question, using observations and an idealized numerical model. We find that the large variability in new primary production is most likely due to the direct effect of variable horizontal exchange rates on phytoplankton reproduction (horizontal Sverdrup mechanism), rather than grazing. Enhanced horizontal exchange flushes phytoplankton from the shallow areas and limits primary production. Horizontal exchange rate seems most sensitive to horizontal density differences between on-shelf and off-shelf waters, which is governed by atmosphere–ocean heat exchange and precipitation. For the primary observational period from 1990 to 2003, a close relationship was found between air temperature in January–April and new primary production. Cold winters produced large density differences, small horizontal exchange rates, and intensive new primary production. Other, less direct, evidence indicates, however, that this relationship may not have been valid in a period before 1990.