Ecosystem uptake and transfer of Sella ﬁ eld-derived radiocarbon ( 14 C) part 2: The West of Scotland

Ecosystem uptake and transfer of Sella ﬁ eld-derived radiocarbon ( 14 C) were examined within the West of Scot- landmarineenvironment.Thedissolvedinorganiccarboncomponentofseawater,enrichedin 14 C,istransported to the West of Scotland where itis transferredthrough the marine food web. Benthicand pelagic biota with var- iable life-spans living in the North Channel and Clyde Sea show comparable 14 C activities. This suggests that mixingof 14 C within theIrish Sea results ina relatively constant northwardsdispersalofactivity.Benthic species in the Firth of Lorn have similar 14 C enrichments, demonstrating that Irish Sea residual water is the dominant sourcetothisarea.Measured 14 CactivitiesinbiotashowsomesimilaritytowesternIrishSeaactivities,indicating that dispersion to the West of Scotland is signi ﬁ cant with respect to the fate of Sella ﬁ eld 14 C releases. Activities measured in commercially important species do not pose any signi ﬁ cant radiological risk.


Radiocarbon ( 14 C)
Radiocarbon ( 14 C) is estimated to be the largest contributor to the collective dose that the global population receives from the nuclear industry (UNSCEAR, 2008), due to its long half-life (5730 years) and ready uptake into the global carbon cycle. In the UK, the Sellafield nuclear fuel reprocessing facility is authorised to discharge waste 14 Ctothe atmosphere and the marine environment and this 14 C is the largest contributor to the collective dose commitment to UK and European populations from the entire nuclear industry (Nuclear Decommissioning Authority, 2015).
Atmospheric releases of 14 C from Sellafield largely dominated total discharges until 1994 when a change in discharge policy and, to a lesser extent, an increase in reprocessing activity resulted in increased marine 14 C discharges, which have continued since that time. The average marine 14 C discharge activity increased from b2 TBq per year, between 1984and 1993, to approximately 8 TBq per year since 1994(BNFL, 1985, MAFF, 1992, RIFE, 1996. Low-level radioactive effluent containing 14 Ci s discharged via pipelines that extend 2.1 km offshore into the Irish Sea. 14 C is released primarily as inorganic carbon and is incorporated into the dissolved inorganic carbon (DIC) component of seawater (Begg et al., 1991, 1992, Begg, 1992, Cook et al., 1995. Through fixation of inorganic carbon, marine photosynthesising organisms derive carbon from the DIC pool (Lalli and Parsons, 1993). Marine calcifying organisms, such as molluscs, also utilise DIC to build their exoskeletons (McConnaughey et al., 1997). These two pathways provide routes for the biological uptake of Sellafield-derived 14 C; however, it is the former that allows 14 C to be transferred throughout the entire food web.
Sellafield 14 C discharges are made in addition to existing "background" inputs of 14 C from natural production and fallout from atmospheric testing of nuclear weapons in the 1950s and early 1960s. Cook et al. (1998) and Tierney et al. (2016) presented near-identical background activities for the UK marine environment over 2 decades (248 ± 1.0 and 249 ± 0.8 Bq kg -1 C for the years 1995 and 2014, respectively). Therefore, marine background 14 C activity in this area is relatively stable and the latter value is used to define the modern 14 C background level in this study. There have been various studies concerned with the fate of Sellafield-derived 14 C within the Irish Sea (e.g. Begg et al., 1992, Cook et al., 1995, 1998, Wolstenholme et al., 1998, Gulliver et al., 2001, Muir et al., 2015andseveral have identified uptake of Sellafield 14 C within the tissues of marine and intertidal organisms, leading to enrichment above ambient background (e.g. Begg et al., 1992, Cook et al., 1995, 1998, while others have established the role of molluscs in 14 Cuptakeandaccumulation within the inter-tidal environment , Muir et al., 2015. However, none have considered the marine ecosystem in detail. There has also been relatively little research on Sellafield 14 C beyond the Irish Sea. The net northerly movement of water through the North Channel dictates the long-term dilution and dispersion of Sellafield marine discharges (Dunster, 1998), with N 99% of Sellafield 14 C dispersed through the North Channel (Gulliver et al., 2001). Using the highly soluble caesium isotopes, 134 Cs and 137 Cs, Jefferies et al. (1973) calculated transit times to the North Channel from Sellafield (then Windscale) in the region of 1.1-1.8 years. A shorter transit time was reported by Kershaw and Baxter (1995) of approximately 1 year, however a study on Sellafield releases of 99 Tc observed a further reduced transit time of 3 months (Kershaw et al., 2004). These studies show that Irish Sea circulation is highly variable and it is difficult to estimate a transit time to the North Channel and beyond, for Sellafield releases. Transit times for 14 C within the DIC fraction of seawater would, nevertheless, be similar to these relatively conservative radionuclides and, over a period of 3 months to a year, sufficient mixing should occur within the Irish Sea for a relatively homogenous activity (on short time scales) to reach the North Channel. From the North Channel, dissolved 14 C is carried around the Scottish coastline by the Scottish Coastal Current (McKay et al., 1986, Hill andSimpson, 1988) and the Fair Isle Current (Turrell and Henderson, 1990) into the North Sea .
Enriched 14 C activities in the DIC fraction of seawater and within the tissue of several marine species have been observed along the Scottish coastline . Similarly, a recent study  demonstrated that despite 14 C activities reducing with distance from Sellafield due to dilution, mollusc shells on the north-west Scottish coastline were enhanced above ambient background.
This research forms the second half of a two-part study examining the uptake and transfer of Sellafield-derived 14 C within the Irish Sea and West of Scotland (WoS) marine ecosystems. In this study, the West of Scotland (Fig. 1) is designated as far-field,relativetoSellafield 14 C inputs. The near-field encompasses the Irish Sea, and is examined in Part 1 (Muir et al., in this issue). Previous studies  have observed Sellafield-derived 14 Cenhancements in selected offshore and intertidal species in the WoS, therefore it is important to consider 14 C uptake within the wider WoS marine ecosystem. The overarching objectives are detailed in Part 1 (Muir et al., in this issue). This study (Part 2. The West of Scotland) investigates, in detail, 14 C activities within the biotic and abiotic components of the WoS marine ecosystem, including that of commercially important fish/crustacean species and the transfer of 14 C through the marine food web. Due to its continuing discharge from Sellafield, long half-life and high bioavailability, it is important to gain an understanding of the transport, uptake and ultimate fate of 14 C within these marine ecosystems, not only with regard to past, current and future authorised 14 C discharges, but also in the unlikely event of a nuclear accident.

Study areas
Samples were predominantly collected from 3 areas in the WoS; the North Channel, the Clyde Sea and the Firth of Lorn.

North Channel
The North Channel connects the northern end of the Irish Sea with the northeast Atlantic Ocean via the Clyde Sea. The channel narrows to 30 km in width and the seabed is characterised by rocky outcrops and localised enclosed deeps such as Beaufort's Dyke (Wilding et al., 2005a). Compared to the Irish Sea, the North Channel is relatively deep, averaging 90 m, and the dyke descends to 312 m (Callaway et al., 2011). Flow rates within the Irish Sea are typically low; however the geomorphology of the North Channel means that the tidal flow here can be relatively fast at up to 1.5 m s −1 . A net northerly flow though the North Channel exists (Ramster and Hill, 1969, Howarth, 1982, Gulliver et al., 2001, but flow direction is seasonally variable and can be reversed (Dabrowski et al., 2010). Due to high flow rates, the central channel substrate is mainly rock and gravel with finer material deposited to the east, proximate to the Great Plateau (Wilding et al., 2005a). Beaufort's Dyke was used as a munitions disposal ground by the UK military from end of World War I until 1972(Fisheries Research Services, 1996 and, perhaps as a consequence, there is a lack of studies on the benthic ecosystem within the North Channel.

Clyde Sea
The Arran Basin is the predominant depositional basin within the Clyde Sea, where deep water can become isolated in summer and renewed during winter when currents carrying dense water flow over the Great Plateau (Wilding et al., 2005b). Sills at the entrances of sea lochs north of the Arran Basin produce a similar effect where loch bottom waters can also become isolated (Edwards and Sharples, 1986). As the water is only exchanged intermittently, the deep benthic environment of the Clyde Sea is particularly vulnerable to anthropogenic impacts (Wilding et al., 2005b). The subtidal environment mainly consists of fine grained layered muds which extend from the Arran Basin into the basins of the sea lochs (Moore, 1931, Wilding et al., 2005b. Benthic communities are characterised by high abundances of heart urchin (Echinocardium cordatum and Brissopsis lyrifera) and brittle stars (Amphiura spp), while bivalves, polychaetes and the burrowing decapod, Nephrops norvegicus (henceforth referred to as Nephrops), are also present (Pearson et al., 1986). Fishing pressure has reduced the volume of fish caught in the Clyde Sea significantly (Thurstan and Roberts, 2010). Historically, the area contained important fisheries for herring (Clupea harengus), cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens) and hake (Merluccius merluccius) but fishing yields have since collapsed (Thurstan andRoberts, 2010, Heath andSpeirs, 2012). Currently, the important remaining fisheries in the Clyde Sea are for Nephrops and scallops (Pecten maximus and Chlamys opercularis) (Thurstan and Roberts, 2010).

Firth of Lorn
The Firth of Lorn is located on the west coast of Scotland to the east and south east of the Isle of Mull. The Corryvreckan whirlpool lies to the south of the Firth of Lorn where a strong tidal race produces flow speeds up to 4.5 m s −1 which has a significant impact on local sediment transport (Howe et al., 2015). The central firth area contains depositional basins at 150 m depth and thick sequences of muds are also found in the many sea lochs that surround the firth (Howe et al., 2015). Sediments within the deep basins support dense fields of crinoids (Leptometra celtica) and large numbers of northern sea-fans (Swiftia pallida)a r e found on sediment-covered stony sea-beds (Davies, 1999). The diversity of habitats here is reflected in the existence of a number of nationallyrare benthic species (Plaza and Sanderson, 1997). Due to its biological richness, the Firth of Lorn is designated as a Special Area of Conservation (SAC). This has resulted in a ban on scallop dredging in the area, previously an important fishery. Nephrops, found in the muddy habitats, is now the single most significant fishery. Although not a major spawning ground, the Firth of Lorn has been identified as an important nursery ground for larval and juvenile fish from a wide range of species (Fox and Lappalainen, 2014).

Methodology
Sampling and analytical techniques utilised are described in detail in Muir et al. (in this issue)) and summarised here in relation to the sites sampled which are shown in Fig. 1 and detailed in Table 1. Seawater, sediment and benthic organisms were collected in the Firth of Lorn (station FoL) in April 2014, on-board the RV Calanus. Plankton samples were collected at the same site in August 2014, together with additional seawater samples. A sampling campaign within the Irish Sea was conducted in June 2014 on-board the RV Prince Madog, and included the North Channel (station NC) where the same suite of sampling techniques was used. Additional North Channel seawater samples were collected from the coastline near to Portpatrick (station PP) in April 2014 and August 2014. A number of fish and crab samples were collected by the Marine Scotland Science's West of Scotland demersal fish survey, from the RV Scotia during November 2014. These samples were predominantly taken from the Clyde Sea (H443 and H444), however, some were also collected further north, between the Isles of Gigha and Islay (H445) and off the west coast of Tiree (H451). The Agri-Food and Biosciences Institute, Northern Ireland (AFBI-NI) collected shellfish samples from off the northern Irish coastline (Areas A and D) in February 2015.

Seawater 14 C biogeochemical fractions
Surface water was collected to measure 14 C activity in the four biogeochemical fractions; dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particulate inorganic carbon (PIC) and particulate organic carbon (POC). Surface samples from 2 m depth were collected on research vessels at stations NC and FoL by pumping 160 l of seawater on board. Additional 160 l samples were collected, at high tide using 20 l carboys, from the North Channel coastline at Port Patrick. The water was filtered through 0.7 μmglassfibre filters with particulate material used for analysis of PIC and POC. Several 500 ml aliquots of filtrate were collected in foil bags for analysis of DIC. Further aliquots of 500 ml were collected in glass bottles for analysis of DOC with 1 ml of (85%) orthophosphoric acid added to liberate DIC and fix the organic carbon content.

Sediment organic content
Sediment cores were retrieved from stations NC and FoL using an OSIL maxi-corer. Cores were approximately 30 cm in depth and sectioned into 1 cm vertical increments which were initially frozen for storage. The sections were later thawed, oven dried at 40°C and ground into a fine powder. The 14 C activity in the organic component of sediment was measured within four depth increments (0-1c m ,5 -6c m ,1 0 -11 cm and the base of the core) in one core from each site.

Biota
Beam trawls and Van Veen grabs were utilised to sample the benthic biota at NC and FoL. A range of organisms was collected and identified, where possible, to species level. Fish and shellfish stock surveys (by AFBI and Marine Scotland Science) using bottom trawls provided additional fish and crab samples. Muscle/soft tissue was subsampled and freeze dried. Where more than one individual of a species was collected, samples were proportionally combined and multiple samples were made if six or more individuals were available. Similarly, where several relatively analogous species had been collected with few individuals of a specific species, the samples were combined (e.g. starfish). Plankton nets were utilised at NC and FoL to collect phytoplankton (80-270 μm mesh size) and zooplankton (N270 μm). Whole plankton samples were freeze dried.

14 C analysis procedure
To measure the 14 C activity, organic samples (DOC, POC, organic sediment, biota) were combusted in sealed quartz tubes according to the method of Vandeputte et al. (1996) and inorganic samples (DIC, PIC) hydrolysed with HCl (1 M) to liberate CO 2 . The gas was cryogenically trapped and purified and graphite was produced from 3 ml subsamples according to the method of Slota et al. (1987). Sample 14 C/ 13 C isotope ratios were measured on the SUERC 250 KV SSAMS or the 5 MV tandem AMS (Freeman et al., 2008) and with quality assurance standards described in Naysmith et al. (2010) and Dunbar et al. (2016). Stable isotope (δ 13 C) ratios were measured offline on a VG SIRA 11 isotope ratio mass spectrometer for calibration of natural fractionation of the measured 14 C. 14 C results were calculated relative to the international standard (oxalic acid II, SRM-4990C) as 14 C activity ratios (fraction modern, F 14 C). Fraction modern results were converted to specific activities (Bq kg −1 C) using the regime for calculating enhanced activity samples described by Mook and van der Plicht (1999). Uncertainties are typically b0.5% of the measured activity.

Seawater 14 C biogeochemical fractions
The 14 C values for the biogeochemical fractions of the surface seawater samples from stations PP and NC are presented in Table 2. The DIC component was enriched in 14 C at both sites and at all 3 sampling dates, in line with the dissolved inorganic form of 14 Cdischargedfrom Sellafield. The POC fraction was also enriched at station NC but marginally depleted at station PP for both the April and August sampling periods. The δ 13 C values of the POC fractions at station PP were lower than that at the NC station, which indicates that a greater proportion of the material at station PP was probably terrestrial in origin. This would explain the reduced activities as terrestrially-derived run-off could potentially contain a significant proportion of "old carbon", resulting in a dilution effect. Surface water collected during June 2014 from station NC was also enriched in 14 C within the PIC fraction. This enrichment in the PIC could result from movement of fine, 14 C enriched material from the intertidal zone as described by Tierney et al. (2016). The PIC fraction was significantly depleted at station PP and the lower δ 13 C values for these samples indicate that a greater proportion of the material was again terrestrial in origin. The single DOC sample containing enough dissolved carbon for 14 C analysis was significantly depleted and had a δ 13 C value typical of terrestrial carbon, indicating significant run-off of "old carbon" from land. 14 C activities measured in the same fractions of seawater from the North Channel in 1989 (Cook et al., 1995) showed similar enrichment in the DIC fraction and depletion in the DOC and POC fractions, despite the increase in Sellafield 14 C marine discharges at this period. Also, at this time, North Channel PIC was depleted in comparison to the slight enrichment observed in 2014 at station NC; however, PIC was depleted in 2014 at station PP. The 14 C activity of the DIC at station PP was also measured in 1995 at the onset of increased Sellafield discharges andwas significantly enriched (430 ± 4 Bq kg −1 C) compared to the activities measured in this study. Sellafield discharges of 14 C were higher in the 12 months prior to sample collection in 1995 (8.7 TBq) relative to the 2014 sample collection (4.8 TBq). As the transit time of discharges from Sellafield to the North Channel is in the order of 3 months to N1 year (Jefferies et al., 1973, Kershaw and Baxter, 1995, Kershaw et al., 2004, the lower activities described here are most likely a direct result of the lower discharged activities in the preceding months.

Sediment organic content
The organic component at all depth increments of station NC sediment were depleted in 14 C( Table 3). The measured activities in the top 11 cm were relatively homogenous (204-210 Bq kg − 1 ) C, while the base activity (29-30 cm) was significantly depleted in comparison (135 Bq kg −1 ). The large variety of benthic species found at this station, (discussed below) demonstrates that this is a highly biologically active site and the relatively homogeneous nature of the surface sediments is likely to be caused to a significant degree by intensive bioturbation. The 14 C activity observed in the surface sediment is depleted, which contrasts with the 14 C enrichment observed in surface water POC at station NC. POC may be rapidly scavenged from the water column and/or the surface sediment resulting in a very low flux and incorporation of Sellafield-derived 14 C into surface sediments. In addition, the build-up of higher activity material could also be masked through effective mixing to depth of 14 C-enriched organic material with significant quantities of old, 14 C-depleted organic material in the sediments, resulting in a dilution effect (see Muir et al., in this issue). Furthermore, physical transport of 14 C enhanced particulate material from this site by currents could also reduce the volume reaching the sediment and this has previously been suggested as an important mechanism for the transport of enhanced fine inorganic material in northern Irish Sea coastal sites .

Biota
The 14 C activities of sampled biota at station NC are presented in Fig.  2. All species analysed were enriched relative to the ambient background and mostly varied between 280 and 330 Bq kg −1 C with whiting (Merlangius merlangus) having a significantly higher activity (413 ± Table 2 Gross and net specific 14 C activities (Bq kg −1 C±1σ)andδ 13 C(‰ relative to VPDB) values from the four biogeochemical fractions of North Channel surface water samples from stations NC and PP. Net activities above the ambient background of 249 ± 1 Bq kg −1 C are in bold. Samples less than this are denoted as 'Depleted'. 2Bqkg −1 C). In contrast, phytoplankton (280 ± 2 Bq kg −1 C) and zooplankton (283 ± 2 Bq kg −1 C) had amongst the lowest 14 C activities. Dab (Limanda limanda) (samples 1 and 2) showed intra-species variation, where the difference between the two samples, each consisting of 3 individuals, was approximately 42 Bq kg −1 C. Similar intra-species differences have been observed for dab in the Irish Sea (Muir et al., in this issue) while other multiple samples of the same species/species group showed little or no intra-species variation. Phytoplankton species have a relatively fast carbon turnover rate and short lifespan, and will readily incorporate 14 C from DIC during photosynthesis. The 14 C activities in DIC, phytoplankton and POC were comparable at station NC illustrating direct uptake of 14 C from ambient waters and implying that the POC is predominantly derived in situ from phytoplankton. The zooplankton sample from station NC was observed to consist largely of copepods (probably Calanus finmarchicus and/or Calanus helgolandicus due to their predominance in this area) with some ctenophores and ichthyoplankton. Although some zooplankton species are carnivorous, copepods feed directly on phytoplankton (Meyer-Harms et al., 1999). That zooplankton 14 C activity is also similar to that of phytoplankton, reflecting the 14 C activity of their primary source of food, and the transfer of 14 C through the food chain.
Although phytoplankton and zooplankton 14 C activities were amongst the lowest and the whiting activity was the highest, there is no obvious trend of increasing 14 C activity moving up the food chain. This corresponds to a general transfer of 14 C from primary producers to higher organisms with no concentration effect, as might be expected. Any variation in 14 C activity more likely derives from variations in the food source and the integration period of carbon uptake. Filter feeders, such as the common cockle (Cerastoderma edule) will incorporate 14 C from the plankton and POC that they ingest (Iglesias et al., 1992). Organisms like polychaete worms, heart urchin (Echinocardium cordatum), brittle stars, Calocaris shrimp (Calocaris macandreae) and the hermit crab (Pagurus bernhardus) are predominantly detritivorous, feeding on the organic material falling from the water column. The relative 14 Cenhancement above background in these species, confirms the supply of 14 C enriched organic material to the sediment surface. Other crab species (Goneplax rhomboides, Atelecyclus rotundatus, Inachus sp.) will predate on smaller organisms as well a feeding on detritus. The sea mouse (Aphrodita aculeata), Nephrops, starfish (Asterias rubens, Crossaster papposus, Luidia sarsii, Asteroidea sp.) and fish species (dab, sole, dragonet (Callionymus lyra), ling (Molva molva), whiting) are predatory, feeding on other benthic organisms. These species have higher 14 Cactivities than the plankton groups indicating that organisms occupying higher trophic levels are integrating 14 C over a longer period of time, including periods of higher ambient activities. For example, many of the analysed benthic species are only locally mobile (e.g., brittle star, sea mouse and starfish). Therefore, their high activities relative to that of phytoplankton are a result of uptake during a period of higher ambient activity, corresponding to transient 14 C enrichment in the DIC fraction of seawater, which was subsequently passed through the food chain. Due to their relatively longer life-span, an integrated higher activity in these species is now observed. Conversely, whiting are highly mobile and the comparatively high 14 C activity in this sample probably results from sampling individuals which had previously foraged in the Irish Sea. Demersal fish within the Irish Sea have 14 C activities N 400 Bq kg −1 C, as do their prey items (Muir et al., also submitted). Therefore, it is likely that station NC whiting had migrated from the more 14 C-enriched Irish Sea.

14 C activities in fish and shellfish survey species
Scallops are filter feeders and are, therefore, likely to have a similar 14 C activity to phytoplankton and to ambient DIC. The 14 C activity of scallops (Fig. 3) collected at Area A (249 ± 1 Bq kg −1 C) was identical to background, whereas scallops from Area D were enriched (283 ± 2Bqkg −1 C). Area A is located on the northern coast of Northern Ireland where the influence of Atlantic Ocean water should be greater and this is demonstrated in the observed 14 C activity being equal to background. Area D, in the North Channel, is affected by a southerly current which carries Atlantic water down the western edge of the North Channel (Bowden, 1980, Dabrowski et al., 2010. Despite there being some Atlantic influence, the effect of Sellafield 14 C discharges is still observable in this sample. The 14 C activities of fish and crab samples from the Marine Scotland Science surveys are also shown in Fig. 3. Samples from the Clyde Sea area (H443 and H444) were 14 C enriched, although fish samples typically had a lower activity here than fish at station NC. Haddock (Melanogrammus aeglefinus), primarily a benthic feeder, and herring (Clupea harengus), a planktivore, had relatively similar activities at station H443 (296 ± 2 Bq kg −1 Cand283±2Bqkg −1 C respectively) indicating little variation in 14 C activity in the water column. In comparison to station NC, whiting at H443 had a significantly lower activity (288 ± 2 Bq kg −1 C), again indicating that station NC whiting had spent time foraging in an enriched area, probably within the Irish Sea. Haddock samples collected further north and west (station H445) were also enriched (286 ± 2 Bq kg −1 C) but had a reduced activity relative to Clyde Sea haddock. Edible crab (Cancer pagurus)a c t i v i t y showed little variation between stations H443 (292 ± 2 Bq kg −1 C), H444 (304 ± 2 Bq kg −1 C) and H445 (288 ± 2 Bq kg −1 C). The activities at these stations are similar to those measured in other crustaceans at station NC, suggesting there was little difference in benthic 14 Cactivity between the North Channel and Clyde Sea.
Station H451 (situated approximately 74 km to the west of the Outer Hebrides) was the most remote sampling area from Sellafield in this study. Only monkfish (Lophius piscatorius)w e r es a m p l e d from this station and the measured 14 Cactivity(250±1Bqkg − 1 C) is indistinguishable from 14 Cbackgroundandreflects the influence of Atlantic waters. Irish Sea residual waters (with enriched 14 C activity) are carried northwards by currents which closely hug the Scottish coastline (McKay et al., 1986, Hill andSimpson, 1988). This result signifies that Sellafield 14 C has little influence in offshore waters dominated by Atlantic water to the west of the Scottish Coastal Current, at least in the benthic environment that monkfish inhabit, and at the time of sampling.

Seawater 14 C biogeochemical fractions
Surface water sample activities (Table 4) showed an enriched DIC 14 C activity at station FoL during the period when benthic biota were sampled in April 2014 and when plankton were sampled in August 2014. In contrast, PIC and POC activities were depleted at both sampling periods. The reduction in DIC activity between April and August demonstrates that the ambient 14 C activity is changeable over short periods of time despite being distant from the discharge source. This could be due to the variation in discharged activity but local current direction influencing the sources of local water will also have an impact on ambient activities. However, station FoL DIC activities are similar to those observed at stations NC and PP. This suggests that at the time of sampling, there was relatively little dilution of DIC 14 C between these two sites after mixing between Atlantic and Irish Sea water in the North Channel, and that there was little further input of Atlantic water to this water mass. DIC sampled at a coastal site further north in Scotland in 1995  showed a comparable 14 C activity (272 ± 4Bqkg −1 C), despite higher discharges in the year preceding sampling. The observed activity could arise from further dilution of residual Irish Sea water with Atlantic water. Although the small dataset from both these studies makes it difficult to draw any firm conclusions on hydrodynamics, it does highlight a general reduction in 14 C activity with distance from Sellafield and the possibility of using Sellafield-derived 14 C as a tracer for water masses in the north of Scotland.
In the case of the PIC, carbon derived from terrestrial sources could affect the activity although the δ 13 C indicates that the material is predominantly of marine origin. This would imply a dilution by PIC of pre-Sellafield origin as described for the intertidal environment in Tierney et al. (2016). In the case of the POC, the April 2014 sample is much more depleted than the August 2014 sample. The April sample has a δ 13 C value that is 3.1‰ lower than the August sample and would therefore imply a greater effect from carbon derived from the terrestrial environment in early spring. Also, the increase in POC activity between April and August probably results from increased primary production over the spring and summer months, converting more 14 C enriched DIC into phytoplankton with subsequent transfer through the food chain resulting in a higher POC activity.

Sediment organic content
Organic sediment 14 C activities at station FoL are all below the ambient background (Table 5). They are again uniform throughout the top 11 cm (208-209 Bq kg −1 C) with a small decrease in activity at the core base, and very similar to station NC for the top 11 cm (204-210 Bq kg −1 C) (Table 4). Again, sediment homogeneity is consistent with intensive bioturbation as a result of the high biological activity at this site. Surface sediment 14 C activity falls between the depleted activities measured in surface water POC in April and August 2014 indicating that this POC material is deposited in the sediment.

Biota
Phytoplankton at station FoL were slightly depleted below background (241 ± 1 Bq kg −1 C) and zooplankton were significantly depleted (227 ± 1 Bq kg −1 C). All other samples were enhanced and relatively uniform in activity (Fig. 4). The reduced phytoplankton 14 C activity at FoL questions the previously stated convention that phytoplankton activity would be expected to reflect DIC activity as is observed at station NC. This could be explained by the phytoplankton sample containing some POC with a depleted 14 C activity. However, the zooplankton 14 C activity, which is significantly depleted relative to that of phytoplankton, suggests that there may be a more complex explanation as this sample, like its equivalent at station NC, was comprised almost totally of copepods which feed directly on phytoplankton. This aspect of the study will require further sampling and analysis in the future as the value of 227 Bq kg −1 Cisdifficult to reconcile with general ocean ambient background activities, although upwelling of older deep water could provide a source for below background 14 C activities.
As benthic organisms were enriched in 14 C in April 2014, it follows that their food source must have also been enriched. Phytoplankton, at the base of the food chain, and zooplankton (dominated by copepods), as grazers of phytoplankton, were therefore likely to be enriched in 14 C prior to when plankton were sampled and 14 C subsequently transferred along the food chain to other organisms. It is possible that plankton and water samples were taken shortly after a change in water mass. If, after a period of stronger Atlantic influence, the water mass became increasingly dominated by Irish Sea residual water, an increase in DIC 14 C activity from background or potentially from below background (depending on the age and 14 C activity of the Atlantic water) to above background would follow. Although this would result in an increase in plankton activity, this would take time to develop, and therefore result in the 14 C enrichment observed in the DIC fraction and depletion in plankton activities at the time of sampling. The fact that other organisms were all enriched in 14 C indicates that, in the longterm, the Firth of Lorn is dominantly supplied by residual Irish Sea water, resulting in these longer-lived organisms integrating 14 C over relatively long periods.
A wide range of benthic organisms were collected at station FoL and results are presented in Fig. 4.Theseapen(Funiculina quadrangularis)is a colonial planktivorous, passive suspension feeder and its 14 C enrichment (267 ± 2 Bq kg −1 C) confirms that primary producers in this area were enriched in 14 C in the period prior to sampling. The edible sea urchin (Echinus esculentus) is an omnivore and could integrate 14 C from grazing on algae or predating upon other invertebrates. A number of species which are predominantly detrivorous were sampled, including the heart urchin, sea cucumber (Holothuroidea sp.), ribbon worm (Nemertea sp.), polychaete worms, spoon worm (Maxmuelleria lankesteri), brittle star species (Ophiocomina nigra, Ophiothrix fragilis and Ophiura albida), burrowing shrimp species (Jaxea nocturna, Callianassa subterranea), and crab species (Pagurus prideaux, Atelecyclus rotundatus, Inachus sp.). The enriched 14 C activities measured in these organisms proves that 14 C enhanced organic material is supplied to the sediment surface and is being scavenged by the large number of detrivores inhabiting this area. The overall extent of Sellafield 14 Cecosystem uptake and biological transfer is shown by the enriched activities found in predatory organisms from station FoL, including the common whelk (Buccinum undatum), sea mouse, starfish species (Asterias rubens, Crossaster papposus, Luidia sarsii), squat lobster (Munida rugose), Nehrops and megrin (Lepidorhombus whiffiagonis).
Small reductions in 14 C activities of biota are observed at station FoL compared to those of station NC, with an average reduction in the enhancement over ambient background of 13%. This confirms that Irish Sea residual water is the dominant source of water to station FoL and so we might expect to see similar enhancements in biota beyond station FoL until significant dilution of the Scottish Coastal Current occurs. Clyde Sea samples were similar to station NC but fish 14 C activities were generally lower. At each site, the activities of organisms are relatively homogenous, despite the large variation in species. The range in species covers significantly different lifespans, different metabolic rates and different feeding behaviours and suggests that the overall Sellafield effect at these sites, particularly at station FoL, is relatively constant. Variable mixing patterns of seawater in the West of Scotland with residual IrishSeawaters,atanygiventime,maycausesmallchangesintheoverall 14 C DIC activity. However, any short-term variations in ambient DIC 14 C activity are likely to be minor in comparison to sites closer to Table 4 Gross and net specific 14 C activities (Bq kg −1 C±1σ)andδ 13 C(‰ relative to VPDB) values from the four 14 C biogeochemical fractions of station FoL surface water samples. Net activities above the ambient background of 249 ± 1 Bq kg −1 C are in bold. Samples with values lower than this are marked as 'Depleted'. n/a denotes insufficient carbon in sample for analysis. Sellafield, particularly in the north-east Irish Sea which presents greater heterogeneity.

Comparison of far-field (West of Scotland) and near-field (Irish Sea) results
Comparing the results presented here with data from the Irish Sea (Muir et al., also submitted) allows us to better understand the scope of transport of 14 C to the WoS and ecosystem uptake in this region. A general reduction in DIC 14 C activity with increasing distance from Sellafield is observed when comparing measurements from the northeast Irish Sea (546 ± 2 Bq kg −1 C) to WoS results. The relative decrease in western Irish Sea DIC activity (264 ± 1 Bq kg −1 C), compared to WoS sites, supports previous work which showed that N 99% of discharged 14 C leaves the Irish Sea through the North Channel (Gulliver et al., 2001). Organic sediment activities at stations NC and FoL are depleted, whereas enriched activities are observed in the north-east Irish Sea (e.g. 298 ± 1 Bq kg −1 C in surface sediment). This shows the greater flux of enriched material to the sediment in the north-east Irish Sea as would be expected at a site much closer to the 14 Csource.
In comparison with organisms obtained from the north-east Irish Sea (station EB), it is apparent that 14 C activities in West of Scotland organisms are significantly reduced (Fig. 5). Although high 14 C activities have been observed in some western Irish Sea (station WB) organisms, most results at station WB are comparable to, or below the activities observed for the North Channel, Clyde Sea and Firth of Lorn stations. The  high activity of whiting collected at station NC is clearly identified as an outlier, however, the median activity in benthic biota from station WB is actually less than the equivalent at station NC. This indicates that the northern extremity of the North Channel is receiving similar or higher 14 C-enriched DIC inputs than the western Irish Sea, as confirmed by the DIC data. Results from the Irish Sea show a much wider range of activities to that of the West of Scotland. It is likely this is due to monthly changes in 14 C discharges from Sellafield having a greater impact on the Irish Sea ambient 14 C activity, ultimately increasing the variability. Transfer and mixing processes within the Irish Sea result in a more homogenous activity being transferred through the North Channel and northwards along the West of Scotland where mixing with Atlantic water can reduce the ambient activity with distance from Sellafield , Gulliver et al., 2001.
Relatively low 14 C activities measured in plankton groups both in the Irish Sea and West of Scotland are identified as being statistical outliers and are depicted in Figure 5 as black dots below the interquartile range. In the Irish Sea this most likely occurs as a consequence of the very low Sellafield 14 C discharge during the sampling period (Muir et al., also submitted). In recent years it appears that highest monthly discharges of 14 C coincide with autumn-winter months (Muir et al., also submitted). Discharges coinciding with plankton blooms during the spring and summer could result in higher organic activities and increased 14 C transfer through the food chain (Cook et al., 1995). It is not clear if the recent discharge policy has followed this protocol or is coincidental, as previously there were no trends in discharge activity. Discharging more 14 C in periods of low primary production will probably result in a net reduction in the overall ecosystem 14 C uptake within the northeast Irish Sea. Due to intensive mixing within the Irish Sea, and the time taken for 14 C to be transported northwards, it is unlikely this would have a similar impact beyond the North Channel. However, it is conceivable that due to reduced uptake within the Irish Sea, higher activity water will be transported north, potentially resulting in increased activities in West of Scotland biota, although this effect remains unconfirmed.
Radiation dose rates have been calculated for the Sellafield critical consumer group for 14 C activities measured in the north-east Irish Sea (Muir et al., also submitted). These dose rates are negligible for 14 Cactivities which are significantly higher in comparison to the 14 C activities observed in commercially important species from the WoS sites. Assuming a WoS critical consumer group has the same consumption rates as the Sellafield critical consumer group (Garrod et al., 2015)and by using the highest activities measured in the WoS for fish (whiting 413±2Bqkg −1 C) and Nephrops (315 ± 2 Bq kg −1 C), the maximum dose received would be 0.59 μSv. This dose is 71% less than the maximum dose measured for the Sellafield critical consumer group (2.05 μSv) from 14 C discharges (Muir et al., also submitted) and does not pose any radiological risk to the public.

Conclusions
Sellafield-derived 14 C is transported to the north-west of Scotland in the form of DIC. 14 C is highly bioavailable as demonstrated from the widespread 14 C enrichments observed in marine organisms. Although sediment activities are depleted in 14 C relative to ambient background, there is a clear pathway of uptake of 14 C by phytoplankton during photosynthesis, followed by transfer to planktivorous organisms and deposition of enriched particulate material. This organic material is rapidly consumed by detritus feeders and subsequently, 14 C is transferred through the entire benthic food web. It is apparent that sedimentation processes must be examined in more detail to determine the fate of 14 C at the sediment-water interface. Any future work should focus on the different organic carbon fractions within the sediment to better understand both the pathways for 14 C re-entry into the marine food and deposition of 14 C.
The extent of 14 C transport and ecosystem uptake is revealed by enriched activities at the Firth of Lorn (approximately 260 km from Sellafield). Although any increase in Atlantic water influence in the Firth of Lorn will reduce ambient 14 C activity, as shown by plankton activities, this area is dominated by inputs from a residual water component from the Irish Sea. However, the overall effect of dilution with Atlantic water is clear and 14 C activities reduce with distance from Sellafield, though a similar order of magnitude of 14 C enrichments in biota can be expected until there is significant dilution of the Scottish Coastal Current. The small dataset of surface water 14 C activities presented in this study shows the potential use of 14 C as a tracer for Irish Sea water and mixing processes in the UK marine environment.
Many of the organisms measured are commercially important species and the findings in this study suggest that 14 C enrichment is likely to be found in other unmeasured species from the same areas. It must be re-stated that the potential 14 C dose received from consumption of seafood in the WoS is negligible, and does not pose any radiological risk to consumers or local populations in the west of Scotland. However, due to its long half-life, high bioavailability and continued release, continued assessment of the fate of 14 C in the environment is important. To this purpose, ongoing work is utilising the data collected across this study to develop a predictive ecosystem model tracing the biological fate of 14 C released into the marine environment which, unlike other discharged radionuclides, cannot be described using a distribution coefficient.