Scottish Sea Farms Ssf) Scapa Flow Ece Estimates 2017

SSF SCAPA FLOW ECE ESTIMATES 2017

SCOTTISH SEA FARMS SSF)

SCAPA FLOW ECE ESTIMATES 2017

Report No. Scapa Flow 013.docx

Anton Edwards

April 2017

For: Anton Edwards CPhys, FInstP Scottish Sea Farms Ltd Duguid's Wark Laurel House Manse Road Laurel Hill Business Park Caputh Stirling Perthshire FK7 9JQ PH1 4JH 01738 710774

SCAPA FLOW 013.DOCX SSF SCAPA FLOW ECE ESTIMATES 2017

SUMMARY

This report outlines the hydrography of the area of Scapa Flow in Orkney, so as to estimate the effect of existing and potential fish farms on local nutrient concentrations via the 'Equilibrium Concentration Enhancement' ( ECE) approach.

Tidal and residual flows to Scapa Flow have been estimated by various methods (tide tables, direct observation and water budgets) to be sufficient to dilute the nitrogen released from existing consented sites and potential sites such that the likely ECE of nitrogen concentration is less than 10 µgN.litre-1. This estimate is based on a conservative assumption of 90% recirculation. The recirculation is expected to be much less than 90%, increasing the flushing of Scapa Flow and reducing the predicted overall ECE further to within the lower part of the range 1 to 10 µgN.litre-1. Local increases of ECE in the east of Scapa Flow may reach similar levels.

Sites in the east of Scapa Flow such as Hunda, Roo Point and Westerbister are removed from the main flows but are well connected to them by an inferred eddy that circulates the water of Scapa Flow in a matter of days. This eddy exchanges with local coastal bays on a time scale of about a day so that the bays are very unlikely to develop local water characteristics different from the main body of Scapa Flow.

An ECE of up to 10 µgN.litre-1 may be compared favourably with various relevant or regulatory standards:

typical background concentrations

a previous Environmental Quality Standard for available nitrogen of 168 gN.litre-1

OSPAR & Water Framework Directive Reference Conditions: in offshore waters such as these (salinity above 34), the DIN (Dissolved Inorganic Nitrogen) reference value is 10M and the threshold 15M. Increases are therefore limited to 5 M (70 gN.l-1).

In the cases and circumstances examined here, predicted ECE increases are therefore insignificant on the scale of the Scapa Flow water body.

Displacement modelling shows that water in the neighbourhood of Hunda North that complies with Environmental Quality Standards is unlikely to reach local embayments in any significant quantity, implying that these embayments are at very low risk of EQS non-compliance. Comparison of displacements of water over regulatory time scales shows that waters from sites at Hunda North and Glimps Holm are very unlikely to meet with any local cumulative effect of raising local medicine concentrations to anything above EQS.

Comparison of modelled depositional footprints at Hunda North and Glimps Holm shows that in this respect the interaction between the two sites is negligible.

SCAPA FLOW 013.DOCX SSF SCAPA FLOW ECE ESTIMATES 2017

CONTENTS

1 INTRODUCTION ...... 4

1.1 SCAPA FLOW REGULATORY ISSUES ...... 4

1.2 THE ECE APPROACH...... 4 2 PHYSICAL BACKGROUND ...... 5

2.1 NON- TIDAL CIRCULATION AND THE FAIR ISLE CURRENT ...... 5 2.2 GEOGRAPHY ...... 6 2.3 HYDROGRAPHY ...... 7 2.3.1 TIDAL RANGE ...... 7 2.3.2 THE TIDAL STREAM ATLAS ...... 8 2.3.3 CURRENTS NEAR GRAEMSAY ...... 9 2.3.4 VOLUME FLOWS ...... 9 2.3.5 A TIDAL FLOW BUDGET ...... 10 2.3.6 CURRENTS WITHIN SCAPA FLOW ...... 10 3 FARM SITES ...... 12 3.1 SITE DETAILS ...... 12

3.2 SITE CURRENTS ...... 13 3.2.1 BRING HEAD CURRENTS ...... 13 3.2.2 TOYNESS CURRENT DATA ...... 13 3.2.3 WESTERBISTER CURRENT DATA ...... 14 3.2.4 HUNDA NORTH ...... 14 3.2.5 ROO POINT ...... 15 3.2.6 ST MARGARET’ S HOPE ...... 16 3.3 SCAPA FLOW CIRCULATION ...... 16 3.3.1 THE INTERTEK MODEL ...... 16 3.3.2 SITE CURRENT SUMMARY ...... 17 3.3.3 FLUSHING TIME ...... 18 3.3.4 CONNECTION OF EAST SCAPA FLOW ...... 18 3.3.5 COASTAL EFFECTS IN THE EAST OF SCAPA FLOW ...... 19 4 PREDICTION OF NUTRIENT INCREASE ...... 21

4.1 NUTRIENT INPUTS ...... 21

4.1.1 SSF SITE OPTIONS ...... 21 4.2 CONCENTRATION ENHANCEMENT ...... 21

4.2.1 TIDAL FLOWS AND RECIRCULATION ...... 21 4.2.2 RESIDUAL FLOWS ...... 21 4.3 ECE ESTIMATES ...... 21

4.3.1 ECE ESTIMATION IN SCAPA FLOW ...... 21 4.3.2 WORST CASE ECE AT MAXIMUM BIOMASS ...... 22 4.3.3 LOCAL EFFECTS ...... 23 4.4 SUMMARY ...... 23 5 EQS COMPLIANCE NEAR HUNDA NORTH ...... 24 5.1 HUNDA NORTH ...... 24 5.2 GLIMPS HOLM ...... 25 6 DEPOSITIONAL FOOTPRINTS NEAR HUNDA NORTH ...... 28

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7 CONCLUSIONS ...... 29 7.1 ECE ...... 29 7.2 EQS ...... 29 7.3 DEPOSITIONAL FOOTPRINTS ...... 29 8 REFERENCES ...... 30

TABLES

TABLE 1: TIDES AT WICK ...... 7

TABLE 2: TIDAL FLOWS IN HOY AND HOXA SOUNDS ...... 8

TABLE 3: TYPICAL TIDAL FLOW ESTIMATES IN HOXA AND HOY SOUNDS...... 10

TABLE 4: EXISTING AND POTENTIAL SITES ( BOLD) IN SCAPA FLOW ...... 12

TABLE 5: BRING HEAD, SUMMARY HYDROGRAPHIC STATISTICS ...... 13

TABLE 6: TOYNESS, SUMMARY HYDROGRAPHIC STATISTICS ...... 13

TABLE 7: WESTERBISTER, SUMMARY HYDROGRAPHIC STATISTICS ...... 14

TABLE 8: HUNDA NORTH, SUMMARY HYDROGRAPHIC STATISTICS ...... 15

TABLE 9: ROO POINT, SUMMARY HYDROGRAPHIC STATISTICS ...... 15

TABLE 10: ST MARGARET’ S HOPE, SUMMARY HYDROGRAPHIC STATISTICS ...... 16

TABLE 11: ECE IN NITROGEN – TIDAL AND RESIDUAL FLOWS; BIOMASS 14600 TONNES ...... 22

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FIGURES

FIGURE 1: KIRK HOPE CATEGORISED AREA ( FROM HTTP:// WWW. GOV. SCOT/ RESOURCE/0050/00507228.PDF) ...... 4

FIGURE 2: SCHEMATIC DIAGRAM OF GENERAL CIRCULATION IN THE NORTH SEA (AFTER TURRELL ET AL. 1992) ...... 5

FIGURE 3: MODELLED CURRENTS UNDER SOUTH- WESTERLY AND SOUTH- EASTERLY WIND; SCALE AS SHOWN ...... 6

FIGURE 4: SCAPA FLOW BATHYMETRY BEFORE CAUSEWAY CONSTRUCTION (DEPTHS IN FATHOMS) ...... 6

FIGURE 5: SCAPA FLOW – ROADS AND CAUSEWAYS ( RED) ...... 7

FIGURE 6: TIDAL STREAMS 5 HOURS BEFORE HW DOVER; NEAP & SPRING ( SPEEDS IN TENTHS OF A KNOT) ...... 8

FIGURE 7: CURRENTS IN HOY SOUND (MARINE SCOTLAND 2010; THE “ LOCATIONAL GUIDANCE” REFERRED TO IN THIS FIGURE IS THAT FOR MARINE ENERGY) ...... 9

FIGURE 8: CARTOON OF EBB TIDE FROM SCAPA FLOW (CURRENTS REVERSE ON THE FLOOD TIDE) 9

FIGURE 9: BODC RECORDED CURRENT MEASUREMENTS 1967-2016: NONE IN SCAPA FLOW . 10

FIGURE 10: EXISTING & POTENTIAL SITES IN SCAPA FLOW ...... 12

FIGURE 11: INTERTEK-METOC: CONNECTIVITY OF DISCHARGE FROM SITES STS3 & ST4 (NO WIND) ...... 16

FIGURE 12: SUMMARY OF RESIDUAL ( MAGENTA) AND TIDAL ( RED) CURRENTS ( NOTE DIFFERENT SCALES) ...... 17

FIGURE 13: SCHEMATIC OF THE SCALE OF POSSIBLE CIRCULATION...... 18

FIGURE 14: SMALL-SCALE FEATURES OF EAST SCAPA FLOW, RESIDUAL CURRENTS, AND TIDAL EXCURSIONS...... 19

FIGURE 15: BAY FLUSHING. BAY DIAMETER D, TIDAL EXCURSION E ...... 19

FIGURE 16: TYPICAL TIDAL EXCURSIONS IN THE SOUNDS OF HOXA AND HOY ...... 22

FIGURE 17: DISPLACEMENTS IN 3 HOURS ( MAGENTA) OR 6 HOURS ( GREEN) FROM HUNDA NORTH 24

FIGURE 18: TIDAL ELEVATION AND CURRENTS AT GLIMPS HOLM, 6 M FROM SURFACE ...... 25

FIGURE 19: GLIMPS HOLM: DIGITIZED ELEVATION AND N-S CURRENT, 6 M FROM SURFACE ...... 26

FIGURE 20: GLIMPS HOLM DISPLACEMENTS AT 3 (GREEN) & 6 (BLUE) HOURS, 6 M BELOW SURFACE ...... 26

FIGURE 21: TYPICAL MODELLED ( DEPOMOD SUITE) DEPOSITIONAL FOOTPRINTS AT HUNDA NORTH BOTTOM LEFT) AND GLIMPS HOLM (TOP RIGHT) ...... 28

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1 INTRODUCTION

1.1 SCAPA FLOW REGULATORY ISSUES

Scapa Flow is not a categorised area within the Locational Guidelines for the Authorisation of Marine Fish Farms in Scottish Waters ( see http://www.gov.scot/Resource/ 0050/00507228. pdf). Figure 1 shows only one such area nearby at Kirk Hope. An assessment of nutrient enhancement for planning purposes or Environmental Impact Assessment and an assessment of the cumulative effect of nutrients released from all sites within Scapa Flow seems appropriate.

Figure 1: Kirk Hope categorised area ( from http://www.gov.scot/Resource/ 0050/ 00507228. pdf)

This report estimates the increase in nitrogen concentration in the water body comprising all existing and potential sites within the general body of Scapa Flow.

1.2 THE ECE APPROACH

The ECE equation was developed by SEERAD Marine Laboratory for the Locational Guidelines. The ECE model is a simply derived mass balance and dilution relation (see http://www.gov.scot/Topics/marine/Fish-Shellfish/ 18716/environmentalimpact/ models) that considers dissolved nitrogen, particulate nitrogen and nitrogen that may have re- dissolved from the seabed, all in relation to water flows through an area. The model takes no direct account of biological or chemical processes. The equation estimates the enhancement of nitrogen from aquaculture above background levels, on the assumption that released nitrogen is conserved and only removed by water flows:

ECE = S M /Q Where: S = Source Rate ( kgN.tonne production-1 year-1) M = Total Consented Biomass ( tonne) Q = Volume Flow Rate ( m3 year-1)

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2 PHYSICAL BACKGROUND

2.1 NON- TIDAL CIRCULATION AND THE FAIR ISLE CURRENT

The main feature of the non-tidal circulation in the area is the Fair Isle current and the general residual flow from the west or north-west into the North Sea (Figure 2).

Figure 2: Schematic diagram of general circulation in the North Sea (after Turrell et al. 1992)

Dooley and MacKay (1975) noted residual speeds of about 0.1 m.s-1 west of Orkney. Dooley (1974) estimated a flow of about 104 km3.year-1 in the Fair Isle current; over the gap between Shetland and north Scotland, this flow translates into a residual current past and through Orkney of a few cm.s-1 to the south-south-east.

Large scale modelling of the shelf seas (Leterme et al. 2008, see Figure 3) reveals the same picture; Figure 3 shows the generally south-eastward, if wind-variable, nature of the flows.

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Figure 3: Modelled currents under south-westerly and south-easterly wind; scale as shown

This general picture of residual flow is the context within which the following site- specific measurements from the fish farms should be seen.

2.2 GEOGRAPHY

Figure 4 shows an early bathymetry before the construction of causeways linking some islands blocked shallow eastern entrance channels to Scapa Flow (Figure 5).

Figure 4: Scapa Flow bathymetry before causeway construction ( depths in fathoms)

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Figure 5: Scapa Flow – roads and causeways ( red)

2.3 HYDROGRAPHY

2.3.1 Tidal Range

The tides at Wick, similar in range to the Scapa Flow area, are summarised in Table 1 National Tides & Sea Level Facility, http://www.ntslf.org/tides/hilo).

Table 1: Tides at Wick

Tide Wick Highest Astronomic Tide 3.97m Lowest Astronomic Tide 0.06m Mean High Water Spring 3.51m Mean Low Water Spring 0.63m Mean High Water Neap 2.78m Mean Low Water Neap 1.43m

Typical tidal ranges in this area thus reach about 3 to 4 metres, with an average of about 2 metres.

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2.3.2 The Tidal Stream Atlas

Currents associated with the rise and fall of the tides are described very roughly in the Admiralty Tidal Stream Atlas (2009). The atlas shows tidal streams every hour. Figure 6 shows examples of the hourly maps in the atlas for this area.

Figure 6: Tidal streams 5 hours before HW Dover; neap & spring (speeds in tenths of a knot)

From the set of such maps it is possible to derive tables of estimated representative flows in Hoy and Hoxa Sounds (Table 2). The representation of flows on the maps is both variable and qualitative, so such estimates are necessarily very rough.

Table 2: Tidal flows in Hoy and Hoxa Sounds

Time from HW Hoy Sound Hoxa Sound Dover Neap and spring ( undifferentiated) Neap and spring ( undifferentiated) Hours Speed m.s-1 Direction Speed m.s-1 Direction 6 ~ 1 South- eastward ~ 0.5 Northward & eddying anticlockwise 5 ~ 1 South- eastward ~ 0.5 Northward & eddying anticlockwise 4 ~ 1 South- eastward ~ 0.5 Northward & eddying anticlockwise 3 ~ 1 South- eastward ~ 0.5 Northward 2 ~ 0.8 South- eastward ~ 0.3 Northward 1 0.6 South- eastward Weak Variable 0 0.6 North-westward Weak Variable 1 Weak North- westward Weak Southward 2 ~ 1 North- westward ~ 0.3 Southward 3 ~ 1.2 North- westward ~ 0.3 Southward 4 ~ 1 North-westward ~ 0.3 Northward 5 ~ 0.8 North-westward ~ 0.3 Northward 6 ~ 0.5 South- eastward ~ 0.5 Northward eddying Typical ( average) ~ 0.8 Inflow/ outflow ~ 0.3 Inflow/ outflow

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2.3.3 Currents near Graemsay

According to http://www.scotland.gov.uk/Resource/Doc/295194/0096885.pdf, spring currents in Hoy Sound near Graemsay (Figure 7) can reach 4 m.s-1 in the most restricted parts of the channel. In Hoxa Sound, spring currents are weaker at about 1 m.s-1.

Figure 7: Currents in Hoy Sound ( Marine Scotland 2010; the “ Locational Guidance” referred to in this figure is that for marine energy)

2.3.4 Volume Flows

Tidal flows in Hoy and Hoxa Sounds oppose each other, so that Scapa Flow fills or drains through the two sounds at about the same time, as in the sketch of Figure 8.

Figure 8: Cartoon of ebb tide from Scapa Flow (currents reverse on the flood tide)

These volume flows may be estimated according to the cross-sectional areas of the sounds: West of Graemsay in the area of the strong currents of Figure 7 the cross-

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sectional area of Hoy Sound is about 2.104 m2 (average depth 10 m; width 2 km); east of Flotta, the cross sectional area of Hoxa Sound is about 6.104 m2 ( average depth 30 m; width 2 km). From Table 2, typical speeds are: Hoy – 0.8 m.s-1; Hoxa – 0.3 m.s-1. Typical flows in these sounds may therefore be estimated as in Table 3.

Table 3: Typical tidal flow estimates in Hoxa and Hoy Sounds

Hoy Hoxa Cross Section, m2 2.104 6.104 Typical speed, m.s-1 0.8 0.3 Typical Flow, m3.s-1 1.6 104 1.8 104 Typical Total Flow, m3.s-1 3.4 104

From this analysis, it appears that the volume flows of Figure 8 are of similar size in each sound. Such estimates are averages over all tidal conditions; neaps will be a little lower; springs a little higher.

2.3.5 A Tidal Flow Budget

In view of the approximate nature of these estimates it is reassuring to estimate them independently.

Rates of fall of sea level in a typical tide of range 2 metres (section 2.3.1) are about 10-4 m.s1. The area of Scapa Flow including the sounds is about 250 km2. A typical tidal outflow or inflow rate is therefore about 250 km2.10-4 m.s1, or 2.5 104 m3.s-1.

This compares well with the combined flows of Table 3, reinforcing a view that the main flows from and to Scapa Flow may be explained mainly by tidal draining and filling through Hoxa and Hoy Sounds, with perhaps a relatively small amount of residual through-flow in the system. (section 3.2.1).

2.3.6 Currents within Scapa Flow

No measurements have been found ( Figure 9) in the current BODC inventory https://www.bodc.ac.uk/data/information_and_inventories/current_meters/search).

Figure 9: BODC recorded current measurements 1967-2016: none in Scapa Flow

Other web resources are dominated by merely qualitative diving surveys. However, it is possible to estimate the size of currents by applying the measured flows to cross sections within Scapa Flow.

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As examples:

A flow of about 2.104 m3.s-1 through the sectional area (40 m x 3 km) near Bring Head may be expected to produce currents typically about 0.16 m.s-1, as are reported by SSF (Bring Head surfis HGdata_analysis_v7.xls, 2015, summarised in section 3.2.1). Once such flows enter Scapa Flow proper, north of Cava, the wider sectional area there means that flows will be generally much less, as is seen at Toyness (section 3.2.2). In the south, a flow of about 2.104 m3.s-1 through the sectional area west of Hunda Figure 10) may be expected to produce currents similar to those of Hoxa Sound - typically about 0.3 m.s-1. It is not realistic to apply this ‘cross-section & flow’ argument to the more intricate areas around Hunda, where currents may be expected to be less (Sections 3.2.3 and 3.2.4). Nor does the argument apply to the Roo Point site (section 3.2.5). These sites, well within Scapa Flow, show lower flows and are also be influenced by local wind-driven flows that, being in the short term a few per cent of wind speed, may exceed the tidal flows Edwards, 2015).

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3 FARM SITES

3.1 SITE DETAILS

Figure 10 shows the locations of the farm sites in the area of the applications.

Figure 10: Existing & potential sites in Scapa Flow

The details of the existing, proposed (St Margaret’s Hope & Hunda North) and potential future site operations are shown in Table 4. At SSF sites marked , some future development may occur subject to appropriate permissions; these increased biomasses are presented here for illustrative purposes only.

Table 4: Existing and potential sites (bold) in Scapa Flow

Site name Lat Long or OS Grid CAR Existing Potential Licence biomass biomass Bring Head 58 54.04N 3 15.85E L/1015854 968 2000 Toyness 58 54.99N 3 07.41E L/1015855 1342.9 2000 Westerbister 58 54.38N 2 57.11E L/1143253 1791.2 1791.2 St Margaret's 58 50.96N 2 59.50E 0 1300 Hope Hunda North 58 51.76N 2 58.05E 0 1700 Ore Bay ND 3120 9440 L/1003962 450 450 West Fara ND 3210 9530 L/1004229 800 800 Chalmers Hope HY 2880 0070 L/1003062 1000 1000 South Cava ND 3227 9880 L/1082725 1511 1511 Lyrawa Bay ND 2980 9900 L/1003960 400 400 Pegal Bay ND 3040 9780 L/1003961 400 400 Glimps Holm ND 4650 9910 L/1122569 1247 1247 Totals 9910.1 14599.2

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3.2 SITE CURRENTS

Currents at all sites have been measured to support their consent applications. Some features of the measurements from SSF sites only are repeated here, together with information about Westerbister taken from Edwards (2015).

3.2.1 Bring Head Currents

Currents at Bring Head are summarized in Table 5. The last row ( blue) is the Pythagorean addition of the two tidal amplitudes.

Table 5: Bring Head, summary hydrographic statistics

Speeds, m.s-1 Surface Net depth Seabed Mean speed 0.168 0.166 0.14 Residual current speed 0.03 0.03 0.027 Residual current direction (°) 304 293 292 Major axis of tidal ellipse (°) 315 ~ 315 320 Residual Flow (Parallel) 0.032 0.013 0.024 Residual Flow (Normal) - 0.007 - 0.031 - 0.012 Tidal Amplitude (Parallel) 0.282 0.204 0.222 Tidal Amplitude ( Normal) 0.043 0.191 0.052 Tidal Amplitude 0.29 0.28 0.23

Currents are similar at all depths, with only a slight decrease near the bottom, perhaps through frictional effects. This suggests that stratification is weak or non-existent and that vertical mixing will be correspondingly uninhibited.

The residual current during the 15 days of measurement was about 0.03 m.s-1 to 300º. Were such a residual to apply to the whole cross section (section 2.3.6) of the sound, this would represent an outflow of about 0.03 x 1.2 x 105 m3.s-1, or 3600 m3.s-1, or about one fifth of the tidal flows estimated in Table 3.

The tidal amplitude is about 0.3 m.s-1, aligned at 315º N, corresponding to a tidal excursion in one cycle of a semi-diurnal tide of about 4 km.

Figure 12 summarises these flows.

3.2.2 Toyness current data

Currents at Toyness (from ToyNess_Surface/Midwater/Bottom-V2_HGdata_v7.xls) are summarised in Table 6. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.

Table 6: Toyness, summary hydrographic statistics

Toyness, Speeds, m.s-1 Surface Net depth Seabed Mean speed 0.076 0.073 0.07 Residual current speed 0.02 0.023 0.027 Residual current direction (°) 193 232 241 Major axis of tidal ellipse (°) 225 225 220

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Toyness, Speeds, m.s-1 Surface Net depth Seabed Residual Flow (Parallel) 0.017 0.023 0.026 Residual Flow (Normal) - 0.011 0.003 0.010 Tidal Amplitude (Parallel) 0.113 0.106 0.093 Tidal Amplitude ( Normal) 0.046 0.04 0.047 Tidal Amplitude 0.12 0.12 0.10

The tidal amplitude at Toyness is about 0.12 m.s-1, aligned at 220º N, corresponding to a tidal excursion in one cycle of a semi-diurnal tide of about 1.5 km. The residual is about 0.02 m.s-1 to the south-west.

Figure 12 summarises these flows.

3.2.3 Westerbister current data

Currents at Westerbister were measured by Xodus (2011a) and are summarised here in Table 7. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.

Table 7: Westerbister, summary hydrographic statistics

Westerbister, Speeds, m.s-1 Surface (8 m) Net depth (10 m) Seabed ( 24 m) Mean speed 0.043 0.035 0.036 Residual current speed 0.024 0.020 0.005 Residual current direction (°) 7 3 121 Major axis of tidal ellipse (°) 10 10 185 Residual Flow (Parallel) 0.024 0.020 0.002 Residual Flow (Normal) - 0.001 - 0.002 - 0.004 Tidal Amplitude ( Parallel) 0.055 0.041 0.048 Tidal Amplitude ( Normal) 0.027 0.026 0.032 Tidal Amplitude 0.061 0.049 0.058

Currents are similar at all depths. This suggests that stratification is weak or non-existent at Westerbister and that vertical mixing is correspondingly uninhibited.

The tidal amplitude at Westerbister is about 0.06 m.s-1, aligned roughly north-south, corresponding to a tidal excursion in one cycle of a semi-diurnal tide of about 1 km. The relatively small residual is about 0.02 m.s-1 to the north and almost non-existent east- west.

Figure 12 summarises these flows.

3.2.4 Hunda North

Currents at Hunda North were measured by Xodus ( 2011b) and are summarised in Table 8. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.

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Table 8: Hunda North, summary hydrographic statistics

Hunda North, Speeds, m.s-1 Surface Net depth Seabed Mean speed 0.044 0.040 0.037 Residual current speed 0.016 0.006 0.005 Residual current direction (°) 88 100 232 Major axis of tidal ellipse (°) 80 80 255 Residual Flow (Parallel) 0.016 0.005 0.004 Residual Flow (Normal) 0.002 0.002 - 0.002 Tidal Amplitude ( Parallel) 0.070 0.067 0.058 Tidal Amplitude ( Normal) 0.023 0.020 0.019 Tidal Amplitude 0.074 0.069 0.06

The tidal amplitude at Hunda North is about 0.0.07 m.s-1, aligned at 80º/ 240º N, corresponding to a tidal excursion in one cycle of a semi-diurnal tide of about 1 km. The residual well above the sea bed is about 0.01 m.s-1 to the east.

Figure 12 summarises these flows.

3.2.5 Roo Point

Currents at Roo Point were measured by Xodus (2011c) and are summarised in Table 6. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.

Table 9: Roo Point, summary hydrographic statistics

Roo Point, Speeds, m.s-1 Surface Net depth Seabed Mean speed 0.052 0.047 0.048 Residual current speed 0.017 0.020 0.031 Residual current direction (°) 211 229 260 Major axis of tidal ellipse (°) 265 260 245 Residual Flow (Parallel) 0.010 0.017 0.029 Residual Flow (Normal) - 0.014 - 0.010 0.008 Tidal Amplitude ( Parallel) 0.075 0.070 0.054 Tidal Amplitude ( Normal) 0.032 0.027 0.032 Tidal Amplitude 0.082 0.075 0.063

The tidal amplitude at Roo Point is about 0.07 m.s-1, aligned at 250º N, corresponding to a tidal excursion in one cycle of a semi-diurnal tide of about 1 km. The residual is about 0.02 m.s-1 to the south-west.

Figure 12 summarises these flows.

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3.2.6 St Margaret’s Hope

Currents at St Margaret’s Hope were measured by Xodus (2011d) and are summarised in Table 10. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.

Table 10: St Margaret’ s Hope, summary hydrographic statistics

St Margaret’ s Hope, Speeds, m.s-1 Surface Net depth Seabed Mean speed 0.035 0.036 0.036 Residual current speed 0.011 0.017 0.019 Residual current direction (°) 77 73 354 Major axis of tidal ellipse (°) 65 70 75 Residual Flow (Parallel) 0.011 0.017 0.003 Residual Flow (Normal) 0.002 0.001 - 0.018 Tidal Amplitude ( Parallel) 0.051 0.049 0.044 Tidal Amplitude ( Normal) 0.021 0.022 0.027 Tidal Amplitude 0.055 0.054 0.052

Currents are very similar at all depths, with only a slight decrease near the bottom, perhaps through frictional effects. This suggests that stratification is weak or non-existent and that vertical mixing will be correspondingly uninhibited.

The tidal amplitude at St Margaret’s Hope is about 0.05 m.s-1, aligned at 70º N, corresponding to a tidal excursion in one cycle of a semi-diurnal tide of about 1 km. The residual is about 0.015 m.s-1 to the south-west.

Figure 12 summarises these flows.

3.3 SCAPA FLOW CIRCULATION

3.3.1 The Intertek Model

Intertek Metoc (2012) modelled Scapa Flow flows to assess ballast water discharge.

Figure 11: Intertek-Metoc: Connectivity of discharge from sites STS3 & ST4 (no wind)

Although these Intertek models do not relate directly to current speed and direction, they contain valuable indications of an anticlockwise circulation in Scapa Flow. The

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distributions of Hydrodynamic Connectivity Index reveal an anti-clockwise connectivity for discharges at the sites shown in Figure 11, one of which is in the east of Scapa Flow close to Westerbister. These results were obtained under conditions of no wind and it may therefore be expected that this anti-clockwise connectivity would be stronger in normal conditions (Edwards, 2015).

3.3.2 Site Current Summary

The main characteristics of the farm site currents from section 3.2 are summarised in Figure 12.

Figure 12: Summary of residual ( magenta) and tidal (red) currents ( note different scales)

In these cases, the tidal flows are roughly parallel to the shore, as expected from continuity considerations ( no flow through the shoreline).

The farm site measurements of residuals, made at different times, nevertheless suggest an anticlockwise circulation. Xodus measurements at Westerbister indicate persistent northward flow (even when calm). The Intertek Metoc modelling in calm conditions similarly suggests anticlockwise circulation. A large-scale circulation may therefore be inferred from all measurements. The imagined circulation is shown in Figure 13.

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Figure 13: Schematic of the scale of possible circulation

The residuals are an order of magnitude smaller than the tidal currents and form an anticlockwise pattern within the main water body (apart from the apparent outflow at Bring head, which is small in comparison with tidal flows (section 3.2.1).

With measured residuals around 0.01 m.s-1 to 0.02 m.s-1, the daily displacement is about 1 to 2 km. This implies a circulation time around the typical dimensions of Scapa Flow of about 10 days. The volume flow around the eddy associated with these residuals may be estimated from typical dimensions (well-mixed depth ~ 20 m; radius ~ 5 km; flow 0.02 m.s-1) as about 2.103 m3.s-1.

3.3.3 Flushing time

Outflows from Scapa Flow estimated in sections 2.3.4 and 2.3.5 are about 25.103 m3.s-1. Scapa Flow including the sounds has a volume of about 5.109 m3 ( note that Wikipedia quotes an erroneous volume of 109 m3). The time scale of flushing is thus 5.109/25.103 seconds), about 2.105 seconds, or 2 to 3 days. This is a low estimate, based on complete flushing. In practice, recirculation of ejected water on each tide will increase the flushing time. For example, 50% recirculation would increase the flushing time to about 5 days. It is expected to be slightly more at neap tides and slightly less at springs.

3.3.4 Connection of east Scapa Flow

The likely circulation time (~10 days) derived from measurement and modelling is therefore comparable with the likely flushing time derived from volume flow considerations (~ 5 days). This comparison suggests that the eastern part of Scapa Flow is reasonably well connected and mixed to the overall circulation and the flows through the Sounds of Hoy and Hoxa.

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3.3.5 Coastal effects in the east of Scapa Flow

Within the eastern coastline of Scapa Flow there are small scale features and embayments such as are shown in Figure 14. These include Echnaloch Bay (EB) and St Mary's Bay ( SMB). It is relevant to estimate how well these bays are connected to the broader flows in Scapa Flow.

Figure 14: Small-scale features of east Scapa Flow, residual currents, and tidal excursions.

Eddy Exchange

The scale of these bays is around one kilometre. Tidal excursions in the east of Scapa Flow are generally parallel to the shore of about one kilometre (per half day, of the semi- diurnal tide). Such bays often contain eddying driven by the cross-mouth flows (of the tidal excursion in this case).

Time scales of exchange may be estimated by reference to the schematic of Figure 15.

Figure 15: Bay flushing. Bay diameter D, Tidal excursion E

In a bay of diameter D the typical eddy circumference is about π.D/2. The excursion across the mouth is E per half day (semi-diurnal tide of 12.5 hours).

The corresponding time scale for exchanges of external flow with the bay is of order

0.5.D/E half days, or 0.25.D/E days.

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In these bays, D ~ 1 km, E ~ 1 km and the exchange time scale is thus about a day.

Residual Exchange

Residuals in the region are around 0.01 m.s-1. The likely time scale of residual flow through such bays is therefore about D/(0.01 m.s-1). With D ~ 1 km, the time scale of residual exchange is about 105 seconds, about a day.

Exchange Summary

Tidal and residual exchanges are both likely to occur on time scales of a day, ensuring that these bays will be well connected to the general circulation of east Scapa Flow and will not develop significantly different local water characteristics in respect of dissolved constituents such as nitrogen or other soluble material.

Currents at all these sites are also driven by and influenced by wind, whose effects have scarcely been examined here but which will enhance exchange and mixing.

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4 PREDICTION OF NUTRIENT INCREASE

4.1 NUTRIENT INPUTS

4.1.1 SSF site options

Potential and existing SSF sites in Scapa Flow are listed in Table 4, together with their potential biomass.

The nitrogen input from this biomass may be estimated from the source rate (section 1.2). With an assumed source rate of 60 kgN.tonne-1. year-1, representing the direct input from excretion and the indirect inputs from decomposition of faeces and seabed inputs:

the total input from the existing consents at full biomass (9900 tonne) is about 18.8 gmN.second-1

the total input from all existing and potential consents at full biomass (14600 tonne) is about 27.8 gmN.second-1.

4.2 CONCENTRATION ENHANCEMENT

The tidal and residual flows both flush Scapa Flow. In relation to the nitrogen input rate, their combined flow determines the increase in nitrogen concentration, the ECE. These flows are considered together in the following section.

Assuming ( Section 3.3) that Scapa Flow mixes well internally by tidal and wind action, the effect of the nutrient inputs on the whole area may be considered and converted to an estimated increase in nutrient concentration over background levels.

4.2.1 Tidal flows and recirculation

Some of the tidal outflows (sections 2.3.4 and 2.3.5) may re-enter Scapa Flow because of the oscillatory nature of tidal flow in the entrance sounds. Section 2.1 suggests that flows outside Scapa Flow are such as to remove much of the outflows but, taking a very conservative view and allowing as much as 90% recirculation, the effective tidal flow through Scapa Flow might be decreased by as much as a factor of ten.

4.2.2 Residual flows

The measured residual flows within Scapa Flow are small relative to tidal flows and are consistent with an anticlockwise circulation within Scapa Flow. At Bring Head the residual flow was less than 20% of tidal flows (section 3.2.1). Elsewhere it was of the order of 10% of the tidal flows. The residuals may therefore play a small part relative to tidal exchange but they enhance circulation within Scapa Flow and add to any tidal exchanges with the exterior.

4.3 ECE ESTIMATES

4.3.1 ECE Estimation in Scapa Flow

If the nitrogen input rate is I (Table 4) and the flow is T (section 2.3.5 and Table 3), the increase in concentration is:

ECE = F.I/T

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where the factor F is a tidal flow recirculation factor ( 1 for no recirculation; 2 for 50% recirculation; 10 for 90% recirculation). Estimates of ECE are shown in Table 11:

Table 11: ECE in Nitrogen – tidal and residual flows; biomass 14600 tonnes

ECE, µgN.litre-1 Flows in m3.s-1 Residual Flow = 0 Residual Flow = 3.103

0% recirculation 3.104 0.93 0.84 (best)

50% recirculation 1.5.104 0.46 1.54

90% recirculation 3.103 9.3 (worst) 4.6

The residual flow has small effect on ECE when tidal recirculation is low. In the case of 90% tidal recirculation, the residual flow is comparable to the tidal flow and therefore decreases the purely tidal estimate of ECE by 50%.

4.3.2 Worst case ECE at maximum biomass

At potential biomass 14600 tonnes, Table 11 shows best and worst cases and various combinations of residual flow and tidal recirculation. The maximum estimate in Table 11 is 9.3 µgN.litre-1, when recirculation is 90% and there is no residual flow. These assumptions of no residual flow and 90% recirculation are very conservative.

Residual flows in the Orkney Islands are widespread and pervasive ( section 2.1) and in the light of section 2.1 are very unlikely to be zero.

Some recirculation of ejected water seems inevitable. However, 90% is a very conservative estimate, for the following reason. The speeds of tidal outflows from Scapa Flow in Hoxa and Hoy Sounds reach about 0.5 m.s-1 and 1 m.s-1 respectively. These correspond to semi-diurnal tidal excursions of about five km (Hoxa) and ten km ( Hoy). Such excursions are shown in Figure 16; they extend well from the Sounds into external adjacent waters flushed by the external tidal flows of section 8 and the flows of section 2.1. Because tidal speeds at Bring Head are about ten times the residual speed ( Table 5), the excursion in Hoy Sound is likely to eject much more water than is ejected or injected by residual flows in the area. From these perspectives, recirculation into Scapa Flow of previously ejected water is very unlikely to reach as much as 90%.

Figure 16: Typical tidal excursions in the Sounds of Hoxa and Hoy

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The worst case ECE of 9.3 µgN.litre-1 therefore represents an upper limit on the likely Scapa Flow ECE; the best case of 0.84 µgN.litre-1 is very unlikely to be bettered unless there are significantly larger residuals. The average ECE of nitrogen in Scapa Flow is therefore likely to be in the range 1 to 10 µgN.litre-1, very probably significantly less than 10 µgN.litre-1.

4.3.3 Local effects

Sites in the East of Scapa Flow are removed from the main flows and may exchange slowly with the main flows. Local increase in nitrogen concentration may therefore exceed that estimated here for the whole of Scapa Flow. The significance of this may be judged from the estimated flows in the anti-clockwise circulation. Section 3.3.2 estimates the residual flow these regions to be about 2.103 m3.s-1, comparable with the flows of Table 11. These sites ( Westerbister, Glimps Holm, Hunda North and St Margarets Hope) comprise less than half the total Scapa Flow biomass. Any local increase in ECE above general Scapa Flow levels is thus likely to be limited to values about half of those of the 0% and 50% recirculation rows of Table 11, and this local effect is therefore not expected to be significant.

4.4 SUMMARY

If existing and potential inputs of Table 4 are applied to typical tidal and residual flows to and from Scapa Flow, the ECE increase in the average nitrogen concentration of Scapa Flow or of the eastern part is likely to be above 1 µgN.litre-1 and rather less than 10 µgN.litre-1. Local increases in the east of Scapa Flow may be of the same order, but smaller, magnitude.

Uncertainties in this conclusion relate to:

Scant current observations in the region Subjective depiction of Hoxa and Hoy Sound flows in the tide tables means that speeds and flows have been only roughly quantified The assumption that Bring Head and Toyness current measurements are relevant to conditions in Hoy Sound; it may be that residual flows in the centre of the channel off Bring Head are greater than measured at the coastal site at Bring Head; the diluting effect of residual flows would be correspondingly greater and the increase in nitrogen concentration less Uncertainty about how much tidal flow is re-circulated; recirculation has been conservatively modelled but is probably very much less, significantly reducing the estimates of increase in nitrogen concentration. These predicted increases are low in comparison with winter nutrient levels of around 160 µgN.l-1 ( values of 10 or 11 µM were quoted by Hydes et al. 2004; also see http://www.eea.europa.eu/data-and-maps/figures/map-of-winter-oxidized-nitrogen- concentrations- observed-in-2005).

An ECE of up to 10 µgN.litre-1 may also be compared with various relevant or regulatory standards:

typical background concentrations a previous Environmental Quality Standard for available nitrogen of 168 gN.litre-1 OSPAR & Water Framework Directive Reference Conditions: in offshore waters such as these (salinity above 34), the DIN (Dissolved Inorganic Nitrogen) reference value is 10 M and the threshold 15 M. Increases are therefore limited to 5 M (70 gN.l-1).

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5 EQS COMPLIANCE NEAR HUNDA NORTH

5.1 HUNDA NORTH

An issue related to the general issue of dispersion of soluble nutrients such as nitrogen is that of medicine compliance via the Environmental Quality Standard EQS). A relevant regulatory model is that used by SEPA to determine the amounts of soluble medicine that may be used (SEPA, 2008). In the model, water leaving a farm is considered to create a roughly elliptical patch of length determined by the tidal excursion, and width determined by lateral diffusion over the time since release; the width is typically much less than the length. A time of three hours is used for the medicine Azamethiphos and six hours is used for Cypermethrin and Deltamethrin. An Environmental Quality Standard ( EQS) for each medicine must be met at the boundary of the patch. For any properly regulated farm operating in compliance with its consent conditions, the EQS will therefore normally be met at a distance from the farm no greater than the tidal excursion.

Motion at Hunda North is roughly east-west. It is interesting to speculate on the fate of water leaving the site. The tidal amplitude ( Table 8) is about 0.07 m.s-1. The corresponding tidal excursion over 6 hours is about 1 km and over 3 hours is less, about 500 m. From this point of view, The EQS for medicine used at Hunda North will normally be met at a distance not exceeding about 1 km from the farm.

However, this general approach ignores the possibility that the ellipse may move in non-tidal manner otherwise than normal flows. It is relevant to ask how far this water might move from the farm in the following three or six hours before it necessarily complies with the relevant regulatory Environmental Quality Standards ( EQS).

Rather than using a general tidal excursion, the Xodus near-surface current record from April 2011 has instead been analysed here to give the total displacements over all possible ( more than 1061) three or six hour periods within the 15-day record, regardless of any assumption about tidal movement. These vector displacements are shown in Figure 17 relative to a starting point in the middle of the site.

Figure 17: Displacements in 3 hours (magenta) or 6 hours (green) from Hunda North

Two viewpoints on the significance of this figure may be adopted.

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Firstly, the calculated total displacements shown here imply that water is generally taken to the east or west. Three hour (magenta) and six hour (green) displacements are similar in direction and shape. A subset of displacements ending in the channel to the east of Hunda is clearly unrealistic - the island lying in the way - and may be disregarded.

Secondly, the regulatory model assumes the long axis of elliptical spreading to be along the tidal axis. If the axial length is determined by mean tidal flows (SEPA, 2008), Figure 17 shows that EQS-compliant water does not reach nearby coastal embayments. If, more prescriptively, the long axis of the ellipse were considered to be determined by actual flows, it would be longer when displacements are larger, thereby offering more dilution and ensuring that EQS compliance is reached before the ellipse extends to the extreme positions of Figure 17.

From either perspective, nearby coastal embayments such as Echnaloch Bay, St Mary’s bay and waters near Lamb Holm are very unlikely to be affected by water from Hunda North at concentrations above EQS.

5.2 GLIMPS HOLM

Compared with other farm sites, Glimps Holm is relatively close to Hunda North. This raises the issue of possible interaction of the near EQS-compliant waters from these two sites.

The hydrography of Glimps Holm was examined by Xodus (2012). The instrumental record had faults near the surface and acceptable ( to SEPA) data were presented from 6 metres below the surface ( and other depths). Figure 18 shows an extract from the Xodus (2012) report.

Figure 18: Tidal elevation and currents at Glimps Holm, 6 m from surface

The main axis of the motion was measured north-south, with lesser motions east-west. This is consistent with the north-south line of the coast at this site.

Original data are not available. Nevertheless, the north-south component and the tidal elevation shown in Figure 18 have been digitised with an accuracy about ±10%. The mean northward speed (blue) and water elevation (green) corresponding to the curves shown in Figure 18 are shown in Figure 19. The tidal signal is clear in the elevation but not in the current.

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Figure 19: Glimps Holm: digitized elevation and n-s current, 6 m from surface

The measured correlation of elevation and current is extremely weak ( correlation coefficient is almost zero at 0.03), meaning that current is mainly a combination of residual motion, wind, turbulence and random motions. The north-south vector displacements over all periods of 3 and 6 hours of this record are shown in Figure 20.

Figure 20: Glimps Holm displacements at 3 (green) & 6 (blue) hours, 6 m below surface

The maximum displacements in Figure 20 over the relevant regulatory periods of 3 or 6 hours are from 1 to 2 km, mainly to the north. These displacements are insufficient to

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take water to a position where it would overlap with the displaced waters (3 & 6 hours; respectively magenta and green positions) shown in Figure 17. Furthermore, in view of the anticlockwise residual circulation of Scapa Flow, it is unlikely that water would flow to the south at Glimps Holm while flowing to the north at Hunda North. These two aspects taken together mean that the likelihood of near-EQS waters meeting from Hunda North and Glimps Holm is very low indeed and may be discounted in any effect of raising local concentrations above EQS.

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6 DEPOSITIONAL FOOTPRINTS NEAR HUNDA NORTH

Parallel to the issue of possible EQS overlap is that of possible depositional overlap. Examples of the typical size and shape of depomod-modelled deposition are shown in Figure 21. These examples are taken from ( Scottish Sea Farms 2016) and Xodus, 2013) and relate to the medicine Slice; other modelled footprints in these two reports are of similar horizontal scales and shapes.

Figure 21: Typical modelled ( depomod suite) depositional footprints at Hunda North ( bottom left) and Glimps Holm (top right)

The separation of these two farms is about 3 km, about five times the size of the footprints. The separation is also about fifty times the horizontal scale of the shoulders” of modelled declining concentration towards the periphery of the footprints. From both these perspectives the footprints may be regarded as very well-separated depositional features whose interactions are negligible.

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7 CONCLUSIONS

7.1 ECE

Tidal and relatively small residual flows to Scapa Flow have been estimated by various methods (tide tables, direct observation and water budgets) to be sufficient to dilute the nitrogen released from existing consented sites and potential sites such that the ECE is above about 1 µgN.litre-1 and rather less than 10 µgN.litre-1. Local increases in the east of Scapa Flow may be of the same order but smaller.

Even at their most conservative, these estimated ECE values are much lower than winter background concentrations, well below a regulatory standard of 168 µgN.litre-1, and well below increases acceptable in relation to OSPAR & Water Framework Directive Reference Conditions

The estimated increases in the average nitrogen concentration of Scapa Flow are therefore insignificant in their likely effects on this water body.

7.2 EQS

Displacement modelling of near-surface flows at Hunda North shows the EQS of soluble medicines released from the site is very unlikely to be exceeded in nearby coastal embayments such as Echnaloch Bay and waters near Lamb Holm. Comparison of displacements of water over regulatory time scales shows that waters of Hunda North and Glimps Holm are very unlikely to meet with any local cumulative effect of raising local medicine concentrations to anything above EQS.

7.3 DEPOSITIONAL FOOTPRINTS

Comparison of modelled depositional footprints at Hunda North and Glimps Holm shows that in this respect the interaction between the two sites is negligible.

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8 REFERENCES

Dooley H D (1974) Hypotheses concerning the circulation of the northern North Sea Journal du Conseil 1974 36(1):54-61 Dooley H D, McKay, D W ( 1975). Herring larvae and currents west of Orkney. Counc. Meet. int. Coun. Explor. Sea C.M.-ICES/H: 43 Pelagic Fish Committee Edwards A ( 2015) Scapa Flow Westerbister ECE Estimates and Hydrography. Report No. Scapa 2015 006 for Scottish Sea Farms, 32pp. Hydes D J, Gowen R J, Holliday N P, Shammon T & D Mills (2004) Winter nutrient N, P and Si) distributions and factors controlling their concentration in north -west European shelf waters Estuar. Coast Shelf Sci 59: 151-161. Intertek Metoc ( 2012) Potential ballast water management policy, habitats regulation appraisal. Appropriate assessment Scapa Flow discharge. Report to Orkney Islands Council Marine Services P1565_rn2788_rev1.pdf. pp 30-31 Leterme S C, R D Pingree, M D Skogen, L Seuront, P C Reid, M J Attrill ( 2008), Decadal fluctuations in North Atlantic water inflow in the North Sea between 1958– 2003: impacts on temperature and phytoplankton populations, Oceanologia, 50 (1), 2008. pp. 59–72. Marine Scotland ( 2010) Locational Guidelines for the Authorisation of Marine Fish Farms in Scottish Waters: http://www.scotland.gov.uk/Resource/ Doc/295194/0104246.pdf Marine Scotland ( 2010) Regional Locational Guidance for Marine Energy http://www.scotland.gov.uk/Resource/ Doc/295194/0096885.pdf Scottish Sea Farms (2016) Hunda North Tech report v1.doc SEPA (2008), Regulation and monitoring of marine cage fish farming in Scotland - a procedures manual. Annex G Models for assessing the use of chemicals in bath treatments v2.2 Turrell W R (1992). New hypotheses concerning the circulation of the northern North sea and its relation to North sea fish stock recruitment. ICES Journal of Marine Science 49: 107-123. Turrell W R, E W Henderson, G Slesser, R Payne & R D Adams ( 1992). Seasonal changes in the circulation of the northern North Sea. Continental Shelf Research 12: 257-286. Xodus (2011a), Report A-30530-S10-REPT-001.pdf Xodus (2011b) Hunda N HG A-30530-S06-REPT-001-R01.pdf Xodus (2011c) Roo Point HG A-30530-S04-REPT-001-A01.pdf Xodus (2011d) SMH HG A-30530-S12-REPT-001-R01.pdf Xodus ( 2012) Hydrographic Analysis Reporting West Glimps Holm 14_377_MAR- WGH_Hydrographic_ Report-162429.pdf Xodus (2013) 14_377_MAR-WGH_Modelling_ Report-162427.pdf

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