SSF SUMMER ISLES ECE ESTIMATES 2018
SCOTTISH SEA FARMS SSF)
SUMMER ISLES: HORSE ISLAND, FADA AND TANERA ECE ESTIMATES 2018
REPORT SUMMER ISLES 002.DOCX
For: Scottish Sea Farms Ltd Laurel House Laurel Hill Business Park Stirling FK7 9JQ
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SUMMARY
This report outlines the hydrography of the area around the Summer Isles, to estimate the effect of existing fish farms at Fada and Tanera, and of a potential fish farm at Horse Island, on local nutrient concentrations via the ' Equilibrium Concentration Enhancement' ( ECE) approach.
Tidal and residual flows near the Summer Isles sites have been estimated to be sufficient to dilute the nitrogen released from existing consented sites at Fada and Tanera, and from the potential site at Horse Island, such that the likely ECE of nitrogen in waters local to the sites and in the surrounding waterbodies is less than about 20 µgN.litre-1.
The estimated values of ECE compare favourably with various relevant or regulatory standards that relate to:
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 circumstances examined here, the predicted ECE values are therefore insignificant within the Summer Isles water bodies.
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CONTENTS
1 INTRODUCTION ...... 6
1.1 SUMMER ISLES REGULATORY ISSUES ...... 6
1.2 THE ECE APPROACH...... 7 2 PHYSICAL BACKGROUND ...... 8
2.1 NON-TIDAL CIRCULATION ON THE WEST COAST OF SCOTLAND ...... 8 2.2 THE SCOTTISH SHELF MODEL ...... 9 2.3 TIDAL RANGE ...... 10 2.3.1 THE TIDAL STREAM ATLAS ...... 11 3 FARM SITES ...... 12 3.1 SITE DETAILS ...... 12 3.2 HORSE ISLAND ...... 13 3.2.1 HORSE ISLAND: CURRENTS ...... 13 3.2.2 HORSE ISLAND: INFLUENCE OF WIND ...... 13 3.2.3 HORSE ISLAND: SPREADING AND DISPLACEMENT ...... 14 3.3 FADA ...... 15 3.3.1 FADA: CURRENTS ...... 15 3.3.2 FADA: INFLUENCE OF WIND ...... 15 3.3.3 FADA: SPREADING AND DISPLACEMENT ...... 16 3.4 TANERA ONE ...... 17 3.4.1 TANERA ONE: CURRENTS ...... 17 3.4.2 TANERA ONE: SPREADING AND DISPLACEMENT ...... 17 3.5 LOCAL RESIDUAL FLOW AT HORSE ISLAND ...... 18
3.5.1 SCALE OF DILUTING FLOW ...... 19 3.5.2 DISPERSION ...... 19 3.5.3 SITE FLUSHING AND ECE ...... 20 3.6 SUMMARY ...... 21 4 CONCLUSIONS ...... 22 5 REFERENCES ...... 23
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TABLES
TABLE 1: TIDES AT ULLAPOOL ...... 10
TABLE 2: EXISTING AND PROPOSED SITES ...... 12
TABLE 3: HORSE ISLAND SUMMARY HYDROGRAPHIC STATISTICS ( 3-8-2012 TO 9-9-2012) ...... 13
TABLE 4: STATISTICS OF 6-HOUR NEAR- SURFACE AND CAGE BOTTOM DISPLACEMENTS ...... 14
TABLE 5: FADA SUMMARY HYDROGRAPHIC STATISTICS ( 3-8-2012 TO 9-9-2012) ...... 15
TABLE 6: STATISTICS OF 6-HOUR NEAR- SURFACE AND CAGE BOTTOM DISPLACEMENTS ...... 16
TABLE 7: TANERA ONE SUMMARY HYDROGRAPHIC STATISTICS ( 3-8-2012 TO 9-9-2012) ...... 17
TABLE 8: STATISTICS OF 6-HOUR NEAR- SURFACE AND CAGE BOTTOM DISPLACEMENTS ...... 18
TABLE 9: ENHANCEMENT IN DISPERSED RESIDUALS OVER ONE DAY ( TANERA 1 & 2 COMBINED) 20
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FIGURES
FIGURE 1: TWO WATER BODIES AROUND THE SUMMER ISLES...... 6
FIGURE 2: CATEGORY 3 AREAS ( YELLOW) IN THE SUMMER ISLES REGION ...... 6
FIGURE 3: SUMMER ISLES SITES AT TANERA, FADA AND HORSE ISLAND ...... 7
FIGURE 4: CURRENT PATHS WEST OF SCOTLAND (AFTER ELLETT, 1994) ...... 8
FIGURE 5: FLOW PARTITIONS WEST OF SCOTLAND (AFTER ELLETT, 1994) ...... 9
FIGURE 6: SURFACE RESIDUALS MAY TO OCTOBER, SSM SUB DOMAIN MODEL, M.S-1 ...... 10
FIGURE 7: TIDAL STREAMS 4 HOURS BEFORE HW DOVER; NEAP & SPRING ( TENTHS OF KNOT) . 11
FIGURE 8: EXISTING AND PROPOSED SITES ...... 12
FIGURE 9: HORSE ISLAND, ALL 6-HOUR NEAR- SURFACE DISPLACEMENTS ...... 14
FIGURE 10: FADA, 6-HOUR DISPLACEMENTS: NEAR- SURFACE & MID DEPTH ( INSET) ...... 16
FIGURE 11: TANERA ONE, 6-HOUR DISPLACEMENTS: NEAR- SURFACE & MID DEPTH ( INSET) ...... 18
FIGURE 12: PROPOSED LAYOUT AT HORSE ISLAND, SHOWING THE MAIN CURRENT DIRECTION .. 19
FIGURE 13: SPREADING OF A PLUME FROM A FARM SITE IN A RESIDUAL CURRENT AT TIME T ..... 19
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1 INTRODUCTION
1.1 SUMMER ISLES REGULATORY ISSUES
The Summer Isles aquaculture sites are positioned within two water bodies Summer Isles and Minch East, Figure 1, from http://map.sepa.org.uk/rbmp/) with an overall status of Good with High confidence in 2012, overall ecological status of Good and overall chemical status of Pass. These are not categorised areas as defined within the Locational Guidelines for the Authorisation of Marine Fish Farms in Scottish Waters (http://www.gov.scot/Resource/ 0052/00529751. pdf).
Figure 1: Two water bodies around the Summer Isles
Nor are there categorised areas listed nearby. Figure 2 is taken from https://marinescotland. atkinsgeospatial. com/nmpi/default.aspx?layers=530 , as the Scottish Government 2018 map of categorised areas. The three areas shown Lochs Broom and Ewe) are low impact risk category 3. An estimate of nutrient enhancement for planning purposes or environmental impact assessment and an assessment of the cumulative effect of nutrients released from the Summer Isles sites is nevertheless appropriate and required.
Figure 2: Category 3 areas (yellow) in the Summer Isles region
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This report estimates the increase in nitrogen concentration near the Summer Isles resulting from the sites shown in Figure 3.
Figure 3: Summer Isles sites at Tanera, Fada and Horse Island
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 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 a region. 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.
The Equilibrium Concentration Enhancement ( ECE) is given by:
ECE = S M /Q kgN.m-3
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 ON THE WEST COAST OF SCOTLAND
The main feature of the non-tidal circulation in the area is a northward residual drift along the west Scottish coast (Figure 4) at typical rates of 105 m3.s-1 ( Ellett, 1994).
Figure 4: Current paths west of Scotland ( after Ellett, 1994)
This flow is distributed over several paths and its partition is sketched in Figure 5. The northward flow in the Minch is around 9.104 m3.s-1. In the typical cross-section of the channel (~ 2.106 m2), this flow corresponds to a mean northward residual current of order 0.05 m.s-1.
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Figure 5: Flow partitions west of Scotland ( after Ellett, 1994)
This general picture of residual flow is the context for the following account of the general hydrography and the site-specific measurements from the fish farms.
2.2 THE SCOTTISH SHELF MODEL
Marine Scotland has led the development of a hydrodynamic model (FVCOM) for the Scottish continental shelf waters called the Scottish Shelf Model ( SSM) http://www.gov.scot/Publications/ 2016/03/8542). A series of six reports has been produced documenting the development of the model. The development of a relevant sub-domain model for the east coast of Lewis and Harris is described in https://data.marine.gov.scot/dataset/scottish- shelf-model-part-4-east-coast-lewis-and- harris-sub-domain/ resource/ 48d628ce- 8aa6.
Figure 6 shows an example of these FVCOM modelled surface residual currents. The surface residuals in the region vary in speed through the year but off the Summer Isles are typically northward at speeds of 0.1 to 0.3 m.s 1.- These speeds are consistent with, although a little higher than, the very general Minch estimate 0.05 m.s-1) of section 2.1.
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Figure 6: Surface residuals May to October, SSM sub domain model, m.s-1
2.3 TIDAL RANGE
The tides at Ullapool, the nearest and relevant national tide gauge, are summarized in Table 1, ( from the records of the National Tides & Sea Level Facility, http://www.ntslf.org/tgi/portinfo?port=Ullapool).
Table 1: Tides at Ullapool
Tide Ullapool
Highest Astronomic Tide 5.87 m
Lowest Astronomic Tide 0.03 m
Mean High Water Spring 5.21 m
Mean Low Water Spring 0.76 m
Mean High Water Neap 3.98 m
Mean Low Water Neap 2.18 m
m Typical tidal ranges are thus about 1.8 to 4.4 metres, with an average of about 2.9 metres.
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2.3.1 The Tidal Stream Atlas
Currents associated with the rise and fall of the tides are described roughly in the Admiralty Tidal Stream Atlas (2009). The atlas shows tidal streams every hour. Figure 7 shows examples of the hourly maps in the atlas for this area.
Figure 7: Tidal streams 4 hours before HW Dover; neap & spring (tenths of knot)
None of the maps in the atlas offers any information about the waters near the Summer Isles. Elsewhere the map representation is variable and merely qualitative. Typical speeds in the Minch west of Eddrachillis Bay are up to a few tenths of one m.s-1. Along- tide to-and-fro displacements corresponding to these currents are typically a few kilometres per tide.
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3 FARM SITES
3.1 SITE DETAILS
Details of existing and potential site operations are shown in Figure 8 and Table 2. The proposed site is south of adjacent islands, in depths of 20 metres or more.
Figure 8: Existing and proposed sites
Table 2: Existing and proposed sites
Site OS Grid CAR Licence Licensed Biomass Potential biomass Fada NB97600780 CAR/L/1002916/V5 600 600 Tanera 1 NB99200770 CAR/L/1002914/V5 625 625 Tanera 2 NB99400740 CAR/L/1002915/V5 625 625 Horse Island E 1972 Total 1850 3822
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3.2 HORSE ISLAND
3.2.1 Horse Island: Currents
Currents at Horse Island were measured over a period of almost 37 days by Scottish Sea Farms ( Report H0817-01, Horse Island Hydrographic Report v1.pdf). The statistics were independently checked for the present report and Table 3 summarises the common results. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.
Table 3: Horse Island summary hydrographic statistics ( 3-8-2012 to 9-9-2012)
Horse Island (Speeds, m.s-1) Near-surface (2 m) Cage Bottom (20 m) Seabed (38 m)
Mean speed 0.046 0.039 0.021
Residual current speed 0.019 0.009 0.004
Residual current direction (°) 4 14 18
Major axis of tidal ellipse (°) 25 35 25
Residual Flow (Parallel) 0.004 0.008 0.017
Residual Flow (Normal) 0.000 - 0.003 - 0.007
Tidal Amplitude (Parallel) 0.064 0.058 0.031
Tidal Amplitude ( Normal) 0.035 0.031 0.018
Tidal Amplitude 0.074 0.066 0.036
Tidal and residual currents at all depths align roughly with the direction of the neighbouring coast of Horse Island. Mean speeds are a few cm.s-1 at all depths. The similarity of current speeds over cage depths suggests credibly that stratification is weak, and that vertical mixing is correspondingly uninhibited.
Tidal amplitude varies little over the depth of the cage but decreases towards the seabed. The tidal amplitude over cage depths is about 7 cm.s-1, corresponding to a semi-diurnal tidal excursion along the coast of about ±1 km. Daily mixing is thus likely to extend significantly along and across the channel east of Horse Island.
Residual currents towards the North along the coast of Horse Island are about two cm.s-1 near-surface and decrease towards the bottom. The mean residual over cage depths is about 1.5 cm.s-1 towards a little East of North, parallel to the coast and corresponding to a daily northwards displacement of about 1.3 km.
3.2.2 Horse Island: Influence of Wind
For regulatory purposes, consent application measurements are taken in a period of low wind. The mean speed of this record is about 4.2 m.s-1. With such speeds, the expected wind-induced surface current is about 0.1 m.s-1 ( 3% wind to current proportionality is typical). This corresponds to daily displacements of typical size a few kilometres, sufficient to contribute significantly to the mixing of waters from the islands into the more open waters of the water bodies.
However, the correlations between easterly and northerly components of wind and current are low, around 10%, implying that at Horse Island itself, wind is less important than tidal and random motions ( and instrumental noise).
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3.2.3 Horse Island: Spreading and displacement
The cumulative near-surface vector displacements over all possible 6-hour periods in the 37-day record are shown in Figure 9, which ignores the blocking effect of any coast. The figure for cage bottom displacements is similar. Table 4 shows the statistics of near-surface and cage bottom displacements.
Figure 9: Horse Island, all 6-hour near-surface displacements
Table 4: Statistics of 6-hour near-surface and cage bottom displacements
Horse Island, 6-hour Displacements, km Surface Cage bottom
Displacement northward 0.43 0.2
Displacement eastward 0.03 0.05
Axial direction (°) 30 45
Standard Deviation N-S, km 0.80 0.67
Standard Deviation E-W, km 0.50 0.50
Such displacement rates imply a time scale of a day or two for cage depth waters to mix with the surrounding coastal waters. These time and length scales are commensurate with those estimated in sections 3.2.2, 3.2.1 and 2.3.1.
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3.3 FADA
3.3.1 Fada: Currents
Currents at Fada (58º 00.791’N 005º 25.635’W) were measured over a period of 15 days in 1999 by Anderson Marine Surveys (2009). Table 5 summarises the results. The last row (blue) is the Pythagorean addition of the two tidal amplitudes.
Table 5: Fada summary hydrographic statistics ( 3-8-2012 to 9-9-2012)
Fada (Speeds, m.s-1) Near-surface (2 m) Cage Bottom (20 m) Seabed (38 m)
Mean speed 0.032 0.037 0.017
Residual current speed 0.012 0.009 0.003
Residual current direction (°) 273 330 232
Major axis of tidal ellipse (°) 185 350 300
Residual Flow (Parallel) 0.000 0.009 0.003
Residual Flow (Normal) 0.012 - 0.003 0.002
Tidal Amplitude ( Parallel) 0.050 0.056 0.031
Tidal Amplitude ( Normal) 0.032 0.034 0.021
Tidal Amplitude 0.059 0.066 0.037
Tidal and residual currents at all depths align roughly east-west. Mean speeds are a few cm.s-1 at all depths. The similarity of current speeds over cage depths suggests that stratification is weak , and that vertical mixing is correspondingly uninhibited.
Tidal amplitude varies little over the depth of the cage but decreases towards the seabed. The tidal amplitude over cage depths is about 6 cm.s-1, corresponding to a semi-diurnal tidal excursion along the coast of about ±800 m. Daily mixing is thus likely to extend significantly throughout the intricate local Fada waters and onwards to more open water.
Residual currents towards the west are about one cm.s-1 near-surface and decrease towards the bottom. The mean residual over cage depths is about 1 cm.s-1 towards the west a little East of North, corresponding to a daily westwards displacement of about 1 km.
3.3.2 Fada: Influence of Wind
For regulatory purposes, consent application measurements are taken in a period of low wind. The mean speed of this record is about 3 m.s-1 ( Anderson Marine Surveys, 2009). With such speeds, the expected wind-induced surface current is about 0.1 m.s-1 ( 3% wind to current proportionality is typical). This corresponds to daily displacements of typical size a few kilometres, sufficient to contribute significantly to the mixing of waters from the islands into more open waters.
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3.3.3 Fada: Spreading and displacement
The cumulative near-surface vector displacements over all possible 6-hour periods in the 15-day record at Fada are shown in Figure 10. Near-surface displacements are shown centred on the site; cage depth displacements are shown separately to the same scale, inset white. The displacement maps ignore the blocking effect of the coast, but displacements are shown in full to give their complete scale.
Table 6 shows the statistics of near-surface and cage bottom displacements.
Figure 10: Fada, 6-hour displacements: near-surface & mid depth (inset)
Table 6: Statistics of 6-hour near-surface and cage bottom displacements
Fada, 6-hour Displacements, km Surface Cage bottom
Displacement northward 0.02 0.2
Displacement eastward - 0.27 - 0.11
Axial direction (°) 274 332
Standard Deviation N-S, km 0.49 0.65
Standard Deviation E-W, km 0.36 0.42
Such displacement rates imply a time scale of about a day for Fada cage depth waters to mix significantly with the surrounding coastal waters.
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3.4 TANERA ONE
3.4.1 Tanera One : Currents
Currents at Tanera One (58º 0.758’N 005º 23.893’W) were measured over a period of 15 days in 1999 by Scottish Sea Farms and were reported by Anderson Marine Surveys ( 2009). Table 7 summarises the results. The last row ( blue) is the Pythagorean addition of the two tidal amplitudes.
Table 7: Tanera One summary hydrographic statistics ( 3-8-2012 to 9-9-2012)
Tanera One (Speeds, m.s-1) Near-surface (2 m) Cage Bottom (20 m) Seabed (38 m)
Mean speed 0.037 0.031 0.035
Residual current speed 0.008 0.007 0.005
Residual current direction (°) 290 250 234
Major axis of tidal ellipse (°) 270 270 270
Residual Flow (Parallel) 0.008 0.006 0.004
Residual Flow (Normal) 0.003 - 0.002 - 0.003
Tidal Amplitude ( Parallel) 0.048 0.046 0.057
Tidal Amplitude (Normal) 0.037 0.026 0.035
Tidal Amplitude 0.061 0.053 0.067
Tidal and residual currents at all depths align roughly with the direction of the neighbouring coast to the south. Mean speeds are a few cm.s-1 at all depths. The similarity of current speeds over cage depths suggests that stratification is weak, and that vertical mixing is correspondingly uninhibited.
Tidal amplitude varies little with depth and is about 5 cm.s-1, corresponding to a semi-diurnal tidal excursion along the coast of about ± 700 m. Daily mixing is thus likely to extend significantly across the whole east-west-width of the site bay, connecting it to flows in Badentarbat Bay to the east.
Westward near-surface residual currents up to about 1 cm.s-1 are directed toward Tanera. This flow is a westward incursion of Badentarbat Bay water and, for continuity, must be matched by an eastern outflow further north in the site bay. This implies that the site bay is very probably flushed by a clockwise eddy, giving a generally northward residual of about 1 cm.s-1 over the site.
3.4.2 Tanera One : Spreading and displacement
The cumulative near-surface vector displacements over all possible 6-hour periods in the 15-day record at Tanera One are shown in Figure 11. Near-surface displacements are shown centred on the site; cage depth displacements are shown separately, inset white. The displacement map ignores the blocking effect of the coast but displacements are shown in full to give their complete scale.
Table 1 shows the statistics of near-surface and cage bottom displacements.
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Figure 11: Tanera One, 6-hour displacements: near-surface & mid depth (inset)
Table 8: Statistics of 6-hour near-surface and cage bottom displacements
Tanera One, 6-hour Displacements, km Surface Cage bottom
Displacement northward 0.06 - 0.05
Displacement eastward - 0.17 - 0.14
Axial direction (°) 290 250
Standard Deviation N-S, km 0.45 0.18
Standard Deviation E-W, km 0.31 0.43
Such displacement scales imply a time scale of less than a day for Tanera cage waters to mix across the southern end of the site bay and hence to the surrounding coastal waters to the East in Badentarbat Bay.
3.5 LOCAL RESIDUAL FLOW AT HORSE ISLAND
In view of the general and measured residual flows at Horse Island it is useful to attribute a notional local residual volume flow to the farm.
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3.5.1 Scale of diluting flow
To typify the Horse Island farm, it is reasonable to use a depth H of twenty metres and an east-west cage grid width W, say 200 metres (Figure 12).
Figure 12: Proposed layout at Horse Island, showing the main current direction
Additional to the physical width of the farms, the effects of horizontal dispersion act to create similar length scales of mixing across flows after release of nutrients.
3.5.2 Dispersion
In simple Fickian diffusion ( Thorpe, 2005; Crank, 1980), the flow rate of dissolved material is proportional to the concentration gradient. The constant of proportionality ( K, the diffusion coefficient) is intrinsic to the medium as in, for example, molecular diffusion in fluids or gases. For regulatory purposes the dispersion coefficient K is usually taken conservatively in Scottish coastal aquacultural regulatory matters to be 0.1 m2.s-1 or more. If the diffusion is isotropic the same in all directions), a patch of material spreads radially, with a typical radius t) at time t, given (in two dimensions) by
t)2 = 4 K.t
The patch area increases at a constant rate and the variance 2 increases in direct proportion to the time. Figure 13 sketches the effluent plume in a residual current U. At the time (t) shown here the width P of the plume is roughly
P W + 2 t) =W + 4 (K.t)0.5
Figure 13: Spreading of a plume from a farm site in a residual current at time t
The notional diluting flow over the depth H after a time t is therefore
Diluting flow = H U P = H U (W + 2 t)) =H U (W + 4 (K.t)0.5)
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If the cage group has a width W about 200 metres, typical values for the plume width P are about 600 metres after a day and about 1600 metres after 15 days. With H = 20 m, the corresponding flow P after one day is about 12000 U m3.s-1.
3.5.3 Site Flushing and ECE
Dispersed residual Flow
The local residual flows of section 3.5.2 are available to receive and dilute the released nutrients. At Horse Island, the diluting flows of Table 3 were derived over a period (37 days) longer the regulatory-required time scale of 15 days that applied to the Fada and Tanera records.
On the assumption that the Horse Island, Fada and Tanera currents typify flows in the area, Table 9 shows the predicted local enhancement for a conservative time scale of only 1 day (t = 8.6.104 seconds). Tanera 1 and Tanera 2 are considered together because they comprise one site lying across the same flows.
Table 9: Enhancement in dispersed residuals over one day (Tanera 1 & 2 combined)
Site Residual, Biomass, Nitrogen input, Notional residual Enhancement, µgN. litre-1 m.s-1 tonne gmN. s-1 Flow, P, m3.s-1
Fada 0.01 600 1.1 120 11
Tanera 1 & 2 0.01 1250 2.4 120 22
Horse Island 0.015 1972 3.8 180 22
Total 3822 7.3 450 17
Main Currents
The above estimates ( Fada, Tanera & Horse Island) are based on dispersed residuals after one day and are conservative. The mean and tidal currents of sections 3.2.1, 3.3.1 and 3.4.1 are about five times greater than the residuals and thus ensure that the local enhancement might be expected to be about a fifth of the values in Table 9. Wind will add to this dilution.
Total System East of Horse Island
Considering the total system east of Horse Island, a greater flow estimate comes from applying residual currents to the channel between Horse Island and the mainland. This approach is reasonable in view of the spreading in Figure 9. With a channel width 1.5 km and cage depth 20 m, the total flow is around 450 m3.s-1. This flow gives an ECE of about 17 µgN.litre-1, summarised in the last line of Table 9
ECE Summary
From these three perspectives, the likely enhancements range from about 3.5 to 17 µgN.litre-1.
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3.6 SUMMARY
Existing and potential inputs of nitrogen to the relevant waterbodies are diluted by tidal flows and mixed around the Summer Isles on a time scale of very few days. The local ECE of nitrogen in the 6-hour and one day dispersed residual flows is likely to be well below 22 µgN.litre-1. Tidal flows and wind-driven currents will reduce these values.
The overall ECE east of Horse Island is unlikely to exceed 17 µgN.litre-1
An ECE of about 20 µgN.litre-1 may be compared with various relevant or regulatory standards:
Typical background concentrations. The estimated values of ECE are low in comparison with west Scottish coastal winter nutrient levels of around 90 gN.l-1. given as < 10 M in http://www.gov.scot/Publications/ 2011/03/16182005/38).
The estimated values of ECE are much lower than a previous Environmental Quality Standard 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). The calculated ECE values, reaching (~ 2 M, 20 gN.l-1) are much less than this.
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4 CONCLUSIONS
Tidal and residual flows at Horse Island have been estimated by various methods to be sufficient to dilute the nitrogen released from existing consented sites and a potential site at Horse Island such that the overall ECE is above about 4 µgN.litre-1 and probably less than 20 µgN.litre-1. Wind effects will increase this dilution.
Even at their most conservative, these estimated ECE values are lower than winter background concentrations, well below a previous 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 the waters around Horse Island are therefore insignificant in their likely effects on the Summer Isles water bodies.
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5 REFERENCES
Admiralty ( 2009) Tidal Stream Atlas
Anderson Marine Surveys ( 2009) Tanera 1 – site and hydrographic survey report. AMSL Report No 09/01.7 – rev2
Anderson Marine Surveys ( 2009) Fada – site and hydrographic survey report. AMSL Report No 09/04.2
Crank, J (1979) The Mathematics of diffusion. Oxford University Press.
Ellett D J (1994) The Oceanographic setting of the Scottish Islands. in Baxter, J M M B Usher eds, The Islands of Scotland: A Living Marine Heritage pp 30-53.
Thorpe (2005) The turbulent ocean, Cambridge University Press.
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