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Oceanographic Pathways and Structure of the Seas Around the British Isles

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Oceanographic Pathways and Structure of the Seas Around the British Isles Oceanographic Pathways and Structure of the Seas around the British Isles Results from the Defra funded project Towards a consensus concerning the extent, strength and persistence of pathways governing the fate of contaminants and nutrient dynamics: AE1225 From early summer (late May) to autumn (mid October), a continuous oceanographic pathway exists from the NW Brittany (Ushant), across the Western English Channel, south along the Cornish coast, around the Lizard Peninsula, along the N Cornish and Devon coasts, across St. Georges Channel, and then around the SW and W coasts of Ireland. These flows occur at the boundaries of the stratified regions and have significant magnitude, of ~0.1 – 0.2 x 106 m3/ s. Such transport remains within its source eco-hydrodynamic region, and the pathway acts as a barrier to limit transport between neighbouring eco- hydrodynamic regions (e.g. stratified to mixed). For example, summer transport between the Celtic Sea and the Irish Sea is very limited, while there is strong transport across their boundary (St Georges Channel), There is thus limited potential for transport of plankton across these pathways, but high potential for transport along them such as Karenia mikimotoi. Northward transport from Brittany – Scotland. The combined pathway from the results of three field programs, with ARGOS drifter tracks overlain on the contours of bottom density. Black arrows indicate geostrophic velocity. Section from Clare Island West Coast Ireland, showing: a) temperature structure, revealing the thermocline and the bottom front, b) the speed of the northward geostrophic flow, estimated from the bottom fronts, c) measured residual speed, once the tides have been removed. The section show the principal features of bottom fronts beneath the main thermocline and a flow associated with these bottom gradients. Independent observations (using Acoustic Doppler Current Profilers ADCPs & drifters) and model results indicate that these geostrophic currents exist, and although there are differences between the results, such as the precise speeds, these differences are small. Scanfish – towed undulator. Do Plankton use this pathway? Harmful algal blooms (HABS) come in various forms, such as species which produce massive blooms such as Karenia mikimotoi or those which are toxic at low concentrations, such as Dinophysis, these and other species are however prevalent along the pathway. The highest occurrences of Dinophysis are along the regional transport pathway, along which favourable conditions exist for growth, primarily that of strongly stratified environments, which act to inhibit turbulence. Incidence of Diarrheic Shell Fish Size of dots indicates number of incidences Poisoning (DSP) due in a 10 year period to the dinoflagellate Dinophysis. Regular occurance of Red tides due to Karenia mikimotoi. Oceanographic pathways in the North Sea In the North Sea, in summer, strong (12 cms-1) organised flows exist from the NE English coast (i.e. North of the Flamborough front), past the northern edge of the Dogger Bank and NE to the Skaggerak. There are also strong flows (10 cms- 1) directed SW along the southern flank of the Dogger Bank. This flow continues anticlockwise around the edge of the stratified region at the south of the Oyster Grounds and in summer, acts to contribute to isolation of the bottom waters there. This isolation, coupled with the relatively shallow depth (and associated productivity), means that oxygen depletion is likely in the bottom waters. Drifter tracks in the central and southern North Sea. Dots mark the daily positions. Large blue arrows indicate the main pathways. Applying improved hydrodynamic models 53 Modelled depth-integrated flow field for the Celtic Sea in July 1998, with drifter tracks (red, each of ~40 days duration). Note that the model replicates well the observed flow from the Cornish coast north to the Gower Peninsula, west across St Georges Channel towards the SE coast of Ireland. Where the density component of flow contributed 52 91% to the net westward flow (Young et al., 2003). N e d u t i t a L 51 50 Satellite tracked drifter being -9 -8 -7 -6 -5 deployed. Longitude W The importance of comparison Good Most modern 3-D baroclinic models replicate measured surface and bottom temperatures typically to better than 0.5°C (1 SD) which is sufficiently accurate for the simulation of many ecological processes. Principle features of the flow fields in the Celtic Sea and North seas are well replicated by the Cefas POM model (as do most other models). Bad Many models, such as POLCOMS, GETM and COHERENS, indicate that freshwater inputs are under-estimated, for example, in the German Bight, where although the horizontal structure is well replicated, the modelled absolute values of salinity are too high. Oceanographic structure in the North Sea: a) GETM modelled temperature, a b) observed temperature; c) GETM modelled salinity, d) observed salinity. Note good simulation of the strong vertical temperature gradients in the stratified region (left of sections a & b) and good replication of salinity structure (c & d) but not absolute salinity. b Biological Modelling Observation Model comparison. Contours of Karenia modelled by c Ifremer, with dots of observed values from Cefas/Irish survey. Results indicate that the bloom is due to the strength d of stratification and reduction in mixing, rather than supply of nutrients (van Houtte Brunier et al., 2006). Structure of the North Sea - why is there high primary productivity? New estimates of primary productivity indicate that 38% of all new productivity in the North Sea occurs along the thermocline during the summer months. The primary nutrient source for this productivity is the bottom water on the UK shelf, which is largely remnant from the winter months. High levels of productivity occur at the thermocline because of the particular interaction between tides, light regime, nutrient pool and bathymetry. The productivity associated with the summer thermocline is sustained for a much longer period than that of the spring bloom and potentially supplies a more reliable source of fixed carbon and energy for higher trophic levels. The increased biomass of chlorophyll resulting from this productivity is not visible from satellite observations. North Sea Average primary productivity, integrated over the entire water column (mg of C/ 56° 2 m /day) for different regions along a transect north of the Dogger Bank (line b). SML = (b) surface mixed layer, CM = deep chlorophyll maximum and BML = bottom mixed layer. (a) 55° ) 40 m Dogger Bank N ( Area Type Entire Water Column SML CM BML Contour e d u Flamborough 2 t i 54° mg of C/m /day % % % t Head a L Stratified (N) 741 35.1 57.8 7.1 53° Frontal (~40 m contour) 1015 44.8 - 55.2 Bank (S) 456 38.1 - 61.9 52° -2° -1° 0° 1° 2° 3° 4° 5° Longitude (W/E) This project has demonstrated that in the stratified regions, the cycling Nitrate ‘turnover times’ (days) for in situ concentrations and for a standard concentration of nitrates and ammonia by phytoplankton in summer is very rapid 3 of NO3 of 5 mmol/m , for the southern N. Sea (well (Weston et al., 2005), at ~1 day in the chlorophyll maximum and ~7 mixed) and north of the Dogger Bank (stratified) days in the surface layer, so that although the summer transport August pathways are strong, direct transport of dissolved nutrients from the NE English coast to the central and southern N. Sea is unlikely. In the Well-mixed waters - In situ 13 ± 17 stratified region the most likely transport mechanism of nutrients is in concentrations 3 particulate form, and while a number of mechanisms can contribute to NO3 at 5 mmol/m such transport, these are weak (because the tides are relatively weak) Well-mixed waters 100 ± 54 and occur over long timescales. This is in contrast to well mixed regions Stratified waters - Surface 7 ± 14 where the cycling of nitrates is much slower (Weston et al., 2004) and because of this significant transport of dissolved nutrients can occur. Stratified waters - Chlorophyll < 1 max. -1 -3 Fluorescence (µg l ) and Density (σt kg m ) 20/8/02 0 Cholorophyll (µg l-1), Density (σ kg m-3) and ToxN (µmol l-1) 0 t -20 -20 ) m ( 1 6 h t p 1 5 e D 1 4 1 3 -40 ) 1 2 m 1 5 1 1 ( -40 Dogger Bank 1 0 1 4 h 9 t -1 1 3 8 (µg l ) p e 1 2 7 D 6 -60 1 1 5 1 0 4 9 3 Production Sites -1 2 8 µg l 1 -60 TOXN 7 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 5 µmol l-1 6 Distance West - East (km) 5 -80 -1 2 µmol l 4 -1 3 3 0.2 µmol l Chlorophyll (µg / l) and density (σt kg / m ) for a section across 2 1 the Oyster Ground (line a) in August 2001. Note the double 0 pycnocline and high levels of productivity along the base of the -100 0 10 20 30 40 50 60 70 80 90 100 110 lower layer. Here, in contrast to the diatom-rich spring bloom, the productivity at depth consists mostly of dinoflagellates, which Section from North of the Dogger Bank (line b) 28th April 2005, thrive in relatively stable, low-energy environments. showing high productivity at the surface and subsurface, with high concentrations of available nutrients in the deep waters beneath the pycnocline. Recent work indicates that as early as mid April, productivity at depth can be greater than at the surface. Productivity experiments from one pair of stations demonstrate that the greatest productivity occurs at depths of ~40 m. Potential impacts of climate change on the UK shelf seas Seasonal stratification is a key factor controlling the timing, location and nature of primary productivity in UK waters.
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