The coastal waters of Ireland are rich in physical features affecting both chemistry and biology. Amongst these are the tidal fronts of the Irish Sea (Le Fevre, 1986) and the Irish Shelf Front on the Atlantic coast lying along the 200m iso bath (Huang et al., 1991). Recently, an upwelling system has been described in the vicinity of the Fastnet Rock (Roden, 1986; Raine et al., 1990). Coastal upwelling systems are ecologically very important and are generally extremely productive, as nutrients brought up to the sea surface can stimulate extensive phytoplankton growth. This paper describes further satellite and ship-based investigations which are currently being carried out to examine the mechanisms driving the upwelling system and its effect on local ecology

Conway, N.

Hoey, M.J.

Joyce, B.

Moloney, M.

Patching, J.W.

Raine, R.

Whelan, D.

Marine Institute Open Access Repository (OAR)

5. THE OCEANOGRAPHY OF SOUTHWEST BRELAND:CURRENT RESEARCH ACTIVITIESR. Rainel, D. Whelan^, N. Conway^, B. Joyce^, M. Moloney^, MJ.Hoey^ and J.W. Patching^Department ofMicrobiology, University College, Galway.^Department ofPhysics, University College, Dublin.5.1 AbstractThe coastal waters of Ireland are rich in physical features affecting both chemistry and biology.Amongst these are the tidal fronts of the Irish Sea (Le Fevre, 1986) and the Irish ShelfFront onthe Atlantic coast lying alongthe 200 m isobath (Huang et al^ 1991). Recently, an upwellingsystem has been described in the vicinity of the Fastnet Rock (Roden, 1986; Raine et al., 1990).Coastal upwelling systems are ecologically very important and are generally extremelyproductive, as nutrients brought up to the sea surface can stimulate extensive phytoplanktongrowth. This paper describes further satellite and ship-based investigations which are currentlybeing carried out to examine the mechanisms driving theupwelling system and its effect on localecology.5.2 Satellite Studies5.2.1 BackgroundThe use of thermal infra-red satellite imagery for the study of marine phenomena wasestablished in the 1970s via the NOAA series of satellites. The development of advanced veryhigh resolution radiometry (AVHRR) permitted images with a 1 km^ resolution that is moreappropriate to marine systems. In the context of Irish coastal waters AVHRR images havebeen used as a technique in the study of the Irish Sea tidal fronts (Simpson and Pingree, 1978),the southwest Ireland upwelling system (Raine et al, 1990) and the Irish Shelf front (Huang etal, 1991).The Coastal Zone Color Scanner, used by previous researchers to plot chlorophyll levels, hasreached the end of its life, having been in operation longer than scheduled. For the immediatefuture there are no provisions to launch into orbit a platform carrying a colour scannerspecifically designed to sense water bodies. Thus, it is of considerable interest to investigatewhether the data produced by existing colour sensors designed for other purposes can besatisfactorily used to retrieve characteristic water parameters such as phytoplankton(chlorophyll) and suspended sediment concentrations.5.2.2 Relative Chlorophyll Concentration Mapping and its LimitationThe technique of detecting chlorophyll using satellite colour imagery depends on the absorptionspectra of pure water and of chlorophyll The radiation absorption curve of chlorophyll is suchthat chlorophyll can be detected by comparing two image bands, one in the blue and one in the//// floating quadrants of a LandSat 188 km wide scene. Each quadrant is therefore approximately/ 62 X62 km in earth area and the quadrant images are approximately 3072 x 3072 pixels whichis equivalent to 9 Mb per image band.For microcomputer application processing, i.e. chlorophyll retrieval, only the two relevant bands(blue and green) were extracted from the original LandSat interleaved data format. Theseextracted bands were then network transferred to micro-computer disk. In order to store theLandSat quadrants on microdisk, it was necessary to divide each into nine sectors, each being1024 X1024 pixels requiring just 1 Mb of storage per band. Chlorophyll retrieval processing ofan image sector, including final median filtering, takes approximately 20 minutes on a 386/38725 MHz PS/2.5.2.4 LANDSAT TM-5 Chlorophyll imagesChlorophyll images for sectors of the southeast and southwest coasts taken in the summer of1990 are shown in Figures 5.1 and 5.2. Although some detail has been lost due to the white-blackshading used for this article, several features are evident.5.2.4.1 Southeast Ireland (Figure 5.1)The Celtic Sea Front, running southeast from the Saltee Islands over to South Wales, supportsenhanced, sometimes extensive, phytoplankton growth. High levels of chlorophyll may be seenin Figure 5.1 at the location of the northwestern end of the front. Both Waterford andWexford Harbour contain increased nutrient levels due to land run-off and industrial anddomestic effluent, and like many estuaries contain high levels of phytoplankton (chlorophyll).The estuary plumes are quite visible on the images, moving to the right emanating from themouths as might be expected from geostrophic considerations, although this could also be dueto tidal movements.5.2.4.2 Southwest Ireland (Figure 5.2)Enhanced chlorophyll levels can be observed around Whiddy Island. This is probably a directresult of nutrient inputs from Bantry town into sheltered waters with restricted circulation.Aberrations due to reflection from the sea bed could be the cause of the highest (white) patchesin shallow inlets. Enhanced levels inside Roaring Water Bay are most likely the result of aprevious upwelling event inside the bay stimulating plant growth. Afront known to exist at themouth of Bantry Bay south-east of Bear Island (see Raine et al., 1990) can be discerned fromthe image, as these ffonts tend to support increased levels of chlorophyll.A contrast in levels of chlorophyll can be made between waters to the west and east of theFastnet Rock lighthouse. Although direct interpretation from the image would indicate that thisis a result of increased headland mixing (around Mizen Head), ground truth studies (LoughBeltra cruise 20/90) indicate this to be a consequence of an upwelling event. Ground truth datafrom Lough Beltra cruise 20/90 which companion these images, together with further imageanalysis may be found in Hoey (1991).Fig. 5.25.3 Studies involving R.V. Lough BeltraLough Beltra cruise 9/91 was the focus ofa collaborative exercise in Bantry Bay funded underan EC MAST programme involving scientists from UCG, Vigo (Spain), Scotland and N. Irelandwhich has the overall aim of elucidating environmental factors which govern phytoplanktonclass dominance in marine systems. The study area and location of stations sampled is shownin Figure 5.3. Methodology may be found in Raine et al. (1990). In addition a WS OceanSystems conductivity-temperature-depth probe was employed, to which was attached aSeaTech in situ fluorometer for the measurement of chlorophyll, used in profiling mode with ahost micro-computer acting as a data logger on deck. Only those results pertinent to the effectofan upwelling event on water circulation in Bantry Bay are presented here, with notes on thecausative mechanisms leading to an exceptional bloom of the red tide organism GyrodinhimCLUYQoluTfi in thearea (see McMahon Qt ctL, this volume).The entire transect of stations along Bantiy Bay were sampled periodically between 15 July and8 August. Temperature distributions along the transect are shown in Figure 5.4; distributionsofdensity closely resembled those oftemperature. Between 15 and 31 July an intrusion ofcoldwater came into the bay reaching the innermost end at Whiddy Island (Station 1). This waspresumably a result ofan upwelling event offshore. Between 31 July and the 8 August this subsurface cold water layer had retreated back down the bay, probably a density-driven currentcaused by relaxation of the upwelling mechanism. Note that the inflow of cold water along thebottom is accompanied by an outflow of the surface layers.Station 3 (Figure 5.3) was the site used for additional experimental work during the cruise, anda high frequency of temperature-salinity profiles was obtained there. Results have beensynthesised into a depth-time plot and are shown in Figure 5.5. After the 31 July the retreat ofthe cooler subsurface layer was remarkably rapid, resulting in an exchange of water equivalentto 70% of the volume of the inner section between Whiddy Island and Sheeps Head within aperiod of 3-4 days. Contemporaneous current meter measurements made by personnel fromthe Dunstaffhage Marine Laboratory, Oban show the bottom layer residual outflow was in theorder 5-10 cm.sec"The bottom outflow observed between 31 July and 3 August was accompanied by an equallystrong inflow of the surface layers. On this occasion the warm surface water flowing into thebay contained a large population of the dinoflagellate Gyrodinhim axireolnm, resulting in surfaceor near-surface concentrations of several hundred thousand per litre (10^-10^ cells.l"^). Thehighest concentration observed was 5.2 x lO^.l'^ at station 11. The increase in concentrationof this organism in the surface layers of Bantry Bay is evident in the counts from samples takenfrom Station 3 (Figure 5.6). The concentrations recorded from this and other stations takenover the cruise were large enough to cause discoloration of surface water, or "red tide", whichwas observed between Shot Head and Roancarrig on 5th August.5.4 DiscussionThe events recorded during Lough Beltra cruise 9/91 have shown that the red tide of 1991,recorded not only in Bantry but also in neighbouring Dunmanus Bay (see McMahon et al, thisvolume), was initiated by a relaxation of an upwelling event resulting in import of warm surfacewaters from offshore containing extensive populations of Gyrodinium anreohim. ThisFig. 5.3rFig. 5.4Fig. 5.5 & 5.6mechanism of water mass exchange is likely to be that causing the inshore red tides in RoaringWater Bay in 1978 and 1979 reported by Roden and others (Roden et a/ 1980;198I; Jenkinsonand rnnnors 1980) The area inside the Fastnet Rock is known to be greatly affected byperiodic upwelhng events (Roden, 1986; Raine et al. 1990) and landward transport of surfacewater containing established populations of G. aureolum Aunng upwelhng relaxatton would inthe hpht of results from the present study, appear to be the operative mechanism. Roden et al.1981) Dointed out that landward transport of offshore communities was likely to be the causeof the red tide of 1979. but were not able to establish asuitable physical mechamsm.,->r.u tn the develoDins aquaculture industry in the area, two crucial questions remain.^ t ""^hv r aureoliim and not other bloom-forming dinoflagellates, and secondly how do theh "^1 mechanisms described above affect the ecology of the dinoflagellates Dinophysis acutaaciminata which are implicated in toxin production and shellfish contamination in thearea?C" dinium aureolum is adinoflagellate which is associated with fronts, for example the CelticS '^ ^front which meanders from Waterford across to South Wales. Implicit in its continuecToccurrence around southwest Ireland is the existence of afrontal system offshore which cansuitably stable to encourage extensive G. aureolum populations. The existence of the IrishShelf front approximately 60 km southwest of Dursey Island is one possible location, althoughas this is a coastal salinity front it is difficult to postulate an adequate nutrient supply for theformation of a bloom. Alternatively, upwelling conditions will produce fronts between theupwelhng and adjacent stratified shelf water. The existence of this type of front is indicated byfthe data ofRoden et al. (1981) a few miles to the south ofSherkin Island.^ It seems most likely Jthat this is the source of the advected blooms. The situation is further complicated by the are^being the location of theboundary between Atlantic shelf and Celtic Sea water.The high flushing rate in Bantry Bay caused by upwelling events, where 70% ofthe Bay watefmay be exchanged with that offshore in about 4 days, has many implications for the marineecology of the area. Amongst these is the occurrence of D. acuta and D acjimuiata. It is ^unfortunate that we still do not know enough of the ecology of these organisms, particularly in fthe offshore realms, to assess the effect of flushing. Further microscopic examinations of 1samples taken in 1991 may yield some insight, but as yet no definitive conclusions may be made /Cbnceming the effect of upwelling on Dinophysis spp.A final comment should be made concerning the periodicity of upwelling/deep water intrusionevents into Bantry and the adjacent bays. As yet, we do not know what mechanisms causeupwelling around southwest Ireland. Data obtained near Whiddy Island by Queens Universitypersonnel (R. Goward, pers. comm.) show that four cold water intrusions occurred between '^May and September 1991. Long-term deployment of current meter and/or thennistor chains in /Bantry Bay would establish the regularity or irregularity of these events. If subsequentcorrelations of their frequency with either meteorological or physical phenomena can bederived, then at last a predictive capability with respect to exceptional blooms of G. aureolum inlate summer around southwest Ireland may be established. There can be no doubt, however,that the combined use of satellite imagery with field studies will prove an important technique inelucidating physical and biological mechanisms in the area.areenwitc. The two bands used for this project were Bands 1 and 2 of the LandSat ThematicMapper (TM-5) imagery (Tassan, 1987). The resultant chlorophyll image maps are segmentedto show distinct relative concentration levels. Statistical techniques have been developed tointerrogate, through image windows, sections of the image to give values for the mean pixelintensity, the mean deviation ofpixel intensities, and the standard deviation of pixel intensities.Imoortant errors may be introduced by instrument noise and by the relatively low number of theIvfilable sivnal quantisation levels. Instrument noise is usually expressed by the signal-to-noiseL Alow signal-to-noise ratio such as is present in the low luminance winter scenes severely!^7,fricts the application of the technique. The analogue radiometric signals from TM-5 arerestncib transmission fro the satellite t the groundconverte however, the dynamic range ofthe signals is such that, when digitised, thehe« range from alevel ofzero to only about 50. This results in amuch-reduced signal-to-noi^rratio and therefore reduces the quality ofthe chlorophyll retrieval algorithm.radiance scenes, the value of the gain can be increased by using pixel clustering withscale factors higher than unity. For example, a 4x4 cluster admits a maximum gain• bv a factor of16. This procedure is effective in reducing the contribution of the errorso^rc^esulting from the relatively few signal quantisation levels, but has no effect on the errorinduced bythe bias level.TM images suffer from narrow striping and wide banding that are exaggerated by thechlorophyll algorithms. In practice, this means the winter scenes retrieved from the UCDarchive do not produce relative chlorophyll mapping ofusable quality. For Irish coastal waters,this is due to the low illumination density and the resultant low signal-to-noise ratio producingthe evident narrow and broad image horizontal striping. Several pixel-by-pixel tests wereexecuted to quantify this effect. For low luminance image scenes it was therefore necessary todevelop and incorporate a localised destriping algorithm. Further, it has been found useful toapply a median smoothing filter to the resultant chlorophyll map. The median filter producesmore even contours of equal chlorophyll concentration and this aids visual interpretation,The excellent TM-5 ground resolution allows one to distinguish details of particular interest incoastal waters. As a LandSat quadrant contains approximately 3072 x 3072 pixels at 30x 30 mper pixel, which equates to a land area of 90 x 90 km. This allows one to distinguish details ofparticular interest in coastal waters, for example headland mixing and river outflows.The relatively long TM-5 repetition cycle of 16 days represents a significant limit to its range ofapplication for water sensing. Inparticular this causes difficulty in the exact matching ofgroundtruth data gathered on scheduled research vessel cruise dates to image data obtained from theclosest satellite cloud-free overpass. For example, in the case of the two 1990 floatingquadrants specially purchased to match the Lough Beltra cruise dates the satellite overpasseswere 21 July 1990 and 4 August 1990 respectively whereas the corresponding cruise dates werethe 23 July 1990 (Southeast Ireland) and 25, 28 and 30 July and 1 August 1990 (SouthwestIreland).5.2.3 Outline of Typical MethodologyThe LandSat images were sourced from the earth station located in Sweden on 9-track tapesthat were loaded onto a VAX workstation. LandSat source images are supplied as fixed orAcknowledgmentsThe remote sensing image analysis for oWorophyU was"^%f\°darse^EneTdurhlg'a sS'p'onsored by the EC MAST programme (DO XII/E;mast 0017 "The control ofphytoplankton dominance ).MASTReferencesTT 1QQ1 The mapping of relative chlorophyll concentration in Irish waters fromH°«y' ^isafm Illlite?majery. Final Report for Department of the Manne, LeesonLane, Dublin. 83pp.wr Cracknell A.P., Vaughan, R.A. and P.A. Davies. 1991. AsateUite and field®"^®'view ofthe Irish keiffront. Cont. Shelf. Res., 11, 543-562.d Connors P 1980. The occurrence of the red tide causing organism,Jenkins^ ai/reo/um Hulbert (Dinophyceae), round the south and west of Ireland inAugust and September, 1979. J. Sherkin Island, 1, 127-146.LeFevre J 1986. Aspects ofthe biology offrontal systems. Advances in Marine Biology, 23,1^-299.Raine R., O'Mahony, J., McMahon, T. and C. Roden. 1990. Hydrography and phytoplanktonof waters offsouthwest Ireland. Bstuarine Coast. Mar. Sci., 30, 579-592.Roden C.M. 1986. An upwelling zone around the Fastnet Rock. Proc. 2nd annual LoughBeltra workshop, G. O'SuUivan, ed., NBST, Dublin.Roden, C.M., Ryan, T and Lennon, H.J. 1980. Observations on the 1978 red tide inRoaringwater Bay, Co. Cork. J. SherkinIsland, 1, 105-118.Roden, C.M., Lennon, H.J., Mooney, E., Leahy, P., and Lart, W. 1981. Red tides, waterstratification and species succession around Sherkin Island, SW. Ireland. J. SherkinIsland, 1, 50-68.Simpson, J. and R. Pingree. 1978. Shallow sea tidal fi-onts produced by tidal stirring. InOceanic Fronts in Coastal Processes, M.J. Bowman and W.B. Bsaias, eds., Springer-Verlag, New York, pp 29-42.Tassan, S. 1987. Evaluation of the potential of the Thematic Mapper for marine applications.Int. J. Remote Sensing, 8, 122-150.

The oceanography of southwest Ireland: current research activities

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The oceanography of southwest Ireland: current research activities

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