Consequencesofcoastalupwellingeventsonphysicalandchemicalpatterns in thecentralGulfofFinland(BalticSea)

摘要

The consequencesofacoastalupwellingeventonphysicalandchemicalpatternswerestudiedinthe central GulfofFinland.Weeklymappingofhydrographicaland-chemicalfieldswerecarriedoutacross the GulfbetweenTallinnandHelsinkiinJuly–August2006.Ineachsurvey,verticalprofilesof temperatureandsalinitywererecordedat27stationsandwatersamplesforchemicalanalyses(PO4 3, NO2 +NO3 ) werecollectedat14stationsalongthetransect.Anordinarydistributionofhydrophysical and -chemicalvariableswiththeseasonalthermoclineatthedepthsof10–20mwasobservedinthe beginningofthemeasurementsinJuly.Nutrientconcentrationsintheuppermixedlayerwerebelow the detectionlimitandnutriclineswerelocatedjustbeloworinthelowerpartofthethermocline.In the firsthalfofAugust,averyintenseupwellingeventoccurrednearthesoutherncoastoftheGulf when waterswithlowtemperatureandhighsalinityfromtheintermediatelayersurfaced.High nutrientconcentrationsweremeasuredintheupwelledwater–0.4mmoll1 of phosphatesand 0.6 mmol l1 of nitrates+nitrites.Weestimatedtheamountofnutrientstransportedintothesurface layeras238–290tonsofphosphorus(P)-PO43 and 175–255tonsofN-NOx for a12mthick,20kmwide and 100kmlongcoastalstretch.Takingintoaccountacharacteristicalong-shoreextensionofthe upwelling of200km,thephosphate-phosphorusamountisapproximatelyequaltotheaveragetotal monthlyriverineloadofphosphorustotheGulfofFinland.ItisshownthatTS-characteristicsofwater massesandverticaldistributionofnutrientsalongthestudytransectexperienceddrasticchanges caused bytheupwellingeventintheentirestudiedwatercolumn.TS-analysisofprofilesobtained before andduringtheupwellingeventsuggeststhatwhilewelledup,thecoldintermediatelayerwater wasmixedwiththewaterfromtheuppermixedlayerwithashareof85%and15%.Wesuggestthatthe coastalupwellingeventscontributeremarkablytotheverticalmixingofwatersintheGulfofFinland. Intrusionsofnutrient-richwatersalongtheinclinedisopycnalsurfacesinthevicinityofupwellingfront wererevealed.Theupwellingeventwidenedtheseparationofphosphoclineandnitraclinewhichin turn preventedsurfacingofnitrate+nitrite-nitrogenduringthenextupwellingeventobservedaweek after theupwellingrelaxation.Asuggestionismadethatsuchwideningofnutriclineseparationcaused by similarupwellingeventsinearlysummercouldcreatefavourableconditionsforlatesummer cyanobacterialblooms.