12th International Conference on Harmful Algae

INTERNATIONAL SOCIETY FOR THE STUDY OF HARMFUL ALGAE12 th <strong>International</strong> <strong>Conference</strong> on Harmful Algae, Copenhagen, Denmark, 4-8 September 2006Measurements of the physical andbiological environment were carriedout in the Ria de Pontevedra(Galician Rías Baixas, Spain) over atwo-week period in June 2005.Fine-scale vertical distributions ofphytoplankton and shear wereobtained using the IFREMERParticle Profiler and Fine ScaleSampler, coupled withmeasurements from a Nortekacoustic Doppler velocimeter, in situobservations of live samples(autofluorescence) and staineddinoflagellates. A sequence ofupwelling-relaxation-upwellingdownwellingevents was observed.Thin layers of Pseudo-nitzschia spp.and other diatoms (up to 30 µgchlorophyll a · L -1 ) developed andpersisted in the pycnocline region,above the cooler (12.5 ºC) nutrientrich(>10µM nitrate) upwelled water,but were vertically displaced anderoded during downwelling. Apopulation of D. acuminata, that hadbeen evident since early March,was never found within the thinlayers but instead was confined tothe warmer surface (0-4m) layersthroughout the entire survey. Thesecells did not perform any verticalmigration. The results showed theneed to define sub-surface thinlayer characteristics suitable forindividual species. In addition, theimportance of the phase of thepopulation growth to determinephysical-biological interactions andbehaviour of Dinophysis in relationto thin layers is highlighted.O.18-04Positive feedback and thedevelopment and persistence ofecosystem disruptive algalbloomsSession: O.18 - Ecology & Oceanography 1Presentation time: 10:10 - 10:30WG Sunda, DR HardisonBeaufort Laboratory, NOS, NOAA,BEAUFORT, NC, United States of AmericaHarmful algal blooms (HABs) haveoccurred with increasing frequencyin recent years with eutrophicationand other anthropogenic alterationsof coastal ecosystems. Many ofthese blooms severely disruptecosystem function, and can bereferred to as ecosystem disruptivealgal blooms (EDABs). Theseblooms are typically caused by toxicor unpalatable species thatdecrease grazing rates byherbivores, and thereby disrupttransfer of nutrients and energy tohigher trophic levels, and decreasenutrient recycling. Many factors,such as nutrient availability andherbivore grazing have beenproposed to separately influenceEDAB dynamics, but interactionsamong these factors have rarelybeen considered. Here we describepositive feedback interactionsamong nutrient availability,herbivore grazing, and nutrientcycling, which can to substantiallyinfluence the dynamics of EDABevents. The positive feedbacksresult from reduced grazing rates onEDAB species, which promote theproliferation of these algae anddecrease grazer-mediated recyclingof nutrients. These effects in turndecrease nutrient availability. Sincemany EDAB species are welladaptedto nutrient-stressedenvironments and many exhibitincreased toxin production andtoxicity under nutrient limitation,positive feedbacks are establishedwhich can greatly increase the rateof bloom development, and promotebloom persistence and adverseeffects on the ecosystem.71