NUTRIENT AND PHYTOPLANKTON RESPONSES TO EXTERNAL

Transcription

1 NUTRIENT AND PHYTOPLANKTON RESPONSES TO EXTERNAL FORCING IN A MEDITERRANEAN COASTAL AREA UNBIAISED BY TERRESTRIAL INPUTS AND LOCAL ACTIVITIES (CALVI, CORSICA) Anne GOFFART 1,*, Jean-Henri HECQ 1 and Louis LEGENDRE 2 1 University of Liège, Belgium & STARESO, Calvi, Corsica, France 2 Laboratoire d'océanographie de Villefranche (LOV), Villefranche, France * A.Goffart@ulg.ac.be Jan 13

2 Objectives To synthesize a long-term high-resolution study of nutrient and phytoplankton bloom dynamics performed between 1979 and 2011 at a permanent station in the Bay of Calvi (Corsica, northwestern Mediterranean) To understand mechanisms controlling the interannual variability of phytoplankton bloom in a context of climate change

3 Impossible d'afficher l'image. Votre ordinateur manque peut-être de mémoire pour ouvrir l'image ou l'image est endommagée. Redémarrez l'ordinateur, puis ouvrez à nouveau le fichier. Si le x rouge est toujours affiché, vous devrez peutêtre supprimer l'image avant de la réinsérer. The studied area : the Bay of Calvi, Corsica, Western Mediterranean 4 kms Open bay and narrow shelf Oligotrophic characteristics Few anthropogenic pressures High control by climate forcing Reference for the WFD Bay of Calvi

4 Time-series : subsurface data Water temperature, wind, phytoplankton and zooplankton from 1979 Nutrients from 1988 High sampling frequency during phyto- and zooplankton blooms (1-7 times per week) A.Goffart@ulg.ac.be JH.Hecq@ulg.ac.be STARESO STAtion de REcherches Sous-marines de l Université de Liège

5 Time-series : results The winter-spring phytoplankton bloom of the Bay of Calvi is characterized by a very large interannual variability ng Chl a l -1 1,5 1,0 0, reaching one order of magnitude from one year to another ng Chl a l -1 0,0 1,5 1,0 0,5 D J F M A M J J A S O N D 2007 What factors and/or processes control this variability? Water temperature Wind intensity Nutrient availability ng Chl a l -1 0,0 1,5 1,0 0,5 0,0 D J F M A M J J A S O N D 2009 D J F M A M J J A S O N D Zooplankton grazing The blue bars indicate the limits of the period of cold water (< 13,5 C)

6 Control of nutrient availability by wind stress In unstratified water column (surface temperature < 13,5 C), surface nutrient enrichment is strongly controlled by wind stress - annual example : 1999 Goffart et al in prep

7 Definition of a winter index intensity : WII WII takes into account the duration of the period of cold water and the frequency of windy days during the period of cold water

8 Control of nutrient availability by WII : time-series results Winter surface nutrient enrichment is strongly and linearly controlled by WII (R 2 = 0,86) Integrated NO 3 (water < 13.5 C) data Integrated NO y = -2, ,73x R 2 = 0, Winter intensity index Goffart et al in prep

9 Control of phytoplankton by WII : time-series results Phytoplankton biomass is strongly controlled by WII : increasing phytoplankton concentrations are observed for low and intermediate WII, while high WII are associated with lower phytoplankton biomasses (polynomial regression : R 2 = 0,70) Integrated Chl a (water < 13.5 C) data Winter intensity index Integrated Chl a (< 13.5 C) Y = M0 + M1*x + M2*x 2 + M3*x 3 M0 M1 M2 M3 R 2 2,12 5,15 10,59-2,08 0, Goffart et al in prep

10 Is zooplankton grazing responsible for lower phytoplankton biomasses associated with high WII? Phyto- and zooplankton biomasses increase concomitantly (linear regression : R 2 = 0,70). This implies that lower phytoplankton concentrations observed at high WII are not due to enhanced zooplankton grazing. Lower phytoplankton biomasses observed at high WII are probably due limitation of phytoplankton growth by reduced exposition to light in a highly mixed water column

11 Trends in WII over the period Winter intensity index WII phyto WII without phyto WII x x Year Goffart et al in prep

12 Conclusions This study, built from three decades of observation acquired in an oligotrophic Mediterranean coastal area, shows the combined effects of 2 key natural forcings (wind intensity and water temperature) on nutrient refueling of surface water, detects consistent patterns in terms of relationships between environmental drivers and response of winter-spring phytoplankton bloom, does not evidence any continuous change in phytoplankton biomass from 1979 but shows a year to year response to climate variation.

13 STARESO Thank you for your attention!