Interannual variability of the Mediterranean trophic regimes from ocean color satellites

Abstract

Abstract. D'Ortenzio and Ribera d'Alcalà (2009, DR09hereafter) divided the Mediterranean Sea into "bioregions" based on the climatological seasonality (phenology) of phytoplankton. Here we investigate the interannual variability of this bioregionalization. Using 16 years of available ocean color observations (i.e., SeaWiFS and MODIS), we analyzed the spatial distribution of the DR09 trophic regimes on an annual basis. Additionally, we identified new trophic regimes, exhibiting seasonal cycles of phytoplankton biomass different from the DR09 climatological description and named "Anomalous". Overall, the classification of the Mediterranean phytoplankton phenology proposed by DR09 (i.e., "No Bloom", "Intermittently", "Bloom" and "Coastal"), is confirmed to be representative of most of the Mediterranean phytoplankton phenologies. The mean spatial distribution of these trophic regimes (i.e., bioregions) over the 16 years studied is also similar to the one proposed by DR09, although some annual variations were observed at regional scale. Discrepancies with the DR09 study were related to interannual variability in the sub-basin forcing: winter deep convection events, frontal instabilities, inflow of Atlantic or Black Sea Waters and river run-off. The large assortment of phytoplankton phenologies identified in the Mediterranean Sea is thus verified at the interannual scale, further supporting the "sentinel" role of this basin for detecting the impact of climate changes on the pelagic environment.

Hydrography and biogeochemistry dedicated to the Mediterranean BGC-Argo network during a cruise with RV <i>Tethys 2</i> in May 2015

“…This pattern is confirmed by the phenology of the underlying phytoplankton dynamics that varies from ultra-oligotrophic regimes in the eastern basin to bloom regimes in the northwestern basin (D'Ortenzio et al, 2009). An extended study on the geographical distribution of these regimes -related to the Mediterranean bio-regionshas revealed significant changes at regional scales during the last decades (Mayot et al, 2016). Indeed, the seasonal cycle of biomass concentration turns out to be a reliable indicator of the response of pelagic ecosystems to external perturbations (Siokou-Frangou et al, 2010).…”

Impact of decadal reversals of the north Ionian circulation on phytoplankton phenology

“…The metrics CHL_Year, CHL_March and CHL_Max refer to the yearly average [Chl a], the March average [Chl a] and the [Chl a] annual maximum, respectively. CHL_March has been defined here because previous studies demonstrated that the Mediterranean Sea trophic situation is highly variable in March, spatially and interannually ranging from oligotrophy to bloom conditions (D'Ortenzio et al, 2009(D'Ortenzio et al, , 2003Mayot et al, 2016). The bloom onset was defined as the date of the maximum growth rate.…”

“…Comparing surface chlorophyll a measured by the CZCS (Coastal Zone Color Scanner;1979-1985 and SeaWiFS (Sea-viewing Wide Field-of-view Sensor;-2000, D'Ortenzio et al (2003) observed just a slight difference in the timing of main bloom events and, during the SeaWiFS period, a recurrent and large patch of [Chl a]. More recently, Mayot et al (2016) studied interannual variability in phytoplankton phenology in the Mediterranean Sea. The authors showed that phytoplankton phenology is extremely dynamic in the north Ionian Sea with significant changes in the annual cycle of surface chlorophyll a concentration from year to year.…”

Impact of decadal reversals of the North Ionian circulation on phytoplankton phenology

“…Comparing surface chlorophyll-a measured by the CZCS (1979CZCS ( -1985 and SeaWiFS (1997SeaWiFS ( -2000 ocean color sensors, D 'Ortenzio et al (2003) observed just a slight difference in the timing of main bloom events and, during the SeaWiFS period, a recurrent and large patch of [Chl-a]. More recently, Mayot et al (2016) showed that phytoplankton phenology is extremely dynamic in the North Ionian Sea with significant 15 changes in the annual cycle of surface chlorophyll-a concentration from year to year. However, the authors did not explain this interannual variability.…”

“…Then, daily averages were computed 20 and the net heat flux was calculated as the sum of the four air-sea fluxes components. The surface buoyancy flux per unit area was computed according to Anderson et al (1996), with coefficients proposed by Gacic et al (2002) CHL_March has been defined here because previous studies demonstrated that in the Mediterranean Sea trophic situation is highly variable in March spatially and interannually ranging from oligotrophy to bloom conditions (D'Ortenzio et al, 2009;D'Ortenzio et al, 2003;Mayot et al, 2016). The bloom initiation was defined as the date of the maximum growth rate.…”