Factors controlling interannual variability of vertical organic matter export and phytoplankton bloom dynamics – a numerical case-study for the NW Mediterranean Sea

R Bernardello,N Bahamon,A Cruzado,I Marinov,D Donis,J G Cardoso

doi:10.5194/bg-9-4233-2012

Abstract

Abstract. Mid-latitude spring blooms of phytoplankton show considerable year-to-year variability in timing, spatial extent and intensity. It is still unclear to what degree the bloom variability is connected to the magnitude of the vertical flux of organic matter. A coupled three-dimensional hydrodynamic-biogeochemical model is used to relate interannual variability in phytoplankton spring-bloom dynamics to variability in the vertical export of organic matter in the NW Mediterranean Sea. Simulation results from 2001 to 2010, validated against remote-sensing chlorophyll, show marked interannual variability in both timing and shape of the bloom. Model results show a tendency for the bloom to start later after cold and windy winters. However, the onset of the bloom occurs often when the mixed layer is still several hundred metres deep while the heat flux is already approaching zero and turbulent mixing is low. Frequency and intensity of wind episodes control both the timing and development of the bloom and the consequent export flux of organic matter. The wintertime flux is greater than zero and shows relatively low interannual variability. The magnitude of the interannual variability is mainly determined in March when the frequency of windy days positively correlates with the export flux. Frequent wind-driven mixing episodes act to increase the export flux and, at the same time, to interrupt the bloom. Perhaps counterintuitively, our analysis shows that years with discontinuous, low-chlorophyll blooms are likely to have higher export flux than years with intense uninterrupted blooms. The NW Mediterranean shows strong analogy with the North Atlantic section within the same latitude range. Hence, our results may also be applicable to this quantitatively more important area of the world ocean.

Highlights

The dynamics regulating the vertical flux of organic matter in the ocean determine the partitioning of carbon between surface and deep ocean and the transfer of organic matter to higher trophic levels
The two Mean Dynamic Topography (MDT) reproduce the main cyclonic circulation in the northern sector, roughly coinciding with the bloom area defined for this study (Fig. 1)
In the RioMDT the northern current is clearly visible along the northern coast from Italy up to the Ibiza channel while in the model MDT it appears somewhat more intense on the Italian coast and starts decreasing along the Spanish coast, in agreement with current measurements of ∼5 cm s−1 off the Ebro delta (Font et al, 1995)

Summary

Introduction

The dynamics regulating the vertical flux of organic matter in the ocean determine the partitioning of carbon between surface and deep ocean and the transfer of organic matter to higher trophic levels. These dynamics affect both climate and the ocean’s ability to sustain fisheries. Primary production of organic matter occurs all year round, with a seasonal bloom at all mid-latitude oceans These blooms are an important component of the total CO2 ocean uptake (Takahashi et al, 2009) which is achieved through the export of organic matter to the deep ocean. Blooms show considerable variability in time, space and intensity, and this variability is related to physical processes that affect vertical mixing.

Methods

Results

Discussion

Conclusion