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

[摘要] Mid-latitude spring blooms of phytoplankton show considerableyear-to-year variability in timing, spatial extent and intensity. Itis still unclear to what degree the bloom variability is connected tothe magnitude of the vertical flux of organic matter. A coupledthree-dimensional hydrodynamic-biogeochemical model is used to relateinterannual variability in phytoplankton spring-bloom dynamics tovariability in the vertical export of organic matter in the NWMediterranean Sea. Simulation results from 2001 to 2010, validatedagainst remote-sensing chlorophyll, show marked interannualvariability in both timing and shape of the bloom. Model results showa tendency for the bloom to start later after cold and windywinters. However, the onset of the bloom occurs often when the mixedlayer is still several hundred metres deep while the heat flux isalready approaching zero and turbulent mixing is low. Frequency andintensity of wind episodescontrol both the timing and development of the bloom and the consequentexport flux of organic matter. The wintertime flux is greater thanzero and shows relatively low interannual variability. The magnitudeof the interannual variability is mainly determined in March when the frequencyof windy days positively correlates with the export flux. Frequentwind-driven mixing episodes act to increase the export flux and, atthe same time, to interrupt the bloom. Perhaps counterintuitively, our analysis shows that yearswith discontinuous, low-chlorophyll blooms are likely to have higherexport flux than years with intense uninterrupted blooms. The NW Mediterraneanshows strong analogy with the North Atlantic sectionwithin the same latitude range. Hence, our results may also be applicableto this quantitatively more important area of the world ocean.

[学科分类] 地球化学与岩石