The Differential Responses of Various Picoplanktonic Groups to Eddies in the Mozambique Channel
Quartly, Graham1; Cameron, Russell2; Zubkov, Mikhail3; Holland, Ross3
1Plymouth Marine Laboratory, UNITED KINGDOM; 2University of Southampton, UNITED KINGDOM; 3National Oceanography Centre, UNITED KINGDOM

The central Mozambique Channel has generally low levels of chlorophyll concentration in its surface waters indicative of its low productivity, with higher chlorophyll concentration and hence greater productivity in the coastal regions, aided by riverine outflow and coastal upwelling. However the channel is host to a complex physical flow regime, with trains of poleward propagating anticyclonic eddies on the western side, and a mix of transient features on the east. These modulate the pattern of chlorophyll concentration as recorded by satellite-borne ocean colour sensors.

A hydrographic cruise through this region in March-April 2006 recorded the in situ concentrations of various groups of picoplankton (plankton of size 0.2-5 microns) using flow cytometry. Using sea surface height data from multiple altimeters (Aviso's DUACS product) plus ocean colour records of surface chlorophyll, we can pinpoint a number of eddies, plus infer where there are strong onshore or offshore currents. Indeed, the availability of some near real-time satellite data enabled us to direct the ship across a cyclonic eddy and make a number of temperature profile measurements across it using CTDs and XBTs. The near-surface flow cytometric counts and estimates of cell sizes can then be placed in the context of a known physical flow pattern. One challenge is the disentangling of spatial and temporal variations, as many of the species exhibit strong diurnal variations in cell size as well as cell concentration. Modelling these variations as simple sinusoidal signals allowed us to study the different responses of cyabobacteria Synechococcus and Prochlorococcus as well as picoplanktonic algae to physical perturbations.