Variations in Remotely-Sensed Phytoplankton Size Structure of a Cyclonic Eddy in the Southwest Indian Ocean

Raymond Barlow,Robert Brewin,Tarron Lamont

doi:10.3390/rs10071143

Abstract

Phytoplankton size classes were derived from weekly-averaged MODIS Aqua chlorophyll a data over the southwest Indian Ocean in order to assess changes in surface phytoplankton community structure within a cyclonic eddy as it propagated across the Mozambique Basin in 2013. Satellite altimetry was used to identify and track the southwesterly movement of the eddy from its origin off Madagascar in mid-June until mid-October, when it eventually merged with the Agulhas Current along the east coast of South Africa. Nano- and picophytoplankton comprised most of the community in the early phase of the eddy development in June, but nanophytoplankton then dominated in austral winter (July and August). Microphytoplankton was entrained into the eddy by horizontal advection from the southern Madagascar shelf, increasing the proportion of microphytoplankton to 23% when the chlorophyll a levels reached a peak of 0.36 mg·m−3 in the third week of July. Chlorophyll a levels declined to <0.2 mg·m−3 in austral spring (September and October) as the eddy propagated further to the southwest. Picophytoplankton dominated the community during the spring period, accounting for >50% of the population. As far as is known, this is the first study to investigate temporal changes in chlorophyll a and community structure in a cyclonic eddy propagating across an ocean basin in the southwest Indian Ocean.

Highlights

Observations in the Indian Ocean have demonstrated that flow around the southern tip of Madagascar is complex [1,2,3]
This increase was due to the influence of approaching smaller cyclonic eddies to the north and east of the MB eddy (Figures 4c,d and 6a) that eventually merged with the Interest in the southern branch of the East Madagascar Current (SEMC) and eddies propagating across the Mozambique Basin (MB) stems from their role in transporting warm, salty Indian Ocean waters poleward into the Agulhas Current system, and their role in the shedding of rings into the Atlantic Ocean at the Agulhas Retroflection [3,32]
The application of the three-component model of Brewin et al [18] to satellite ocean colour data has proven to be useful for tracking the surface phytoplankton community within a cyclonic eddy as it propagated across the MB over a four-month period from mid-June to mid-October 2013. This is the first study to investigate the temporal changes in community structure within an eddy that moves across an ocean basin in the southwest Indian Ocean, and useful quantitative information was obtained on the variability of chlorophyll a levels and the proportions of micro, nano, and picophytoplankton

Summary

Introduction

Observations in the Indian Ocean have demonstrated that flow around the southern tip of Madagascar is complex [1,2,3]. Previous studies have suggested that the southern branch of the East Madagascar Current (SEMC) bifurcates, with a portion of the flow retroflecting and persisting east [4,5], and the remainder of the flow continuing west across the northern part of the Mozambique Basin (MB) and feeding into the Agulhas Current system (Figure 1), mainly in the form of mesoscale eddies [6,7,8,9,10]. A more recent investigation has shown that the SEMC flow dissolves into a series of cyclonic and anticyclonic dipole eddy pairs (Figure 1), and that there is no retroflection or westward jet [3]. Recently it has been shown that the number of cyclonic eddies exceeds the anticyclonic eddies generated southwest of Madagascar [9]. Analysis of water mass characteristics of these dipolar structures has shown that the Remote Sens. 2018, 10, x FOR PEER REVIEW cyclonic eddies consist of waters with Mozambique Channel (MC) properties, while the anticyclonic eddies asprterroucpcoetumrtriepessr,hiwsaeshdislehootfhwwenaantthteiarctsyctfhlroeonmcicycetldhodeniieScsEeadMrdeCiceos[m6c,po1nr1iss]ies.dt of of waters waters with from

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