Contrasting hydrography and phytoplankton distribution in the upper layers of cyclonic eddies in the Mozambique Basin and Mozambique Channel

Abstract

Hydrographic data collected in cyclonic eddies in the Mozambique Channel and  Basin revealed notable differences in temperature and salinity at a depth of 100 m, the upper mixed layer, the nitracline depths, and vertical distribution of chlorophyll-a (Chl-a). Differences in temperature and salinity did not show any consistent   patterns. In contrast, the differences in the upper mixed layer, nitracline depths and the vertical Chl-a profile appeared to be driven by combined effects of eddy  dynamics (i.e. shoaling of isopleths) and the seasonal variation in light availability and mixing conditions in the upper layers. Cyclonic eddies studied during austral spring and summer in the Mozambique Channel exhibited shallower upper mixed layers and nitracline depths, and deeper euphotic zones. Distinct subsurface Chl-a maxima (SCM) were associated with the stratified conditions in the upper layers of these eddies. In contrast, a cyclonic eddy studied during mid-austral winter in the Mozambique Basin had a shallower euphotic zone, deeper upper mixed layer and uniform Chl-a profiles. Another eddy sampled in the Mozambique Basin toward the end of winter showed a less pronounced SCM and roughly equal euphotic zone and upper mixed layer depths, suggestive of a transition from a well-mixed upper layer during winter to stratified conditions in summer.

Keywords: chlorophyll-a, euphotic zone, fluorescence profile, hydrographic survey,  mesoscale, nitracline, phytoplankton biomass, seasonal variation, subtropical surface waters, thermodynamics, western Indian Ocean