Phytoplankton dynamics in the southern New Caledonian lagoon during a southeast trade winds event

During the Echolag cruise (13 February–8 March 2007), the effect of 16–30 knot trade winds on abundance, community structure and dynamics of phytoplankton was investigated in the southern part of the New Caledonian coral reefs and surrounding oceanic waters. In this area, the coral reefs form two horn-like structures (hereinafter referred to as ‘horn reefs’), oriented south-east, and separated by a relatively deep valley. Three repeated samplings at one week intervals of a 32 station array showed that trade winds induce surface water cooling and significant enrichment of mid- and bottom waters above the reef shelf. This enrichment appeared as the result of a general rise of the oceanic thermocline and nitracline due to the combination of trade winds and tidally-generated internal waves between the horn reefs. It was accompanied by a factor of 1.5 and 2.3 mean increase for the chlorophyll concentration and picoplankton–nanoplankton cell number, respectively. From diel variations of in vivo chlorophyll a fluorescence and hourly sampling, phytoplankton gross growth rate was estimated to be 1–1.35 divisions per day and was quasi balanced with phytoplankton mortality for a 24 h period.