Island Mass Effect around the Marquesas Islands

The Marquesas Islands are characterized by the presence throughout the year of a remarkable biological enrichment which is attributed to the Island Mass Effect (IME). This enhancement of productivity and biomass can extend up to 800km downstream of the islands and is mainly noticeable west of the archipelago and more marked in the austral summer. While these enrichments are particularly visible by satellite, there is very little in situ data to conclude on 1) the biophysical processes involved, i.e. hydrodynamic conditions associated with the presence of the archipelago (coastal upwelling, eddies formation), terrigenous inputs controlled by wind and rain regimes, non-limiting micro-nutrients, influence of equatorial upwelling, and 2) the structure and .functioning of the planktonic ecosystem.

The objective of the MOANA-MATY cruises is to understand through in situ measurements the physical and biogeochemical processes at the origin of the IME in the Marquesas archipelago as well as the nature of enrichment and its impact on the higher trophic levels. In this study area, the different hypotheses put forward on the origin and nature of enrichments can only be confirmed by a detailed study of i) hydrodynamics and hydrology, ii) biogeochemistry and iii) structure and dynamics of phyto- and zooplankton communities. Two cruises have been scheduled at different seasons: the first cruise MOANA-MATY 2018 was conducted in the cool season (Sept-Oct 2018) and the second one MOANA-MATY 2019 in the warm season (Feb.-March 2019). The sampling strategy selected takes into account both the north-south and inshore-offshore gradients, the marked contrast between the enriched zone in the wake of the islands and the non-enriched zone upstream and the zones of strong turbulence.