Micronekton diel migration, community composition and trophic position within two biogeochemical provinces of the South West Indian Ocean: Insight from acoustics and stable isotopes
Résumé
Spatial distribution, community composition and trophic roles of micronekton (crustaceans, fishes and squids)
were investigated in the Indian South Subtropical Gyre (ISSG) province and the East African Coastal province
(EAFR), by combining acoustic surveys, mid-water trawls and stable isotope analyses from scientific cruises
conducted in 2009 and 2010. Mesopelagic micronekton performed diel vertical migrations in both provinces,
from deep (400–740 m) to surface (0–200 m) layers at dusk and in the opposite direction at dawn, with some
species migrating below 740 m. The EAFR province was more dynamic than the oligotrophic ISSG province, with
enhanced eddy activity and enhanced yearly productivity. The active enrichment mechanisms in the EAFR, in
terms of available primary production, led to high micronekton acoustic density (as a proxy of micronekton
abundance) and large micronekton weight and abundance estimates from trawl data. Particulate organic matter
in the EAFR exhibited greater enrichment in 13C and 15N compared to the ISSG and, consequently, tissues of
selected micronekton organisms in the EAFR were more enriched in 15N (higher δ15N values). In both provinces,
micronekton encompassed a wide range of isotopic niches, with large overlaps between species. Micronekton
and swordfish in the EAFR had an overlapping range of δ15N values, contrasting with the ISSG province where
swordfish were two trophic levels higher than the sampled micronekton. Our results provide some evidence that
the combined action of riverine input and the dynamics of eddies might influence productivity in the EAFR, and
hence the abundance of micronekton and the enrichment of tissues in 15N, compared to the oligotrophic ISSG
province.