New Study Redefines Marine Boundaries in the Southwest Atlantic

Researchers have newly established the vertical boundaries of the subtropical ocean region along the South American coastline. They discovered that the upper boundary of the mesotrophic zone is actually located in shallower waters than previously thought.

Researchers at the Federal University of São Paulo (UNIFESP) and the University of São Paulo (USP), funded by FAPESP, have accurately defined the vertical boundaries of marine environments in the Southwest Atlantic for the first time. This area includes the offshore and coastal zones along the Atlantic coast of South America.

The study is reported in an article published in the journal Marine Environmental Research.

The main finding is the upper boundary of the mesophotic zone, the “middle light” region between the brightly lit ocean surface and the darkest depths. The lower limit of the mesophotic zone is the furthest that sunlight can penetrate the ocean.

Previous studies put the upper limit of this zone at a depth of 30 m, but the authors’ measurements of light penetration and fish inventories showed it to lie at a depth of between 15 m and 18 m in the subtropical coastal region.

A Spotted moray (Gymnothorax moringa), at the bottom of the frame, surrounded by a shoal of Tomtate grunts (Haemulon aurolineatum), Sea chubs (Kyphosus spp.), and Horse-eye jacks (Caranx latus), with one Squirrelfish (Holocentrus adscensionis), at Saco da Banana near “Snake Island,” properly called Ilha da Queimada Grande. Credit: LabecMar-UNIFESP

“Besides the amount of light, which at this depth is only 10% of surface light incidence, we detected a different fish fauna, as well as DOI: 10.1016/j.marenvres.2024.106527

The study won a prize for the best oral presentation in early May at the Brazilian Reef Meeting (EReBra) held in Niterói (Rio de Janeiro state).

The study was funded by the São Paulo Research Foundation.