The upper boundary of the Southwest Atlantic mesophotic zone is now considered to be closer to 15-18 meters, according to a Brazilian study conducted in So Paulo that has challenged the 30-meter estimate and highlighted significant differences in species and ecology at this depth.
The subtropical ocean region along the South American coastline has been redefined by scientists. They discovered that the upper boundary of the mesotrophic zone is actually occupying shallower waters than previously believed.
The vertical boundaries of marine environments in the Southwest Atlantic region of South America have been accurately defined for the first time by researchers at a small research group funded by FAPESP and a USP research center. This area includes both offshore and coastal areas along the South Atlantic coast.
A report on the study is published in the journal Marine Environmental Research.
The central region, also known as the "middle light" zone, is where sunlight can only reach the ocean's surface at a maximum depth.
Although some earlier research has estimated the deepest limit of this zone to be 30 m, the authors' measurements of light penetration and fish stocks indicate that it is situated at a depth between 15 m and 18 m in the subtropical coastal region.
A group of Tomtate grunts, Sea chubs, and Horse-eye jacks, as well as a Spotted moray and a Squirrelfish flanked by the Navarro raptors at the base of the photograph, complete the scene.
During her research for a master's degree in biodiversity and marine and coastal ecology at the Institute of Marine Sciences (IMAR-UNIFESP) in Santos (So Paulo state, Brazil), Maisha Gragnolati, the lead author of the study, discovered a diverse range of fish fauna and migratory species that move between shallow and mesophotic habitats.
The researchers claim that the classical definition (30 m) in the scientific literature differs from their conclusion (18 m) because the majority of previous studies conducted in tropical areas above the Tropic of Capricorn, while the Southwest Atlantic is predominantly subtropical (under this line).
Research on coral reefs is typically limited, while rocky reefs are more prevalent in subtropical regions and exhibit diverse chemistry with light and living organisms, as noted by Fábio Motta, a professor at UNIFESP and LabecMar.
The research was part of a project funded by FAPESP's BIOTA program, which focuses on the application of science to the management of marine protected areas, encompassing the management of public use, such as the biodiversity of subtropical mesophotic reefs, with Motta taking the lead as the primary investigator.
The scientists conducted studies on 12 rocky reefs in Laje de Santos State Park, the Central Coast Marine Environmental Protection Area, and Tupiniquins Ecological Station in So Paulo, examining temperature, depth, and light penetration, as well as inventorying landforms and fish species around islands.
The researchers employed BRUVs, equipped with waterproof cameras on tripods and a long arm holding a sardine in hand, to explore fish groups in shallow and mesophotic habitats at depths between 6 m and 43 m. They recorded video for an hour and used an international ocean database to estimate the amount of light that entered the area and analyze the fish species found.
Gragnolati observed that the direct impact of light on primary production results in a smaller number of organisms that rely on light for photosynthesis, and that there were no herbivorous fish in the area.
The video footage was analyzed using software that recognized the fish species, and was used to count and measure the fish, as well as estimate their relative abundance and quantify the biomass in that area, using software.
By dividing them by diet (carnivore, herbivore, or omnivore) and their role in the fishing area, the findings showed that the mesopotamian zone had a 73% lower diversity than the shallow zone, with a group of eight species making up half of the total divergence.
In the shallow zone, the Tomtate grunt (Haemulon aurolineatum) was the species most frequently observed, along with the Red porgy or Common seabream (Pagrus pagrus) and the Sand perch (Diplectrum formosum).
The study found that areas where fishing is prohibited, including Laje de Santos State Park, had 2.5 times the species richness and eight times the target species biomass, as evidenced by the ecological effects of such areas.
While the So Paulo coast is renowned for its marine protection, only 5.7% of Brazil's territory is considered endangered due to fishing.
The scientific method of marine biology, which includes the statistical analysis of reef fish assemblages and the revelation of new boundaries of mesophotic ecosystems in the subtropical Southwestern Atlantic, was the subject of an innovative study conducted in 2024.
At the Brazilian Reef Meeting (EReBra) in Niterói (Rio de Janeiro state) in early May, the study was recognized for the best oral presentation, and a prize was awarded to the team.
The So Paulo Research Foundation made a contribution to the study.