Scenario-Based Productivity and Potential Long-Term Carbon Retention for Mesophotic Kelp Habitats in the Southwestern Atlantic

Mesophotic kelp habitats may contribute to marine carbon cycling, yet their productivity and long-term carbon retention remain poorly constrained. We assessed a depth-constrained analytical support domain for the endemic and endangered Brazilian kelp Laminaria abyssalis across 45–70 m on the southwestern Atlantic shelf using a published species distribution model, high-resolution bathymetry, biomass observations, and literature-based productivity scenarios. Accounting for within-cell bathymetric variation yielded 12,514.60 km2 of depth-constrained analytical support within non-NA SDM coverage. The biomass dataset contained 23 observations aggregated into 19 raster cells. Leave-one-out validation showed that inverse distance weighting did not outperform a non-spatial mean (RMSE = 75.25 vs. 74.02 kg km−2; r = −0.389, p = 0.100); therefore, no regional biomass or biomass-derived productivity surface was produced. Using the only depth-overlapping Laminaria productivity analog (22.32 g C m−2 yr−1), the full-occupancy area-integrated NPP scenario was 279,326 t C yr−1, with an empirical transferability envelope of 4242–1,074,367 t C yr−1. At a long-term retention fraction of 0.11, the retained-carbon equivalent ranged from 467 to 118,180 t C yr−1. These values are assumption-dependent screening estimates, not measurements of export, burial, sequestration, or carbon-credit potential. Field measurements of occupancy, local productivity, transport, burial, and permanence are required before carbon-accounting claims can be supported.