Macroalgae caging facilitates nitrogen nutrients monitoring in tropical oligotrophic environments

In oligotrophic tropical environments, low concentrations of dissolved nutrients often limit the detection of anthropogenic pressures when using spot hydrological measurements in seawater samples. This study evaluates the potential of macroalgae caging experiments to monitor nutrient inputs in coastal areas. Two common genus and species of macroalgae, Dictyota spp. and Tricleocarpa fragilis, were collected at a control site, then caged and exposed for 10 days in six sites, located in the Petit Cul-de-Sac Marin Bay (Guadeloupe, Lesser Antilles). These sites have been chosen due to their contrasted exposure to mineral pollution pressures, coming from urban, industrial, agricultural and/or wastewater flows. Macroalgae samples were then analyzed to measure their nitrogen isotopic composition (δ¹⁵N), elemental concentrations (%N and ratios C:N) and biochemical composition for Dictyota spp. (proteins, carbohydrates, lipids concentrations). These results were compared between exposed and control sites, but also between seasons (dry and wet seasons). Results showed an enrichment in δ¹⁵N at the most impacted sites, whereas C:N ratios displayed significant spatial variations among sites and seasons. In Dictyota spp., these isotopic and elemental changes were associated with increased protein concentration and caloric content. By integrating isotopic, elemental, and biochemical responses, macroalgae caging experiment emerges as a sensitive, reproducible, and operational tool for monitoring diffuse nutrient flows in coastal tropical environments.