Spatial variations of nutrient and trace metal concentrations in macroalgae across blue carbon habitats of the Saudi Arabian Red Sea

Macroalgae play a crucial role in blue carbon ecosystems, yet their elemental compositions in the Red Sea are not well documented. This study examined the concentrations of 22 elements in 161 macroalgae blade samples from 19 species (5 unidentified) across 3 phyla in the Saudi Arabian Red Sea. Macroalgae blades collected from coral reef habitats exhibited higher concentrations of K, As, and Sr compared to those from seagrass meadows, but had lower levels of total nitrogen (TN), Na, Mg, Al, P, S, Cr, Mn, Fe, and Zn. These differences may be attributed to the limited or absent sediment in coral reef habitats, as sediment in seagrass meadows acts as a trap for heavy metals, influencing element accumulation. In the Southern Red Sea, macroalgae blades showed lower Sr levels but higher total organic carbon (TOC), TN, P, and Cd than those in the North, a trend influenced by nutrient inflows from the Indian Ocean and the semi-enclosed structure of the Red Sea. The study found that macroalgae blades had lower mean TOC and TN content than seagrass and mangrove leaves from the same research cruises but higher levels of Al, Cr, Ni, Cu, Zn, As, Mo, Cd, and Pb. Among the surveyed red, green, and brown macroalgae blades, Halymenia sp., Caulerpa taxifolia, and Dictyota sp. had the highest TOC, TN, and P contents, while Amphiroa fragilissima, Udotea flabellum, and Padina sp. had higher trace metal contents. Notably, some macroalgae blades, including red macroalgae A. fragilissima, green macroalgae Halimeda tuna, U. flabellum, and brown macroalgae Padina pavonica, Turbinaria ornata, collected from Al Birk, Al Lith, Thuwal, Yanbu, and Al Wajh exhibited concerning levels of Cr (~10.9 mg kg^-1) and Ni (~27.2 mg kg^-1), surpassing toxicity thresholds. These findings emphasize the urgent need for effective pollution mitigation measures to protect these marine ecosystems.