Contribution of heterotrophs to secondary calcification in marginal reefs of the Mexican Pacific

Background. Sclerobionts (e.g., calcareous algae, bryozoans, polychaetes, mollusks, and barnacles) produce reef calcium carbonate (CaCO3). Their contribution is key to maintaining positive CaCO3 balances, especially in marginal reefs. Objective. To compare the production of CaCO3 by sclerobionts in two marginal reefs of the Mexican Pacific: Las Gatas (LG), in Zihuatanejo Guerrero Bay, and La Llave (LL), in Bahía de Los Angeles (Gulf of California). Methods. CAUs (Calcification/Accretion Units) were used to promote sclerobiont recruitment during two deployment times: 6 and 15 months. Results. The calcification rate was high at six months and then decreased due to rapid colonization and initial growth, followed by a decrease over time. Sclerobionts deposited 1.2 ± 0.4 kg CaCO3 m-2 yr-1 in LG, which represents 7% of the production of branching corals in the Mexican South Pacific (17.2 kg m-2 yr-1), while in LL, they deposited 2.1 ± 0.7 kg CaCO3 m-2 yr-1; equivalent to 20% of the production of massive corals in the area (10.1 kg m-2 yr-1). The groups that deposited most CaCO3were mollusks and bryozoans in LG (up to 0.65 ± 0.16 kg m-2 yr-1) and barnacles in LL (up to 2.32 ± 0.35 kg m-2 yr-1). Conclusions. These results highlight the role of heterotrophs as secondary calcifiers both in LG, a site impacted by anthropogenic activity, and in LL, an area with low anthropogenic impact but high biological productivity associated with upwellings. This finding implies that the environmental conditions at the study sites limit the calcification of primary calcifiers (i.e., corals and CCA) but promote that of secondary calcifiers, with potential geomorphic repercussions