A Baseline Survey of Red Algae (Rhodophyta) at Three Coastal Localities in Northern Luzon, Philippines

Rhodophyta are ecologically and economically important components of marine macroalgal assemblages, yet comparable site-level information from geographically separated coastal localities in Northern Luzon remains limited. This study documented Rhodophyta composition, site occurrence, and diversity at Brgy. Daddao, Calayan, Cagayan; Brgy. Canaoay, San Fernando, La Union; and Brgy. Didaddungan, Palanan, Isabela. Each locality was surveyed once during August-September 2025 using three 30-m line transects with fixed observation points at 10, 20, and 30 m. Taxa were identified primarily from macromorphological characters using regional references, with names cross-checked against AlgaeBase and representative vouchers retained. Thirty-one Rhodophyta taxa representing 9 orders and 14 families were recorded. During the survey, Calayan had the highest observed richness (24 taxa) and thallus count (253), followed by Palanan (7 taxa; 104 thalli) and San Fernando (5 taxa; 11 thalli). Shannon diversity index was highest in Calayan (H' = 2.921), Pielou's evenness was highest in San Fernando (J' = 0.961), and Simpson's dominance was highest in Palanan (D = 0.263). Twenty-seven taxa were recorded at only one surveyed locality, three occurred at two localities, and Tricleocarpa cylindrica occurred at all three. The marked site-to-site turnover is consistent with spatial heterogeneity reported in Philippine macroalgal surveys, but the present single-period design does not identify its environmental causes. Physicochemical measurements were therefore treated as contextual observations rather than predictors. The study provides a voucher-backed, Rhodophyta-focused baseline for future seasonal, spatial, habitat-based, and taxonomic monitoring in Northern Luzon