Nutrient status, blue carbon stocks, and community perceptions in the seagrass meadows of Puttalam Lagoon, Sri Lanka: A socio-ecological assessment under monsoonal forcing

Abstract

Seagrass meadows are critical coastal ecosystems that provide nursery habitats, regulate nutrients, and sequester organic carbon. Despite their importance, tropical lagoons in Sri Lanka remain poorly studied. This study assessed seagrass diversity, seasonal nutrient dynamics, sediment organic carbon stocks, and community perceptions in the Puttalam Lagoon. Field surveys were conducted across four sampling campaigns representing the Northeast and Southwest monsoons between 2023 and 2025. Seagrass sampling was obtained by placing quadrats with three replicates; macronutrient analyses included total dissolved phosphorus, dissolved inorganic phosphorus, total dissolved nitrogen, and dissolved inorganic nitrogen; both vegetated (27) and non-vegetated (3) samples were extracted, using 0.6 m soil core; and thirty local stakeholders were interviewed. Five seagrass species were identified, with Enhalus acoroides dominant. Species densities were higher in the Northeast monsoon (mean 65.3 plants m⁻²) than in the Southwest (43.8 plants m⁻²). Seawater nutrients showed a phosphorus-dominated regime (N:P ratios < Redfield), as a result of agricultural runoff and aquaculture inputs. One-way ANOVA test confirmed no statistically significant differences (P > 0.05) in sediment carbon stocks in three sampling stations, the highest at the lagoon mouth (204.4 ± 44.4 Mg C ha⁻¹) and lowest at Dutch Bay (32.5 ± 16.3 Mg C ha⁻¹). Community surveys revealed moderate awareness of seagrass benefits for fisheries but limited recognition of carbon storage role. Findings highlight the ecological and socioecological importance of Puttalam Lagoon’s seagrass meadows. Protecting these habitats requires integrated monitoring of carbon and nutrient dynamics together with community-based conservation strategies to enhance resilience and blue-carbon benefits.