Use of δ15N and macroalgae as indicators of the level of anthropogenic intervention in the Colombian Pacific.

Coastal ecosystems are highly vulnerable to nutrient-driven eutrophication from anthropogenic sources such as urbanization, wastewater discharge, and industrial development, among others, which alters their ecosystem services. In order to determine nitrogen sources, the nitrogen isotopic composition (δ¹⁵N) was analyzed in the macroalgae Boodleopsis verticillata and Bostrychia spp., collected between 2014 and 2016 at four localities with different degrees of anthropogenic disturbance: Valencia – Very Low Intervention (MBI-VA), Chucheros – Low Intervention (BAI-CHU), San Pedro – Moderate Intervention (MOI-SP), and Piangüita – High Intervention (ALI-PI) in the Colombian Pacific. The δ¹⁵N values ranged between 0.3 and 2.4‰ in MBI-VA, 1.8 and 3.4‰ in BAI-CHU, 2.3 and 5.5‰ in MOI-SP, and 2.3 and 10.16‰ in ALI-PI. Since the assumptions of normality and homogeneity of variances were not met (p < 0.05), a non-parametric Kruskal–Wallis test was applied, revealing significant differences in δ¹⁵N among localities (p < 0.0001). Dunn’s test indicated that MBI-VA and BAI-CHU differed significantly from MOI-SP and ALI-PI (p < 0.05). Three nitrogen sources were defined: atmospheric deposition, oceanic waters, and wastewater. Both species (B. verticillata andBostrychia spp.) showed a decreasing gradient of atmospheric deposition (87% ± 3% to 52% ± 7% and 82% ± 6% to 21% ± 11%, respectively) from MBI to ALI, in contrast to an increase in oceanic waters (8% ± 4% to 37% ± 13% and 12% ± 7% to 38% ± 21%) and wastewater contributions (5% ± 2% to 12% ± 6% and 7% ± 3% to 41% ± 12%). This pattern was more evident in Bostrychia spp., suggesting greater sensitivity to variations in nitrogen sources. Linear regression between δ¹⁵N and nitrate concentration yielded coefficients of determination of R² = 0.71 for B. verticillata and R² = 0.89for Bostrychia spp., indicating that isotopic variability was explained by nitrate. The potential of macroalgae as bioindicators of anthropogenic intervention in coastal ecosystems of the Colombian Pacific is suggested.