Localized inshore warming, acidification, and elevated particulate organic matter across a coupled mangrove,...

Global declines in mangroves, seagrasses, and corals threaten the provisioning of ecosystem services to coastal communities. However, potential feedbacks between these ecosystems are poorly understood owing to a lack of studies exploring functional links between these frequently coupled nearshore tropical ecosystems. To better understand these links, we sampled seawater temperature, salinity, and pH in addition to particulate organic carbon, particulate organic nitrogen, and their respective stable isotope composition (δ¹³C and δ¹⁵N) monthly from July 2018 to June 2019 at sites ranging from inshore to offshore in La Parguera Natural Reserve, Puerto Rico. We observed persistent, year-round warmer, more acidic, and higher particulate organic matter seawater at the most inshore Bioluminescent Bay station compared to sites further offshore. Particulate organic carbon values ranged from a maximum of 764 mg/m³ at the most inshore station to a minimum of 53.96 mg/m³ at the most offshore station while particulate organic nitrogen ranged from 166 mg/m³ at the most inshore station to 6.39 mg/m³ at the most offshore station. Biogeochemical variability across spatial scales largely followed gradients in biological and physical controls, with limited differences between the more offshore coral reef sites and the most intense biogeochemical modification occurring in the shallowest and most nearshore stations. Limited temporal variability was observed for most parameters except for seasonal variations in temperature, salinity, and pH. Particulate organic matter stable isotope composition were slightly enriched at inshore stations and suggested a mixture of primarily marine allochthonous and autochthonous organic matter sources. The importance of heterotrophy for coral resilience to warming and acidification suggests this localized nutrition-rich albeit warmer and acidified waters in nearshore mangrove and seagrass ecosystems is an important functional link to nearshore corals with the potential to modulate coral resilience to ocean warming and acidification.