Exploring the effects of nutrients, carbon, and water darkening on coastal phosphorus bioavailability

Coastal ecosystems are increasingly influenced by the lateral transport of organic matter, where pigmented dissolved organic carbon (DOC) contributes to water darkening and affects nutrient dynamics. These changes coincide with rising dissolved organic phosphorus (DOP) inputs, which have implications for eutrophication and carbon cycling. However, it is unclear how the bioavailable DOP (BDOP) pool responds to the individual and interactive ecosystem-level effects of water darkening, increased DOC, and higher inorganic nutrient concentrations. To explore these interactions, we conducted bioassays to estimate BDOP in a fully factorial mesocosm experiment manipulating the supply of inorganic nutrients, labile DOC (glucose) and pigmented compounds causing darkening. Results showed that while labile DOC had limited influence on bioavailable BDOP, nutrient enrichment increased BDOP in clear water. In darkened waters, added inorganic phosphorus persisted largely in its inorganic form, reflecting decreased conversion to BDOP. These findings reveal the complex interplay between light availability, organic matter inputs, and phosphorus bioavailability. By highlighting the impact of water darkening on nutrient and carbon dynamics, this study underscores the need for integrated management approaches to mitigate eutrophication and support ecosystem resilience across the terrestrial-aquatic continuum.