Nutrient, carbon, and darkening impacts on coastal dissolved phosphorus bioavailability—a mesocosm study

Abstract Coastal eutrophication results from increased riverine loads of inorganic nutrients, including phosphorus (P), which may co‐occur with increased dissolved organic carbon (DOC) loading. These DOC molecules are often pigmented, causing water darkening, but they also contain dissolved organic P (DOP), which could exacerbate eutrophication. However, it is unclear how the bioavailable DOP (BDOP) pool responds to the individual and interactive effects of increased DOC, higher inorganic nutrient concentrations, and water darkening in coastal ecosystems. To explore these interactions, we conducted bioassays to estimate BDOP in a fully factorial mesocosm experiment manipulating the supply of labile DOC (glucose), inorganic nutrients and pigmented compounds that cause darkening. Whereas the evidence for labile DOC (glucose) effects on BDOP was weak, inorganic nutrient enrichment caused increases in BDOP concentrations in clear‐water mesocosms. By contrast, in experimentally darkened waters, the addition of inorganic P did not contribute to BDOP but mainly persisted in its inorganic form. Our results suggest that water management efforts aimed at preventing or reversing coastal darkening could increase the removal of excess inorganic P from the water due to light‐enhanced algal uptake. However, the total dissolved bioavailable P pool may not decrease but rather shift from dominance by inorganic to organic forms. Therefore, mitigating both coastal darkening and eutrophication in these ecosystems is essential for reducing total bioavailable P to a level that supports their ecological balance and functionality.