Abundance and variation of colloidal organic phosphorus in riverine, estuarine, and coastal waters in the northern Gulf of Mexico

The abundance of colloidal organic phosphorus (COP) and colloidal inorganic phosphorus (CIP) was quantitatively determined using an ultrafiltration permeation model for riverine, estuarine, and coastal waters in the northern Gulf of Mexico. Dissolved inorganic phosphorus (DIP) was the dominant species in Mississippi and Pearl River waters, while dissolved organic phosphorus was dominant in marine environments. The abundance of COP was highest in the Pearl River (up to 88%), followed by the Mississippi Bight (~67%), and the Bay of St. Louis (~59%), but was lowest in the Mississippi River (41–50%). These variations highlight the roles of terrestrial inputs, autochthonous production, and anthropogenic activities in controlling the COP abundance in different aquatic environments. In the Gulf of Mexico, COP abundance generally decreased with increasing depth and coincided with chlorophyll a, reflecting the production of COP by phytoplankton and remineralization during downward transportation in the water column. The colloidal organic C: P molar ratios were substantially higher than the Redfield ratio but consistently lower than those of the bulk dissolved organic matter except for the Mississippi River, indicating a diagenetically fresher or younger COP pool. The percentage of CIP in the DIP pool was generally negligible or very low (≤3%) except in the Pearl River, where CIP abundance was as high as 27–47%, likely contributed from colloidal soils, minerals, and iron oxyhydroxides. We hypothesize that high COP abundance and seasonal P‐limitation play an important role in regulating the biogeochemical cycling of P and the development of hypoxia in the northern Gulf of Mexico.