Phosphorus Speciation and Sedimentary Phosphorus Release from the Gulf of Mexico Sediments: Implication for Hypoxia.

Phosphorus (P) is an important macronutrient that can limit primary productivity in coastal marine ecosystems. This study provides a quantitative determination of the abundance and chemical speciation of P and potential anaerobic P-release rates from northern Gulf of Mexico (GOM) sediments. Seven sediment cores were collected from the shelf, slope and deep-sea sediments in the northern GOM. A sequential extraction method was used to operationally separate the P pools into readily available-P, Fe/Al-P, alkali extractable organic-P, Ca/Mg-P, and residual-P. The Ca/Mg-P was the most abundant form of sediment P (79.0 ± 9.4% of TP). Northern GOM sediments were found to release 0.02-4.4 mg kg^-1 d^-1 of bioavailable-P under anoxic sediment condition. The P-release rate had significant positive correlation with the Fe/Al-P fraction, indicating this relatively small pool of TP (∼0.4-10.2%) is contributing to the water column nutrients during hypoxia events. However, future increasing ocean acidification can potentially remobilize the major P fraction (Ca/Mg-P), releasing a much larger amount of available-P into the overlying water column. The findings of this study suggest that internal sedimentary P loading contributes substantial bioavailable-P to the P-limited water column which leads to increased primary production in the GOM, creating a positive feedback mechanism for hypoxia. Therefore, the action plan to reduce the GOM hypoxia zone needs to take the internal sediment P loading into account when managing for watershed nutrient loading.