Gulf of Mexico Sediment Phosphorus Fractionation: Implication for Hypoxia

Abstract:

Phosphorus (P) is one of the limiting nutrients in coastal environments, especially in river- dominated deltaic regions with large amount of inorganic nitrogen inputs from the river. The principal source of P to coastal waters is suspended particulate matter from river discharge, a large portion of which is deposited on the continental shelves. The bottom sediments thus can be an important source of P to the overlying water column depending on the concentrations and chemical forms of P. This study provides a quantitative determination of the abundance and chemical speciation of P from the shelf, slope and deep sea sediments of coastal Louisiana. A sequential extraction method (SEDEX) was used to operationally separate the P pools into readily available P, Fe/Al-bound P, alkali extractable organic P, Ca/Mg-bound P, and residual/refractory P phases. Samples were analyzed for total metals (Ca, Mg, Fe, Al) and total organic carbon to quantify their association with different forms of P. In general, TP was well correlated to the concentrations of Mg (p<0.05). The concentrations of available P, alkali extractable organic P, and refractory P higher in deep sea sediments, while the shelves sediments had higher concentrations of Fe/Al-bound P. Total phosphorus (TP) concentrations ranged between 425 to 1883 mg/kg. Available P was the smallest pool (<1% of TP), while Ca/Mg-bound P was the most abundant (~80% of TP) pool. Phosphorous sedimentary fractions can provide us with the insights into burial, digenesis and residence time of P in the coastal sediments in the northern Gulf of Mexico. The flux of P from bottom sediments provides an additional source of P to the nitrogen-rich water column that can stimulate algal growth, ultimately contributing to hypoxia in coastal Louisiana.