Phosphorus Reservoirs in the St. Lawrence Upper Estuary

Abstract

A first-order phosphorus (P) budget is presented for the St. Lawrence Upper Estuary. The upwelled intermediate waters of the Laurentian Trough contribute 90% of the dissolved P source to the estuary whereas the St. Lawrence River carries in most of the particulate P. The dissolved P flux to the estuary (~80 000 t∙yr⁻¹) is about 10 times larger than the particulate one (~5000 t∙yr⁻¹, more than half being detrital apatite). However, the dissolved and particulate estuarine P reservoirs are of the same order of magnitude (~3000 t) because of the short residence time of the water masses relative to that of the solid matter. This rapid water mass turnover is responsible for the conservative behavior of dissolved orthophosphate (PO₄) throughout the estuary; dissolved PO₄ concentrations are not significantly affected by the biogeochemical exchange reactions involving particulate P. Approximately 500 and 300 t PO₄∙yr⁻¹ are respectively adsorbed on the flocculating iron hydroxide colloids and desorbed from marsh deposits. Marsh macrophytes are the main P consumers in the estuary (~100 t∙yr⁻¹), but detectable PO₄ changes attributable to this biological uptake are confined to the areas near the marshes.