The oxygen isotopic composition of phosphate in marine and estuarine systems: A tracer for phosphate sources and cycling

Phosphorus (P) is an essential macro-nutrient required for life and may ultimately limit oceanic primary productivity over geologic timescales. Yet, little is known about the biogeochemical cycling of P. Because P has only one stable isotope it cannot be used as an isotopic tracer for sources and cycling. However, the most common mineral form of P is orthophosphate in which P is bonded to four oxygen (O) atoms. These P--O bonds are resistant to inorganic hydrolysis and do not exchange oxygen with water without biological mediation. Thus, systematic variability of the oxygen isotopic composition of phosphate (delta18Op) can potentially be used to determine phosphate sources and the extent of phosphate cycling in aquatic ecosystems.; The first part of this dissertation describes the development of a methodology to routinely extract dissolved inorganic phosphate (DIP) from natural water samples and from phosphatic compounds present in particulate organic matter (phytoplankton, sediments and soils) for oxygen isotopic analysis via high temperature pyrolysis mass spectrometry. In the second part of this dissertation, this methodology is applied questions of phosphate biogeochemical cycling. Results from these studies demonstrate the utility of the delta18 Op isotopic tracer. First, in a seasonal investigation of San Francisco Bay, an estuary that has been impacted by both urban and agricultural runoff, the delta18Op is used as an effective tracer for sources of phosphate. The delta18O p and salinity produce a two end-member mixing model for each month with slight excursions representing additional local phosphate sources. This approach has also worked well in Elkhorn Slough, a small nature reserve in a largely agriculturally dominated landscape. Transects along the slough and at different times throughout the tidal cycle demonstrate variations in phosphate sources in space and time. In contrast, a two year time series investigation of the Monterey Bay, an oceanic setting, shows seasonal variations in the delta 18Op which are associated with changes in phosphate cycling. These applications of delta18Op as a tracer for phosphate sources and cycling are the first of their kind and will improve scientific knowledge of the biogeochemical cycling of phosphate within marine and freshwater ecosystems.