A time series investigation of the oxygen isotopic composition of dissolved inorganic phosphate in Monterey Bay, California

Abstract

We present a 2-yr time series of the oxygen isotopic composition of dissolved inorganic phosphate (d 18 O p ) in Monterey Bay, California. d 18 O p can serve as a proxy for phosphate utilization because its approach toward isotopic equilibrium is a function of the degree of biological phosphate cycling. When phosphate is extensively cycled by the biological community in the euphotic zone, the d 18 O p approaches isotopic equilibrium with surrounding water. Our results indicate that d 18 O p in the upper water column is consistently out of isotopic equilibrium with respect to seawater and fluctuates between the equilibrium value and the isotopic signatures of phosphate sources. This suggests that either phosphate is not extensively utilized or that the input of new phosphate to the bay exceeds the requirements of the biological community. d 18 O p is variable and linked to episodic upwelling events and the biotic response to these events. The greatest percent of phosphate oxygen exchange, and thus the greatest phosphate utilization relative to input, occurs at the locus of upwelling (Sta. M1), followed by the offshore (M2) and nearshore (C1) stations, respectively. During the highly productive upwelling season, phosphate turnover is greater than in the winter. Episodes of higher phosphate turnover, as indicated by peaks in d 18 O p , occur simultaneously throughout the upper 200 m of the water column and at all three stations, indicating that these events affect the bay on a large scale. d 18 O p data also suggest that deep water (.500 m) may be a source of phosphate to the euphotic zone in Monterey Bay.

Phosphate oxygen isotope ratios as a tracer for sources and cycling of phosphate in North San Francisco Bay, California

“…We used the October 2002 Station 19 value as the oceanic end‐member value because the January, June, and August 2004 transects did not extend to Station 19 and the salinity values measured at Station 18 are suggestive of some degree of freshwater influence during these periods. The values for salinity, δ 18 O p , and phosphate concentration observed in October 2002 at Station 19 are similar to values observed in January, June, and August 2004 in the coastal waters of Monterey Bay, 100 miles south of the San Francisco Bay, suggesting that this end‐member is indeed representative and is relevant for all of the transects (all seasons) [ McLaughlin et al , 2006].…”

Phosphorus availability and turnover in the Chesapeake Bay: Insights from nutrient stoichiometry and phosphate oxygen isotope ratios

“…One of the most important processes is the reversible reaction between orthophosphate and pyrophosphate catalyzed by pyrophosphatase, which exchanges all O atoms in PO 4 with ambient water O, resulting in a temperature dependent equilibrium fractionation [ Blake et al ., ; Longinelli and Nuti , ]. This isotopic equilibration between PO 4 and water has been observed in many environments as an indicator of biological P turnover (i.e., PO 4 uptake, intracellular cycling, and PO 4 release into environment) [ Kolodny et al ., ; McLaughlin et al ., , ; Paytan et al ., ]. Moreover, many extracellular enzymes catalyze reactions that involve breaking of P bonds and cause kinetic isotopic effects [ Blake et al ., ; Colman et al ., ; Liang and Blake , ], providing a potential means to identify P sources and transformations.…”

Phosphate oxygen isotopes: Insights into sedimentary phosphorus cycling from the Benguela upwelling system

“…First, oxygen isotope equilibration between P i and water, caused by a cascade of phosphorylation and dephosphorylation reactions during microbial P i metabolism, and second, kinetic isotope fractionation by extracellular, hydrolytic cleavage of P i from OM, that preferentially releases isotopically light P i thereby driving the isotope signature away from the equilibrium (Blake et al, 2001). Pioneering field studies of marine, estuarine, and riverine water d 18 O P have disentangled the balance of P i release and uptake (Colman et al, 2005;McLaughlin et al, 2006a), and identified external P i sources (McLaughlin et al, 2006b). This new isotope biosignature should thus prove useful for unraveling P i dynamics in marine sediments, where the pore water P i pool constitutes the interface between release of P i by OM mineralization, microbial uptake and release of P i , and interactions of P i with the mineral solid phase.…”