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

We present a 2-yr time series of the oxygen isotopic composition of dissolved inorganic phosphate (δ¹⁸O_p) in Monterey Bay, California. δ¹⁸O_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 δ¹⁸O_p approaches isotopic equilibrium with surrounding water. Our results indicate that δ¹⁸O_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. δ¹⁸O_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 δ¹⁸O_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. δ¹⁸O_p data also suggest that deep water (>500 m) may be a source of phosphate to the euphotic zone in Monterey Bay.