Contribution of river phosphate variations to apparent reactivity estimated from phosphate-salinity diagrams

Estuarine mixing of dissolved inorganic phosphate (DIP) was simulated in the low salinity region of the Delaware Estuary. The study was designed to evaluate the extent to which variations in measured river DIP concentration can explain apparent estuarine reactivity in the low salinity region of the estuary inferred from DIP-salinity diagrams. For 12 simulation periods beginning in January 1987 and ending in January 1988, average DIP reactivity inferred from simulated DIP-salinity diagrams was 0•38 μM, with values ranging between − 1•7 (removal) and 0•74 (release) μM. On six of 12 occasions, the inferred reactivity of DIP from the simulated mixing diagram was > 10% of the river DIP concentration. A relative change of 0•6 and 2•0% per day in river DIP was found to produce definite non-linear effects (>I 0%) for periods of increasing and decreasing DIP, respectively. For two dates, the incorporation of simulated mixing into the interpretation of field data actually changed the direction of inferred reactivity. Our study clearly documents, as previously shown by others, the danger of inferring estuarine reactivity of DIP from a DIP-salinity diagram without including knowledge of temporal variations in river DIP concentration.