The fate of phosphate in an in situ Lagrangian addition experiment in the Eastern Mediterranean

The Eastern Mediterranean is the largest oceanic ecosystem that is phosphate-limited. To determine the impact of a transient input we executed a phosphate addition experiment in the surface waters of the Cyprus Eddy (33.3°N 32.3°E), and compared the ecosystem response with surrounding unperturbed water. A tracer, sulphur hexafluoride (SF6), added with the phosphate, enabled tracking of the patch when phosphate concentration declined to detection limits, and provided quantitative estimates of mixing, dilution and patch volume. The patch expanded to >400 km2 over 9 days with a lateral diffusion rate of 23±2 m2/s that was consistent with previous tracer releases in eddies. Mixed layer phosphate concentration was ∼110 nmol/l immediately post-release, and declined to <5 nmol/l after 6 days. A phosphate budget was developed using SF6 as a proxy to discriminate between dilution and biological pathways, with dilution resulting in loss of ∼75% of added phosphate from the patch centre by day 3. Non-conservative phosphate loss was largely due to biological incorporation into particulate-P, of which 50% accumulated at the patch centre whilst the remainder was removed by lateral dilution by day 3. Non-conservative phosphate loss at the patch centre was 15–15.5 nmol/l by day 4, which was equal to the cumulative biological P uptake of 15.6 (±5.6) nmol/l P and concurred with two other independent estimates of P uptake. This closure of the phosphate budget infers that the transfer of added P to mesozooplankton and higher consumers was not significant within the timescale of the experiment, despite the observed biomass increase that followed phosphate addition. Although patch dilution significantly reduced phosphate concentration and particulate accumulation, and so the apparent biological response to the added phosphate, analysis suggests that lateral mixing would not prevent bacterial biomass accumulation at the growth rates observed, suggesting that another factor such as grazing was responsible.