[A-CG45] Biogeochemical linkages between the surface ocean and atmosphere

[ACG45-06] Experimental results of the effect of rainwater addition on surface primary productivity in the northwestern North Pacific in spring

*Kazuhiko Matsumoto¹, Fumikazu Taketani¹, Koji Sugie¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:rainfall, P-E parameter, nutrients, metal toxicity

In oligotrophic subtropical waters, the increase in surface primary productivity due to deposition of atmospheric nutrients associated with rainfall has previously been confirmed by the shipboard observations conducted before and after rainfall. In this study, to evaluate the effect of rainwater on primary productivity in detail, we conducted the follow-up experiments in the northwestern North Pacific during the spring season by adding rainwater to the seawater collected by a bucket at the surface and examined changes in photosynthetic parameters using the photosynthesis-irradiance (P-E) response curve. Since rainwater falling in this area contains not only macro-nutrients such as nitrogen and phosphorus, but also many metals that affect phytoplankton physiology, such as iron and copper, we also prepared the metal-removed rainwater through the metal adsorption column. Addition of rainwater without metal removal to the seawater collected in stratified water masses with shallow mixed layers showed a decrease in primary productivity, suggesting inhibition by metal toxicity due to copper. On the other hand, although nutrients in the seawater had already been depleted at the time of sampling, the addition of rainwater with metal removal did not result in an increase in primary productivity, probably due to nutrient deposition from heavy rainfall caused by the passage of a typhoon about a week earlier. Instead, samples one day after rainwater addition, collected in a water mass with a developed mixed layer, showed a marked increase in chlorophyll-a, even without metal removal treatment. The results of the rainwater addition experiment suggest that rainwater does not necessarily increase primary productivity, but has various effects on phytoplankton, depending on the chemical composition of rainwater and seawater, as well as the mixing conditions of the water mass.