The Impact of Typhoon “Mangkhut” on Surface Water Nutrient and Chlorophyll Inventories of the South China Sea in September 2018

The influence of the exceptionally strong typhoon Mangkhut on the availability of nutrients and changes in primary production were studied in the northern South China Sea in September 2018. A tight station grid was sampled to analyze major nutrients, chlorophyll_a, particulate and dissolved organic carbon and nitrogen. Based on interpolated profiles, nutrients and organic matter budgets were determined for the upper 100 m of the water column prior to and after Mangkhut's passage. An upper layer of 100 m was found to reflect the important changes by the typhoon. Considerable differences between the on‐shelf, shelf edge and the deep‐sea stations were determined. Nitrate and phosphate increased by about 80% and 36% on the shelf, respectively, and both by almost 40% at the shelf edge. The open deep‐sea part of the study area reflects some deviating results that may be caused by just displacement of water or by mixing water of different origin. However, right on Mangkhut's track on the shelf even contact between surface waters and bottom waters was enabled, increasing phosphate and silicate, but declining nitrate. The inventory of organic carbon of the upper 100 m of the study area (138,000 km2) of 92 Gmol had increased within a few days after the typhoon's passage by 5 Gmol on the shelf and about 2 Gmol in the shelf edge area. Chlorophyll_a doubled during our stay and might have reached a factor of 3 increase in the subsequent time by nitrate supply and excess phosphate.Plain Language Summary: The influence of the super typhoon Mangkhut on the waters of the northern South China Sea was studied in September 2018. Nutrients and organic material were measured on 63 stations from the Chinese research vessel HAI YANG DI ZHI SHI HAO. Amounts of nutrients and biogenic matter were calculated for the on‐shelf, shelf edge and deep‐sea stations for the pre‐ and post‐Mangkhut period. An important finding was that the stations of the different areas, on‐shelf, shelf edge and the deep‐sea appeared to be differently impacted by Mangkhut. Even differences between the stations right on its track and in the other parts of the study area were found. In general, nutrients were supplied in enormous amounts and caused immediate algae growth. Moreover, enough nutrients were supplied to support algae growth for a couple of weeks. In summary, it was found that Manghut's upper water column mixing and shifting caused an almost tripling of primary production compared to the normal situation.Key Points: The typhoon Mangkhut clearly impacted the water column differently on the continental shelf, at the shelf edge and in the deep sea. On Mangkhut's track a maximum nitrate supply of 162 mmol m−2 was caused by induced upwelling at the shelf edge. The chlorophyll inventory of 2.8 Gg was almost tripled by contributing 4.7 Gg estimated from an additional nutrient supply.Federal Ministry of Education and Research, BMBF http://dx.doi.org/10.13039/501100002347National Natural Science Foundation of China http://dx.doi.org/10.13039/501100001809https://doi.pangaea.de/10.1594/PANGAEA.936352https://doi.pangaea.de/10.1594/PANGAEA.93609