Features of the physical environment associated with green tide in the southwestern Yellow Sea during spring

摘要

Massive green tides caused byUlva prolifera in the Yellow Sea have occurred every summer since 2007 and have caused huge economic losses for local governments. The Subei (North Jiangsu Province, China) Shoal, with its large-scalePorphyra aquaculture, has been regarded as the most important source ofU.prolifera for green tides. To reveal the physical mechanisms of floating and drifting algae in this area, the characteristics of the current, the temperature, the salinity and suspended particulate matter (SPM) in the southwestern Yellow Sea, especially in the Subei Shoal, were studied. The topography of the radial sand ridges in the Subei Shoal constrains the features of the currents and causes net longitudinal and latitudinal movements. The longitudinal net movement is a dominant dynamic factor that can bringU.prolifera into offshore waters. The amount of gas that is produced by algae during photosynthesis determines whetherU.prolifera can float well on the sea surface after it is disposed into the water fromPorphyra aquacultural apparatus. The Subei Shoal is characterized by a high turbidity, which can result in significant light attenuation and affect the photosynthesis together with the buoyancy of aU. prolifera in the water. According to satellite remote sensing data from 2012, the three-month-averaged surface SPM (April, May and June) in the Subei Shoal was 140 mg/dm3, and the north of the Subei Shoal (the north of 34.5°N), it was 11 mg/dm3. According to the monthly averaged surface SPM in April, the transparency in the Subei Shoal was only 0.1 m, but it often exceeded 2.0 m outside of the Subei Shoal. The results explain why the floating ability ofU.prolifera increases significantly once the green algae drifted outside the Subei Shoal.