Relationship of mesozooplankton and population variations of jellyfish (Aurelia aurita) in Masan Bay, Republic of Korea.

Abstract

Masan Bay is a representative semi-enclosed bay in Korea. Masan has been developed as a coastal industrial complex since the 1970s. There is a serious contamination from municipal and industrial sewages, due to the inflow of eight rivers around the area. However, the self-cleansing process of nature is retarded as the circulation of water is slow. As a result, eutrophication, phytoplankton bloom, and the formation of layer, which has a low level of oxygen, have occurred frequently and damaged the ecosystem. In addition, there has been often the presence of jellyfish in a large quantity during the summer. This study investigated the temporal variation of jellyfish population and focused on the relationship between jellyfish and mesozooplankton communities, which are the prey of jellyfish, in Masan Bay from 2004 to 2006. The digestion time of jellyfish was measured using jellyfish's gut contents in the field and the lab. Artemia, which is a prey of jellyfish, was used in the lab. The biggest size of jellyfish was found in February, while ephyrae, which were 2-3 mm in diameter, were present from March to May in Masan Bay. An ephyra grew faster after May and became an adult by June. The size of adult jellyfish maintained until December. The population density of jellyfish reached 3000 inds/100m3 in March and April when ephyra appeared, whereas it decreased as the size of ephyra increased after April. However, the population density of jellyfish was still high (around 600 inds/100m3) in the summer of 2005. The abundance of mesozooplankton related inversely to the abundance of jellyfish. The gut contents of jellyfish were mainly copepods, while the seasonal variation of amount of gut contents was various. The digestion time was measured in the lab as an ingested prey (Artemia) was disappeared from the stomach of jellyfish. It took approximately 30-60 minutes. The presence of jellyfish in a massive quantity caused the decrease in the copepod population. This may trig