High benthic foraminiferal diversity in polluted Busan North Port (Korea)

Surface sediments were collected from Busan North Port (southern coast of Korea) and the outer port area (OPA) in February and August 2009 to ascertain the distribution of benthic foraminifera and assess controlling environmental factors. Ninety-eight species (81 calcareous and 17 agglutinated), 40 of which had live representatives, were identified. Busan North Port can be divided into distinctive regions according to different depositional environments and to the degree of influence from pollution sources, such as nearby streams and industrial facilities around the inside north port (INP) and outside north port (ONP). The OPA is characterized by inner continental-shelf environments. These areal distinctions corresponded well to cluster- and principal component-analysis results based on foraminiferal-assemblage and heavy-metal data. The degree of organic enrichment and heavy-metal concentration in the sediments were major factors explaining the distributions of the foraminiferal assemblage. The low abundance of benthic foraminifera in the INP compared to the OPA seems to be directly related to heavymetal toxicity caused by high Cu, Zn, and Pb concentrations. The dominant occurrence of pioneer species Cribroelphidium excavatum and Trochammina hadai appears to indicate the adverse effects of heavy metals and high levels of organic matter on benthic foraminiferal distribution in the INP. Species diversity, however, was unusually high in Busan North Port across all pollution levels, and can possibly be explained in two ways. Short water-residence time in the inner port and maintenance of high salinity through active seawater exchange may reduce the effects of heavy metals and possibly make the area hospitable to a variety of species. High diversity may also be attributed to "rare short-lived" species that favor unstable conditions. The response of the benthic foraminiferal community to the unstable environment seems to be a succession of these "rare short-lived" species rather than a predominance of a few opportunistic ones. The uncoupling of species diversity and pollution level observed in Busan North Port might not be uncommon, and caution should be exercised when benthic foraminiferal diversity is used as an index of relative environmental health. Presence of indicator species and/or size distribution in the foraminiferal assemblage should be considered as well as species diversity to correlate the community structure with environmental change and pollution level.