Historical record of the effects of anthropogenic pollution on benthic foraminifera over the last 110 years in Gamak Bay, South Korea

Abstract. This study investigated the historical record of the effects that anthropogenic pollution has had on benthic foraminifera over the last 110 years in the semi-closed Gamak Bay. The evidence consisted of geochemical data including ²¹⁰Pb concentrations and benthic foraminiferal assemblages acquired from core sediments (western, eastern and northwestern areas). Various records of benthic foraminiferal assemblage in the northwestern area were suitable as the standard for variation regarding pollution history. In the period between 1906 and 1964 (the pre-urbanization period), Gamak Bay was composed of Ammonia beccarii-Elphidium advenum-Elphidium clavatum assemblage, except for the northwestern area with A. beccarii-Buccella frigida-E. advenum assemblage, and may have remained mostly unpolluted. Although the northwestern area did not show a difference in the species composition of the benthic foraminifera, it may be polluted to some degree due to stagnant sewage supplied from a small village that had formed before city construction in the hinterland, as shown from the species diversity of 1.37, with a total number of benthic foraminifera (TNBF) of 704 individual and total organic carbon/total sulfur (C/S) of 2.63.

The benthic foraminiferal assemblage of the northernmost area between 1965 and 1987 (the urbanization period) rapidly varied from E. somaense-A. beccarii-B. frigida assemblage, through A. beccarii-B. frigida-E. advenum, B. frigida-A. beccarii-E. subarcticum, T. hadai-E. subarcticum-B. frigida, to A. beccarii-E. subarcticum-T. hadai assemblage with a diversity of 1.8, TNBF of 244, C/S of 2.05 on average. During this period, it was characterized by an increase in abundance frequency in T. hadai, and E. subarcticum, which are known as bioindicators of eutrophication and organic pollution, respectively, and rapid variation of benthic foraminiferal assemblage. These may have been caused by an increase in the influx of sewage from Yeosu City, which was constructed at the hinterland of the northernmost area in Gamak Bay, as shown from the sedimentation rate of 1.0 cm/y. Pollution during the urbanization period may have been restricted to the northwestern area, and it did not diffuse to the surrounding area.

The E. subarctum, A. beccarii-E. subarcticum-T. hadai and E. subarcticum assemblages with diversity of 1.35, TNBF of 562, C/S of 2.33 were sequentially distributed in the northwestern area from 1988 to 2014 (the aquaculture period), and this is characterized by the high abundance frequency of E. subarcticum of 51 % and high sedimentation rate of 1.75 cm/y caused by biodeposits discharged from mussel farming (Mytilus galloprovincialis) since the 1980s. The organic pollution materials originated from and deposited by biodeposits may contribute to the continuous deterioration and variation in the benthic ecological environment by means of pollution storage. During this period, benthic foraminiferal assemblages in the northwestern area are correlated with the E. subarcticum-A. beccarii assemblage of the eastern area where oyster farming has taken place, and it is composed of E. subarcticum (35.4 %) and A. beccarii (15.5 %) with a TNBF of 1787 individuals, species diversity of 2.18, C/S of 4.8 and a sedimentation rate of 0.95 cm/year. It is clear that the northern area seriously progressed in pollution compared to the eastern area, although the species composition is somewhat similar between the two areas. It may be caused by an overabundance and excessive deposition of the organic matter through an over-supply from mussel farming as well as the oval-shaped bottom physiography and very slow current speed. During the transition from the pre-urbanization to urbanization period, and aquaculture period in the northwestern area, the processes of variation in the benthic foraminiferal assemblages may represent the transition from oxic to anoxic environmental conditions. The western area with A. beccarii-E. advenum-E. clavatum assemblage, however, was unpolluted over the last 110 years. These differences in the degree of pollution and benthic foraminiferal assemblages between the areas in Gamak Bay may be caused by the physiography and current movements of the bay.