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Analytical Studies on Seawater Characteristics and Seawater Pollution

HOME > Marine Science Reports > Analytical Studies on Seawater Characteristics and Seawater Pollution
From November 2006 to March 2008, a study was conducted on the chemical characteristics and distribution of pollutants in Dokdo’s marine environment. Scientists collected data on the concentration of dissolved inorganic nutrients in the seawater; heavy metals in surface seawater, sediment cores and in mussels; and the PAH in the atmosphere and in sediments.
Figure 1. A map of research areas in the seas of Dokdo
Figure 1. A map of research areas in the seas of Dokdo
It was found that the concentration of nitrates (including nitrite), phosphate and silicate increased with depth, where nitrates and phosphate reached their peak at a depth of 750 to 1000 meters. The concentration of silicate peaked at a much deeper 2000 meters. Ammonia was more concentrated in the surface layer than in the intermediate or lower layers of seawater where concentration levels were below 0.5μM. A ‘low-nutrient environment’ with lower concentrations of nutrients existed within 50 meters of the surface in the winter, but expanded to 150 meters in the spring. Up to a depth of 200 meters, there exist several different environments: the low-nutrient layer; a layer in which nutrient concentration increases steeply; a layer right below the thermocline in which nutrient concentration remains quite stable; and a layer in which nutrient concentration increases with increased depth. As such, the dissolved inorganic nutrients in the Dokdo seas vary according to the season and according to the influence at different depths of cold currents and warm currents flowing through the area.
Figure 2. Vertical profiles of nutrients in the study areas
Figure 2. Vertical profiles of nutrients in the study areas
Figure 3. Seawater structure and characteristics in accordance with nutrient concentrations in the research areas
Figure 3. Seawater structure and characteristics in accordance with nutrient concentrations in the research areas
The distribution of dissolved heavy metals in the surface seawater varies by element and by season, but in general concentrations were lower in the direct vicinity of Dokdo and to its east, while they were higher to the west. The correlation coefficient was usually higher among Mn, Co, Mi, Cu and Cd. Overall, with the exception of Cd, the concentration of dissolved heavy metals in this study showed a decrease in concentrations compared to the past (1999-2000). As for particulate metals, the concentration was lower in seas to the south of Dokdo and higher in the vicinity and north of Dokdo. As for the DM(dissolved metal)/TM(total metal = dissolved metal + particulate metal) ratio, the ratio for Fe was quite low due to the high concentration of Fe in suspended particulate matter, but in all other cases, the concentration of dissolved metals was higher than that of particulate metals. In particular, for Co, Ni, Cu, Cd, Zn and Pb, the DM/TM ratio was over 80%.
Figure 4. Distribution of dissolved heavy metals in the surface seawater of the seas of Dokdo
Figure 4. Distribution of dissolved heavy metals in the surface seawater of the seas of Dokdo
In sediment cores, the concentration of Fe, Mn, Co, Ni, As, Pb and Hg increased sharply at surface layer. For Mn the concentration at the highest layer of the core was 13,960 ppm - more than 40 times the amount found in the lower levels. This distribution may be attributed to diagenesis within sediments, where high concentrations of Pb at the surface layer are most likely due to the influence of an artificial input through atmospheric pathways.
Figure 5. Vertical profiles of metals in a sediment core of the seas of Dokdo
Figure 5. Vertical profiles of metals in a sediment core of the seas of Dokdo
Mussels (Mytilus coruscus) collected near Dokdo had shell lengths of 48.8 to 148 mm, and the shell length had a linear positive correlation to total weight and tissue weight. As for metal concentrations in the mussels, Al, Fe, Co, Ni, As, Cd, Pb and Hg were highest at St.3 (Seodo, West Island), Mn, Cr and Cu were highest at St.2 (Dongdo, East Island), while V and Zn were highest at St.1 (the Sea Lion Rock). Concentrations of As and Cd had positive correlations to shell length, but concentrations of all other metals decreased in the larger shells.
Data from December 2006 and March 2007 on PAH concentrations in marine sediments near Dokdo show us that in dry weight the minimum was 4.0 ng g-1 and the maximum 79.6 ng g-1, for an average of 28 ng g-1 - similar to PAH measurements carried out on deep sea sediments in the Ulleung Basin and the seas of the Central Pacific. These distributional characteristics tell us that PAH concentrations are not influenced by a specific source of pollution, but that these PAHs come from non-point source pollutants that have travelled long distances through the air or on sea currents and are distributed according to the particle size in sediments. Analysis of the PAH source recognition index shows that the PAHs found in sediments in the seas of Dokdo travelled long distances to the East Sea after having originated from the combustion of coal and wood. PAHs dissolved in seawater (particulate phase 0.05-0.07 ng L-1, average 0.06 ng L-1; dissolved phase 18.9-58.4 ng L-1, average 33.7 ng L-1) were observed in higher concentrations on the coast of Dokdo and in the seas north of the island, but were still considerably lower than in other coastal environments in Korea and around the world. Data collected in March 2007 tells us that atmospheric PAH concentrations (vapor phase 7.74 ng L-1, particulate phase 2.60 ng L-1) in the atmosphere of Dokdo were 1/3 of PAH concentrations measured at the same time on the southern coast of Korea, and only 1/8 and 1/20, respectively, of the average PAH concentrations found in the potential pollution sources of Seoul and the large cities of China. Still, a considerable amount of PAHs (527 ng m-2 day-1) seems to be entering the Dokdo area through a process (of dry deposition) where PAHs fall to the sea surface after being absorbed into particles such as dust that exist in the air.
Figure 6. The dry deposition rate of PAHs in the seas of Dokdo (March 2007) and a diagram on the multi-medium movement of materials PAHs are entering the Dokdo area through dry deposition (527 ng m-2 day-1). The amount of inbound PAHs is expected to rise in the winter when northwest winds become stronger.
Figure 6. The dry deposition rate of PAHs in the seas of Dokdo (March 2007) and a diagram on the multi-medium movement of materials PAHs are entering the Dokdo area through dry deposition (527 ng m-2 day-1). The amount of inbound PAHs is expected to rise in the winter when northwest winds become stronger.