동해 남서해역에서 용존태 유기 질소 화합물이 식물플랑크톤의 생장에 미치는 영향

Abstract

The East Sea is a semi-enclosed marginal sea of the western North Pacific Ocean, and has been regarded as an ideal ‘Miniature Ocean’ for studying large-scale oceanic processes. Although this area is characterized by high chlorophyll-a and phytoplankton abundance, the dissolved inorganic nitrogen (DIN) : dissolved inorganic phosphorus (DIP) ratio in mixed layer is much lower than Redfield ratio (16:1). To understand the high production in DIN-limited condition, we investigated the distributional characteristics of biological and chemical factors in the southwestern part of East Sea on May, September 2014 and October 2015. In addition, the utilization of nitrogen compounds by dominant species such as Chaetoceros debilis, Leptocylindrus danicus, Prorocentrum minimum and Chroomonas mesostigmatica under DIN-limited condition was examined. Water masses showed a seasonal difference. This characteristic is due to the difference in inflow intensity between Tsushima Warm Current and North Korea Cold Currents. In the vertical profiles of DIN and DIP, the concentrations were low in the surface layer and increased with water depth, however the concentrations of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) had an inverse correlation with those of dissolved inorganic nutrients. Although DIN ： DIP ratio of all water masses (below 500 m) was similar to Redfield ratio (16), in mixed layer the ratios had a seasonal differences, which are 6.0 on May, 2.0 on September and 4.1 on October, respectively. This indicated that the growth of phytoplankton are controlled by DIN. Moreover, DON proportion of dissolved total nitrogen (DTN) was account for ca. 86% in the mixed layer. Changes in community of phytoplankton were found with seasonal changes. The environmental factors such as water temperature and the water masses influenced on the distribution in each season. In May 2014, the number of species was dominated by diatoms such as C. debilis, L. danicus. In September 2015, increases of dinoflagellates including were P. minimum observed, possibly due to increase in water temperature. In October 2015 the phytoplankton community was also dominated by the diatoms, characterized by high abundances in May and September 2014. The highest concentration in size fractions of Chlorophyll-a (Chl-a) were observed in May, and during all study period nano-plankton fraction was the major contributor. The primary productivity (P.P) ranged from 133 to 225 mgCm-2day-1 in September 2014 and from 501 to 1148 mgCm-2day-1 in October 2015. High primary productivity was also found in nano-plankton. Therefore, the major contributor to chlorophyll-a and primary productivity is the nano-plankton in southwestern part of East Sea. In the utilization experiment of DON, the dominant species, C. debilis, L. danicus, P. minimum and Ch. mesostigmatica, were able to grow using DON (i.e. DFAA and urea) as well as DIN (i.e. nitrate, nitrite and ammonium). This indicates that the dominant species are also able to grow using DON under DIN-limited conditions. In the growth kinetics experiment, nitrogen sources were nitrate and ammonium represented as DIN, glycine and urea that is main component of DON in East Sea. Half-saturation constant (Ks) of C. debilis from Monod equations was estimated, with 1.62 µM in nitrate, 6.97 µM in ammonium, 3.36 µM in glycine, 0.55 µM in urea, respectively. Also, Ks of L. danicus was estimated, with 5.21 µM in nitrate, 4.57 µM in ammonium, 3.80 µM in glycine, 1.94 µM in urea, respectively. Ks values were 1.23 µM in nitrate, 7.23 µM in ammonium, 0.57 µM in glycine and 6.74 µM in urea for P. minimum. Ks of Ch. mesostigmatica at each nitrogen compounds was 0.96 µM in nitrate, 2.28 µM in ammonium, 3.40 µM in glycine and 0.58 µM in urea. These results indicate that the dominant species have higher affinity to DON, rather than DIN. Also, In uptake of experiments of nitrogen compounds, the dominant species were rapidly uptake DON, rather than DIN. Uptake rate of C. debilis for each nitrogen compounds was 0.037 pmol cell-1 day-1 in nitrate, 0.023 pmol cell-1 day-1 in ammonium, 0.175 pmol cell-1 day-1 in glycine and 0.049 pmol cell-1 day-1 in urea. The uptake rate of L. danicus was estimated, with 0.290 pmol cell-1 day-1 in nitrate, 0.982 pmol cell-1 day-1 in ammonium, 0.863 pmol cell-1 day-1 in glycine and 0.205 pmol cell-1 day-1 in urea, respectively. Uptake rate of P. minimum was 0.284 pmol cell-1 day-1 in nitrate, 0.232 pmol cell-1 day-1 in ammonium, 0.957 pmol cell-1 day-1 in glycine and 0.349 pmol cell-1 day-1 in urea. Uptake rate of Ch. mesostigmatica were appeared to 0.103 pmol cell-1 day-1 (nitrate), 0.062 pmol cell-1 day-1 (ammonium), 0.482 pmol cell-1 day-1 (glycine) and 0.033 pmol cell-1 day-1 (urea). Thus, high concentration and contribution of DON in the southwestern part of East Sea and the availability of DON under DIN-limited condition identified from this study is related to survival strategy to maintain a high primary productivity in the under low DIN : DIP ratio of East Sea.