Growth dynamics of the deep-water Asian eelgrass, Zostera asiatica, in the eastern coastal waters of Korea

Abstract

Among the seagrasses that occur along the coast of Korea, Zostera asiatica inhabits the deepest depth; however, to date, there is limited information on its ecology. This study presents the first quantitative data on the seasonal growth dynamics of Z. asiatica in Korea. We measured seasonal growth and morphological characteristics, as well as environmental factors, including underwater irradiance, water temperature, salinity and nutrient concentrations of the water column and sediment pore water, bimonthly from July 2012 to May 2015. Underwater irradiance showed clear seasonal trends, increasing in the spring and summer and decreasing in the fall and winter, ranging from 2.4 ± 0.2 mol photons m-2 d-1 in November 2012 to 12.8 ± 1.3 mol photons m-2 d-1 in July 2014. Water temperature also followed a strong seasonal trend similar to underwater irradiance, ranging from 9.8 ± 0.1°C in January 2013 to 20.5 ± 0.2°C in September 2013. Nutrient availability fluctuated substantially, but there was no evidence of distinct seasonal variations. Shoot density, biomass, leaf productivity, and morphological characteristics of Z. asiatica exhibited significant seasonal variations: maximum values of these variables occurred in summer, and the minima were recorded in winter. Total shoot density was highest (218.8 ± 18.8 shoots m-2) in July 2012 and lowest (106.3 ± 6.3 shoots m-2) in January 2013. Total biomass ranged from 182.6 ± 16.9 g dry weight (DW) m-2 in January 2015 to 310.9 ± 6.4 g DW m-2 in July 2014.Areal leaf production was highest (4.9 ± 0.0 g DW m-2 d-1) in July 2012 and lowest (1.4 ± 0.2 g DW m-2 d-1) in January 2013. The optimum water temperature for the growth of Z. asiatica was between 16-19°C. Growth of Z. asiatica was more strongly correlated with underwater irradiance than water temperature, suggesting that light is the most important factor determining seasonality of Z. asiatica at the study site.