Spring phytoplankton community response to an episodic windstorm event I in oligotrophic waters offshore from the Ulleungdo and Dokdo islands, Korea

Abstract

We investigated the phytoplankton distribution and its relationship to environmental factors at 40 stations in oligotrophic waters offshore from the Ulleungdo and Dokdo islands (hereafter Ulleungdo or Dokdo) in Japan/ East Sea, prior to and following an episodic windstorm event. Nutrient addition bioassay experiments (control, + N, + P, and + NP, in both the presence and absence of added Fe) were also conducted to investigate the growth response of the phytoplankton assemblage and its nutrient consumption, using surface seawater collected from stations 36 and 40, which are in the vicinity of the Dokdo. Field measurements showed that the surface water temperature ranged from 13.33 degrees C to 16.18 degrees C and the salinity ranged from 34.03 to 34.55. The nitrate + nitrite, phosphate, and silicate concentrations varied from 0.07 to 2.22 mu M, 0.01 sM to 0.19 mu M and 0.76 to 6.93 mu M, respectively. The Chl-a concentration varied from 0.36 to 15.97 mu g L-1 (average 2.66 +/- 3.261.mu M), but was significantly higher in Zone III-a (Dokdo) than in Zone I-b (between Ulljin and Ulleungdo, prior to the windstorm), Zone I-a (between Ulljin and Ulleungdo, following the windstorm), and Zone II-a (Ulleungdo) = 17.438, p < 0.001; ANOVA). Diatoms and Raphidophyta were the dominant phytoplankton types. Following episodic windstorm events the abundance of the raphidophyte Heterosigma akashiwo was maintained at high levels in the offshore oligotrophic area around the Ulleungdo and Dokdo, particularly in Zone III-a (F = 16.889, p < 0.001; ANOVA). In the algal :bioassays conducted with and without added Fe, the in vivo fluorescence values in the + N and + NP treatments were higher than in the control and the + P treatments, which-suggests that plankton biomass production was stimulated by N availability. In the + N and + NP treatments, H. akashiwo typically dominated in the initial, logarithmic, and stationary growth phases. The growth rate of the phytoplankton community in the presence of added Fe was not statistically different (p > 0.05) from that in the treatments without added Fe. The results suggest that in this area natural phytoplankton communities, including those dominated by H. akashiwo, respond rapidly to pulsed nitrogen loading events. The episodic windstorm event probably resulted in vertical mixing that brought nutrients into the euphotic upper layer. The results suggest that such events are important in triggering spring phytoplankton blooms in potentially oligotrophic waters, such as those in the vicinity of the Dokdo in the Japan/East Sea.