유해 와편모조류 Cochlodinium polykrikoides와 규조류 Skeletonema sp.의 종간경쟁에 미치는 용존태 질소 및 인의 영향

Abstract

We evaluated the effects of dissolved nitrogen and phosphorus on the interspecific competition between harmful dinoflagellate Cochlodinium polykrikoides and diatom Skeletonema sp.. We examined samples by using field observations and laboratory experiments. In addition, we simulated the population dynamics of C. polykrikoides and Skeletonema sp. applying a mathematical model to a mixed culture of these species. We used this model to analyze interspecific competition between phytoplankton at Jaran Bay during the summer. For our field observations, we obtained samples from three stations in Jaran Bay during May 2013 to April 2014. We monitored the prevailing environmental conditions and assessed the phytoplankton appearances. We found that the dominant species during the study period were diatoms; moreover, we did not observe C. polykrikoides. We showed that the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) ratio during the simmer of 2013 July to September was <16, implying that DIN was the limiting factor for phytoplankton. On the other hand, dissolved organic nitrogen (DON) accounted for approximately 70% of dissolved total nitrogen (DTN). Thus, DON may be important for the spreading species and for the survival strategy of phytoplankton under DIN-limited conditions. We investigated the utilization of dissolved nitrogen and dissolved phosphorus under laboratory condition. We showed that, C. polykrikoides and Skeletonema sp. were able to grow by utilizing DON and dissolved organic phosphorus (DOP), as well as DIN and DIP. The half-saturation constants (Ks) of urea and glycerophosphate (glycero-P) were 3.94 μM and 3.43 μM, respectively, for C. polykrikoides and 1.57 μM and 0.64 μM, respectively for Skeletonema sp.. The maximum uptake rates (ρmax) of urea and glycero-P were 6.44 pmol cell-1 hr-1 and 9.75 pmol cell-1 hr-1, respectively, for C. polykrikoides and 0.042 pmol cell-1 hr-1 and 0.097 pmol cell-1 hr-1, respectively, for Skeletonema sp.. Our results indicate that Skeletonema sp. has a higher affinity for dissolved organic nutrients such as urea and glycero-P than does C. polykrikoides. We conducted competition experiments with C. polykrikoides and Skeletonema sp., to obtain data for our population dynamics model simulation. We showed that in media supplemented with DIN and DIP, Skeletonema sp. was the dominant species; this species attained a density of approximately 1.0×103 cells mL-1. We conducted the experiment by using three different DIN/DIP ratios(<16, 16, and >16). In our population dynamics of model simulation of C. polykrikoides was only marginally affected by the DIN/DIP ratio was >16; in addition, the cell density of Skeletonema sp. was higher than tha of C. polykrikoides under all of the investigated conditions. The simulation model was run based on the prevailing environmental conditions at Jaran Bay during summer. The results revealed that Skeletonema sp. dominant species. However, the computed cell density of this species was higher than the actual peak density observed at Jaran Bay during summer, probably because the model did no take into account the effects of predation. We conducted sensitivity analyses by doubling and halving the parameters. We showed that the cell density of C. polykrikoides was only marginally affected by any of the investigated parameters. On the other hand, Skeletonema sp. was markedly influenced by the changes in most of the parameter values. Our results indicate that C. polykrikoides has large cellular pools of nitrogen and phosphorus and these pools may enable this species survive in rapidly changing environments. Hence, the utilization of DON and DOP by C. polykrikoides and Skeletonema sp. may contribute to crucial survival strategy and dominance strategies under DIN-limited and DIP-limited conditions. However, in comparison with C. polykrikoides, Skeletonema sp. has a higher capacity and affinity for dissolved organic nutrients such as urea and glycero-P, hence, Skeletonema sp. has an advantage under conditions of interspecific competition with C. polykrikoides. Thus, on basis of its physiochemical properties, Jaran Bay is a more suitable environment for Skeletonema sp. than for C. polykrikoides.