## 4 Discussion
HA extracted from _Spartina_-derived humic nitrogen also stimulated the growth of estuarine and marine species through photochemical and microbial regeneration rather than direct uptake [11]. HA extracted from the Yeongsan River also increased the growth of _Raphidocelis subcapitata (formerly Pseudokirclureiella subcapitata) (Chlorophyta)_, a common test species for aquatic toxicology because of its complexation reactions with Fe [10]. The direct HS uptake of Miozoa (dinoflagellates) through pinocytosis has also been reported [26; 27]; however, this has not been considered in the present study because dinoflagellates did not appear at the study sites. HS can also affect phytoplankton growth by changing light availability in the water column through light absorption, especially at high concentrations [28; 29]. The Yeongsan River estuary study sites showed high turbidity, where the Secchi disk depth was low (0.88 \(\pm\) 0.30 m) during water sampling. The chl \(a\) levels increased as the concentration of HA increased from 2.5 to 10 ppm (Figure 4), suggesting that HA addition probably changed the light regime slightly to the extent of no effect on phytoplankton growth during incubation. No apparent effect of HS on the light regime has also been reported for a shallow lake with a similar latitude to the study sites, where the algal community was dominated by phytoplankton growing well at low light levels [30]. The intracellular inhibitory effects of HA on photosynthesis have also been studied but remain controversial. Extracellular and stimulating effects are now emphasized more than before [31]. In this context, HA was probably capable of stimulating phytoplankton growth, possibly by increasing the regeneration or algal uptake rate of P in the mesocosm experiments. Further studies are required to elucidate the mechanisms of P regeneration and algal uptake of P based on the interaction with HS in the estuary. Recently, the concentrations of orthophosphate have decreased significantly in the freshwater zone of the Yeongsan River estuary because of the improved P treatment of municipal sewage, although DIN nutrients did not decrease significantly [19]. However,the significant decrease in orthophosphate concentrations did not lead to the mitigation of algal blooms. The N:P ratios increased and resulted in more potential P limitation than before. The algal bloom magnitude and frequency have considerably increased, particularly after the construction of weirs in the upper region of the freshwater zone [32]. Furthermore, HS fractions are expected to increase in the estuary, considering the significant increase in COD relative to BOD in the freshwater zone [32]. Therefore, in the freshwater zones of the estuary, a reduction in HS, as well as dissolved inorganic nutrient (P) levels, may be required to control algal biomass, especially that of nanosized phytoplankton, such as _Stephanodiscus_ sp., which are sensitive to HS inputs. Thus, the management of HS based on a better understanding of HS behavior may be crucial in mitigating algal blooms in eutrophic estuaries, such as the Yeongsan River estuary, that are subjected to anthropogenic disturbance by engineered structures such as weirs and sea embankments.