## 4 Discussion  
In estuaries, nutrient inputs from river discharge are a major source of inorganic nutrients, especially NO\({}_{2}\)\({}^{-}\) + NO\({}_{3}\)\({}^{-}\) and DSi [17]. Riverine nutrients contribute to the production of new phytoplankton in coastal areas [18]. In the Yeongsan River estuary, nutrients are supplied mainly from riverine inputs that are affected by discharge water from urban wastewater treatment [19]. Despite the trophic state of the estuary being classified as hypereutrophic during the wet seasons based on Nurnberg's criteria [20] because of riverine inputs, the freshwater zone has been reported to be subject to P limitations during dry seasons [21] similar to other freshwater systems. A significant increase in chlorophyll \(a\) resulted from the addition of phosphate in the mesoccosm experiments conducted in the freshwater zone of the estuary. In this study, DIN and DSi in situ reached high levels and generally decreased downstream, whereas orthophosphate (DIP) levels were relatively low (Table 1), which could be attributed to the effect of riverine inputs. Seasonally, the ambient nutrient concentrations of DIP and DSi were relatively low during the dry season, especially in May 2009, compared  
Figure 6: Nutrient concentrations, including nitrite + nitrate (NO\({}_{2}\)\({}^{-}\) + NO\({}_{3}\)\({}^{-}\)), ammonium (NH\({}_{4}\)\({}^{+}\)), orthophosphate (PO\({}_{4}\)\({}^{-}\); DIP), and dissolved silica (DSi), over the incubation periods of experiments conducted in January and March 2010.  
to the wet season in August 2009. DIP was especially low (\(\leq\)0.40 \(\upmu\)M) in May 2009. This resulted in a high N:P molar ratio of ambient nutrients available to phytoplankton (>16:1, Redfield ratio), suggesting that phytoplankton growth at the sampling sites was potentially limited by P.  
HS are mixtures of complex chemical compounds with different functional groups and molecular configurations that exert abiotic and biotic interactions in water [22]. Among these, HS are known to modulate P uptake by phytoplankton by stimulating or inhibiting P regeneration resulting from alkaline phosphatase activity ( microbiologically mediated) and UV light photorelease in the epilimnetic water of lakes [23]. HS also interact with orthophosphates bound to high molecular weight HS aggregates in the presence of Fe [24]. Colloidal aggregates can release orthophosphates into water via displacement reactions. Additionally, Fe can be transferred to larger molecular weight fractions to form HS-Fe-P complexes through complexation reactions, and orthophosphate can be released into the water via UV radiation, as reviewed by [PERSON] [22]. However, the importance of HS as stimulators or inhibitors depends on water systems, and further studies are needed to elucidate their interaction with orthophosphate and microbes in water [9; 22]. In this study, HA addition resulted in a significant increase in phytoplankton biomass through the seasons, especially nanosized chl \(a\), possibly by stimulating (not inhibiting) the regeneration and uptake of P by phytoplankton. This can be inferred from the observation that, unlike other nutrients, such as DSi (Table 4), no significant decrease in DIP levels resulted from the significant increase in chl \(a\) levels. The positive algal response was also similar to that of phytoplankton to DIP addition during the dry season in mesocosm experiments conducted in January and April 2011 [21]. In the present study, among the phytoplankton taxonomic groups, the nanosized diatoms, _Stephanodiscus_ sp. which is dominant during the cold and dry seasons, responded to HA addition, whereas no significant response of _Stephanodiscus_ sp. to DIP addition was observed [21]. [PERSON] et al. [25] reported that HS naturally stimulated the growth rates of marine benthic diatoms, probably because of the enhanced bioavailability of trace metals or other nutrients.