## 1 Introduction  
Humic substances (HS) are complex mixtures of biogenic, heterogeneous, and refractory organic compounds with high molecular weights and yellow to black colors that have been extensively transformed after their production in terrestrial and aquatic environments [1]. HS can be categorized into humic acids (HA; insoluble below pH 2), fulvic acids (soluble at any pH), and humin (insoluble in water) based on their solubility. HS are a major component of material cycling in aquatic systems (especially freshwater) accounting for 50-80% of the dissolved organic matter (DOM) in freshwater [2].  
Although HS have been characterized as compounds recalcitrant to microbial degradation, dissolved HS have been established to interact with freshwater microorganisms and affect microbial growth in freshwater and estuarine systems [3, 4, 5, 6]. For instance, the biomass of heterotrophic bacteria incubated in a culture based on humic lake water was double that of bacteria incubated in clear lake water [7], and pelagic bacterial yield increased with increasing humic content [8], indicating that HS form an important carbon source for heterotrophic bacteria. However, the behavior or ecological effect of HS on pelagic microbes in estuaries, including phytoplankton, is not well understood owing to the chemical complexity of HS and the highly limited information available for these systems [9].  
It has been reported that HS addition stimulated the growth of _Pseudokirchneriella subcapitata_ in cultures owing to the supply of bioavailable Fe from HS [10]. This species is used as a phytoplankton test species in aquatic toxicity bioassays. Humic nitrogen (N)derived from _Spartina_ has also been reported to be taken up by estuarine phytoplankton isolates in non-axenic and axenic cultures, suggesting that humic N is an important source for phytoplankton growth [11]. The cultures in the studies were designed to be limited by nutrients such as Fe or N. However, it has not been fully explored how phytoplankton communities above the species level, especially in eutrophic estuaries (in situ), respond to HS addition.  
The Yeongsan River estuary, one of the five major river estuaries in South Korea, is located in the southwestern part of the country (34\({}^{\circ}\) N, 126\({}^{\circ}\) E; Figure 1). Temperate estuaries are affected by Asian monsoons that record high rainfall in summer but low rainfall in other seasons. A sea embankment was formed in 1981 at the river mouth, which physically divided the system into freshwater and seawater zones. In 2012, two weirs were constructed in the upper regions of the freshwater zone, and subsequently harmful algal blooms developed more frequently [12]. The freshwater reservoir within the sea embankment is eutrophic compared to other freshwater systems in adjacent areas [13]. Freshwater is discharged into the seawater zone in wet seasons, affecting its trophic status and phytoplankton dynamics [14; 15; 16]. The aim of the present study was to investigate the response of the phytoplankton communities in situ in terms of size and taxonomic structure to the addition of a generic HS (commercially available) in the freshwater zone using mesocosm experiments. It was hypothesized that phytoplankton communities do not respond to HS addition because the dissolved inorganic nutrients available to phytoplankton remain relatively sufficient in the freshwater zone of the estuary.  
Figure 1: Sampling stations in the freshwater zone of the Yeongsan River estuary (**A**) and incubation bath (**B**) used in the mesocosm experiments to simulate the natural environments by circulating estuarine water inside the bath using an electric pump. Two baths were interconnected for the experiments.