**4. Discussion**

#### *4.1. Spatial and Seasonal Variation of Chl-a*

Figure 13 illustrates the water temperature data from the sampling sites in order to investigate the seasonal variation of the concentration of Chl-a in detail; the results of the inversion of the concentration of Chl-a; and the water level measured at the Xingzi hydrological station.

**Figure 13.** Relationship between Chl-a concentration, water temperature, and water level in Poyang Lake.

The comparison in Figure 13 suggested that, regarding the spatial distribution of Chl-a, the highest concentrations of Chl-a in Poyang Lake are mainly distributed near the channel in the north of the lake, which connects it to the Yangtze River, in the places where the Ganjiang, Fuhe, Xinjiang, Raohe, and Xiushui rivers flow into the lake, and in the waters near to the shore of the lake. These observations can be attributed to the transport of various pollutants into the lake by the Ganjiang, Fuhe, Xinjiang, Raohe, and Xiushui rivers, and to human activities in waters near to the lake shore [37]. Sand mining occurs during the whole year in the northern channel that connects the lake with the Yangtze River [16]. Consequently, a large amount of sewage is discharged from sand dredgers, and sand mining activities disturb the lake bottom, releasing large amounts of nutrients; both of these result in a high level of nutrients in the lake water, which in turn leads to an increase in the concentration of Chl-a [38]. The relatively high Chl-a concentration observed in the waters near the inlets of the five aforementioned rivers is mainly due to the large amount of pollutants carried by these rivers [39]. Large amounts of domestic and industrial wastewater are discharged into the lake due to the large numbers of people who live near the shores of Poyang Lake and the acceleration of industrialization and urbanization that has taken place in recent years; this discharge is also an important factor behind the increase of Chl-a concentration in the lake.

The temporal variation of Chl-a concentration in Poyang Lake is related to the lake's unique hydrological characteristics. From Figure 13, the Chl-a concentration of Poyang Lake is positively correlated with water temperature and water level. This finding is similar to the analysis of Zheng et al [37]. In the study area, summer is part of the wet season and, accordingly, the water level reached its yearly maximum in August 2015. In the wet season, the increase in water level causes Poyang Lake to enter a relatively stable state, with the water flow speed reducing and the water temperature increasing. These two factors are highly conducive to the growth of algae in the water body, which causes the concentration of Chl-a in the lake to increase. This can explain our observation that summer is the season with the highest concentration of Chl-a. On the other hand, the water level of Poyang Lake begins to decline in autumn, the water flow speed increases, and the Yangtze River begins to flow into the lake, due to the fact that the level of the river is higher than that of Poyang Lake [40]; consequently, the water temperature begins to decline in autumn, which, in turn, causes the Chl-a concentration to reduce to levels that are significantly lower than those in summer. Winter is the season with the lowest water level and the lowest water temperature in Poyang Lake. The combination of these two factors inhibits the growth of algae in the lake, which in turn causes the concentration of Chl-a to reach its lowest yearly value during this season.

#### *4.2. Spatial and Seasonal Variation of TSM*

We compared the concentration of TSM calculated while using the retrieval model, the water temperature measured at the sampling sites, and the water level measured at the Xingzi hydrological station in order to investigate the seasonal variation of the TSM concentration in Poyang Lake (Figure 14).

**Figure 14.** Plot showing the TSM concentration, water temperature, and water level of Poyang Lake.

Figure 12 concluded that the concentration of TSM was high in the channel that connects the north of Poyang Lake to the Yangtze River, and in the main central channel of the lake. This can be attributed to human activities, such as shipping and sand mining [16]. The TSM concentration in the Junshan Lake area (the south part of Poyang Lake) was at a low level throughout the year and changed little throughout the year. The concentration of TSM near the inlets of the Ganjiang, Fuhe, Xinjiang, Raohe, and Xiushui rivers changed greatly throughout the year. This can be attributed to the di fference in the flow speed of these five rivers throughout the year [41].

Regarding the temporal variation of the concentration of TSM, the mean concentration was the lowest in summer (August 2015) [42]. At that time, water temperature and water level of Poyang Lake both reached their highest yearly levels. At this time, the flow speed of the lake was relatively low [41]. Although the concentration of TSM was high in the lake's main central channel, which can be attributed to the impact of sand mining activities, the TSM concentration in most of the other areas of the lake was low and relatively homogenous. The highest TSM concentration that was observed in Poyang Lake in this study was 254.43 mg/L, and it was observed in autumn (October 2015). At this time, the water level of Poyang Lake began to retreat, and the water level of the Yangtze River was higher than that of Poyang Lake. The backflow of water from the Yangtze River into Poyang Lake and the influence of sand mining activities increased the concentration of TSM in Poyang Lake [27]. The highest TSM

concentrations were mainly observed in the channel that connects the north of the lake to the Yangtze River, and in the central part of the lake. In winter month (January 2016), the water temperature and the water level reached their lowest yearly levels. At this time, the TSM concentration ranged from 0 to 201 mg/L. This can be attributed to human activities, such as shipping and sand mining, which disturb the sediment at the lake bottom and thereby lead to an increase in TSM concentration [27]. In January 2016, the highest concentrations of TSM were observed in the lake's main central channel and in the channel that connects the north of the lake to the Yangtze River.
