User:Rosemary Guo/Lake Tai

Background InformationLake Tai is a shallow fresh water lake which located along the Yangtze River. It has 2,425 km2 surface areas but only 2.12 meter average depth, which is one of the largest fresh water in China. Also, it located in subtropical monsoon climate, which have the four distinctive seasons. There are hot and wet in summer, and cold and dry in winter. Lake Tai is a polymictic lake, which is frequent cycles of mixing and stratification, which have the fewer seasonal differences compared with the deep lake in the similar latitude. Recently, Lake Tai has a quite serious problem on algea bloom due to increase of nutients, such as nitrogen and phosphorus, due to the industrial sewage discharge from humen activities, which lead to a negative effect for the ecosystem of Lake Tai.

The algae composition in different periods

Lake Tai has had a very serious problem with the eutrophic effect and algae bloom since 1997 and has become an important investigation place for eutrophication in China. Through the monitoring of climates and nutrients elements in Lake Tai from 1995 to 2007, researchers got the data about the biomass and composition of phytoplankton in interannual and different seasons. As a result, they found both climate and nutrients elements played a very important role in algae bloom and the algae composition in different periods (5 periods in total). From 1992 to 2017 (which cover first two periods), cyanobacteria is the dominant phytoplankton in Lake Tai. In addition, diatoms and cryptophytes also contributed much biomass during 1992-1995, and filamentous chlorophytes joined in from 1996 to 2007. On the other hand, through 2008 to 2010, the abundance of cyanobacteria had decreased. However, the amount of large diatoms and samll, fast-growing cryptophytes increased at that period, and their biomass become higher than the abundance of cyanobacteria. From 2011 to 2014, cyanobacteria were abundant in Lake Tai and contributed to the most of biomass again, and its biomass were twice larger than the previous periods. the abundance of large diatoms and small-colonial green algae were further increased, but the abundance of cryptophytes were decreased. In the last period (2015-2017), the total biomass of phytoplankton grew rapidly, especially for cyanobacteria (dominating in four seasons) and diatoms (focus on winter and spring). One thing may need to be noticed that cyanobacteria could produce cyanotoxins, which is a toxin that can lead people sick. In addition, the biomass of cyanobacteria increased in whole year, and the biomass of larger diatoms mainly increase in winter and spring. Usually, the peaks of biomass of phytoplankton are appear in summer.

Nutrients (N) Change in Lake Tai

Increasing nutrients is one of the most important contributors for eutrophication in Lake Tai and causes pollution. Tracking the amount of nutrients, such as nitrogen (N) and phosphorus (P), can help people know more about eutrophication in Lake Tai. The increase amount of nutrients will increase chlorophyll a concentration in phytoplankton, so it promote the growth of phytoplankton (biomass increase). As a result, the eutrophication gets worse and worse. In one study, researchers collected and analyzed the water quality of Lake Tai and the wastewater treatment plants around it to figure out the trends and causes of nitrogen pollution in Lake Tai. Consequently, they found that, in a year, the total nitrogen (TN) concentration is high in winter and spring but low in summer and fall. Also, the highest concentration value appeared in March, and the lowest values appear in September. In addition, since 2001, the TN concentration increased continuously and reached the peak in 2006. After that, the TN concentration started to decline. In spatial variation, the average TN concentration is lower in the areas which are near the center of the lake than the areas which are near the shore, and the highest average TN concentration value appeared in the northern shore of Lake Tai. It indicated that the pollution of Lake Tai was impacted from external factors, such as discharge of the wastewater treatment plants and spreading to the center of lake Tai gradually. In addition, not only N and P, but also some micronutrients could affect the abundance of phytoplankton in Lake Tai, such as copper and iron.

The Source of N & P in Lake Tai

The main way that lead nutrients flow into Lake Tai is the water discharged from rivers. Knowing the forms and sources of nitrogen and phosphorus in Lake Tai combined with the variable reasons for eutrophication of Lake Tai and the critical loads of variable nutrients for the lake may help people find out the effective solution to increase water quality of Lake Tai. Thus, Wang et al. used the MARINA-Lake model (Model to Assess River Inputs of Nutrients to seAs) to measure the dissolved inorganic and organic N and P discharges from rivers to Lake Tai and used PCLake model to test how much nutrients exported from rivers exceeds critical loads. As a result, researchers found that in 2012, the river output contributed 61 kton of total dissolved nitrogen (TDN) and 2 kton of total dissolved phosphorus (TDP) to Lake Tai. In these output, 90% of TDN were diffuse sources exported from rivers, 40% of the diffuse source of river export came from synthetic fertilizers. For TDP, point sources accounted for 48% of the total TDP drained from river to Lake Tai in 2012, and sewage systems are the main sources of pollution for that. In addition, the actual output of river nutrients exceeds dramatically over the critical loads of Lake Tai, and the river export need to decrease about 46%-92% to back to the critical load of TDN. Also, the river export of TDP need to decrease about 55%-92% to meet critical load s.

Change of Food Web due to Eutrophication Eutrophication also can change the structure of the food web in Lake Tai. For the long term eutrophic effect in Lake Tai, its food web was simplified and lost the barycenter of energy exchange. From the 1960s to 1990s, about 14 trophic species in lake Tai were removed from the food web, and in the 2000s, there were less than 50 trophic species left. Also, the ratio of predator and prey decreased continuously, which indicated the decreasing amount of predators. And the decreasing link density of the food web suggested the declining species interactions in Lake Tai through the long term of Eutrophication. However, the connectivity properties of the food web were not changed too much due to the reduction. In addition, the energy exchanges barycenter of food web trended to the lower level in Lake Tai. The eutrophic effect caused the extinction or weakness of the top species in Lake Tai due to the lack of energy and enough food sources. Thus, the role of intermediate species in the food web was more weighted because of the lack of predators (top species) and plenty of food (algae bloom due to Eutrophication). In addition, the biomass of producers and herbivores, as well as lower trophic level s, increased dramatically during Eutrophication, but the biomass in upper trophic levels had not particularly changed. Moreover, due to the shortening of food chains, the ability of the food web that transfer energy to the upper levels was also decreased.

Solutions for Eutrophication in Lake Tai

The Chinese government also came up with some strategies to solve the eutrophication in Lake Tai. They get inspiration from fish that created a bionic platform which simulates the filters on the gills of the silver carp to remove algae and established a 12-meter-long, 11-meter-wide floating platform to do that work. Also, they set up a machine to remove the lake-bed sediment which is rich in nitrogen and phosphorus, the main chemicals for algae outbreaks. The last way is that scientists invented a powder that can aggregate algae as a large stone, which can transform the soluble phosphorus into insoluble calcium phosphate turns. Also, the phytoplankton may condensed at the same time.