Estimation of Organic Pollution of the Jinyaoshi Lake Based on the Diatom Assemblage of the Lake Sediments

. From 2015 to 2019, samples of surface mud from the bottom of the artificial lake in Tianjin Normal University were collected for diatom analysis and identification in May and June. 1296 diatoms were identified and divided into 25 species. DAIpo (Diatom Assemblage Index to organic water pollution) was calculated: 56 in 2015, 76 in 2017, 58 in 2018 and 63 in 2019. In 2017, it was β-oligosaprobic, and in 2015, 2018 and 2019, it belonged to α-oligosaprobic. The Jinyaoshi Lake is an artificial lake where groundwater is exposed by artificial excavation, and its water source mainly comes from groundwater, then by precipitation. It shows that the pollution degree of the Jinyaoshi Lake was not serious since 2015, and its pollution level was α or β-oligosaprobic.


Introduction
Diatoms are abundant in water (including lakes, rivers, oceans, etc.) and are easy to sample. Diatoms are of great significance to the environment. It is a great indicator of environmental assessment. Watanabe, T. et al. (1986) published DAIpo, a comprehensive evaluation value of organic pollution based on attached diatoms. Diatoms were divided into three groups: saprophilous taxa, eurysaprobic taxa, and saproxenous taxa. The relative percentages of the three species are calculated to obtain the water pollution degree, DAIpo. By calculating the DAIpo, the organic pollution degree of water can be numerically calculated, and the pollution status of different water or different periods of the same water can be compared (Watanabe,et al 1986a(Watanabe,et al ,b,1988Asai and Watanabe,1995). In this study, Watanabe and other methods were used to evaluate the pollution degree of artificial lake in Tianjin Normal University in the southwest of Tianjin in different years.

Materials and methods
The West Coast Plain of Bohai Bay is located in the subsidence zone of North China which has been sinking continuously since Cenozoic. Most of the land on the plain is lower than 2.5m, the terrain is flat, and the slope is only 1/20000~1/30000, which is the lowest and flat place in the coastal area of North China. The surface is covered by extensive Quaternary coastal alluvial, lack deposits or marine deposits. The research area is located in the Jinyaoshi Lake of Tianjin Normal University in the southwest of Tianjin (Fig. 1).
From 2015 to 2019, samples of surface mud from the bottom of the Jinyaoshi Lake in Tianjin Normal University were collected for diatom analysis and identification in May and June. Diatom slices were made in the laboratory and identified under 400 times optical microscope. More than 200 diatoms were counted in each sample, and the percentages of diatoms of different species and genera were calculated, the diatom map was made. According to the tolerance of different diatoms to the pollution, the turbidity index was calculated to evaluate the water pollution degree. Diatom was identified according to Watanabe et al. (2005). Diatoms were divided into saproxenous taxa (A) and saprophilous taxa (B), and then DAIpo were calculated according to their proportions.
B: The sum of the relative frequency (%) of all saprophilous taxa (#) appearing at the survey site.
The criteria of water quality determined by DAIpo are shown in Table 1.   Fig. 2).

Evaluation of water quality of the Jinyaoshi Lake
Diatom analysis showed that 25 species of diatoms were found in the surface mud of the  Based on diatom analysis of the surface mud of the Jinyaoshi Lake in Tianjin Normal University, the DAIpo obtained in 2015 and 2017-2019 was calculated, showing that the average value is 63. The pollution level belongs to α-oligosaprobic, indicating that the pollution is not serious.