Characteristics of Diatom Assemblages in Surface Sediments of the new Yongding River and Haihe River Estuaries, Tianjin

Diatom analysis was conducted on the surface sediments from the estuaries of the new Yongding River and Haihe River in Tianjin. The results showed that the main diatom assemblages in the new Yongding River was Navicula marina-Auliscus caelatus-Coscinodiscus hirosakiensis-Coscinodiscusoculusiridis and Auliscus caelatusCoscinodiscusoculus-iridis-Cyclotella meneghiniana-Cyclotella stylorum in the Haihe River. There was saprophilous taxa, Cyclotella meneghiniana, in both estuaries indicating that the two estuaries were polluted to a certain degree.


Introduction
The Tianjin plain is bounded by the Haihe River, and there are many oyster reefs in NEE direction to the north of the Haihe River. Most of the oysters that make up the reef grow vertically and develop in layers. Meanwhile, there are many horizontal layers in the upper half of the reef [1] . Oyster reefs grow in the environment from subtidal to intertidal, which is of great significance to restore the position of the ancient estuarine shoreline location, restore the paleogeography, paleoecology and the ancient sea surface. There are five cheniers to the south of the Haihe River, which represent the position of the ancient coastline during the stagnant period of the five regressions since the middle Holocene. Estuaries between land and sea are affected by river and tidal current. It is an important place to study sea level and ancient coast change. Based on the diatom analysis of modern estuarine sediments in Tianjin plain, the characteristics of diatom assemblage in the estuary area were found out, which provides basic data for the judgment and restoration of estuarine paleoenvironment.

Geological setting and methods
Tianjin plain is located on the west coast of Bohai Bay, where the terrain is low and flat, and the surface elevation is only 1-2m. Many rivers, such as the new Yongding River and the Haihe River, which are developed in Yanshan Mountains and Taihang Mountains, flow into the Bohai Sea in Tianjin (Fig. 1). The tidal range in this area is large (the average tidal range in Tanggu is 3.02 m), and the intertidal shoal is developed. There are 11 buried oyster reefs in the north of the Haihe River and 5 cheniers in the south of the Haihe River. The surface mud samples were taken from the estuaries of the two main rivers, the new Yongding River (117°57′21″E, 39°12′58″N) and the Haihe River (117°42′53″E, 38°59′22″N). 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.   18 diatom species were found in sediments of the new Yongding River estuary, of which 55.4% were marine species (M), 44.1% were brackish water species (B), and 0.5% were freshwater species (F). The main diatoms were Navicula marina (19.6%) belonging to indicators of a mud flat in saline water (E1), Auliscus caelatus(10.8%)belonging to indicators of a sand flat in saline water, Coscinodiscus hirosakiensis(19.6%)and Coscinodiscusoculus-iridis(13.2%)belonging to indicators of outer bay, and Thalassionema nitzschioides(10.3%)living in inner bay. The content of freshwater species was low, and only one species of freshwater planktonic species, Stauroneis phoenixcentron, was found.

Results and discussion
10 diatom species were found in sediments of the Haihe River estuary, of which 45.4% were marine species (M), 52.2% were brackish water species (B), and 2.4% were freshwater species (F). The main diatoms were Auliscus caelatus(41.5%) belonging to (D1) indicators of a sand flat in saline water, Coscinodiscusoculus-iridis(8.2%)living in outer bay, Cyclotella meneghiniana(10.6%)belonging to indicators of a river mouth (H) and Cyclotella stylorum(9.2%)living in inner bay. The content of freshwater species was low, and only one species of freshwater planktonic species, Stauroneis phoenixcentron, was found. Freshwater diatom, Gomphonema intracatum (2.4%), was found, which belongs to benthic diatoms in freshwater (F).
In addition, Cyclotella meneghiniaan (H) accounted for 5.9% and 10.6% respectively in the sediments of the new Yongding River and the Haihe River estuaries. Cyclotella meneghiniaan is a kind of turbid species, which mainly lives in alkaline water, indicating that there were different degrees of pollution in the estuary of the new Yongding River and the Haihe River, and the pollution degree of the Haihe River was greater than that of the new Yongding River.

Conclusion
Based on the analysis of diatoms in the sediments of the new Yongding River and the Haihe River estuaries, it was concluded that there were more species of diatom in the new Yongding River estuary than in the Haihe River estuary. Navicula marina (19.6%) and Auliscus caelatus (10.8%) accounted for 19.6% and 10.8% respectively in the intertidal zone of the new Yongding River estuary, while the proportion of Auliscus caelatus in the Haihe River was 41.5%. The phytoplankton species Cyclotella meneghiniaan accounted for 5.9% and 10.6% respectively in the sediments of the new Yongding River and the Haihe River. The pollution degree of the Haihe River estuary was higher than that of the new Yongding River.