Abstract
As a semi-closed bay with narrow bay mouths, the distribution of nutrients in Zhanjiang Bay was different from bays with open bay mouths and rivers with large flows. It is important to study the water quality of Zhanjiang Bay to determine the impact of human activities on this semi-closed bay. Based on field survey data in spring, the spatial distribution of nutrients and other physico-chemical parameters was investigated, in order to study the geochemical characteristics of nutrients in semi-closed bays. Higher nutrient concentrations were observed in the inner and outer bays, but lower concentrations were observed at the bay mouth. With other analyses of physico-chemical parameters, the higher nutrient concentrations in the inner bay originated mainly from the diluted freshwater input from local developments and rivers. With the strong flow that exists along the western coast of Guangdong Province, the higher dissolved inorganic nitrogen (DIN) and SiO3–Si concentrations along the outer bay may be influenced by discharge from local cities in western Guangdong Province. There was stronger phytoplankton assimilation at the bay mouth, which resulted in reduced nutrient concentrations in this area. Although the hydrographic characteristics between the inner bay and outer bay were significantly different, the distribution of chlorophyll-a (Chla) levels was similar. However, we found significantly low dissolved oxygen (DO) and high apparent oxygen utilisation (AOU) consumption levels in the inner bay, and high DO and low AOU levels in the outer bay, which suggested that decomposition was more important than photosynthesis in the closed bay, even in spring during the phytoplankton bloom.
Highlights
The marine ecosystem in the coastal zone is complex and changeable, where various environmental factors affect and restrict each other
The distribution pattern of salinity in surface water was similar to the bottom water in the inner bay and outer bay
Similar to the distribution pattern of salinity, there was a significant difference in temperature between the inner bay and the outer bay, where the inner bay exhibited higher values
Summary
The marine ecosystem in the coastal zone is complex and changeable, where various environmental factors affect and restrict each other. Trace elements necessary for the growth of phytoplankton (i.e., N, P, and Si) in seawater affect the level of marine primary productivity and the structure of the ecological environment. Those human-induced elements in the environment are becoming a serious problem for coastal areas around the world [1,2,3]. Anthropogenic inputs have resulted in a series of adverse effects that include eutrophication, harmful algal blooms, and seasonal hypoxia [4,5,6]. Strong public concerns about eutrophication have resulted in increasing levels of scientific study to evaluate it
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