Spatiotemporal Urea Distribution, Sources, and Indication of DON Bioavailability in Zhanjiang Bay, China

Abstract

In marine environments, urea is an important component of the biogeochemical cycle of nitrogen. The autochthonous and allochthonous sources (rivers, aquaculture, waste water input, etc.) of urea play a key role in urea cycles in adjacent coastal waters. Because urea is a specific marker to trace the sewage fluxes in coastal waters, we investigated urea associated with terrestrial source input and coastal water in Zhanjiang Bay (ZJB) during the time from November 2018 to July 2019, and the spatiotemporal urea distribution and the bioavailability of dissolved organic nitrogen (DON) based on urea concentration in the ZJB were explored. The results showed that the urea enrichment in coastal water was mainly due to discharge from urban sewage systems, rivers, and coastal aquaculture. The concentration of urea ranged from 1.14 to 5.53 μmol·L−1, and its mean value was 3.13 ± 1.02 μmol·L−1 in the ZJB. The urea concentration showed a significantly different seasonal variation in the ZJB (p < 0.05), and the highest and lowest concentrations were found in November 2018 and April 2019, respectively. Its high value appeared in the north and northeast of the ZJB, which were polluted by coastal aquaculture and agriculture fertilizer utilization. The range of urea concentration of terrestrial source inputs in the ZJB was 1.31–10.29 μmol·L−1, and the average urea concentration reached 3.22 ± 0.82 μmol·L−1. Moreover, the total urea flux surrounding the ZJB was 2905 tons·year−1. The seasonal terrestrial source of urea flux contributions had significant seasonal variation in wet, normal, and dry seasons (p < 0.05). The ZJB was subjected to a large flux of urea by estuaries and sewage outlet discharges. The seasonal urea concentration in all stations (>1 μmol·L−1) indicated that urea in the ZJB may have a bioavailable DON source. As a bioavailable nitrogen source, the ability of terrestrial source-derived urea to increase eutrophication should not be ignored in ZJB.

Spatiotemporal Dissolved Silicate Variation, Sources, and Behavior in the Eutrophic Zhanjiang Bay, China

“…Thus, river estuaries' and sewage outlets' monitoring stations were used to quantify the fluxes of DSi transported from the river estuaries and sewage outlets to the coastal water. The annual riverine and sewage outlets flux of DSi used here were estimated using the equation [41,42]-the calculation formula of the sea flux is as follows:…”

“…There are more than 5 small seasonal rivers and sewage outlets carrying various amounts of water into the coastal water. Among these rivers are the Suixi River, Nanliu River, Lvtang River, Wenbao River and Hongxing river, of which the largest is the Suixi River, located at the top of ZJB [41,42]. However, with the rapid socioeconomic development of Zhanjiang city, most of these rivers have become polluted with agricultural, industrial, and domestic wastewater from the land-based sources adjacent to the area of ZJB.…”

“…During the past three decades, the frequency of red tide outbreak in ZJB increased significantly. The frequency of red tide outbreak was two in 2005, which had the biggest accumulative area of red tide outbreak, with 310 km 2 [42]. Data from the monitoring stations of seawater and survey stations of the estuaries and sewage outlets of terrestrial input were collected in ZJB ( Figure 1).…”

“…Water sampling was chosen with respect to the physical-chemical-biological status of the hydrological and land-based source input's spatial distribution characteristics in ZJB ( Figure 1). Sampling of land-based sources and seawater was conducted in the same seasonal period [42]. Monitoring stations of seawater in ZJB were selected in accordance with the method specified in the Specification of Oceanographic Survey [47].…”

“…The results show that the high DSi concentration in different seasons was mainly in April and June 2019, located in the north and northeast of ZJB ( Figure 2). Due to the influence of local aquaculture activities, sediment discharge, diatom phytoplankton's influence, microbial growth and metabolism, precipitation and anthropogenic impacts in ZJB, the seasonal DSi variation was significantly different in the coastal waters [42,53]. The range and high value of DSi concentration in ZJB was much lower than in Shenzhen Bay, Hangzhou Bay and Huanghe, but the mean and maximum DSi concentration were higher than those in Rongcheng Bay, the Bay of Bengal, Sanggou Bay, Jiaozhou Bay, Daya Bay, and other coastal waters in China (Table 2) [54][55][56][57][58][59][60][61][62][63][64][65].…”