Influence of sedimentation rate on the environmental capacity of total nitrogen in the water of Bohai Bay, China

Abstract

Measuring environmental capacity of total nitrogen (TN) in marine water has become an effective indicator of how much TN is entering the ocean from various sources. However, the TN environmental capacity balance is affected not only by external input but also by removal of TN from water. This removal occurs by numerous mechanisms, among which sedimentation is one that should not be ignored. The influence of sedimentation rates on the environmental capacity of TN in the water body in the northern area of Bohai Bay was studied by investigating the total nitrogen contents of both water and sediments independently. The radioactivity of lead and cesium isotopes (i.e., 210Pb and 137Cs) that were present in columnar sediment samples was measured for time periods ranging from 48 to 72 h using a high-purity germanium detector. A constant rate of supply model (CRS) was used to calculate sedimentation rates. The results indicate that 5.81 × 106 t of sediment are removed from the water column annually through precipitation. Based on the concentration of TN in the studied sediment samples, we calculate that a total of 3.34 × 103 t of nitrogen are removed from the water via sedimentation every year. According to the target TN concentrations for the marine environment that were adopted for the study area for 2025 and 2030, the amount of TN removed from water by sedimentation could remove 62.08% and 64.85%, respectively, of the TN introduced to the marine environment by allowable amounts of anthropogenic discharge into the northern area of Bohai Bay in 2025 and 2030. The high sedimentation rate in Bohai Bay can be expected to remove half of all nitrogen currently entering the water in this area. This demonstrates that when the environmental capacity of TN in ocean water is used to determine the total amount of nitrogen that can be permissibly discharged from land sources, the capacity of the environment to “self-purify” via sedimentation should be considered.