Characteristics of vertical mixing in a sea-cage farm and its environmental influences in a strong tide system: A case study in the Nanji Archipelago, East China Sea

Abstract

Vertical mixing is an important aspect of the sea-cage hydrodynamic since it is directly related to the reduction of the potential environmental risks of sea-cage farms. Previous studies have focused on the enhancement of vertical mixing due to the water blockage of the sea cage through numerical simulations and laboratory experiments. However, these experimental settings inevitably deviated from the field hydrodynamic environments where vertical mixing was contributed by not only the water blockage of the sea cage but also some natural dynamic processes. This paper investigated the vertical mixing in a sea-cage farm located in a strong tide system through an in-situ method. The results indicate that the mixing could be divided into three regimes, namely, the bottom-boundary-layer regime (BBL regime), the regime generated by the baroclinic process during the currents reversal (CR regime), and the regime mainly located under the cage bottom (CB regime). The first two regimes were generated by natural dynamic processes, while the latter one was caused by the water blockage of the sea cage as illustrated in previous studies. Highlight of this paper was the identification of the interaction between the BBL and CB regimes, i.e., the interaction between mixing driven by natural dynamic processes and the artificial sea cage. The mixing in the bottom layer was significantly enhanced by this interaction and can maintain strong for a longer duration than that generated by natural dynamic processes alone. The nutrients concentration and the size distribution of the suspended particulate matter (SPM) were chosen to illustrate the environmental influences of the vertical mixing in the sea-cage farm. The nutrient release from the sea bottom was enhanced by the local vertical mixing, and the concentrations of NO2−, PO43−, and SiO32− increased when the mixing was strong. The principal variation in the NO3− concentration was independent of vertical mixing but was controlled by the alternation of the water masses. The size variation of the SPM was negatively correlated with the intensity of the vertical mixing. The results in this paper inspire us to create a suitable vertical-mixing environment in future sea-cage farms by taking advantage of the interaction between the mixing generated by the artificial sea cage and natural dynamic processes, thereby reducing the potential environmental risk and guaranteeing the sustainable development of the sea-cage farming.