Effect of bioturbation of Urechis unicinctus on the diffusion flux of nitrogen and phosphorus at the sediment-water interface

Bioturbation refers to the biological reworking of soils and sediments by benthic animals, especially macrobenthos (e.g Venerupis philippinarum, Nutaliaolivacea, and Echinocardium cordatum, etc.). Previous studies have shown that bioturbation activity could enhance the migration and transformation of inorganic nitrogen with different forms at the sediments-water interface on the tidal flat, and increase the benthic nitrogen release rate, thus affecting the chemical characteristics of the sediment. In this study, Urechis unicinctus was selected as the research object to explore their effect of bioturbation on the diffusion flux of nitrogen and phosphorus at the sediment-water interface. Chinese Penis Fish, U. unicinctus, is an echiuran species with solid environmental adaptability, and is widely distributed in North Korea, Japan, and northern China, especially near intertidal and subtidal zones along the Bohai Sea coast of China. Usually,U. unicinctus inhabit in U-shaped burrows in sediments in intertidal areas and feed on filtering organic particles in the water. Furthermore, it has high economic value and nutritional value as aquatic products. Currently, the research of U. unicinctus mainly focuses on Reproductive biology, tolerating mechanism for sulfide, extract the active component, ecological health cultivation, etc. However, very few studies can be found about the impact of U. unicinctus bioturbation on the biogeochemical process of biogenic elements. To understand the ecological role of U. unicinctus in the sedimental environment, the effect of biological disturbance of U. unicinctus on the diffusion of nitrogen and phosphorus at the sediment-water interface was studied with indoor experiment. The U. unicinctus were divided according to four areal densities: low density (LD, 500 ind/m²), medium density (MD, 2 500 ind/m²), high density (HD, 8 300 ind/m²) and control group (CO, no worms), respectively. Each of this treatment group has five replicates. The 20-day indoor experiment were carried out in tanks during November-December 2020.The results showed that the release rate of NH₄⁺ contents from sediment to water ranged from 10.6 to 765.3 μmol/(m²·d), and it increased to the maximum and then decreased gradually with time elapsed. Compared to the control group, the average nutrient release rate increased by 39%, 111%, and 257% in the LD, MD, and HD treatment groups, respectively. Significant differences were found for NH₄⁺flux between the LD, MD, HD groups and the CO group (P<0.05). The values of NO₃⁻+NO₂⁻flux ranged from −172.05 to 208.63 μmol/(m ².d) for all the experiment, but the average flux values were reduced by 8% and 32%, and 81%, respectively, in the LD, MD, and HD treatment groups. Further, the values of PO₄⁻ flux ranged from −7.85 to 6.42 μmol/(m ².d) in the treatment groups during the entire period. There was a significant difference in PO₄⁻-P flux between the MD and HD groups and the CO group from day 2 to day 14 during the experiment (P<0.05). It turned out that NH₄⁺ is the main dissolved inorganic nitrogen (DIN) that affected by the biological disturbance of U. unicinctus. Taking into account the influence of U. unicinctus's metabolism and ammonia excretion, the NH₄⁺ diffusion flux is calculated by subtracting the ammonia excretion. Although the NH₄⁺ diffusion flux has a slight decrease, the trend of change over time remains unchanged. The results indicated that although the ammonia excretion effect of U. unicinctus has a certain contribution to the diffusion of NH₄⁺ from the sediment to the water body, the dominant factor still comes from the bioturbation effect of U. unicinctus. In addition, the U. unicinctus promoted the diffusion of DIN from sediments to bottom water, with an obvious density effect. This work provides support to evaluate the potential ecological role of the U. unicinctus in the integrated multi-trophic aquaculture (IMTA) system.