Vertical distribution of the ions in overlying and interstitial waters and their diffusion fluxes in intensive hybrid snakehead aquaculture enclosures

Intact overlying water and interstitial water in sediments were collected by Peeper devices from intensive hybrid snakehead (Channa maculata♀×C. argus♂) aquaculture enclosures in Nansha District, Guangzhou. The concentration of NH₄⁺-N, NO₃^--N, NO₂^--N, PO₄^3--P, SO₄^2--S and Fe²⁺ were measured by miniaturised photometrical method using a microplate reader. All measured ions showed strong vertical distribution patterns, and the depth related samples were divided into three groups, overlying water group, surface layer sediment group (water-sediment interface group) and lower layer sediment group with distinct differences. Diffusion fluxes of the ions were calculated by the Fick's First Law. NH₄⁺-N, NO₂^--N, PO₄^3--P and Fe²⁺ were released from the sediment to overlying water, while NO₃^--N and SO₄^2--S in the overlying water were absorbed into the sediment. Among them, NH₄⁺-N and SO₄^2--S have much higher exchange fluxes than the other ions relatively. And NH₄⁺-N was the main endogenous pollution ions in the hybrid snakehead aquaculture enclosures with the diffusion fluxes of 76.432, 111.631 and 209.835 mg/(m²·d) respectively.