Enrichment of organic carbon increases the flux of phosphorus from sediment in mariculture ponds

Abstract

Environmental pollution associated with aquaculture activities is one of the major bottlenecks encountered in the sustainable development of fisheries. Accumulating organic carbon (org-C) is an inevitable but undesirable consequence of aquaculture activities, and its degradation influences nutrient flux. However, neither the variations in org-C accumulation nor its impact on phosphorus (P) dynamic has been clearly untangled and reliably quantified so far. Therefore, we carried out a field survey to examine the influences of org-C accumulation on sediment P release in mariculture ponds. The results suggested that org-C contents varied considerably between ponds, being significantly lower in monoculture ponds with bivalve shellfish than in polyculture ponds with bivalve shellfish, shrimp, crab, and fish, likely due to the less-artificial-feeding practice and the function of bivalve shellfish as “environmental cleaners.”Sediment P in mariculture ponds mainly occurred in iron-bound P (44%) and organic-P (hot NaOH-extractable organic + acid-soluble organic P = 56%), while calcium-bound P (CaP) only contributed a tiny proportion of about 1%. The positive correlation between P-flux from sediment and org-C suggested that org-C enrichment can stimulate sediment P release. Further analysis of the linear mixed model combined with piecewise structural equation modeling indicated that the two main forces driving P release - enzymatic hydrolysis release and anoxic desorption - operate with sediment org-C being a significant factor (accounted for 75% of the variation in P-flux). At the same time, other factors such as alkaline phosphatase activity, pH, and CaP may play minor roles in affecting P release. Our findings indicated that org-C accumulated in the aquaculture ponds might trigger sediment P release, posing a potential risk of contaminating the downstream water bodies during tailwater discharge.