Particle size-dependent partitioning of phosphorus in aquaculture pond and estuarine systems

Nanoparticle-bound phosphorus (P) is critical for nutrient cycling in aquatic environments, but its behavior across contrasting aquatic systems remains elusive. A comparative study of P load and speciation on particles in water columns was conducted in eutrophic aquaculture pond and Chesapeake Bay estuarine systems. Particle size separation, Hedley's sequential extraction, and microscopic observations were performed to characterize particle size-dependent distribution and speciation of P in water columns. For both aquatic systems, particles shared similar morphologies and P partitioning patterns. However, aquaculture ponds exhibited significantly higher particle loads and greater presence of colloid- and nanoparticle-associated P. Specifically, over 70 % of both inorganic P (P_i) and organic P (P_o) enriched in particles with sizes smaller than 100 nm in ponds. It is the concentration of particulate and dissolved P_o that controls the extent of algal blooms in P-rich aquaculture ponds. These P_o fractions are mainly derived from suspended colloidal and nanoparticulate matter instead of large algae (>1,000 nm). Seasonal comparisons revealed that both diversity and bulk P (BP) concentrations were greater in summer than winter in both pond and estuarine systems. In the estuarine system, P was mainly present in the form of Si/Al-bound species, whereas in aquaculture ponds, Fe- and Ca-bound P forms were more prevalent. This suggests that P in estuarine systems primarily originates from internal sediment release, while P in ponds is largely derived from external inputs, such as fish feed. Correlation analysis further indicated that soluble reactive phosphorus (SRP) concentrations in ponds were significantly associated with water hardness and dissolved oxygen (DO) levels. Increased DO availability may promote the formation of metal phosphate minerals, thereby reducing free orthophosphate in the water column. The Ca-bound nanoparticulate P-often overlooked in traditional P assessment-was found to be a dominant form in high-P aquaculture ponds. This fraction has the potential to suppress algal bloom development by promoting microbial degradation of P_o and limiting the availability of bioactive P_o.