Uncovering nutrient regeneration, transformation pattern, and its contribution to harmful algal blooms in mariculture...

The prevalence of harmful algal blooms (HABs), especially in mariculture waters, has become a concern for environmental and human health worldwide. Notably, the frequent occurrence of HABs relies upon a substantial supply of available nutrients, which are influenced by nutrient recycling. However, nutrient regeneration, transformation pattern, and their contribution to HABs in mariculture waters remain largely unknown. In this study, by combining field investigation and incubation experiments from June to September 2020, the temporal variations in nutrients and algal composition were revealed. In addition, the nutrient regeneration and assimilation rates in the water column during two continuous algal blooms were measured. The results indicated that organic nutrients, which were the dominant components, strongly stimulated nutrient regeneration. High regeneration rates were observed, with dissolved inorganic nitrogen (DIN) and phosphorous (DIP) regeneration rates ranging from 0.25 to 2.64 μmol/L·h and 0.01 to 0.09 μmol/L·h, respectively. Compared to the direct uptake of organic nutrients, the rapid regeneration of inorganic nutrients played a vital role in sustaining continuous algal blooms, as regenerated DIN contributed 100 % while regenerated DIP contributed 72-100 % of the algal assimilation demand. Furthermore, the redundancy analysis and inverse solution equations indicated that different N transformation patterns and utilization strategies occurred during Heterosigma and Nannochloris blooms. The shorter N recycling pathway and faster NH₄⁺ supply rates provided favorable conditions for the dominance of Nannochloris over Heterosigma, which had a preference for the uptake of NO₃^-. In conclusion, we propose that nutrient regeneration is a key maintenance mechanism underlying the maintenance of continuous algal blooms, and different N transformation patterns and utilization strategies regulate algal communities in mariculture waters.