Unregulated use of ammonium-based inorganic fertilizers in large-scale commercial tropical eucheumatoid seaweed farming: evaluating physiological and biochemical consequences through controlled land-based hatchery cultivation

Depleted or limited available macronutrients is considered a major constraint in eucheumatoid seaweed farming, particularly in intensively farmed areas. Without baseline data and scientific advice, some farmers use commercial inorganic fertilizers to enhance growth and shorten cultivation periods. In the southern Philippines, this is done by briefly immersing seaweeds into a fertilizer solution, then allowed to batten on nutrient-rich surface moisture until partially dessicated before out planting. Despite its prevalent practice, this process lacks empirical evidence from a robust experiment. Therefore, this study investigated the effects of using ammonium-based commercial fertilizers, i.e., ammonium phosphate (NH₄)₃PO₄ and ammonium sulfate (NH₄)₂SO₄, on the eucheumatoid Kappaphycus alvarezii var. tambalang under controlled, land-based cultivation. Fertilizers were applied intermittently every 5 days at 8.0 g L⁻¹. Both treatments significantly improved growth (7.182 ± 0.203 % d⁻¹ and 6.448 ± 0.182 % d⁻¹ for (NH₄)₃PO₄ and (NH₄)₂SO₄, respectively) of K. alvarezii. Moreover, photosynthesis (0.126 ± 0.022 mg O₂ gFW⁻¹ h⁻¹ and 0.136 ± 0.038 mg O₂ gFW⁻¹ h⁻¹, respectively), nitrate reductase activity (0.039 ± 0.007 μmol NO₂⁻ gFW⁻¹ h⁻¹ and 0.046 ± 0.007 μmol NO₂⁻ gFW⁻¹ h⁻¹, respectively) and nitrogen-based biochemical constituents, including protein, phycobiliproteins, and chlorophyll a were elevated under fertilizer treatments. Protein was highest in ammonium sulfate-treated samples, while carbohydrate declined in both fertilizer treatments. However, carrageenan yield and quality did not differ significantly across treatments. These results provide a physiological basis for assessing the effectiveness of fertilizer application in enhancing the productivity of eucheumatoid seaweeds.