Impacts of aquaculture nutrient sources: ammonium uptake of commercially important eucheumatoids depends on phosphate levels

In an integrated multitrophic aquaculture (IMTA) system, seaweeds serve as extractive species that utilize excess nutrients, thereby reducing the risk of eutrophication and promoting sustainable aquaculture. However, the use of excessive fish feeds and the resultant faecal waste as nutrient streams can contribute to variations in nitrogen and phosphorus levels (e.g., primarily NH 4 + and PO 4 −3 ) in the surrounding area and this may impact the physiology of the integrated seaweeds, particularly on how these species take up inorganic nutrients. In this study, the effect of different PO 4 −3 levels on NH 4 + uptake of the three commercially important eucheumatoids Kappaphycus alvarezii , Kappaphycus striatus and Eucheuma denticulatum was examined under laboratory conditions. Seaweed thalli ( n = 4) were incubated in seawater media containing 30 µM NH 4 + , and 0, 0.5, 1.0, 1.5, 3.0 or 5.0 µM PO 4 −3 for 1 h under a saturating light level of 116 ± 7.13 µmol photons m −2 s −1 inside a temperature-controlled laboratory. Species-specific responses to PO 4 −3 levels were observed. For K. alvarezii , maximum NH 4 + uptake (17.8 ± 1.6 µmol gDW −1 h −1 ) was observed at 0.5 µM PO 4 −3 and the uptake rate declined at higher PO 4 −3 levels. For K. striatus , NH 4 + uptake increased with increasing PO 4 −3 levels, with maximum N uptake (6.35 ± 0.9 µmol gDW −1 h −1 ) observed at 5.0 µM PO 4 −3 . For E. denticulatum, maximum NH 4 + uptake (14.6 ± 1.4 µmol gDW −1 h −1 ) was observed at 1.0 µM PO 4 −3 . Our results suggest that among the three eucheumatoid species, the NH 4 + uptake of K. striatus persists even at high levels of PO 4 −3 . However, our results also showed that K. striatus had the lowest range of NH 4 + uptake rates. These results should be taken into consideration when incorporating eucheumatoids in the IMTA system, where PO 4 −3 levels significantly vary in space and time.