Responses of bloom forming and non-bloom forming macroalgae to nutrient enrichment in Hawai‘i, USA

Macroalgal blooms of Ulva lactuca and Hypnea musciformis have been problematic in shallow coastal waters around agricultural and urbanized regions of Maui, Hawai‘i for decades. Observations have highlighted the correspondence between these blooms and elevated nutrient levels from the adjacent land-use, however little evidence exists regarding the effects of nutrient enrichment on the blooming and non-blooming macroalgae in the area. To determine if elevated nutrient levels influence H. musciformis physiology, we conducted a nutrient enrichment (+N, +P, and +N+P) experiment and measured growth, photosynthetic status, and pigment absorbance. Phycobilin pigments were significantly reduced in the no addition and +P treatment and maintained in those with N additions, suggesting that H. musciformis can use phycobilins to store N. We conducted a second, larger experiment with additions of secondarily-treated wastewater effluent on the bloom forming species Acanthophora spicifera, H. musciformis, and U. lactuca and the common non-bloom forming species, Dictyota acutiloba. All samples were initially depleted of potential N stores and measured for growth, photosynthetic status, and N uptake rates; H. musciformis and U. lactuca were also assessed for micro nutrient uptake, % tissue N, and δ15N values. Growth rates of D. acutiloba, H. musciformis, and U. lactuca increased with increasing % wastewater effluent addition and concentrations of TN and NO3− and those of the bloom forming species were 2-fold higher. All species increased photosynthetic capacity and saturation irradiance with increasing % wastewater effluent addition and concentrations of TN and NO3−. U. lactuca was the most sensitive to low N conditions, evidenced by declines in light capturing efficiency. All species utilized a substantial amount of N over 24 h. H. musciformis and U. lactuca also (1) utilized micro nutrients: iron, manganese, molybdenum, and zinc, (2) decreased % tissue N in low N conditions, (3) increased % tissue N in response to elevated N conditions, and (4) expressed elevated δ15N values with increasing additions of wastewater effluent. These results demonstrate that in Hawai‘i, the bloom forming species H. musciformis and U. lactuca, have similar physiological responses to decreased and increased nutrient levels.

► We assessed the response of four macroalgae to increasing wastewater additions. ► With N enrichment, Hypnea and Ulva grew 2 times faster than the other species. ► Ulva was the most sensitive to low N conditions, evidenced by declines in α. ► Hypnea and Ulva % tissue N was representative of the surrounding N concentration. ► Hypnea and Ulva δ15N values reflected the level of wastewater effluent exposure.