Growth and Nutrient Assimilation Rates in Macroalgae Under Varying Ambient N:P Ratios

Since the past few decades, nutrient dynamics in the Yellow Sea have undergone continuous change, specifically, an increase in the nitrogen:phosphorus ratio. This phenomenon has significantly influenced the relative growth rate (RGR), biomass production, and nutrient contents of macroalgae. This study investigated the response of four macroalgal species native to the Yellow Sea: Ulva pertusa, Ulva prolifera, Gracilariopsis longissima, and Grateloupia filicina. They were cultured in triplicate under six ambient N:P ratios—4:1, 8:1, 12:1, 16:1, 20:1, and 24:1. We observed that high N:P ratios (16:1–24:1) significantly limited the RGR and nutrient assimilation rate. However, an 8:1 ratio was optimal for macroalgal species. With an increase in N:P, the RGRs of U. pertusa, U. prolifera, and G. longissima were reduced by >50%, but only by 17% in G. filicina. Higher P storage capacity and lower tissue N:P and C:P ratios revealed that the green macroalgae U. pertusa and U. prolifera were more adaptable to high N:P ratios than the red macroalgae G. longissima and G. filicina. Furthermore, the assimilation rates of C, N, and P for all macroalgae except G. filicina declined significantly with increasing N:P ratios. These findings provide key insights into the differential susceptibility of macroalgal species to nutrient imbalances and highlight the importance of species-specific responses in understanding the ecological impacts of changing nutrient ratios.