Mind your methods: acidification degrades total nitrogen and stable isotopic values within calcified marine macroalgae

Abstract

ABSTRACT Nitrogen and carbon are commonly used to determine nutrient regimes and trophic structures within marine ecosystems. Macroalgae are convenient for assessing nutrient conditions via stable isotopes and tissue nutrient levels because of their ability to absorb and integrate ambient nutrients over extended time periods. Calcified macroalgae, such as Halimeda and Udotea spp, are common constituents of tropical marine ecosystems, making them ideal candidates for nutrient-based and food web analyses. However, calcified genera require acidification to remove calcium carbonate to accurately determine δ13C and percentage of N (by weight); the overall effect of acidification on the tissue nutrients and stable isotopes of calcified genera is unresolved. Individuals of Halimeda kanaloana (n = 10) and Udotea geppiorum (n = 9) were collected from Maui, O‘ahu, and Lāna‘i. Each specimen was split into two samples and either decalcified using liquid-phase HCl (acidified) or left unaltered (control). We found that liquid-phase HCl acidification resulted in significantly lower percentage of N in both Halimeda kanaloana and Udotea geppiorum. Whereas δ13C values in acidified samples of both species were predictably lowered, the δ15N in acidified U. geppiorum was significantly increased. Acidification may have unpredictable consequences on both the percentage of nutrients in calcified algal tissue and their δ15N, suggesting that the use of acidification in calcified algal nutrient studies may produce erroneous conclusions. Analysing two sets of samples as calcified (for δ15N) and acidified (for δ13C) would eliminate these errors. However, the use of calcified macroalgae to assess percentage of N should be avoided.