Influence of light intensity and nutrient source on δ13C and δ15N signatures in Ulva pertusa

Stable isotopes are increasingly used to infer sources of nutrient enrichment and trophic linkages in coastal marine systems, although the utility of these tools often depends upon a predictable expression of δ15N and δ13C signatures by primary producers. Accordingly, we examined how tissue δ15N and δ13C values change in the common coastal marine alga Ulva pertusa Kjellman under contrasting light and nutrient treatments. In Expt 1, we manipulated nitrogen and phosphorus enrichment and light availability in a factorial design and found that: (1) δ13C values in the tissue of U. pertusa depended upon interactions between light and nutrient availability, and there was no clear, overarching relationship between tissue δ13C values and growth rate; and (2) these effects yielded a substantial (11.6‰) range of variation in δ13C values. In Expt 2, we manipulated natural light (shaded versus unshaded) and nitrogen form (nitrate versus ammonium) in a factorial design and found that (3) δ15N of U. pertusa tissue was closely tied to δ15N of source nitrogen under all treatments and (4) δ15N differences between high and low light treatments were largest when U. pertusa was supplied with ammonium (3.7‰), relative to nitrate (0.8‰). The variation in δ13C values has implications for studies that use stable isotopes to infer trophic relationships in coastal marine environments, where gradients in nutrient concentration and light availability are common. The comparatively small range of δ15N values expressed in U. pertusa supplied with nitrate confirms that this species represents a good proxy for δ15N of biologically available nitrogen in nitrate-dominated coastal seawater.