Patterns in the δ13C and δ15N signature of Ulva pertusa: Interaction between physical gradients and nutrient source pools

Abstract

Field surveys and laboratory experiments were used to investigate the influence of the physical environment on variability in δ13C and δ15N signatures of Ulva pertusa, an abundant macroalgae inhabiting the low salinity layer (LSL) of Doubtful Sound, a New Zealand fjord. Field surveys revealed significant spatial variability in δ13C (‐18% to ‐12%) and δ15N (0% to 6%). δ13C was enriched at high irradiance sites and depleted at the fjord’s wave‐exposed entrance. δ15N signatures increased from 0% at the fjord head where freshwater influence is greatest to an oceanic signature of 6% at the fjord entrance. δ15N also increased by up to 4% between 2‐m depth and the LSL‐seawater interface (4‐m depth); this pattern was less pronounced near the ocean. During laboratory experiments, δ13C of U. pertusa became significantly enriched under high levels of irradiance (>50 mmol quanta m−2 s−1). When exposed to high irradiance, increases in water motion rapidly depleted δ13C signatures by as much as 5%. Variability in δ13C of U. pertusa in Doubtful Sound is largely a function of the light regime, which influences rates of photosynthesis and in turn the algae’s dependence on HCO3−, an enriched source of carbon. However, increased water motion at the fjord entrance counteracts the influence of irradiance, leading to enhanced flux of CO2 and depleted δ13C signatures. Variation in δ15N of U. pertusa is less dependent on the physical environment and instead is driven by the source pool signature, which in turn varies between freshwater and marine sources of nitrogen.