Patterns in the d¹³C and d¹⁵N signature of Ulva pertusa: Interaction between physical gradients and nutrient source pools
Cornelisen, Christopher D., Stephen R. Wing, Kim L. Clark, M. Hamish Bowman, Russell D. Frew, and Catriona L. Hurd
Limnol. Oceanogr., 52(2), 2007, 820–832

Field surveys and laboratory experiments were used to investigate the influence of the physical environment on variability in d¹³C and d¹⁵N 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 d¹³C (-18% to -12%) and d¹⁵N (0% to 6%). d¹³C was enriched at high irradiance sites and depleted at the fjord’s wave-exposed entrance. d¹⁵N signatures increased from 0% at the fjord head where freshwater influence is greatest to an oceanic signature of 6% at the fjord entrance. d¹⁵N 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, d¹³C 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 d¹³C signatures by as much as 5%. Variability in d¹³C 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 d¹³C signatures. Variation in d¹⁵N 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.