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 δ 13 C and δ 15 N 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 δ 13 C (‐18% to ‐12%) and δ 15 N (0% to 6%). δ 13 C was enriched at high irradiance sites and depleted at the fjord’s wave‐exposed entrance. δ 15 N signatures increased from 0% at the fjord head where freshwater influence is greatest to an oceanic signature of 6% at the fjord entrance. δ 15 N 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, δ 13 C 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 δ 13 C signatures by as much as 5%. Variability in δ 13 C 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 HCO 3 − , an enriched source of carbon. However, increased water motion at the fjord entrance counteracts the influence of irradiance, leading to enhanced flux of CO 2 and depleted δ 13 C signatures. Variation in δ 15 N 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.

Bibliographic Information