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

Field surveys and laboratory experiments were used to investigate the influence of the physical environment on variability in delta C-13 and delta N-15 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 delta C-13 (-18 parts per thousand to -12 parts per thousand) and delta N-15 (0 parts per thousand to 6 parts per thousand). delta C-13 was enriched at high irradiance sites and depleted at the fjord's wave-exposed entrance. delta N-15 signatures increased from 0 parts per thousand at the fjord head where freshwater influence is greatest to an oceanic signature of 6 parts per thousand at the fjord entrance. delta N-15 also increased by up to 4 parts per thousand between 2-m depth and the LSL-seawater interface (4-m depth); this pattern was less pronounced near the ocean. During laboratory experiments, delta C-13 of U. pertusa became significantly enriched under high levels of irradiance (> 50 umol quanta m(-2) s(-1)). When exposed to high irradiance, increases in water motion rapidly depleted delta C-13 signatures by as much as 5 parts per thousand. Variability in delta C-13 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 delta C-13 signatures. Variation in delta N-15 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.