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.

Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification

“…The response of coralline macroalgae to OA under the fluctuating treatment was stronger than would be predicted under static conditions alone, as the absolute growth rates were even further reduced by the additive negative effects of diurnal fluctuations in pH. In addition, no other diagnostic that provides a measure of organism fitness was influenced by pH treatment, except organic dC 13 , which increased under both lower mean pH treatments, indicating an increase in the use of diffusive CO 2 [35,36]. Increased variability in pH did not act to ameliorate the longer term effects of OA (at least over 40 days) as was hypothesized, but amplified OA's negative influence on growth.…”

Distribution and Trophic Importance of Anthropogenic Nitrogen in Narragansett Bay: An Assessment Using Stable Isotopes

“…Some of the variability in δ 13 C of the macroalgae may be due to differences in the ability of different species to take up HCO À 3 (e.g., Sand-Jensen and Gordon 1984;Maberly 1990;Maberly et al 1992;Raven et al 2002), local differences in physical environments that can effect irradiance (e.g., Cornelisen et al 2007), as well as small scale variability in water pH (e.g., Menéndez et al 2001; Table 2). On a larger scale, it appears that our assumption that pH in the bay would follow the general north-south gradient in chlorophyll and primary production led us astray.…”

Topographic shading and wave exposure influence morphology and ecophysiology of Ecklonia radiata (C. Agardh 1817) in Fiordland, New Zealand

“…Nevertheless, variation in these regional averages for the environmental conditions provided the basis for statistically explaining 60% of the variation in morphology and 84% of the variation in light saturation kinetics as indicated by variance in I max . These models provide a good approximation of relative climatological conditions among sites because of the intense spatial variability in both light and wave exposure in the Fiordland region (Cornelisen et al 2007). …”