Stable isotope analysis of polar seaweeds: Assessing productivity and response to environmental change on seasonal and multi-annual time scales

Phaeophyta (i.e., brown seaweeds) are significant primary producers in high-latitude environments, serving as a key nutritional source to fauna and carbon sink. Despite being the dominant biomass source in polar regions, they have largely been overlooked in carbon assessments, trophic ecology, and biogeochemical studies. Our understanding of how these ecosystems will respond to climate change is limited, based on a handful of studies that are primarily Arctic focussed. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis of macroalgae has often been used as a tool to assess nutrient sources, energy transfer and photosynthetic mechanism but has rarely been applied to polar macroalgae. The rapid environmental change in both poles has the potential to shift the isotopic baseline. Our current understanding is poor, to our knowledge only 28 studies have published biogeochemical assessments of Antarctic macroalgae, half of which are from the South Shetland Islands in the northern West Antarctic Peninsula. Yet biogeochemical data can provide a wealth of information regarding nutrient source changes, light dynamics, productivity and nutritional quality. Larger species, such as Himantothallus grandifolius (Antarctic) or Saccharina latissima (Arctic) can provide seasonal or even multi-annual data through incremental stable isotope analysis along macroalgal blades. Changing productivity rates can be tracked through δ¹³C values, fluctuating due to sea ice break out, carbon demand and growth requirements. Over 20 specimens of H. grandifolius and Arctic kelps have been collected over several field trips to the Antarctic Peninsula, East Greenland coastline and Svalbard for δ¹³C and δ¹⁵N analysis; forming the largest biogeochemical dataset for polar macroalgae to date. Large variations > 15 ‰ were recorded for the Antarctic species H. grandifolius from a single organism, a significant variation when considering trophic level shifts are on a scale of ~ 3–5 ‰. Cyclical trends in productivity were also identified in several specimens with wider implications for shifting isotopic baselines of primary producers in response to environmental change on seasonal and multi-year time scales. Strong seasonal responses in δ¹³C are linked to sea ice and fluctuating light conditions with increased run off through glacial melting. Nitrogen was found to vary between sub-tidal and inter-tidal species, as well as incrementally along blades of larger species. New nitrogen sources may be introduced to remote polar regions as increased tourism increases the risk of wastewater and pollutant inputs to these fragile ecosystems. Macroalgae could become an ideal tracer in coastal environments where nutrient sources can be assessed at varying time scales.

Our incremental approach provides high resolution isotopic data with the capacity to generate seasonal to multi-year records from an understudied ecosystem. Polar environments are set to change in unprecedented ways, the shifting isotopic baseline has repercussions for the wider food web, ecosystem structure and functioning that macroalgae play a key role in.