Carbon and nitrogen zooplankton isoscapes in West Antarctica reflect oceanographic transitions

Abstract

Antarctic marine ecosystems are spatially and temporally dynamic. Regional climate change is significantly altering the patterns and magnitudes of this dynamism with cascading impacts on biogeochemistry, productivity, and food web architecture. Isoscapes (or isotopic maps) provide a valuable analytical framework to characterize ecosystem processes and address questions about trophic dynamics, animal movement, and elemental cycling. Applications of stable isotope methods to Antarctic ecosystems are currently limited by a paucity of information on geospatial isotope characteristics within the Southern Ocean. In response, we have created the first empirically derived zooplankton isoscapes for West Antarctica based on analysis of bulk nitrogen (N) and carbon (C) isotope values (δ 15 N and δ 13 C, respectively) in 94 zooplankton specimens from the Drake Passage, West Antarctic Peninsula (WAP), and Amundsen and Ross Seas. The zooplankton δ 15 N values increased by 3 ‰ from north of the Polar Front (3.3 ± 0.6 ‰) to the Ross Sea (6.2 ± 0.8 ‰), reflecting a productivity gradient across this region. Abundant open water polynyas in the Amundsen and Ross Seas exhibit strong nitrate drawdown, resulting in more 15 N-enriched phytoplankton and zooplankton relative to those from the generally less productive WAP and Drake Passage. Zooplankton δ 13 C values decreased by 3 ‰ from north of the Polar Front (-24.2 ± 0.9 ‰) to the Ross Sea (-27.5 ± 1.6 ‰), likely driven by decreasing sea surface temperatures with increasing latitude. Our isoscapes are a valuable first step in establishing isotopic spatial patterns in West Antarctica and are critical for addressing numerous ecosystem questions.

Trophic position and foraging ecology of Ross, Weddell, and crabeater seals revealed by compound-specific isotope analysis

“…The δ 13 C and δ 15 N values of a consumer are the isotopic values of primary producers, the base of the food web, with modifications resulting from each trophic transfer, when energy is transferred from one trophic level to another (e.g. Lorrain et al 2009, Graham et al 2010, Jaeger et al 2010, Brault et al 2018. These modifications are predictable and described further below (Minagawa & Wada 1984).…”

“…Provided the primary processes driving variation are known, bulk δ 13 C values of an animal can give valuable information on its foraging habitat. The Southern Ocean is known to have considerable spatial variation in baseline δ 13 C (δ 13 C baseline ) values (Rau et al 1982, Quillfeldt et al 2010, Brault et al 2018. Several studies have observed decreasing δ 13 C values with increasing latitude, with offsets between about 55 and 79°S of approximately 3 ‰, largely reflecting variations in sea surface temperature (Rau et al 1982, Graham et al 2010, Quillfeldt et al 2010, Magozzi et al 2017, Brault et al 2018.…”

“…If variations in these factors are well understood, then the bulk δ 15 N values of a consumer can provide insights into its foraging region and trophic position (Post 2002). As with carbon, substantial spatial variations in δ 15 N baseline values occur in the Southern Ocean (DiFiore et al 2006, Somes et al 2010, Jaeger et al 2010, Brault et al 2018. Much of the Southern Ocean has low δ 15 N baseline values, likely due to less complete nutrient drawdown (Somes et al 2010, Brault et al 2018.…”

“…As with carbon, substantial spatial variations in δ 15 N baseline values occur in the Southern Ocean (DiFiore et al 2006, Somes et al 2010, Jaeger et al 2010, Brault et al 2018. Much of the Southern Ocean has low δ 15 N baseline values, likely due to less complete nutrient drawdown (Somes et al 2010, Brault et al 2018. However, areas near the Antarctic continent have extensive coastal open water polynyas, which are often productivity 'hot spots' (Arrigo & van Dijken 2003).…”

“…However, areas near the Antarctic continent have extensive coastal open water polynyas, which are often productivity 'hot spots' (Arrigo & van Dijken 2003). These 'hot spots' experience more complete nutrient drawdown and, thus, higher δ 15 N baseline values (DiFiore et al 2006, Brault et al 2018. Indeed, coastal regions may have δ 15 N baseline values that are up to ~3 ‰ higher than those in open ocean regions (DiFiore et al 2006, Brault et al 2018.…”