Niche metrics suggest euryhaline and coastal elasmobranchs provide trophic connections among marine and freshwater biomes in northern Australia

Abstract

Tropical elasmobranchs could play significant roles in connecting coastal and river ecosystems, yet few studies have explored the trophic ecology of elasmobranch species that may link these biomes. We investigated the trophic niches of 7 such species in northern Australia during the tropical monsoonal wet and dry seasons, using stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes (SI), and fatty acid (FA) biomarkers taken from muscle tissue. Both SI and FA metrics suggested significant niche partitioning between species, with 2 distinct guilds: a marine food web based on epiphytes and seagrass (low δ¹³C), and an estuarine/freshwater food web with a seston base (higher δ¹³C). A large overlap in SI niche areas and higher mean trophic positions (4.1-4.8) were evident in species accessing marine diets (Carcharhinus leucas, Rhizoprionodon taylori) when compared with species predominantly feeding in estuaries (3.2-3.6; Glyphis garricki, G. glyphis). Across all seasons, G. garricki had the greatest FA niche space, and variable overlap with 2 other species (R. taylori, C. leucas). Although limited seasonal effects were apparent for individual FA biomarkers, SI niche metrics revealed greater niche areas and inter-specific partitioning during the dry season for 3 species. Subtle differences in niche metrics derived from SI and FAs were likely due to disparate turnover times, and the statistical approach of each metric (2-dimensional versus multi-dimensional). Collectively, our analyses suggest that these tropical coastal and euryhaline elasmobranchs consume prey from a range of sources to provide trophic connections across marine, estuarine and freshwater biomes.