Within and among-site variability in δ13C and δ15N for three estuarine producers, Sporobolus virginicus, Zostera capricorni, and epiphytes of Z. capricorni

Abstract

Carbon and nitrogen stable isotope ratios of three estuarine autotrophs (the seagrass, Zostera capricorni and its epiphytic algae, and the saltcouch grass Sporobolus virginicus) were measured within and among sites in Southern Moreton Bay, Southeast Queensland. For all taxa, isotope ratios were significantly affected by the position of a plant within a site. Carbon signatures of S. virginicus were more enriched at upper elevations (−14.3‰), and more depleted at the lower edge (−15.0‰). Small but significant differences for S. virginicus were also found between edge (−14.7‰) and interior (−14.9‰) positions. Z. capricorni was more depleted in 15N at edge (5.9‰) positions than the interior (6.2‰). The seagrass epiphytes varied along the elevation gradient, being more depleted in 13C at the upper (−19.7‰) than the lower (−19.3‰) edge. This small within-site variation (< 1‰) may result from differences in the physical characteristics among the sites that influence the productivity of plants and thereby their isotope ratios, but would not preclude the use of carbon and nitrogen stable isotopes in small-scale food web studies. At a larger scale, isotope ratios differed significantly among sites separated by several kilometres and the range of this variation was greater for all taxa than at the within-site scale. Differences among sites are probably due to variation in nutrient source and hydrodynamics.