Extreme 15N Depletion in Seagrasses

Estuaries and Coasts November 2016, Volume 39, Issue 6, pp 1709–1723 | Cite as Extreme 15N Depletion in Seagrasses AuthorsAuthors and affiliations M. E. M. WaltonI. Al-MaslamaniN. HaddawayH. KennedyA. CastilloE. S. Al-AnsariI. Al-ShaikhM. Abdel-MoatiM. A. A. Al-YafeiL. Le Vay Open Access Article First Online: 09 May 2016 3 Shares 1.4k Downloads 1 Citations Abstract Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8 ‰ δ15N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ15N values ranging from +2.95 to −12.39 ‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ15N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ15N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ15N values were correlated with Halodule uninervis δ15N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant 15N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ15N values. This study reports some of the most extreme, negative δ15N values ever noted for seagrass (as low as −12.4 ‰) and some of the most highly spatially variable (values varied over 15.4 ‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients. KeywordsNutrients Eutrophication Seagrass health Seagrass biomass Redfield ratio Stable isotopes  Communicated by:Dennis Swaney Electronic supplementary material The online version of this article (doi: 10.1007/s12237-016-0103-3) contains supplementary material, which is available to authorized users. Introduction Although only 0.1 to 0.2 % of the ocean seabed is occupied by seagrass, it forms a highly productive ecosystem that provides a wide variety of very important functions, such as: protecting shorelines against erosion; trapping and cycling nutrients; providing food for coastal food webs and, creating habitats for an extensive range of wildlife, including commercially important species of fish and crustaceans (Thresher et al., 1992; Loneragan et al., 1998; Nagelkerken, 2009; Kennedy et al., 2010). The sensitivity of seagrass species to environmental conditions means they can be used as indicators of ecological processes. For example, elemental stoichiometry and spatial patterns in elemental content of phytoplankton and higher plants (Redfield ratio) have been used to investigate their nutrient status and this approach can also be applied in seagrass (Duarte, 1990) to generate hypotheses about the nature of spatial and temporal variation in ecosystem processes (Fourqurean et al. 1992; Fourqurean and Zieman 2002). Stable isotopes are also being increasingly used as tools to investigate ecosystem processes, such as determining sediment source, nutrient cycling and carbon limitation, as well as their use as indicators of environmental threats to, and stresses on, seagrass ecosystems (Barrón et al., 2004; Kennedy et al., 2004, 2005; Papadimitriou et al., 2005a, b, 2006; Frederiksen et al., 2006; Walton et al., 2014). Research investigating natural and anthropogenic environmental threats has focused on establishing the extent of natural variations in the elemental (C, N, S) and stable isotope composition (δ13C, δ15N, δ34S) of seagrass, validating its ability to act as a tracer of anthropogenic nutrient sources and in determining the limits of our ability to recognise anthropogenic impacts from natural environmental change (e.g. Udy et al., 1999; Papadimitriou et al., 2006). However, the success of this type of approach is strongly dependent on understanding the underlying processes that affect stable isotope values. The interpretation of stable isotope values is often li (...truncated)