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.
Highlights
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; Estuaries and Coasts (2016) 39:1709–1723Loneragan et al, 1998; Nagelkerken, 2009; Kennedy et al, 2010)
This study reports some of the most extreme, negative δ15N values ever noted for seagrass and some of the most highly spatially variable
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)
Summary
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; Estuaries and Coasts (2016) 39:1709–1723Loneragan et al, 1998; Nagelkerken, 2009; Kennedy et al, 2010). Stable isotopes are 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 The interpretation of stable isotope values is often limited by a lack of knowledge with regard to either the abiotic/biotic factors that define the incorporation of stable isotopes into their biogeochemical repositories or the range of biogeochemical processes that can subsequently alter any isotopic signature
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