Extreme 15N Depletion in Seagrasses

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 ‰ δ 15 N. 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 δ 15 N 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 δ 15 N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ 15 N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ 15 N values were correlated with Halodule uninervis δ 15 N 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 15 N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ 15 N values. This study reports some of the most extreme, negative δ 15 N 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.

Decrypting stable‐isotope (δ13C and δ15N) variability in aquatic plants

“…Frequency histograms of δ 13 C and δ 15 N signatures in aquatic plants thriving in the sampled water bodies. Described ranges for freshwater macrophytes, terrestrial plants and marine macrophytes based on the ranges published by different authors (Benedict, ; Finlay & Kendall, ; Hemminga & Mateo, ; Keeley & Sandquist, ; Mancinelli, Vizzini, Mazzola, Maci, & Basset, ; Milligan et al., ; Raven et al., ; Smith & Walker, ; Walton et al., ) are depicted below each stable isotope axis…”

“…Considering all data together, the signature of d 13 C displayed a significant negative relationship with DIC (Table 2, Figure 4). However, if we analyzed this relationship for each functional group separately, only the groups with species completely submerged showed a significant negative linear relationship with DIC of the water (submerged stoneworts: R 2 = 0.64, p-value = .000; rooted submerged vascular plants: (Benedict, 1978;Finlay & Kendall, 2007;Hemminga & Mateo, 1996;Keeley & Sandquist, 1992;Mancinelli, Vizzini, Mazzola, Maci, & Basset, 2013;Milligan et al, 2010;Raven et al, 2002;Smith & Walker, 1980;Walton et al, 2016) are depicted below each stable isotope axis The geomorphological type of the water body showed a significant relationship with d 15 N ( Table 2) | 1813 between 90 and 100% of natural coverage and no or little agricultural coverage.…”

“…Some studies have emphasized the wide ranges of δ 13 C and δ 15 N in aquatic plants, particularly in continental waters (Finlay & Kendall, ; Keeley & Sandquist, ; Milligan, Pretzlaw, & Humphries, ), and to a minor extent in marine angiosperms (Hemminga & Mateo, ; Raven et al., ; Walton et al., ). However, to our knowledge there have been no attempts to examine the relative influence of environmental conditions (extrinsic factor) and of functional groups (intrinsic factor) on δ 13 C and δ 15 N variability in continental aquatic plant species from stagnant water bodies.…”

“…Some studies have emphasized the wide ranges of d 13 C and d 15 N in aquatic plants, particularly in continental waters (Finlay & Kendall, 2007;Keeley & Sandquist, 1992;Milligan, Pretzlaw, & Humphries, 2010), and to a minor extent in marine angiosperms (Hemminga & Mateo, 1996;Raven et al, 2002;Walton et al, 2016).…”

Towards spatial management of fisheries in the Gulf: benthic diversity, habitat and fish distributions from Qatari waters

“…Intertidally and in shallow waters, these muddy habitats can be extremely productive due to the formation of dense algal mats dominated by cyanobacteria and diatoms that support coastal food webs and commercially important species including both fish and shrimp (Al-Zaidan et al 2006). Seagrass beds are another highly productive habitat with above-ground biomass of as much as 900g DW m -2 on the east coast of Qatar (Walton et al 2016), more than ten times than reported for this species from elsewhere in the world (Duarte and Chiscano 1999). Macroalgae can form dense beds where hard substrata provide point of attachment, and form an important component of the detrital loop (Sheppard et al 2010).…”