Abstract
Abstract Seasonal variation of structural, physiological and growth indicators and the metabolome of the seagrass Cymodocea nodosa, as well as biogeochemical conditions of underlying sediment were studied in two meadows growing at increasing distance downstream from a fish farm in the Aegean Sea in order to assess seagrass performance under stress. Horizontal rhizome production decreased significantly with proximity to the fish farm (0.67 and 1.57 g DW m−2 d−1 close and far from the fish-farm, respectively). This coincided with observed effects on ecophyiological indicators, such as rhizome nitrogen, leaf carbon and leaf δ13C, which were elevated with proximity to the fish-farm. Seasonality was shown by some indicators being elevated in either in the warm (C of all tissues and leaf δ34S) or the cold period (N of all tissues). Growth promoting metabolites (sucrose, fructose, myo-inositol, heptacosane, tetracosane, stigmasterol, catechin and alpha-tocopherol) were lower close to the zone, whereas metabolites involved with stress-response (alanine, serine, proline, putrescine, ornithine, 3,4-dihydroxybenzoic acid and cinnamic acid) were higher. We found that growth-promoting metabolites were positively correlated with horizontal rhizome production, whereas the metabolites related to stress were negatively correlated. Metabolomic fingerprinting of seagrass provides opportunities for early detection of environmental degradation in marine ecological studies.
Highlights
Seagrasses are indicators of environmental deterioration, as meadow declines often point to disturbances that affect the entire ecosystem (Orth et al, 2006)
Pore water ammonium and sediment chromium reducible sulfur (CRS) pools and total nitrogen (TN) were elevated by 78%, 73% and 170%, respectively, at Station A compared to Station B (Table 1)
organic carbon (OC), inorganic carbon (IC), δ15N and δ34S sulfide pools) were not affected by station, except for sediment δ13C and acid volatile sulfur (AVS) pools, but this depended on season. δ13C signal was highest during the warm period, and AVS pools had an interaction effect between station and season, where the biggest variation between stations was during the cold period
Summary
Seagrasses are indicators of environmental deterioration, as meadow declines often point to disturbances that affect the entire ecosystem (Orth et al, 2006). Traditional ecological indicators involve nutrient availability in water and sediment (Fourqurean et al, 1992; van Katwijk et al, 2011), structural variables like shoot density and biomass (Agawin et al, 1996; Ibarra-Obando et al., 2004), physiological variables such as elemental and isotopic composition of seagrass tissue (Christianen et al, 2012; M Pérez et al, 1991), and growth such as rhizome production (Duarte et al, 1994), all of which have been identified as good indicators of seagrass stress (Roca et al.2016).
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