Carbon cycling and bacterial carbon sources in pristine and impacted Mediterranean seagrass sediments

Abstract

Stable carbon-isotope ratios of bacterial biomarkers were studied in Mediterranean seagrass meadows and macroalgae communities to identify the sources of organic carbon used by the sediment bacteria. Bacteria δ 13 C ratios in pristine sediments vegetated by the seagrass Posidonia oceanica were either similar to the seagrass signal or slightly enriched, suggesting that seagrass detritus was of major importance as a bacterial carbon source. There was a shift in bacterial carbon sources in anthropogenic impacted P. oceanica meadows towards seston and macroalgae. The net primary productivity was reduced in these meadows, whereas the rates of mineralization as measured by sulfate reduction rates were enhanced in the sediments. This effect on mineralization was probably due to the input of less refractory organic matter compared to seagrass detritus, which enhances the bacterial decomposition of organic matter. In the fast growing seagrass Cymodocea nodosa meadow, the bacterial carbon sources consisted of a mixture of seagrass detritus and seston, and the mineralization rates were much higher compared to the P. oceanica meadows, indicating that these carbon sources were more labile and easily decomposed by the bacteria. A similar pattern was found in the macroalgae bed with Caulerpa prolifera, where the rates of mineralization were high, similar to findings in organic impacted fish farm sediments. Here C. prolifera detritus was the most important carbon source and accounted for an increase in sediment organic content. The possible impacts of a shift in bacterial carbon sources due to nutrient loading are discussed in relation to the performance of P. oceanica in carbonate sediments.

Organic carbon metabolism and carbonate dynamics in a Mediterranean seagrass (Posidonia oceanica), meadow

“…TP content in P. oceanica and unvegetated sediments in Magaluf Bay was 2.9 and 2.4 mmol C g 21 dw, respectively (Holmer et al 2003). Sulfate reduction is significantly higher in P. oceanica sediments than in bare sediments in Magalluf, and total reducible sulfur concentrations in P. oceanica sediments are slightly higher than in bare sediments (Holmer et al 2004). Pore water sulfides concentrations of both communities in Magalluf Bay are not reported by Holmer et al (2003), but in three of the sites of Mallorca Island from this study, higher sulfide concentrations were found in sediments under P. oceanica than in unvegetated sediments.…”

“…Bare sediments in Magalluf Bay had lower POC and PON content (0.19% and 0.21% POC; 0.017% and 0.030% PON) compared to P. oceanica meadows (0.26% and 0.38% POC; 0.031% and 0.041% PON; Holmer et al 2003Holmer et al , 2004. TP content in P. oceanica and unvegetated sediments in Magaluf Bay was 2.9 and 2.4 mmol C g 21 dw, respectively (Holmer et al 2003).…”

“…Particulate organic carbon (POC) and nitrogen (PON) content and total phosphorus content (TP) in Magalluf Bay in autumn 2000 and May and June 2001 are reported in Holmer et al (2003Holmer et al ( , 2004. Bare sediments in Magalluf Bay had lower POC and PON content (0.19% and 0.21% POC; 0.017% and 0.030% PON) compared to P. oceanica meadows (0.26% and 0.38% POC; 0.031% and 0.041% PON; Holmer et al 2003Holmer et al , 2004.…”

Effect of environmental factors (wave exposure and depth) and anthropogenic pressure in the C sink capacity of Posidonia oceanica meadows

“…Indeed, the second highest C org burial rate in this study (40 6 3 g C org m 22 yr 21 ) was observed in the sediments from Portocolom, where a higher C org sedimentation rate compared to other meadows (i.e., Sta. Maria and Magalluf) has been reported in the past due to high nutrient and organic inputs from the surrounding town, boating activity and a fish farm (Holmer et al 2004;Vaquer-Sunyer et al 2012). Symptoms of eutrophication have been observed in Es Port bay during the summer period (Marba et al 2006; Supporting Information Table SI 1) where sedimentation rates exceeded those measured at the pristine meadows elsewhere in Cabrera National Park (e.g., Sta.…”

“…Symptoms of eutrophication have been observed in Es Port bay during the summer period (Marba et al 2006; Supporting Information Table SI 1) where sedimentation rates exceeded those measured at the pristine meadows elsewhere in Cabrera National Park (e.g., Sta. Maria) (Supporting Information Table SI 2; Holmer et al 2004). In addition, an increase in the accumulation of allochthonous carbon in coastal sediments as a consequence of human pressure increase has been already reported in previous studies (Savage et al 2010), including studies conducted in seagrass meadows (Macreadie et al 2012).…”