Temporal changes in the amino acid contents of particulate organic matter sedimenting in the Bay of Banyuls-sur-Mer (northwestern Mediterranean)

Abstract

We monitored gross sedimentation rates (GSR) together with organic carbon, total hydrolysable (THAA) and enzymatically available (EHAA) amino acid contents of the particulate organic matter collected within 2 sediment traps moored during a 4 yr period at a shallow station (18 m) in the Bay of Banyuls-sur-Mer (northwestern Mediterranean). GSRs were between 0.26 and 470.55 g DW m-2 d-1 They were low during spring and summer, and usually higher but much more variable during fall and winter. Organic carbon concentrations were between 1.21 and 6.94% DW, versus 3.36 and 29.16 mg g-1 DW, and 0.54 and 14.44 mg g-1 DW for THAA and EHAA, respectively. EHAA/THAA ratios were between 13.91 and 65.78%. These 4 parameters showed an annual periodicity and correlated negatively with GSR, which supports the role of resuspension in controlling sedimentation. THAA and EHAA spectra were both dominated by glycine, aspartic acid, glutamic acid and alanine. However, there were consistent differences between the 2 kinds of spectra. EHAAs were enriched in threonine, glutamic acid, leucine and aspartic acid; and depleted in arginine, lysine, glycine and histidine. EHAA spectra showed fewer temporal changes than THAA spectra. In all cases but one, significant changes in THAA spectra were related to glycine, aspartic acid, glutamic acid and threonine, and were indicative of the presence of degraded material, probably due to resuspension. We computed degradation indexes based on both THAA and EHAA spectra. THAA degradation index was between -0.318 and 1.363 versus 0.051 and 1.006 for EHAA. Both of these indexes showed an annual periodicity and correlated positively (although weakly) with EHAA/THAA ratios. None of them correlated negatively with GSR. This last result suggests that EHAA/THAA ratios constitute a better index of the lability of particulate organic matter than degradation indexes.