Bioavailable organic matter in surface sediments of the Nazaré canyon and adjacent slope (Western Iberian Margin)

The distribution of bioavailable organic matter in surface sediments of the Nazaré submarine canyon and adjacent open slope was investigated. The concentration of chlorophyll a (chl a), phaeopigments (phaeo), chloroplastic pigment equivalents (CPE) and total hydrolyzable amino acids (THAA) decreased with increasing water depth, and were in general higher within the canyon (specially in the upper regions) than on the open slope. The concentrations were low on the canyon walls, increasing towards the canyon axis. The chl a:phaeo ratio, degradation index (DI), asp:β-ala and glu:γ-aba ratios were highest in the upper canyon, and similarly low in the deeper canyon and along the open slope. On the canyon axis and walls these lability indices were similar. chl a:OM ratio indicated that the quality of the bulk organic matter in the upper canyon was higher than on the slope and deeper canyon regions. Bioavailable organic matter enters the canyon through the upper region; it is transported down canyon by the tide circulation, where it is dispersed across a bigger area under a more refractory state. Flume experiments demonstrate that arborescent foraminifera and polychaete pellet mounds, as found in the head of the canyon; increase deposition of phytodetritus under critical shear velocities by a 50%.