Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: The case of the Ría de Vigo (NW Iberian Peninsula)

Opal concentrations in dry bulk sediment were measured in 43 sampling stations in the Ría de Vigo at 0–1, 1–2, 4–5, 10–11 and 14–15 cm sediment depths. The amount of opal was correlated with the biogenic silicon flux to the sediment obtained by means of a box model. The biosiliceous flux to the sediment (annual mean) is in good agreement with the content of opal found in the uppermost oxic layer, revealing that pelagic primary production is the main factor controlling the biogenic silica content in surface sediments. This correlation is illustrated by the equation: Opal (wt.%) = 0.103BSiF + 0.414 (R2 = 0.95) where BSiF is the biogenic silicon flux to the sediment in g Si m−2 year−1. In the innermost part of the ría, i.e. San Simón Inlet, the content of opal is higher than predicted by the equation, since other biogenic silica sources to the sediment may be involved, as benthic diatoms proliferation or freshwater diatoms input. Elevated percentages recorded in the inner ría in the subsuperficial sediment respond to the establishment of suboxic–anoxic conditions that enhance the preservation of opal. The input of faecal pellets to the surface sediment derived from mussel rafts also controls the opal distribution and concentration in the ría.