Effect of internal waves on near-surface chlorophyll concentration and primary production in the Nazaré Canyon (west of the Iberian Peninsula)

Internal tidal wave propagation introduces vertical motions within the water column, especially near the pycnocline, forcing water particles to undergo upward and downward motions. Since neutrally buoyant phytoplankton cells are usually passive, these cells can be significantly displaced in the vertical (tens of metres). In this paper we investigate the effects of large-amplitude internal tidal waves on chlorophyll vertical distribution in the region of the Nazaré Canyon west of Portugal, using in-situ and multi-sensor satellite data. The results confirm the occurrence of enhanced levels of chlorophyll detected by a satellite ocean colour sensor that are spatially correlated with internal tides. It is shown that internal tidal waves expose phytoplankton to larger light intensities. Maximum exposure is localised where the internal tide crests occur at noon, but an average increase occurs along the wavelength of a progressive semi-diurnal internal tide during a full day, due to the exponential attenuation of light in the water column. Hence, internal waves can increase the amount of carbon uptake in comparison with an unperturbed water column. The results suggest that internal tidal waves can make a significant impact on biological processes such as primary production, with the generation of local-scale (35 km×10 km) patchiness of enhanced production on 3–4 day timescales.