Annual benthic irradiance levels at the Northwestern Cuban shelf

Abstract

This study, aimed at determining the average annual benthic light irradiance in the northwestern shelf to later calculate its macrophytobenthos total primary production, is the first and only one ever conducted on this subject in Cuban waters to date. Organic mud constitutes 39% of all bottom types, sand is 37%, and the remaining 24% is made up of carbonate mud, hard bottom, rubble, coral, and their combinations. The water average depth is 7.9 m (2 to 28 m range), with an average color 6 (2 to 14 range) in the Forel-Ule scale. Average water temperatures are 28.1 ºC at bottom, and 28.4 ºC at 0.5 m depth, indicating a well-mixed water column; the average bottom water currents speed of 0.116 m s−1 is unremarkable. Visible down to bottom in 73% of all stations, the average Secchi disk depth is 7.1 m (90% transparency) at the remaining 27% stations. The value of the diffuse attenuation coefficient of down welling irradiance “k” is affected by the hour of day, seasons, water color, runoffs, distance from the coast, and depth where it is determined. With an average surface-to-bottom “k” (k0−bottom) value of 0.13 m−1, this shelf water is 34% less transparent than nearby open ocean water. The bottom light illumination is 49% of that at surface, allowing an instantaneous irradiance energy at bottom of 0.31 kW m−2, which is almost twice the average energy most abundant macrophytobenthos species require to reach maximum photosynthesis rates.