Continuous underwater light measurement near Helgoland (North Sea) and its significance for characteristic light limits in the sublittoral region

Abstract

Underwater irradiance was measured at intervals of 20 min for one year at 2 water depths (2.5 and 3.5 m below M.L.W.S.) and in 3 spectral regions in the sublittoral region of the rocky island of Helgoland. Data are presented for spectral and total irradiance at water depths ranging from 2 to 15 m (below M.L.W.S.). 90% of the total annual light reaching sublittoral habitats is received during the period from April to September, when Jerlov water type 7 (occasionally water type 5) dominates. During the other half of the year, the water is very turbid, and transparency is so low that long dark periods occur even at moderate water depths. The total annual light received at the lower kelp limit (Laminaria hyperborea), at 8 m water depth, is 15 MJ m−2 year−1 or 70 E m−2 year−1, which corresponds to 0.7% of surface irradiance (visible). At the lower algal limit (15 m water depth) these values are 1 MJ m−2 year−1 or 6 E m−2 year−1, corresponding to 0.05% of surface irradiance. These data are similar to measurements at the same limits in several different geographical areas, and may determine the depth at which these limits occur.