Influence of spring-neap tidal cycles on the light available for photosynthesis by benthic marine plants

Abstract

An underwater light-measuring station was established off Helgoland (southern North Sea) to provide continuous measurements of underwater irradiance at three wavelengths. During autumn and winter 1990, in spite of variations in weather conditions, there was a strong two-weekly cycle of total daily irradiation, with peaks when low tide occurred at around midday (neap tides at Helgoland) and troughs when high tide occurred at this time (spring tides). The amplitude of this cycle was much greater than could be accounted for by the difference in tidal height at midday, but could be explained if light penetration through the water was greater during neap tide series than during spring tides. However, no association between water clarity and the spring-neap cycle was apparent in April-June 1991. -from Authors