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

An underwater light-measuring station, consisting of 3 sensors at each of 2 depths (4.5 and 5.5 m below MLWS), was established off Helgoland (southern North Sea) to provide continuous measurements of underwater irradiance at 3 wavelengths (452,552 and 653 nm, isolated by interference filters). The percentage transmittance of each wavelength through l m of seawater was calculated from the readings at the 2 depths. Irradiance and transmittance were recorded every 75 s during 3 periods of 2 mo between August 1990 and June 1991. During autumn and winter 1990, in spite of variations in weather conditions, there was a strong 2-weeMy 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. This hypothesis was supported by the transmittance measurements, and by wind speed data and Secchi disc depths from the same site. However, no association between water clarity and the spring-neap cycle was apparent in Apnl-June 1991. Nevertheless, the variable relationship between underwater and surface irradiance through the spring-neap cycle needs to be taken into account in any attempt to estimate benthic primary productivity from surface light data.