A note on seabed irradiance in shallow tidal seas

Light falling on the seabed influences the behaviour of animals and the growth of plants. In shallow tidal seas, the irradiance falling on the seabed depends upon the height of the tide as well as the solar elevation. This note examines the interaction between these two cycles. Observations in the Menai Strait show a springs-neaps cycle of seabed irradiance. At spring tides, when high water occurs at midday, seabed darkening occurs in the middle of the day, and peak irradiance occurs in the morning and afternoon. The peaks do not occur exactly at low water, but at a time between low water and midday. The exact time of the daily peak depends upon the attenuation coefficient: the higher the attenuation, the closer the maximum is to low water. At neap tides, there is just one peak in seabed irradiance in the middle of the day. A simple model of seabed irradiance is able to reproduce these features. When the model is run for a whole year (with high water springs at midday) it shows a springs-neaps cycle of daily mean irradiance. The amplitude of the cycle decreases exponentially with depth. The maxima in this cycle are at neap tides at the beginning and end of the year, but as the days lengthen, the maximum moves away from neap tides and after a few weeks occurs instead at spring tides. The reason is that when the days are longer the morning and afternoon spring tide peaks become more important. The timing of the switch in the cycle depends upon the attenuation coefficient (it occurs earlier in the year the higher the attenuation). No such switch is observed when the model is run for locations with low water springs at midday: at these places there is also a springs-neaps cycle of daily mean seabed irradiance, but the maxima always occur at spring tides.