The light climate at the seabed of shallow, tidal seas is extremely variable, and light is, therefore, often the most important abiotic factor pertaining to primary production in coastal benthic ecosystems. The influence of the tide on seabed light, and thus photosynthesis, has received limited attention in the scientific literature, despite being potentially very substantial. In this thesis, three physical aspects of tidal modulation of seabed light are investigated: (1) temporal (i.e., springs-neaps) patterns induced by the tide in daily total seabed light; (2) the net effect of the tide (e.g., amplification or reduction) on seabed light integrated over a range of timescales; and (3) the potential of the tide to modify the spectrum of seabed light. The implications of
these three aspects for benthic algae are explored theoretically. Observations of seabed light were made over springs-neaps cycles in winter and summer
at the Menai Strait (North Wales, U.K.). At this turbid, macro-tidal site, low waters occur at about midday and midnight during neap tides, and at about 6am and 6pm during spring tides. Neap tides hosted the largest daily totals of seabed light in both seasons. A ‘switch’ to spring tides hosting the largest daily totals in the summer months (by virtue of both morning and evening low waters occurring within the longer daylengths), as predicted by Bowers et al. (1997), did not occur. The general increase in turbidity in the strait at spring tides, and the marked increases at low waters of spring tides, were shown to be responsible. Observations at the Bay of Brest (Brittany, France), a macro-tidal site diametrically opposite to the Menai Strait in terms of the times of low waters at neaps and springs, indicated a peak at springs throughout the year. Analytical and graphical tools were developed that permit the nature of springs-neaps patterns in daily total seabed light and the occurrence (or non-occurrence) of seasonal ‘switches in sense’ to be predicted for sites of interest. Comparison of measured light levels with literature
values for the saturation of photosynthesis in example algal species indicated that corresponding springs-neaps cycles may be induced in daily total benthic primary production.The net effect of the tide on seabed light integrated over daily intervals was described the-oretically by Bowers and Brubaker (2010). Observations of the effect from the Bay of Brest (Brittany, France), made using a novel mooring configuration, showed good agreement with pre-
dictions based on the existing theoretical framework, confirming that the important parameters are the time of low water, tlw , the tidal amplitude,
b , the diffuse attenuation coefficient, k PAR, and the daylength, L. The results of a simple numerical modelling study indicated that depth-integrated annual totals of irradiance and photosynthesis are amplified by the tide in the Bay of
Brest, despite annual total photosynthesis being reduced by the tide at the shallowest sub-tidal positions, owing to light-saturation and photoinhibition effects. The results also suggested that differences in the areal extent, primary productivity, and vertical zonation of benthic algal com-munities from site to site may be partially attributable to differences in the values and temporal
behaviour of tlw, b kPAR, and L between the sites, through their influence on seabed light. Time-series observations of the spectral distribution of light at the seabed, made during winter at the Menai Strait, revealed that the most strongly attenuated wavelengths (i.e., blue and red in coastal waters) were preferentially amplified by the tide over a springs-neaps cycle. Further analysis indicated that the observed ‘tidal flattening’ of the spectrum of time-integrated
irradiance would not significantly benefit one example species more than another (i.e., one red alga, one brown) on the basis of their different pigment compositions and photosynthetic action spectra. This was considered to be true at the mean depth of the observations, but it was predicted that at shallower depths the effect would confer a photosynthetic advantage on brown algae such as species of kelp, perhaps contributing to their dominance amongst benthic
vegetation in the shallow sub-tidal zone. This study has shown that the tide has significant and predictable effects on the temporal behaviour, quantity and spectral quality of seabed light, which should be accounted for when modelling benthic marine primary productivity, interpreting the results of ecological field studies, and predicting the response of coastal benthic ecosystems to environmental change.