A study of the underwater light field available to macro-algae and its potential effect on intertidal macro-algae zonation
Inter-tidal macro-algae communities often exhibit consistent broad vertical distribution patterns of species. The factors that set the distribution limits of these patterns have long intrigued marine ecologists, and several hypotheses have been proposed to explain this phenomenon; however, no general conclusion has yet been drawn. Of all the factors limiting the occurence and development of photosynthetic organisms in aquatic environments, light is the most crucial and variable, and its role in the zonation of intertidal macro-algae is an unresolved issue. This thesis addresses the problem of whether the characteristics of the underwater light field available to intertidal macro-algae could influence species distribution. Semi-diurnal in situ measurements of the underwater light field were carried out at three sites on the south-west coast of England (United Kingdom)); the Hamble Estuary, Southampton Water and Swanage Bay. The tide is shown to significantly affect the water optical properties, and the tidal range and water transparency combine to produce important intensity and spectral fluctuations of the underwater light field available to intertidal macro-algae. To circumvent the limitations of in situ field studies a mathematical model was developed to investigate the effect of tidal features on the availability of underwater light to intertidal macro-algae across the compete tidal cycle. The results reveal the importance of the range and phase of the tide, and identify two significant zones within the intertidal zone which might depict where certain macro-algae flourish. Finally, the effect of vertical height of "transplanted" macro-algae in the intertidal zone and their pigment content was studies in situ. The results of this study were inconclusive but produce the foundation for future work. This study demonstrated that intertidal macro-algae are subjected to major intensity and spectral fluctuations in underwater irradiance, and there are two major zones within the intertidal zone which could potentially affect the zonation of some species.