The Vertical Distribution of Attached Marine Algae in Malta

Abstract

Light has long been considered the main factor controlling the distribution of attached sublittoral marine algae, since both the total amount and the spectral composition of the ambient illumination changes markedly with increased depth (Jerlov 1951; Levring 1947). It has been clearly established by laboratory experiments that the different pigmentation of the green, brown and red algae allows these plants to use light of different colours for photosynthesis with varying efficiency (Klugh 1931; Levring 1947; Haxo & Blinks 1950; Fork 1963), light energy absorbed by the various accessory pigments probably being transferred directly to the chlorophyll system present in all these algae. Such observations are in agreement with the hypothesis of complementary chromatic adaptation. This hypothesis was first suggested by Engelmann (1883), and was based on the results of his bacterial method for the determination of photosynthetic action spectra of filamentous algae; it was later set out formally by Gaidukov (1903) who presented many of his earlier results on blue-green algal culture as confirmation. It was proposed that these laboratory experiments could explain the occurrence of red, algae at lower depths than other algae in coastal waters and that the red pigments of these algae represented 'adaptation' to the green light predominant in such waters. In situ experiments on photosynthesis have been carried out by several workers on algae in northern coastal waters (Gail 1922; Printz 1939; Levring 1947), all of whom found the maximum depth for detectable photosynthesis in these waters to be between 30 and 40 m, and also that the red algae were best suited to conditions near this depth. However, the differences between the three algal types were not great and Levring concluded that several other factors were involved in addition to the spectral composition of the ambient submarine light. One of the most prominent of such factors was light intensity, which supports the earlier view of Harder (1923) that both wavelength and intensity of light are important. Such work did much to reconcile the two, early, opposing hypotheses, concerning the influence of light on vertical algal zonation, viz. (1) complementary chromatic adaptation effects due to changes in spectral composition, and (2) a direct effect of light intensity (Berthold 1882; Oltmanns 1892). Only since the advent of the aqualung has direct observation of the sublittoral vegetation been really feasible, and little information is as yet available. It seems that in northern coastal waters the massive brown Laminariales are usually dominant down to at least 20 m, with an undergrowth of red species, and that these give way to coralline