IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 178

Recent miniaturization and computer-control facilitate measurements of both
fluorescence and oxygen exchange in the field or even in the water column 45,54-55 .
Measurements at the growth site are advantageous, since the transport of the specimen
to the laboratory may cause artefacts due to thermal stress and changes in irradiance and
salinity. Recently an underwater PAM instrument has been developed (Diving PAM
underwater fluorometer, Walz, Effeltrich, Germany) which allows to measure the
quantum yield of fluorescence under water on site.
Macroalgae have a distinct and fixed pattern of vertical distribution in their
habitat 56 ranging from the supralittoral (above high water mark) through the eulittoral
(intertidal zone) to the sublittoral zone where they are never exposed to air. One
important factor controlling the abundance and species distribution of algae is solar
exposure which ranges from the bright solar radiation at the surface and in rock pools to
shaded habitats in crevices or under overhanging rocks where light exposure is strongly
attenuated.
Photosynthetic production strongly depends on depth and varies with the species:
surface-adapted macroalgae such as the brown Cystoseira, Padina and Fucus or green
Ulva and Enteromorpha have maximal oxygen production close to the surface 39,53 while
deeper water algae thrive best deeper in the water column (the green algae Cladophora,
Caulerpa , most red algae) 40-42 . This is even more obvious in algae adapted to shaded
habitats in crevices, under overhanging rocks or in the understorey of kelps 39,57 . In most
species studied, respiration is inhibited to a far smaller degree than photosynthesis.
Almost all macroalgae show a pronounced photoinhibition after various times of
exposure to unfiltered solar radiation at the surface at least at high zenith angles 39,58-60 .
Even algae growing in rock pools, where they are exposed to extreme solar irradiances,
show photoinhibition during local noon 59-62 . Algae adapted to deep water or shaded
conditions are inhibited even faster and stronger when exposed to direct solar
radiation 40 . The next question is: how long does it take an organism to recover from
photoinhibition after exposure? The recovery time depends on the species and its depth
adaptation. Surface-adapted species recover much faster from photoinhibition than low
light-adapted algae. Also the time required for recovery strongly depends on the degree
of inhibition. Algae adapted to shaded conditions or deep water may not recover at all or
only partially after massive inhibition indicating that they experienced chronic rather
than dynamic photoinhibition.
From an ecological standpoint it is even more relevant to follow the
photosynthetic quantum yield in specimens exposed at their natural growth site over a
whole day 39-42,48 . Most of these studies show a strong decrease during local noon and
high values in the morning and evening especially when the tidal changes are not very
pronounced. In habitats with high tidal changes as in the Atlantic or Pacific the
photosynthetic quantum yield strongly depends on the depth of the water column above
the organisms. From this it is obvious that we have to reconsider our conception of the
pattern of maximal photosynthesis in supralittoral and subtidal macroalgae. These algae
show optimal photosynthetic quantum yield either early in the morning and evening
hours during low tide or when high tides coincide with high solar angles, resulting in a