Conclusions
453
and some macroalgae are able to translocate and store their internal
nutrient resources for further use. In seagrasses, internal recycling or
reabsorption of nutrients from the old leaves contributes nearly one
fourth, or under oligotrophic conditions more than one-half of their
annual nitrogen demands (e.g. Patriquin 1972; Borum et al. 1989;
Hemminga et al. 1991; Pedersen and Borum 1992).
Field observations available so far, as well as numerous experimental
works on the effects of nutrients on marine primary producers, suggest
that the typical sequence of changes in benthic vegetation under eutrophication conditions cannot be attributed to increasing nutrient levels
alone. Other indirect, direct or secondary effects and feedback mechanisms, triggered by the proliferation of fast growing nutrient opportunists
as a direct response to increased nutrient levels, must become effective in
the further progress of eutrophication (e.g. Harlin and Thorne-Miller
1981; Twilley et al. 1985; Vogt and Schramm 1991; Sand-Jensen and
Borum 1991; Neundorfer and Kemp 1993). Such effects have recently
been summarized and comprehensively discussed by Duarte (1995).
Among the indirect effects, a key role in the observed changes in
benthic vegetation is obviously played by the deterioration of the light
climate under eutrophication conditions. Enhanced growth of epiphytic
algae and phytoplankton results in shading of the rooting or attached
benthic plants confined to the bottom. A comparison of the light requirements of seagrasses, macro algae and phytoplankton (Duarte 1995) shows
that benthic plants, although competitively superior to phytoplankton
under low nutrient supply, are outcompeted by epiphytes and by the
free-floating phytoplankton or unattached macro algae when light availability decreases. One reason is that since the latter group grows on the
benthic plants or floats above them in the water column, they occupy
positions more favourable for light harvesting. In addition, the photosynthetic capacity, efficiency, and consequently growth rates of phytoplankton and monostromeous or finely branched macro algae are significantly higher. This is because of their higher proportion of photosynthetically active cells compared to seagrasses and perennial macrophytes
(Fig. 3).
A result of the deterioration of the light climate under eutrophication
is that the depth distribution limits of benthic macrophytes move upward, in response to their minimum light requirements for growth: macroalgae can grow down to depths receiving 0.12-1.5% of surface light,
whereas seagrasses require at least 11% (Luning 1985; Duarte 1991;
Markager and Sand-Jensen 1992). The changes in depth distribution may
considerably reduce the area inhabited by macrophyte communities, and
thus their biomass and reproductive potential for recruitment. Along the
453
and some macroalgae are able to translocate and store their internal
nutrient resources for further use. In seagrasses, internal recycling or
reabsorption of nutrients from the old leaves contributes nearly one
fourth, or under oligotrophic conditions more than one-half of their
annual nitrogen demands (e.g. Patriquin 1972; Borum et al. 1989;
Hemminga et al. 1991; Pedersen and Borum 1992).
Field observations available so far, as well as numerous experimental
works on the effects of nutrients on marine primary producers, suggest
that the typical sequence of changes in benthic vegetation under eutrophication conditions cannot be attributed to increasing nutrient levels
alone. Other indirect, direct or secondary effects and feedback mechanisms, triggered by the proliferation of fast growing nutrient opportunists
as a direct response to increased nutrient levels, must become effective in
the further progress of eutrophication (e.g. Harlin and Thorne-Miller
1981; Twilley et al. 1985; Vogt and Schramm 1991; Sand-Jensen and
Borum 1991; Neundorfer and Kemp 1993). Such effects have recently
been summarized and comprehensively discussed by Duarte (1995).
Among the indirect effects, a key role in the observed changes in
benthic vegetation is obviously played by the deterioration of the light
climate under eutrophication conditions. Enhanced growth of epiphytic
algae and phytoplankton results in shading of the rooting or attached
benthic plants confined to the bottom. A comparison of the light requirements of seagrasses, macro algae and phytoplankton (Duarte 1995) shows
that benthic plants, although competitively superior to phytoplankton
under low nutrient supply, are outcompeted by epiphytes and by the
free-floating phytoplankton or unattached macro algae when light availability decreases. One reason is that since the latter group grows on the
benthic plants or floats above them in the water column, they occupy
positions more favourable for light harvesting. In addition, the photosynthetic capacity, efficiency, and consequently growth rates of phytoplankton and monostromeous or finely branched macro algae are significantly higher. This is because of their higher proportion of photosynthetically active cells compared to seagrasses and perennial macrophytes
(Fig. 3).
A result of the deterioration of the light climate under eutrophication
is that the depth distribution limits of benthic macrophytes move upward, in response to their minimum light requirements for growth: macroalgae can grow down to depths receiving 0.12-1.5% of surface light,
whereas seagrasses require at least 11% (Luning 1985; Duarte 1991;
Markager and Sand-Jensen 1992). The changes in depth distribution may
considerably reduce the area inhabited by macrophyte communities, and
thus their biomass and reproductive potential for recruitment. Along the