68
tween nutrient levels and algal excretion. In most cases excessive excretion is found at the end of or after the algal bloom. Very often
grazing on the algae and consumption of the excretion products have
taken place during the experiments, and cell breakage together with
other experimental weaknesses occured, in addition to the difficulties
caused by uneven distribution of organisms and nutrients. All this,
together with the fact that nutrient concentrations as well as phytoplankton species composition are often not determined, make data collected in fie~d experiments and in nature more or less useless in the
present context. Many of the problems involved have been pOinted out
by Berman and Holm-Hansen (1974); Sharp (1977) and Wangersky (1978)
among others.
At this point credit should be given to several recent workers, especially Wiebe and Smith (1977), Juttner and Matuschek (1978), Lancelot (1979) and Larsson and Hagstrom (1979, 1982) who have developed
new and useful methods and provided more evidence to demonstrate that
excretion of organic matter by natural phytoplankton is important under
special circumstances. These contributions do, however, not shed new
light on the role of. nutrient stress on the release processes, mainly
because they were not specially designed for this aspect.
CONCLUSION
We know that excretion of organic compounds by planktonic algae
differ between species in regard both to the nature of the compounds
and the quantities released. Nutrient levels and composition undoubtedly have strong influence on the release processes.
It is quite clear
that some algal species will excrete most of their photosynthate to
the surrounding water when the cells have plenty of light, are nutrient
limited (N and P) and grow slowly. The quantities released under such
circumstances may be quite significant. The dominating component of
the excreted material will very likely be carbohydrates. We do not
know how widespread this character is among commonly occurring microscopic algae since only a limited number of species have been properly
investigated.
We know that N- and P-limited phytoplankton occur in lakes, fjords
and coastal waters in temperate
and boral zones. It is therefore
reason to believe that excretion of organic matter may be quite significant in such waters, provided the species composition of the phyto-
tween nutrient levels and algal excretion. In most cases excessive excretion is found at the end of or after the algal bloom. Very often
grazing on the algae and consumption of the excretion products have
taken place during the experiments, and cell breakage together with
other experimental weaknesses occured, in addition to the difficulties
caused by uneven distribution of organisms and nutrients. All this,
together with the fact that nutrient concentrations as well as phytoplankton species composition are often not determined, make data collected in fie~d experiments and in nature more or less useless in the
present context. Many of the problems involved have been pOinted out
by Berman and Holm-Hansen (1974); Sharp (1977) and Wangersky (1978)
among others.
At this point credit should be given to several recent workers, especially Wiebe and Smith (1977), Juttner and Matuschek (1978), Lancelot (1979) and Larsson and Hagstrom (1979, 1982) who have developed
new and useful methods and provided more evidence to demonstrate that
excretion of organic matter by natural phytoplankton is important under
special circumstances. These contributions do, however, not shed new
light on the role of. nutrient stress on the release processes, mainly
because they were not specially designed for this aspect.
CONCLUSION
We know that excretion of organic compounds by planktonic algae
differ between species in regard both to the nature of the compounds
and the quantities released. Nutrient levels and composition undoubtedly have strong influence on the release processes.
It is quite clear
that some algal species will excrete most of their photosynthate to
the surrounding water when the cells have plenty of light, are nutrient
limited (N and P) and grow slowly. The quantities released under such
circumstances may be quite significant. The dominating component of
the excreted material will very likely be carbohydrates. We do not
know how widespread this character is among commonly occurring microscopic algae since only a limited number of species have been properly
investigated.
We know that N- and P-limited phytoplankton occur in lakes, fjords
and coastal waters in temperate
and boral zones. It is therefore
reason to believe that excretion of organic matter may be quite significant in such waters, provided the species composition of the phyto-