eLearning

When such conditions occur for several days in a
row during cloudless conditions, high UV exposure
can have devastating consequences for phyto-
plankton and bacteria trapped above the near-
surface thermocline (Milot-Roy and Vincent 1994;
Xenopoulos 1997; Xenopoulos and Bird 1997).
Bottle-scale nutrient experimentswere also flawed
(Schindler 1971, 1988). It is common to inject
bottles with small amounts of nutrients before
incubation. If injection of a nutrient causes an
increase in production over unmodified reference
bottles, it is assumed to be limiting in the natural
system. In the early days of eutrophication research,
it was commonly assumed that limiting nutrients
identified by such techniqueswere those that should
be controlled to reduce eutrophication. Bottle-scale
nutrient additions indicatewhat is limiting to phyto-
plankton at the time of incubation, but do not
necessarily indicate the cause of the limitation,which
may be inputs of other nutrients, toxins, or nonna-
tive organisms. For example, following fertilization
of a lake with phosphorus, bottle experiments com-
monly indicate that nitrogen is limiting. Also, small
enclosures can exclude key ecosystem processes.
For example, in experimentally fertilized Lake 227,
carbon limitation was indicated in bottle experi-
ments following additions of phosphorus and nitro-
gen, whereas in the whole lake, exchange of CO2
between the atmosphere and lake water was suffi-
cient to allow algae to multiply in proportion to
phosphorus additions (Schindler and others 1972;
Schindler 1988).
Such misinterpretations of bottle-scale experi-
mentswere at least partly responsible for the contro-
versy over which nutrients should be controlled to
reduce the eutrophication problem, delaying nutri-
ent management in some jurisdictions. Bottle and
small mesocosm experiments indicated that many
culturally eutrophied lakes were carbon limited
(Kerr and others 1970; Kuenzel 1970; Lange 1970),
leading to the erroneous conclusion by some that
eutrophication could not be controlled without
controlling carbon [see Legge and Dingeldein (1970);
reviewed by Edmondson (1991)].
However, early whole-lake experiments (Schindler
and others 1971, 1972, 1973) clearly showed that
even the most severely carbon-limited lakes devel-
oped intensive algal blooms when fertilized with
phosphorus and nitrogen. The invasion of CO2 from
the atmosphere allowed carbon to be taken up in
proportion to nitrogen and phosphorus. The results
of smaller-scale experiments were an artifact of
restricted invasion of carbon dioxide fromthe atmo-
sphere to lakes (Schindler 1971, 1988; Schindler
and others 1972; Schindler and Fee 1975). But easy,