Effects of pH and predation by
larvae on the
plankton of a shallow and acidic forest lake
HASEEB MD. IRFANULLAH AND BRIAN MOSS
Jones Building, School of Biological Sciences, The University of Liverpool, Liverpool, U.K.
1. Eutrophic acid lakes are not common. Delamere Lake in Cheshire, U.K. is shallow and
acid (mean pH 4.5) with a very high phytoplankton crop (mean 290
. Rotifers were dominant in the pelagic waters but
small cladocerans (
occasional in the littoral waters.
larvae were the top predators in this
±shless lake. Two mesocosm experiments were carried out in which pH and
populations were manipulated.
2. Progressively higher concentrations of
were maintained in the elevated pH
treatments (pH 6 and 8;
< 0.001) with increased amounts of a
the end of the experiment. Highest species richness was seen at ambient pH. Thus the low
pH of Delamere Lake alone did not control the structure of the phytoplankton community.
showed signi±cantly higher abundance at pH 6 than in other pH
< 0.001). Species richness of rotifers was unaffected by pH.
3. Most Cladocera were
. Although never seen in the open lake,
appeared in all the pH treatments. Low pH did not control small Cladocera abundance in
Delamere Lake, but probably hampered reproduction in
. Negative correlations
in the mesocosms (
however, indicated the potential of large-bodied daphniids in controlling phytoplankton.
4. Neither different combinations of
instars (none, instars 1 and 2 and instars 3–5)
nor different densities of instars 3–5 (0.15, 0.5 and 1.0 L
) had a negative impact on
remained unaffected in the experiment, perhaps because of its large
because of its high reproductive rate compensating predatory losses.
5. Very low pH in Delamere Lake might suppress
by hampering its reproduction.
may be vulnerable to invertebrate predation even at low predator
density in the lake.
: acid lakes,
, eutrophication, unusual
Acid lakes are often considered to be less productive
than circumneutral systems and usually have low
nutrient loadings (Olsson & Pettersson, 1993), but
recent studies suggest that the total biomass of
producers and grazers in an acid lake can be similar,
if not greater, than that in circumneutral lakes in the
vicinity or than in pre-acidi±ed conditions (Havens &
Carlson, 1998; Fischer, Frost & Ives, 2001; Findlay,
2003). The low nutrient concentrations in acid lakes
may be explained by the local geology, which is often
of igneous or other poorly weathered rocks (Olsson
& Pettersson, 1993; Kopa
, 1995). Low species
in acid lakes (Schindler