Daphnia-pH#2 - Freshwater Biology (2005) 50, 19131926

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Effects of pH and predation by Chaoborus 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. SUMMARY 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 l g chlorophyll a L ) 1 ), dominated by Dictyosphaerium pulchellum . Rotifers were dominant in the pelagic waters but small cladocerans ( Alona guttata , Chydorus sphaericus and Scapholeberis mucronata ) were occasional in the littoral waters. Chaoborus Favicans larvae were the top predators in this ±shless lake. Two mesocosm experiments were carried out in which pH and Chaoborus populations were manipulated. 2. Progressively higher concentrations of D. pulchellum were maintained in the elevated pH treatments (pH 6 and 8; P < 0.001) with increased amounts of a Chlamydomonas species at 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. Keratella quadrata showed signi±cantly higher abundance at pH 6 than in other pH treatments ( P < 0.001). Species richness of rotifers was unaffected by pH. 3. Most Cladocera were C. sphaericus . Although never seen in the open lake, Daphnia pulex appeared in all the pH treatments. Low pH did not control small Cladocera abundance in Delamere Lake, but probably hampered reproduction in Daphnia . Negative correlations between chlorophyll a concentrations and Daphnia in the mesocosms ( r 2 ¼ 0.215, P < 0.05), however, indicated the potential of large-bodied daphniids in controlling phytoplankton. 4. Neither different combinations of Chaoborus 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 ) 1 ) had a negative impact on Cladocera. Daphnia pulex remained unaffected in the experiment, perhaps because of its large size, and C. sphaericus because of its high reproductive rate compensating predatory losses. 5. Very low pH in Delamere Lake might suppress Daphnia by hampering its reproduction. Consequently, Daphnia may be vulnerable to invertebrate predation even at low predator density in the lake. Keywords : acid lakes, Daphnia , Dictyosphaerium pulchellum , eutrophication, unusual Introduction 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 `c ˇek et al. , 1995). Low species diversityatalltrophiclevelsis,however,quitecommon in acid lakes (Schindler et al.
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Daphnia-pH#2 - Freshwater Biology (2005) 50, 19131926

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