Limnol. Oceanogr., 44(3, part 2), 1999, 757–773

Fossil pigment analyses and 19 year-long historical records were used to quantify whole-lake algal response to changes in optical and chemical properties following experimental acidification of Lake 302 with H2SO4 (south basin, 302S; 1981–1989) or HNO3 (north basin, 302N; 1982–1986) and HCl (1987–1989). Undisturbed sediments were collected by freeze-coring, sectioned in approximately annual intervals, and analyzed for fossil carotenoids, chlorophylls, and derivatives by high performance liquid chromatography. Concentrations of fucoxanthin (diatoms, chrysophytes, some dinoflagellates) were correlated with algal standing crop (r 2 5 0.67, P , 0.05; 1978–1989) and increased 6-fold following acidification of Lake 302S with H2SO4 from pH 6.6 to 5.0, consistent with observed reductions in dissolved organic carbon (DOC) from 7 to 4.5 mg liter21, improved water clarity, and increased biomass of deep-water chrysophytes. However, fucoxanthin concentrations declined to baseline values in sediments from 1988 to 1990, concomitant with severe acidification to pH 4.5, continued DOC loss (,1.5 mg liter21) and an estimated 8-fold increase in the penetration of UVb radiation (UVR-b). Increased penetration of ultraviolet radiation (UVR) was recorded also by increased relative abundance of pigments characteristic of UVR-transparent environments. In contrast, pigments from green algae (Chl b, pheophytin b, lutein-zeaxanthin) doubled during acidification with H2SO4, while those from cryptophytes (alloxanthin) were unaffected and diatoxanthin from diatoms declined. Patterns of ubiquitous b-carotene, Chl a, and pheophytin a suggested that total algal biomass increased ;200– 400% by the mid-1980s, but declined to near-baseline under severe acidification. Variance partitioning using redundancy analysis captured 80–83% of variation in fossil chlorophylls and carotenoids and suggested that the direct effects of pH were greater (;50% of total variance) than those of irradiance (;12%), but that ;20% of variance was attributable to factor interactions. Fossil concentrations of pigments from green algae and diatoms increased ;100% following acidification of Lake 302N to pH 6.1, but there were few signals of deep-water blooms, possibly because DOC remained 3.5–5.0 mg liter21. Such complex interactions between pH, DOC, and light may help explain the high variability of algal biomass response to lake acidification. Lake acidification can impact algal communities through both biotic and abiotic pathways (Fig. 1). To date, most research has focused on the direct effects of pH or associated factors (e.g., metals) on members of aquatic food webs (reviewed by Stokes 1986). Laboratory and field experiments 1 Present address: Abisko Scientific Research Center, Box 62, S981 07, Abisko, Sweden. Acknowledgments This research was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) research grants to P.R.L. and J.P.S. Department of Fisheries and Oceans and NSERC also supported P.R.L. with a visiting fellowship. Additional support for R.I.H. from University of Regina. We thank R. Hesslein, D. W. Schindler and R. Vinebrooke for constructive reviews. demonstrate that algal growth can decline with pH (e.g., Stokes 1981; Vinebrooke 1996). In contrast, field surveys document that acidification results in species replacements (Turner et al. 1987; Howell et al. 1990; Nichols et al. 1992), although not necessarily a decline in production (Shearer and DeBruyn 1986; but see Turner et al. 1987). In principle, trophic interactions within the food web could lead to either increase or decrease phytoplankton and periphyton abundance, depending on whether predators or prey species are more strongly influenced by acidification events (e.g., Schindler et al. 1985; France et al. 1991; Appelberg et al. 1993). Acidification also reduces the availability of dissolved inorganic carbon (DIC) and photosynthesis by periphyton (e.g., Turner et al. 1994, 1995a,b,c; Vinebrooke 1996). However, benthic biomass may not decline because severe acidification often leads to the development of loosely attached