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Algal responses to dissolved organic carbon loss and pH decline during whole‐lake
acidification: Evidence from paleolimnology

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 (r2 = 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 liter−1, 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 liter−1) 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 β‐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 liter−1. Such complex interactions between pH, DOC, and light may help explain the high variability of algal biomass response to lake acidification.