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

Abstract

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 H 2 SO 4 (south basin, 302S; 1981-1989) or HNO 3 (north basin, 302N; 1982-1986) and HCl (1987HCl ( -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 ϭ 0.67, P Ͻ 0. 05; 1978-1989) and increased 6-fold following acidification of Lake 302S with H 2 SO 4 from pH 6.6 to 5.0, consistent with observed reductions in dissolved organic carbon (DOC) from 7 to 4.5 mg liter Ϫ1 , 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 (Ͻ1.5 mg liter Ϫ1 ) 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 H 2 SO 4 , while those from cryptophytes (alloxanthin) were unaffected and diatoxanthin from diatoms declined. Patterns of ubiquitous ␤-carotene, Chl a, and pheophytin a suggested that total algal biomass increased ϳ200-400% 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 (ϳ50% of total variance) than those of irradiance (ϳ12%), but that ϳ20% of variance was attributable to factor interactions. Fossil concentrations of pigments from green algae and diatoms increased ϳ100% 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 Ϫ1 . 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

Effects of drought‐induced acidification on diatom communities in acid‐sensitive Ontario lakes

“…Threecategory VPA was performed to quantify the additional unique and interactive effects of water chemistry fluctuations on diatom communities in order to assess whether the effects of climatic variability and acid deposition on diatom assemblages are mediated via alterations in water chemistry. Details of the computations required for two-and three-category VPA are presented by Leavitt et al (1999) and Hall et al (1999), respectively.…”

“…VPA uses direct gradient analysis (e.g., canonical correspondence analysis, redundancy analysis) to quantify the proportion of total variation in diatom community composition over time into that which can be explained by unique effects of each explanatory category alone, that which can be explained by covariation among the categories, and that which cannot be explained (i.e., variation that is due to other (unmeasured) factors or noise; Hall et al 1999).…”

“…We used standardized a priori selection criteria developed by Hall et al (1999) to ensure similar numbers of statistically significant variables per category. Specifically, we selected only variables that accounted for significant amounts of variation in fossil diatom assemblages (␣ ϭ 0.05) based on a series of RDAs constrained to a single explanatory variable at a time, and assigned them to one of the three explanatory categories (acid deposition, climatic variability, water chemistry).…”

Climatic control of ultraviolet radiation effects on lakes

“…Fossil pigments were isolated and quantified using standard high-performance liquid chromatography (Leavitt et al 1997(Leavitt et al , 1999. Algal abundance was measured as changes in fossil concentrations (nmol pigment g Ϫ1 dry sediment) of chemically stable carotenoids, including ␤-carotene (all algae), alloxanthin (cryptophytes), diatoxanthin (diatoms), my- xoxanthophyll (colonial cyanobacteria), and lutein-zeaxanthin (chlorophytes and cyanobacteria).…”

“…Similar stratigraphic profiles were obtained when fossils were expressed as accumulation rates or as organic matter-specific concentrations. Sediment organic content was determined as the percentage mass loss on ignition (LOI) at 500ЊC for 1 h (Leavitt et al 1999).…”