This research evaluated the effects of acidification upon littoral algal associations in studies conducted at the Experimental Lakes Area in northwestern Ontario, Canada. The investigations included: experimental whole-lake sulfuric and nitric acidification, experimental whole-lake nutrient additions, and surveys of lakes varying in nutrient concentrations. Two principal littoral algal associations were studied: epilithon (association on rock surfaces), and metaphyton (algal community associated with, but unconstrained by, a surface).

Nutrient controls of littoral and planktonic algal photosynthesis differed greatly due largely to the diffusive resistance to benthic uptake of dissolved inorganic carbon (DIC). Epilithic net photosynthesis declined as a result of acidification altering the carbon cycle. With a parallel increase in epilithic respiration, the energy balance within epilithon became unfavourable (i.e. respiration as a fraction of 'gross' photosynthesis increased) causing decline of this association. This dysfunction serves as an early warning of metabolic imbalance in lakes, which are more sensitive to acidification than previously thought.

Filamentous green algae (FGA) of the Zygnematales proliferated in the littoral zone initially as periphyton, and later as metaphyton, as acidification progressed. FGA had high photosynthetic capacity, displaying an energy balance similar to unacidified epilithon. Their growth was controlled negatively by density-dependent feedback, and positively by light intensity, DIC, water movement, and temperature. FGA biomass varied seasonally, being at a minimum in spring, and reaching an annual maximum in early fall.

FGA blooms, sometimes reaching 30 cm in thickness, affected several aspects of the littoral zone both positively and negatively. Large intra- and interannual variability diminished their ability to compensate for acidification-induced oligotrophication otherwise seen in the littoral zone. The FGA were sometimes the largest epilimnetic phosphorus-containing compartment. Their nitrogen dynamics caused both lake-wide acidification (spring and summer) and alkalinization (fall). Blooms attenuated light available to other phytobenthos by as much as 90%. The FGA provided seasonal habitat for animals, but respiration- and decomposition-related oxygen depletion posed potential risks for inhabitants. FGA in acid lakes will likely proliferate further as human activities release the algae from several growth limitations by increasing nutrient availability (e.g. CO2), by increasing water temperatures, and by extending their growing season.