Summarizing presentations at a symposium on air pollutant effects on aquatic ecosystems, this document includes an overview of U.S. research programs, atmospheric emissions and deposition, cycling processes, and effects on plants and animals. Current U.S. research addresses ecosystem classification and inventory by sensitivity to acidity, interactions between watershed terrestrial and aquatic ecosystems, and effects. Atmospheric input to aquatic ecosystems may be gaseous or particulate and by wet and dry deposition. Particle dry deposition is most predictable; gaseous dry deposition models exist but are not yet reliable for regional scale. Linear correlation may exist between air sulfate content and deposition acidity. Source apportionment has been used to relate deposition to emission sources. Radioactive isotope studies of cycling processes within aquatic ecosystems have shown that particulate metals are usually removed from the aqueous phase by sedimentation rather than with outflowing water. A chronological record of atmospheric inputs to aquatic ecosystems may be found in carefully taken sediment samples.