Understanding the relative importance of top-down and bottom-up regulation of ecosystem structure is a fundamental ecological question, with
implications for fisheries and water-quality management. For the Laurentian Great Lakes, where, since the early 1970s, nutrient inputs have
been reduced, whereas top-predator biomass has increased, we describe trends across multiple trophic levels and explore their underlying drivers.
Our analyses revealed increasing water clarity and declines in phytoplankton, native invertebrates, and prey fish since 1998 in at least three
of the five lakes. Evidence for bottom-up regulation was strongest in Lake Huron, although each lake provided support in at least one pair of
trophic levels. Evidence for top-down regulation was rare. Although nonindigenous dreissenid mussels probably have large impacts on nutrient
cycling and phytoplankton, their effects on higher trophic levels remain uncertain. We highlight gaps for which monitoring and knowledge should
improve the understanding of food-web dynamics and facilitate the implementation of ecosystem-based management.