Marine sediments tell it like it was.

W e live in a constantly changing environment. Although some changes are natural and therefore beyond our control, there is no question that many anthropogenic activities have dramatically altered our planet. The ‘‘human footprint’’ is now discernable around the globe, so much so that Crutzen (1) proposed a new geological epoch (the Anthropocene) to delineate the recent human-dominated era of Earth’s history. The list of environmental problems that can be related to cultural activities continues to grow, including ecosystem changes linked to climatic warming from elevated greenhouse gas emissions, acidic precipitation, industrial pollutants, invasions of exotic species, as well as many other environmental insults. While new problems continue to be recognized, perhaps the oldest and most commonly reported water quality problem is anthropogenic eutrophication, or the overfertilization of waters by excessive nutrient inputs (especially nitrogen and phosphorus). Typically, anthropogenic nutrient sources can be divided into point and nonpoint sources. Point sources include those with easily defined origins, such as a sewage pipe or nutrient-rich industrial eff luent from a factory. Nonpoint or diffuse sources, such as agricultural runoff from fertilized fields, are more difficult to control. Nutrients are of course essential for biological growth; however, excessive enrichment can result in numerous undesirable effects, such as increased algal blooms (including some toxic forms), excessive growths of certain aquatic macrophytes and periphytic algae (with the displacement of other biota), taste and odor complaints, decreased oxygen levels in deeper waters (which can eventually result in fish kills and the extirpation of some benthic species), and major shifts in food web structures and biogeochemical changes in both the water column and sediments. Loss of ecosystem services, as well as declines in fisheries and tourism, can also have many social, health, and economic ramifications. Daunting challenges become evident, however, when one attempts to track and study these environmental problems, because monitoring data are rare or nonexistent for most aquatic ecosystems. Most environmental assessments are done only after a problem has been identified. Therefore, without long-term observations, how can environmental scientists, water quality managers, and policy makers be certain that ecosystems have been detrimentally affected by human activities and that current observations are not simply reflecting ‘‘natural’’ conditions and/or falling within the boundaries of natural variability? In this issue of PNAS, Kidwell (2) proposes one method to answer these questions: by examining compositional differences between living and dead mollusk assemblages in a suite of estuaries and lagoons.