Diatoms – single-celled algae – are abundant in both fresh and salt waters and are among the most common types of phytoplankton. By some estimates diatoms are responsible for one-fifth of the photosynthesis on earth. Diatom cell walls are hard due to a high proportion of silica. Their geometric shapes are diverse and beautiful, and they are incredibly durable, persisting for hundreds of millions of years in the right matrix. Because they have different ecological requirements, the kinds, mixture and relative abundance of diatoms is an exceptional tool for the study of ancient marine and freshwater environments and their change through time.
Christof Pearce of Aarhus University, working with colleagues at Utrecht University and the Geological Survey of the Netherlands, studied the diatoms preserved in a 79cm-long core of peat sediment from Highlands Hammock state park in central Florida to reveal a 2,500-year history of the area's hydrology. They discovered a new diatom species, Aulacoseira coroniformis, which proved important for reconstructing aspects of the site's ecological history.
The newly recognised diatom has been around for a while, occurring both in the earliest layers of the sediment column as fossils, and lives today in swamps of the south-eastern US. The new species showed up in previously published scanning electron micrographs, but it was misidentified. This previous failure to distinguish A coroniformis means that the only unequivocal ecological information comes from the Highlands Hammock site. The species was the dominant diatom there for more than 1,700 years, from 550BC until AD1200.
The new species is acid-loving with an optimal pH of five. Thus, its presence reveals a good deal about the water conditions in which it is found. Close to the core site, contemporary A coroniformis were found attached to leaves of the yellow water lily, Nuphar lutea. In contrast, A nygaardii prefers a pH of about seven. A coroniformis are heavy in comparison to other diatoms and only join surface plankton when turbulent currents lift them.
The diatoms in the column sorted themselves into four more or less distinct zones. The earliest, dating from 2,511 to 700 years before present, was dominated by A coroniformis and accompanied by sponge spicules, which suggests a shallow, acidic and permanently flooded wetland. Today, the site is seasonably flooded by a stream that is fed from a small lake whose level varies with precipitation.
Two subsequent zones, each marked by a sift in kinds and frequency of diatoms, date from 700 years ago until about 1957. During these two, A coroniformis was replaced by various epiphytic diatom species, indicating shallower water and shortened periods of flooding, and a dramatic change in the system in the 13th century.
Zone four is abrupt and apparently reflects changes due to human impact on the site. While other factors may have contributed, this upper zone is correlated with "improvements" in the park made in the 1930s and 1940s, including elevated roadways, dams and a number of canals built to permanently flood a hydric hammock in the centre of the park. This wetland illustrates the sudden, profound and unintended impacts that humans can have on ecosystems.
• This article was amended on 10 September 2014 to correct the spelling of Christof Pearce's name.
