Reading Rodeo Lagoon's Past in Sediment Cores: Diatoms, Clams and Salinity

Sieving a sediment core from Rodeo Lagoon, fall 2020. NPS / Yi-Chuan Luk

What did a place look like in the past, and how much have people changed it? At Golden Gate National Recreation Area, scientists from the park and the U.S. Geological Survey (USGS) are asking that of Rodeo Lagoon — using sediment cores.

Windows into the past

A sediment core is a cylinder of whatever makes up the ground — mud, sand, dirt or gravel. Sediment washing off the land collects in low spots such as lakes and lagoons and settles in layers. Changes between layers often record changes in the environment: nutrients, salinity, floods, fire or algae blooms. Sometimes the layers even hold the organisms that lived there.

Why Rodeo Lagoon

The lagoon has seen several fish die-offs and water quality problems in recent years, but no one knows how unusual they are. Cores can help reconstruct its past: how salty it was, and who lived in it.

• Diatoms — tiny single-celled algae with silica walls that preserve well — are one clue. Different species prefer different salinities, so USGS can read past saltiness from the diatoms in each layer.
• Filter feeders such as clams and mussels eat floating phytoplankton and help keep water clear. Knowing the lagoon's conditions when native mussels lived there would help efforts to bring them back and improve water quality.

First results

In fall 2020, scientists collected about 21 four-inch cores from across the lagoon. Most held shell fragments.

A clam of the invasive Limecola balthica–petalum complex, identified with help from Moss Landing Marine Labs. NPS / Darren Fong

The good news: the lagoon once supported large numbers of clams. The bad news: they were an invasive species, the Limecola balthica–petalum complex. The project continues.

Sources

Based on Laurel Teague (science communication intern, San Francisco Bay Area Inventory & Monitoring Network), "Scientists Use Sediment Cores to Look Into the Past at Rodeo Lagoon" (December 2020), National Park Service; a work of the United States government in the public domain; rewritten in hubnx's own words.