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A new analysis of California’s ancient shoreline at the peak of the last ice age roughly 20,000 years ago revealed a very different coastline than what exists today.

The University of California research team discovered that Southern California was largely devoid of estuaries, which are among the most productive ecosystems on the planet and homes to specially adapted plants and animals that live where saltwater and freshwater meet.

“The ancient shoreline was in a different place during the ice age,” said Ryan Hechinger, a professor at Scripps Institution of Oceanography at the University of California San Diego and a coauthor of the study. “The shoreline was different as sea level was over 400 feet lower because so much ocean water was tied up in the massive polar ice caps and glaciers.”

Back then, the animals and plants in estuaries were forced to live in two estuary “havens” separated by over 965 kilometers (600 miles), one to the north near Morro Bay on California’s central coast and the other much farther to the south in the middle of Baja California, Mexico.

To determine where ancient estuaries were located, the researchers developed a numerical model based on the presence of current estuaries and land slope. They were then able to pinpoint where the ancient shorelines existed at different points in time by overlaying information about past sea levels onto maps of the ocean floor. This investigation allowed the researchers to determine the locations of where estuaries could have formed.

The study, recently published in the journal Proceedings of the Royal Society B, also analyzed the genes of several estuarine fishes, such as the California killifish and shadow goby, to find a signature of the refuges (and their subsequent expansion) in the fishes’ population genetic structure. This analysis showed that ancient habitat restriction strongly influenced modern-day genetic patterns of these coastal species.

“The results are surprising to scientists since until now they typically thought that species sought refuge from glaciers by living in the south, and then expanded northwards at the end of the ice age,” said former University of California Los Angeles researcher Greer Dolby, the lead author of the study.

Estuaries form under certain conditions where the ocean meets the coastline especially where the slope is very shallow or nonexistent. At the ice age’s peak, sea level went down below the gently sloping continental shelf to the steeper continental slope, which is why few estuarine habitat existed in most areas at that time. It turns out that during the ice age, much of the Southern California coastline was too steep to to form estuaries.

“In this case, the steep coastline eliminated a lot of estuary habitat in the middle, leaving refuges in the north and south,” said Hechinger. “Species emerged from both of these refuges to recolonize the middle region.”

After the ice age, sea levels rose as glaciers melted. The resulting flooding formed the estuaries seen today in Southern California. Animals and plants colonized these new habitats from both the northern and southern refuges and the once-isolated estuarine fishes interbred, mixing their genes.

The novel habitat modeling approach developed by the research team in this study could be used in other areas around the world to help forecast where estuaries will be in the future as sea levels continue to rise from climate change.

“In fact, this process is relevant for estuaries anywhere in the world,” said UCLA researcher and study coauthor Dave Jacobs.

– Annie Reisewitz