Tides don't always flush water out to sea, study shows
Phys.org - Wed 11 Sep 00:10 GMT

By area, tidal flats make up more than 50 percent of Willapa Bay in southwest Washington state, making this more than 142-square-mile estuary an ideal location for oyster farming. On some parts of these flats, oysters grow well, filling their shells with deli…

  In a paper published online July 26 in the journal Estuarine, Coastal and Shelf Science, researchers at the University of Washington and the University of Strathclyde report that the water washing over the Willapa Bay tidal flats during high tides is largely the same water that washed over the flats during the previous high tide.

  This "old" water has not been mixed in with "new" water from deeper parts of the bay or the open Pacific Ocean, and has different chemical and biological properties, such as lower levels of food for creatures within the tide flats.

  The team, led by Jennifer Ruesink, a UW professor of biology, employed oceanographic modeling and water quality readings to show that high-tide water flowing over the Willapa Bay flats can take as many as four tidal cycles—or about two days—before it is fully replaced by "new" water.

  Credit: Wheat, Banas and Ruesink, 2019, Estuarine, Coastal and Shelf Science "Previously, there had been this belief that when water drains off of tide flats or out of a bay, currents and wind mix that water up," said lead author Eli Wheat, a UW instructor in the College of the Environment who conducted this study as a doctoral student in the UW Department of Biology.

  To determine water turnover rates in Willapa Bay, Ruesink and Wheat partnered with Neil Banas, an oceanographer at the University of Strathclyde in Glasgow, who modeled water "residence times" and circulation in Willapa Bay using data on the bay's depth profile, the rivers that feed into it and its outlet to the Pacific Ocean.

  The model predicted that high-tide waters over the flats have residence times ranging from zero to four tidal cycles—depending on location in the bay—before it is fully replaced by "new" water from deeper channels.

  Temperature varied primarily according to the tidal cycle, while variations in salinity and chlorophyll throughout Willapa Bay were more consistent with their model of water residence times.