Light and nutrients are the staples of every phytoplankton production recipe, but wind and tides in the Delta affect phytoplankton production too, say researchers.
Several projects in the lower Sacramento River and Delta have been exploring strategies for increasing the quantity and quality of food for migratory and resident fish. As part of a broader whole-ecosystem experiment that added nitrogen into the Sacramento River Deep Water Ship Channel, Leah K. Lenoch and colleagues at USGS and UC Davis looked into
the channel’s hydrodynamics to explore whether the environmental conditions there hold promise for increasing the quantity of phytoplankton (the microscopic algae at the base of the food web). They knew that turbid and well-mixed waterways, including parts of the San Francisco Estuary and Delta, are not very productive because phytoplankton don’t get enough light and are at the mercy of invasive clams. However, where the water column is stratified, phytoplankton can grow in the upper layers, where they are isolated from clam grazing and there is more light. They monitored the channel for 78 days during the summer of 2019 to understand how water mixing (vertical, lateral, and longitudinal) happens in the deepwater channel.
Lenoch and Jon Burau, another author of the study, say they were surprised by how much the wind—which blows in alignment with the Channel—affected the water column. “The channel is a lot more energetic than we thought,” says Burau. Nutrients added to the channel dispersed quickly due to the tidal currents, and wind mixed the water column vertically in an average of just one hour and six minutes. Such quick mixing doesn’t support greater phytoplankton growth before they are plunged into the dark and are eaten by clams. Periods of longer stratification were only observed on five days, when the wind speeds were low which reduced mixing. Burau now wants to study other processes happening in the DWSC, from how the morphology of the channel banks can contribute to fish food to how the organisms living at the bottom of the channel use this artificial habitat. Lenoch highlights that climate change will likely change the DWSC and Delta hydrodynamics, due to warmer days and nights and weaker winds creating more prolonged periods of stratified water and, potentially, more fish food. This only raises another question: will native fish be able to tolerate a warmer Delta?