Compared to most Californians, I’ve got some pretty deep roots – I’m a sixth-generation Bay Area resident, so the weather here is normal to me. A lot of my friends and neighbors are more recent transplants to the area and there’s something I hear them say pretty frequently – “I love California, but I miss having real seasons!”
Sure, the changing of the seasons isn’t especially dramatic in our Mediterranean climate: once-a-season snowfall on the Bay Area’s highest peaks makes the news. But that doesn’t mean we don’t have real seasons – just ask a leopard shark!
You’re probably wondering what the heck I’m talking about, so let’s back up a little. Our climate is characterized by dry, warm summers and mild, wet winters. This uneven pattern of rainfall means that the amount of fresh water entering San Francisco Bay changes throughout the year. The salinity of the Bay may increase or decrease depending on these seasonal changes.
So, what is “salinity” and how is it measured? Salinity is the amount of salt in the water, and it’s measured as a ratio of parts of salt per parts of water, by weight. The Pacific Ocean along our coast is about 35 parts of salt per thousand parts of water. We abbreviate “parts per thousand” as ppt. While the ocean’s salinity stays relatively constant, San Francisco Bay’s salinity changes in response to the amount of fresh water entering the Bay during different times of the year.
During the summer and fall, the creeks and rivers entering the Bay are at their lowest flows, so there is not as much fresh water to mix with the salt water entering through the Golden Gate. Around the Dumbarton Bridge near MSI, observed salinity tends to be around 25 – 30 ppt.
During the winter and spring, rivers and creeks are delivering much more water into the Bay, so the salinity goes down: near MSI, observed salinity can drop to 10 ppt. This isn’t always the case – sometimes we receive far less rain than normal, and the salinity will remain high year-round.
If you’re a fish, these dramatic changes in your environment can be quite challenging! Some fish, like sharks, are adapted to saltier water. Other fish, like tule perch, are adapted to fresher water. As the salinity of San Francisco Bay changes, these fish must respond by moving to different parts of the bay where the water is more to their liking.