DISSOLVED OXYGEN IN AN URBAN TIDAL ESTUARY - OAKLAND CA'S LAKE MERRITT 2022-2023

Lake Merritt, a brackish water tidal estuary in the center of Oakland CA, has long suffered from low water quality. It was declared an “impaired body of water” by the U.S. E.P.A. in 1997 due to low dissolved oxygen (DO) and trash. Water quality has improved in subsequent years through city and regional programs to reduce trash and employ green infrastructure to deal with runoff. In August 2022, Lake Merritt suffered a catastrophic fish kill caused by a Harmful Algal Bloom (HAB) of the phytoplankton species Heterosigma akashiwo. DO plummeted to near zero at both the top and bottom of the water column. In order to understand the conditions that might spark a HAB and possibly predict a recurrence, Oakland Public Works, with the help of Laketech.com and the Lake Merritt Institute, installed water quality monitoring buoys that provided 24/7 monitoring of DO, temperature, salinity, chlorophyll and turbidity.

Our project addressed the need for additional information about spatial variation and seasonal DO changes at mid-lake and near the shore stations. It addressed the benefits of involving the local community, especially youth, in the science of stewardship of the lake, encouraging participants to pursue science and technology careers. We used a calibrated sonde to measure water quality parameters at mid-lake and shore stations from August 2022 to the present.

We found that:

• Dissolved oxygen varied with location along the shore as well as with depth in mid-lake.
• During daytime hours of monitoring, the water quality objective of 5 parts per million dissolved oxygen was nearly always met.
• Dissolved oxygen did not approach zero at both the top and the bottom of the water column during our monitoring.

We conclude that community monitoring can contribute to understanding the conditions surrounding HABs and benefit community and youth development.