Dissolved Oxygen (DO) is an essential ecosystem health indicator in coastal environments. Regular monitoring can help us predict low DO, which can stress, displace or kill organisms such as fish larvae, shellfish, and other aquatic organisms. In 2024 we used Onset Corporation HOBO U26 dataloggers with in-situ dissolved oxygen sensors and HOBO U24 salinity dataloggers deployed side by side. In 2025 we plan to incorporate the new MX801 datalogger in our monitoring, which offers improved accuracy and records both salinity and DO within the same datalogger.
Sensors are deployed on a milk crate frame and fixed to the top with zip ties so that they are logging data one foot above the sea floor. We monitor near bottom of the estuary because limited water circulation and organic matter decomposition can cause DO levels to be lower in deeper water.
The high productivity in estuaries causes these frames to attract a diversity of aquatic organisms both sedentary and mobile. Clean and unobstructed sensors are essential to maintain reliable data. For example, excess of attached algae on the frame can artificially augment photosynthesis. Respiration of attached tunicates or other organisms near or on on the dataloggers can reduce influence DO readings
Members of the COMBB team clean the sensor faces and frames weekly. This ensures sensor faces are not obscured by fouling eliminates fouling influence on the environment around the logger. In addition to logger and frame cleaning, our team calibrates the loggers weekly using a handheld YSI ProDSS probe. These calibrations identify faulty dataloggers, validate the datalogger readings and account for any minor sensor drift.
Separate Onset HOBO dissolved oxygen logger (left) and salinity logger (right) deployed in Buzzards Bay. In the future we plan to deploy newer generation HOBO dataloggers that combine dissolved oxygen and salinity measurements.
We deploy dataloggers attached to weighted plastic milk crates that keep loggers approximately 25 cm off the estuary bottom.
Fouling of both the crates and dataloggers by algae and by other marine organisms compromises measurements if they are not removed regularly. We determined that loggers and crates need to be cleaned once per week to avoid the effects of fouling.