Keeping Tabs on HABs

Harmful algal blooms (HABs) are a perennial threat to public health and coastal economies dependent on fishing and tourism. Nationwide, these blooms are becoming more frequent, making the need to understand and manage them a priority for all coastal states. This multi-Reserve study has positioned the National Estuarine Research Reserves System-Wide Monitoring Program to support algal bloom research, management, and education through enhanced high frequency, in situ chlorophyll a monitoring.

Why this Study

Chlorophyll a is often used as a proxy for phytoplankton in studies of the food web and harmful algal blooms. Traditionally, this photosynthetic pigment is measured by extracting it from water samples in the lab—the current practice used by the Reserve System-wide Monitoring Program. Such monthly grab samples are valuable when assessing long-term trends, however, they are not sufficient to track short-term changes.

Recent advances in optical sensor technology have made high frequency, in situ measurement of chlorophyll a possible. Yet, while there is a relationship between these and extracted measurements, there are inconsistencies that—prior to this project—had not been tested. As a result, high frequency, monitoring has not been incorporated into the System-Wide Monitoring Program even though its use would enhance the program’s value by allowing Reserves to respond to local and national needs for algal bloom research, management, and education

Informed communities are resilient communities. Now, more than ever, communities need continuous, high quality data to help them respond to harmful algal blooms and other hazards.


With funding from the NERRS Science Collaborative, scientists from 12 biogeographically diverse Reserves compared fluorescence measurements taken by the YSI EXO Total Algae sensor to extracted chlorophyll concentrations processed in the lab. They explored possible sources of error in fluorescence measurements, including temperature, turbidity, and fluorescent dissolved organic matter (FDOM). They also looked at how best to predict extracted chlorophyll a from the suite of YSI EXO sensors. Ultimately, they synthesized their work into recommendations on whether and how to include high-frequency, in situ chlorophyll a monitoring in the System-Wide Monitoring Program.

What We Learned

The study confirmed a relationship between fluorescence detected by YSI EXO TAL sensors and extracted chlorophyll measurements, but the strength of that relationship varied by Reserve. Temperature, turbidity, and fluorescent dissolved organic matter all influenced sensor readings.

With a proper understanding of respective strengths and limitations, both in situ and extracted metrics can be informative. Each approach has merits that complement the NERRS mission. Extracted measurements contribute to understanding long-term change because they can be compared with historical data. High-frequency measurements are useful to assess short-term variability, allowing us to explore, for example, how chlorophyll changes with tides, from day to night, seasonally, and after storms. Both approaches strengthen Reserves as reference sites and living laboratories for research on topics ranging from harmful algal blooms and coastal acidification to ecosystem metabolism and the drivers of primary production.

High-frequency, in situ chlorophyll data also can support management, education, and aquaculture, particularly when the monitoring stations are telemetered. Near real-time chlorophyll data could support early detection of, and rapid response to, algal blooms; aquaculturists could use this data to assess food availability for their filter feeder crops; and teachers could integrate it with efforts to educate students on data literacy, primary production, and food web dynamics.

MonitoringKeeping Tabs on HABs