Case Study: Yolo Bypass

"Scientists are learning that factors including the ratio of nitrogen to phosphorus, organic nutrient supply, temperature, and light attenuation, play important and interacting roles in determining HAB composition and toxin production.

The precise drivers of toxin production are still an active area of research."

(US EPA 2019)

Quantifying HABs with α-Chlorophyll

Much of the data currently available on HABs in Stockton is not robust or accurate* enough to effectively analyze possible relationships between HAB severity and measured water quality data. Alternative means of measuring HABs must be explored in order to gain a deeper understanding of their causes.

* The currently available water quality and HABs data for Stockton is very qualitative, derived from low-resolution satellite imagery, and/or incomplete.

Chlorophyll is responsible for plants' green colors and signifies the presence of photosynthetic organisms. α-chlorophyll ("alpha"-chlorophyll) is a specific form of chlorophyll that can be used as a HABs indicator because of the observed relationship between its concentration and the amount of blue-green algae in the water. Therefore, measurement of α-chlorophyll concentration with spectroscopic tools is the most common quantitative means of measuring HAB growth.

Image: structure of α-chlorophyll

(Keith et. al. 2012)

The table on the left provides a standard by which the severity of a HAB can be assessed using α-chlorophyll concentration.

There is currently no data on α-chlorophyll concentration in Stockton's waterways. Because of this, analysis has been performed on a different dataset: the Interagency Ecological Program (Yolo Bypass Fish Monitoring Program, 1998 - 2022).

Below, the relationships between various water quality parameters and α-chlorophyll concentrations in Sacramento's waterways are displayed graphically and analyzed. However, keep in mind that these water quality parameters are not direct causes of HABs and that there may be other factors at play that have not yet been accounted for.

Flow Rate

Literature has previously found connections between decreased flow rate and the ability for algal blooms to grow. The graph below illustrates this connection; the most severe and frequent blooms only occur at flow rates at or near zero.

Water Temperature

Previous research links increased water temperatures and HAB occurrences. Increase in water temperature leads to an increase in “nutrient availability, solubility, and cycling in aquatic and terrestrial ecosystems”, which promotes algae growth (Coffey et al. 2018).

One important result of this association is that atmospheric temperature increases as a result of climate change will also increase the frequency and severity of HABs.

Water temperature rises and falls with the seasons, peaking in summer.

HAB severity appears to peak 1-2 months before the peak of water temperature.

Turbidity

Turbidity is the measure of the "cloudiness" or amount of dissolved solids in a water sample. These solids can include sediment, organic materials, and plankton.

There is an obvious association between a severe HAB and high turbidity, since HABs themselves directly increase the turbidity of the water by clouding it.

HABs and turbidity are closely related across time for the same reason. This confirms the validity of the dataset as established by prior research (Liu et al. 2023). This could provide an avenue for estimating α-chlorophyll concentration in bodies of water using turbidity and therefore increasing the amount of data available.

This plot displays turbidity against water temperature with HAB severity color-coded.

The density of dots indicating blooms is relatively consistent across the water temperature axis; however, more severe HABs indicated by green dots are concentrated at higher water temperatures. This suggests that water temperature is more closely tied to HAB severity than to HAB frequency.

Dissolved Oxygen

The amount of dissolved oxygen in water determines the amount of life a body of water can sustain.

An inverse association was observed between HAB severity and dissolved oxygen. This is because HABs consume oxygen; as a result, it is more difficult for other life to form in water bodies contaminated by HABs, which reduces biodiversity.

This graph illustrates a feedback loop; once dissolved oxygen levels are high, algal blooms grow very quickly. This depletes the dissolved oxygen in the water and decreases the severity of the blooms, which in turn allows dissolved oxygen levels to rise again and so on.

This plot displays water temperatures against dissolved oxygen levels with HAB severity color-coded.

HABs appear to be more severe at lower dissolved oxygen levels but more frequent when dissolved oxygen is higher. This aligns with the known association between dissolved oxygen in bodies of water and their ability to sustain larger volumes of life.

pH

pH is a measurement of the acidity or basicity of a body of water. The further away a body of water is from a neutral pH of 7, the more difficult it is for other forms of life to grow in the water.

HAB contamination is associated with an increase in the pH of water bodies. This has the potential to negatively impact plants and animals living in the water.

The association between pH and HABs is less apparent in this case. Further analysis is required to parse the relationship between the two and potentially use it as a means of predicting HABs.

HABs are most frequent at a pH around 7.7, close to a neutral pH; however, HABs are most severe at a pH of about 8.1.

This relationship is likely at least partly a result of cyanotoxin emission by HABs that increases the amount of basic material in the water.

Conductivity

The conductivity of a body of water is closely related to its salinity. Both measurements indicate the amount of inorganic material in a body of water.

The data shows that HABs are positively associated with conductivity, indicating that HABs increase the amount of inorganic material in a body of water.

Unlike with temperature, where HAB intensity tended to peak after temperature, HAB intensity appears to peak before conductivity. This association warrants further investigation as a potential means of predicting rather than simply measuring HAB formation.

Conductivity has a very close relationship with the severity of HABs; the vast majority of severe HABs are associated with conductivity levels above 200 S/m. This strong association is also a promising factor in the potential predictive power of conductivity for HABs.

These higher conductivity levels are also associated with less frequent HABs.

Conclusions

This table presents a summary of our most important findings for each parameter analyzed.

It is important to note that there are far more factors involved in HAB formation and water quality than have been analyzed here. Other relationships and effects are likely at play in HAB formation.

Regardless, these findings provide useful context to HAB formation and water quality measurements and open up questions for further research.

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