Lab 12.3

Lab 12.3 – Ash Plume and Algal Bloom

Fundamental concept: Describe variations in atmospheric conditions indicative of wildfire ash plumes and identify the arrival date/time of the plume over the Endurance Array. 
Estimated time to complete: 30 minutes
Data skills Preparation: Lab 2
Materials needed: None

As we have learned through the past two lessons, in 2020 east winds pushed the ash plume from the 2020 Labor Day Fires over the coastal Oregon ocean. The duration of the ash plume over the ocean was long enough for ash to deposit into the ocean. Now we are interested in the impact of ash deposition, and the 2020 Oregon Labor Day Fire event provides an excellent opportunity to investigate the impact of wildfires on phytoplankton.

Recall that oceanographers use measurements of the photosynthetic pigment, chlorophyll a, to estimate the abundance of phytoplankton in the water. High chlorophyll values indicate many phytoplankton in the water, whereas low chlorophyll values indicate the presence of only a few. At times a phytoplankton population can grow very quickly to very large numbers, causing what is commonly called an algal bloom.

Most of the time, algal blooms simply fuel the food web, which is especially important in offshore waters and the open ocean. However, when an algal bloom is too large, it can become harmful to marine ecosystems. First, as the algal bloom is followed by an algal die-off, the decomposition of the algae by aerobic respiration uses oxygen from the water column. This can result in a reduction of oxygen available to fish and other aerobic organisms, causing suffocation. Second, some species of algae produce toxins that can cause harm or death to marine organisms and humans, and are referred to as harmful algal blooms (HABs). As a result of the importance of algal blooms for oceanic food webs, as well as potential negative issues associated with them, scientists are very interested in understanding what causes blooms to form and persist in ocean systems.

There can be many potential factors that contribute to the formation of an algal bloom, such as light availability (which we already discussed), but inputs of nutrients are a primary factor. Wildfire ash is known to contain a variety of important nutrients including carbon, nitrogen, phosphorus, and iron (Sanchez-Garcia et al. 2023). Given the high concentration of nutrients in wildfire ash, scientists are both interested and concerned about the potential for ash deposition to stimulate the growth of phytoplankton and the formation of an algal bloom. As shown in the previous lab section (CDOM and backscatter), ash from the 2020 Labor Day fire entered the water column adding nutrients.

How is productivity measured? 

There are many parameters that can be used to measure productivity, or the amount of plankton, in the water. In this exercise, we will focus on chlorophyll a and dissolved oxygen.

• Chlorophyll a: The OOI array uses a fluorometer to measure chlorophyll a concentration in the water through a process called fluorescence. Fluorescence occurs when certain molecules absorb light—often invisible ultraviolet light—and then emit it as visible light. The intensity of this emitted light is directly proportional to the amount of chlorophyll present, which in turn reflects the concentration of phytoplankton in the water.
• Dissolved Oxygen (DO): The OOI surface moorings of the Endurance array use oxygen optodes to measure the amount of oxygen dissolved in water, crucial for the survival of aquatic organisms and an important indicator of water quality. Variations in dissolved oxygen levels can reflect changes in biological activity, such as photosynthesis (production) and respiration (consumption), as well as physical processes like mixing, upwelling, cooling and warming.

Study the graphs below. Note the increases in both chlorophyll and DO upon the arrival of the smoke plume. Is it likely that there was a sharp increase in primary productivity in the hours immediately following the arrival of the plume?

 

Likely, this is not the case. So why do the sensors show DO and Chlorophyll  increase so rapidly when the ash plume arrives? The explanation is likely more mundane. Both DO and  Chlorophyll use optical sensors. Because of the presence of ash in the water column, it is probable that we are seeing false readings during the deposition event, and that the sharp increase in these parameters is an artifact from the smoke plume itself. Recall that vegetation was burning in the fire, and the plume likely contained ash and residual organic material from the plants themselves. Therefore, the immediate spikes in chlorophyll a and DO that occur on the day the smoke plume arrives over the mooring should be disregarded, and we need to look for a more subtle response in the days and weeks to follow, when the plankton has had time to reproduce

Productivity in the post-depositional environment:

Now that we understand why the arrival of the smoke plume over the Endurance Array may have created a false signal of an algal bloom, let’s shift our focus to the post-deposition period. We’ll examine the data from the days and weeks following the plume’s arrival—when it delivered carbon and other micronutrients—to look for evidence of a real increase in primary productivity.

Quick Check Questions

 

Interpretation Questions

Use the interactive graphs to answer the questions below.

 

Select Chlorophyll a as the parameter for Graph 1 and select post-depositional period in the blue boxes just below the graphs.

1. What is the date and time of the lowest chlorophyll reading at the start of the post-depositional period?
2. What is the date and time of the highest chlorophyll concentration in the post-depositional period?
3. Calculate the percent change in chlorophyll concentrations between the dates. Show your work.
4. Now select dissolved oxygen as the parameter for Graph 2. Describe the correlation between DO and chlorophyll. Is this the result you would expect? Explain why or why not.
5. Manipulate the other parameters presented in the graph above. What additional evidence suggests that ash deposition stimulated phytoplankton growth?

Reflection Question

1. As discussed in 12.0, the occurrence of wildfires is increasing globally. Given what you learned about the result of the Oregon and Australia fires, make two predictions about how wildfires will impact the ocean environment.