Lab 12.5

Lab 12.5 Biological Response During Deposition

Fundamental Concept: Examine biological response variables during ash deposition event

Estimated time to complete: 20-30 minutes

Data skills preparation:

Materials needed: None

As we have learned through the past few lessons, in 2020 westward winds pushed an 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. This gives us a unique opportunity to investigate the impact of wildfire ash deposition on some of the smallest marine organisms, phytoplankton.

At times, phytoplankton can grow very quickly to very large numbers, causing what is referred to as an algal bloom. There can be many potential factors that contribute to the formation of an algal bloom, such as light availability, but inputs of nutrients are also 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.

Most of the time, algal blooms simply fuel the ocean food web, which is especially important in offshore waters and the open ocean. However, sometimes algal blooms can be harmful to marine ecosystems. As very large algal blooms die-off, their decomposition by aerobic respiration removes oxygen from the water column, causing suffocation to fish and other aerobic organisms. In other blooms, the blooming species of algae is capable of producing toxins that can cause harm or death to marine organisms and humans. These types of blooms are often referred to as harmful algal blooms (HABs) or ‘red tide.

To investigate the impact of the 2020 Labor Day Fire ash plume on phytoplankton, we will consider three parameters that are commonly used to measure phytoplankton productivity:

Video Comprehension Questions:

1. How large was the algal bloom that resulted from the 2019 Australia wildfires?
2. What is the limiting nutrient in the ocean environment?
3. Was the amount of carbon stored in the phytoplankton bloom approximately equal to, greater than, or significantly less than the amount of carbon released from the 2019 Australia wildfires.
4. In 1961, what bird species was responsible for biting at least 8 people?
5. What type of plankton was responsible for producing domoic acid, a neurotoxin that altered the behavior of the birds and caused them to attach people?
6. What caused the plankton to bloom in the Santa Cruz incident of 1961?

Key Terms:

• Primary Productivity: Primary productivity refers to the rate at which plants produce biomass per unit area, or, the amount of new organic matter created by primary producers in an ecosystem.
• Chlorophyll a: The OOI array uses a fluorometer, a type of optical sensor, 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, measured by a optical sensor, called a fluorometer, 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 (optical sensors) 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.
• Backscatter: Recall lab 12.4 – the OOI array uses optical sensors to measure backscatter. These sensors emit light (around 600-700 nm) and then measure the amount of light scattered back toward the sensor after hitting particles in the water. These measurements estimate particle concentration in the water, which can be related to biological biomass, sediment, or organic matter presence.

Quick Check (Parameters):

Orientation Questions:

Now study the three graphs below, and answer the following orientation questions:

Figure 12.5.1

Figure 12.5.2

Figure 12.5.3

1. What is the approximate peak concentration in chlorophyll and dissolved oxygen?
2. Do all three productivity parameters peak at the same time? Approximately, what date?

Interpretation Questions:

1. How many hours on September 8th did it take for the chlorophyll to go from below 2 µg/L to its peak concentration (use your best judgement)
2. How likely is it for this sharp of an increase in primary productivity to occur in the hours immediately following the plume arrival?

Explanation:

It is actually highly unlikely that the ash plume immediately triggered a significant increase in the three productivity parameters examined. Physiologically, phytoplankton do not have the capacity to  grow and accumulate that quickly. So, why do these parameters show DO and chlorophyll increased so rapidly when the ash plume arrives? 

The explanation is likely more mundane. All three parameters use optical sensors to make their measurements. Recall that vegetation was burning in the fire, and the plume likely contained ash and residual organic material from the plants themselves. Ash particles that were deposited in the water column can block light or coat the sensor surface, which may reduce the amount of light reaching the sensor or distort the sensor’s ability to detect it properly. In addition, certain chemical compounds in the ash might absorb or scatter light in ways that confuse the sensor. As a result, the readings likely do not indicate an immediate algal bloom on the day the ash plume arrived. Rather, is likely that the optical sensors were unable to take accurate measurements and relying on this data could cause us to draw misleading conclusions about phytoplankton productivity. 

Therefore, these immediate spikes in chlorophyll a, DO, and backscatter 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 these productivity metrics in the days and weeks to follow to properly understand the impact of wildfires on phytoplankton.