Lab 12.3 – Observing Changes the Water Column
Fundamental concept: Describe the variations in the water column during the period associated with the wildfire smoke plume and investigate the correlation between the wildfire and these variations.
Estimated time to complete: 20-30 minutes
Materials needed: None
In this activity, you will explore data from the Oregon Offshore Surface Mooring to identify changes within the water column, including sea surface temperature (SST), backscatter, and Fluorometric Dissolved Organic Matter (fDOM), in response to the ash plume.
Figure 12.3.1
Charred debris collected in CalCOFI seawater samples. Photo: Rasmus Swalethorp
When wildfires occur, ash and other particulate matter are released into the atmosphere. These materials can be transported over long distances by wind and eventually settle in water bodies, impacting the water column and marine ecosystems. Indicators of ash entering the water supply include increased turbidity (see Figure 12.3.1), changes in pH, elevated levels of CDOM, presence of specific chemical markers, variations in backscatter, and changes in dissolved oxygen.
In this lab, we will dive deeper into a few key parameters: backscatter, sea surface temperature, and fDOM. By examining these parameters, you will learn how environmental disturbances impact the water column.
Key Parameters
Backscatter: Refers to the scattering of light or other radiation by particles in the water back towards the direction from which it came. This phenomenon is used to measure the amount of particles, including phytoplankton, in the water.
Ash particles increase the concentration of suspended particles in the water, which enhances the backscatter signal. An increase in particle concentration results in more light being scattered back to the sensor. By analyzing backscattered light, scientists can estimate the concentration and distribution of these particles, providing insights into nutrient levels and overall particle dynamics in the ocean.
Sea Surface Temperature (SST): Sea Surface Temperature is a crucial parameter in oceanography, serving as an essential indicator of the thermal state of the upper ocean. SST influences a wide range of oceanic and atmospheric processes, including weather patterns, ocean currents, and marine ecosystems. Changes in SST can affect the levels of dissolved oxygen in the water, impacting marine life and ecosystem health, as well as the distribution and behavior of marine organisms, the formation of tropical cyclones, and the global climate system.
Wildfires release ash and particulate matter into the atmosphere, which can be transported over long distances by wind. When these particles form a dense smoke plume, they can block sunlight from reaching the ocean surface. This reduction in solar radiation can lead to a decrease in SST in the affected areas. The cooling effect occurs because the smoke plume increases the albedo (reflectivity) of the atmosphere, reflecting more sunlight back into space and reducing the amount of heat absorbed by the ocean.
Fluorometric Dissolved Organic Matter (fDOM): Dissolved organic matter (DOM) are dissolved organic compounds found in aquatic systems. Within the DOM pool, colored dissolved organic matter (cDOM) are the fraction of compounds that absorb light. Some compounds will absorb ultraviolet light that fluoresces under UV or blue light, this pool is called fluorometric dissolved organic matter (fDOM). Generally, fDOM originates from organic molecules associated with phytoplankton or supplied by terrestrial sources.
Monitoring for fDOM involves making measurements using an optical sensor called a fluorometer which measures the intensity of emitted light from fDOM, providing an estimate of fDOM concentration in the water. Elevated levels of fDOM can indicate runoff from land or the presence of decaying plant material, including ash from wildfires. As such, fDOM can be used as a proxy to estimate the dissolved organic mater in the water.
This diagram illustrates the relationship between DOM, CDOM, and FDOM—showing how dissolved organic matter includes light-absorbing (CDOM) and fluorescent (FDOM) components. (Harvey)
Quick Check Questions
Video Quick Check:
Interpretation Questions
Use the interactive graphs to answer the questions below:
Recall from Lab 12.1 that net solar irradiance decreased as a result of the smoke blocking solar radiation. Sea surface temperature (SST) is another change observed in the water column related to this. View the graphs above and observe the changes in both SST and NETSIRR that occurred as a result of the wildfires.
- Set the first graph to Net Solar Irradiance (NETSIRR) and graph two to sea surface temperature. What correlation between net solar irradiance and sea surface temperature can be observed in the graph above?
- Now select the parameters in Graph 1 to Backscatter and fDOM in the second graph. What type of correlation is shown? Explain.
Application Questions
- How did the levels of fDOM change in the days following the arrival of the ash plume, and what might this indicate about the presence of ash and organic matter in the water column?
- On what date did the ash plume from the 2020 Labor Day Fire begin to impact the water column, as indicated by changes in backscatter, fDOM, and sea surface temperature. Does this correlate with you findings in 12.1?
Reflection Question
- How might the introduction of ash from wildfires into the water column impact marine ecosystems? In your response, consider both abiotic factors (such as water quality and nutrient availability) and biotic factors (which could include phytoplankton, invertebrates, or fish).
