Water Quality: Chlorophyll a

In this month’s exploration of water quality, we are going to talk about chlorophyll a. You might already be familiar with chlorophyll a because it’s what makes plants green. Chlorophyll a is a photosynthetic pigment that plants use to absorb sunlight, which they turn into sugar. By measuring chlorophyll a, we can get an idea of how much phytoplankton is in the water. This is especially important if we’re trying to track algal blooms. Algal blooms are when any aquatic photosynthetic organisms begin to increase their populations in a way that may be harmful for an ecosystem.

A throwback to your highschool biology class, chlorophyll a is the photosynthetic pigment that makes plants look green.

When it comes to proper levels of chlorophyll a, there is no one right amount. Rather, we use this parameter as an indicator for phytoplankton populations. Too much chlorophyll a suggests an algal bloom, which suggests nutrient loading. This means that the water is impaired due to too many nutrients like nitrogen and phosphorus. The big thing to keep in mind here is that, just like the tomatoes growing in your garden, phytoplankton is nutrient limited. This means, it can only grow so long as nutrients like nitrogen and phosphorus are available. If you’re not getting enough growth, you might add fertilizer to your garden. Nitrogen and phosphorus in our wastewater and runoff act as fertilizer for aquatic plants. So when we have high levels of these nutrients, we tend to see higher algal growth. Why is that a bad thing you might ask? Well, when the algae forming that bloom begin to die, the decomposition of those cells by bacteria uses up a lot of oxygen and can cause hypoxia (no oxygen) and dead zones, which we’ll talk more about in a future article. This whole process is referred to as eutrophication and means that something is out of balance in our natural system.

Phytoplankton are a direct food source for adult menhaden and eastern oysters, as well as food for zooplankton, which are food for a variety of other animals.

However, phytoplankton are the base of the aquatic food web. Without a healthy phytoplankton population, we would not have the robust fisheries that we do in the Chesapeake Bay. From feeding oysters to zooplankton to menhaden, phytoplankton are a major food source for important species either directly or indirectly.

So, when it comes to the Chesapeake Bay Watershed Agreement’s goals for chlorophyll a, we’re left with the following statement:

“Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall not exceed levels that result in ecologically undesirable consequences—such as reduced water clarity, low dissolved oxygen, food supply imbalances, proliferation of species deemed potentially harmful to aquatic life or humans or aesthetically objectionable conditions—or otherwise render tidal waters unsuitable for designated uses.”

The actual numeric chlorophyll a level is going to be highly variable depending on the season and proximity to land (ie. a tributary vs. main stem Chesapeake Bay). According to the Environmental Protection Agency, Washington D.C. has 25 ug/L as a seasonal average listed as a target chlorophyll a level for July 1st through September 30th. Virginia has 10 to 15 ug/L listed at the chlorophyll a target for the James River in the spring and 10 to 23 ug/L listed for summer. As a point of reference, when chlorophyll a levels reach 40 ug/L, this is considered an algal bloom.

We also tend to see chlorophyll a between 0 to 5 ug/L in open water systems such as in parts of the ocean and the open waters of the Gulf of Mexico. However, along coastlines, chlorophyll a levels increase due to increasing nutrients coming off the land. This is a really important connection, that land based nutrients can drive estuarine and oceanic productivity and helps explain why estuaries like the Chesapeake Bay are important to our global aquatic ecosystem. We also tend to see higher nutrient levels in colder waters, such as the Artic and Antartic. This is because water temperatures in these regions tend to be similar, no matter the depth, which allows for better mixing to occur. This means that nutrients are less likely to settle out of the surface water, making them available for phytoplankton to use, rather than sinking to the bottom and being trapped below the depth at which phytoplankton can survive (due to needing sunlight for photosynthesis).

Click the image to see up to date chlorophyll a levels. Note that grey contour lines on this map show 0.01, 0.1, 1, and 10 mg/m3 to help the eye gauge values.

Finally, when it comes to chlorophyll a, we can measure this parameter in a couple of different ways. One is through the use of a YSI (which we introduced last month) that measures fluorescence. Because chlorophyll a is a photosynthetic pigment, it reacts to light. A YSI that is able to measure chlorophyll a does so by shining a beam of light of a specific wavelength into the water and then measuring the higher wavelength light which is emitted as a result of the fluorescence process. This allows for a real time chlorophyll measurement. However, as mentioned last month, YSIs are expensive.

Other methods of measuring chlorophyll might be less expensive by are time consuming and require a trained technician to process samples. These include spectrophotometry and fluorometry, both of which require the collection of a water sample. The sample is then filtered to concentrate the photosynthetic organisms, then the chlorophyll is extracted from the organisms into an acetone solvent. From there, the chlorophyll in solvent can be read by a spectrophotometer, high-performance liquid chromatography, or a fluorometer, all of which will calculate the quantity of chlorophyll in the original sample.

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