Turbidity, Water Clarity, and the Disk of Vision

NASA Earth Observatory image by Joshua Stevens, using MODIS data from LANCE/EOSDIS Rapid Response.

This week in the lab we have been focused on helping our undergraduate wrap up her research project. The semester ends in just a few weeks, and she will present her work next weekend at an undergraduate research symposium, so we've been drafting, editing, and finalizing her poster presentation. Her project focuses on the tolerance ranges of model groups to turbidity in the Mississippi Sound and Bight, so I thought I would spend this week's blog explaining her research, its importance, and how we will be putting her efforts to work after she finishes her time in the lab.

Turbidity is a physical characteristic of a liquid that describes its transparency. The more turbid a water sample is, the more cloudy that water is, because of suspended solids. Some examples of suspended solids that increase turbidity in the ocean are: phytoplankton, food scraps from messy eating, and sediment. Turbidity is a useful measurement because it helps explain survivorship of visual predators and filter-feeding prey animals. If the water is full of suspended solids, a visual predator is likely going to spend additional energy during foraging to find their prey compared to when the water is clearer. Similarly, if the water is full of suspended solids, filter-feeding animals spend more time and energy filtering out non-food items and they may have difficult clearing their gills during this process. While there are a few methods to measure turbidity, scientists often use the secchi disk - the titular disk of vision - as a turbidity approximation. A secchi disk is a disk that hangs from a string, rope, or measuring tape, and the disk is divided into four quadrants, alternating white and black colors. As a researcher lowers the disk into the water, they are evaluating the depth at which they can no longer see the disk, known as the secchi depth. The secchi depth provides an approximation of water clarity, where deeper secchi depths mean clearer, less turbid waters. A challenge with the secchi depth measurements is that environmental factors, including cloud cover, reflectance, wave action, can steeply influence these measurements. Our undergraduate is using the frequency of occurrence of model groups across secchi depth readings to determine the optimal water clarity for the groups. She is then creating response curves, like the salinity and temperature curves I've previously generated, to describe habitat suitability using water clarity or secchi depth as an environmental variable. While these secchi depth readings don't provide the most exact measurements of water quality, these data help inform our understanding of habitat requirements for our model groups.

So why are turbidity and physical water quality features important aspects to include in our ecosystem models? When river waters enter the ocean, they carry sediment particles that were picked up along the way, leading to high quantities of suspended sediments in the water column. As you can see in the NASA satellite image here, the Mississippi River expels lots of sediment in the birdsfoot delta of Louisiana, but you may also notice sedimentation that is visible in Lake Pontchartrain in the upper left corner of the image. As Bonnet Carré Spillway openings increase in frequency, more sediment will be diverted into the Mississippi Sound and Bight from the Mississippi River, which will affect the physical water quality, making it harder for some of the model groups to survive and thrive. Incorporating our undergraduate's response curves and tolerance ranges will allow us to provide more information on environmental parameters that affect the health, wellbeing, and habitat use of our 74 model groups.

Stay tuned to hear more about our research in the fish ecology lab in next week's blog.

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