Evaluating Sediment Resuspension and Internal Nutrient Loading in Tappan Lake Using Remote Sensing, Acoustic Doppler Current Profiler Observations, and EFDC+ Modeling
This study integrates remote sensing, acoustic Doppler current profiler observations, and EFDC+ modeling to demonstrate that wind and flow-driven sediment resuspension in Tappan Lake's mid-lake zone is the primary driver of internal nutrient loading and persistent algal blooms, independent of external watershed inputs.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Freshwater lakes and reservoirs are often seen as static bodies of water, but they are dynamic systems where water, sediment, and nutrients constantly interact. A major challenge in managing these waters is understanding why algae blooms persist even when the surrounding land is kept clean. While it is well known that runoff from farms and cities can dump excess nutrients into a lake, causing green scums of algae, scientists have long suspected that the lakebed itself acts as a hidden reservoir. Over decades, sediments at the bottom can soak up nutrients like phosphorus and nitrogen. When conditions change, these stored nutrients can be released back into the water, feeding algae from below rather than from above. This internal recycling is difficult to see because it happens underwater, hidden from satellite cameras that only look at the surface and from simple water samples taken at a single point. Understanding this hidden mechanism is crucial because if the problem is coming from the bottom, simply cleaning up the rivers feeding the lake might not be enough to stop the blooms.
Researchers focused their attention on Tappan Lake, a medium-sized reservoir in Harrison County, Ohio. Despite the surrounding area being mostly forested with limited agricultural runoff, the lake suffers from recurring algae blooms that degrade water quality and threaten recreational use. To solve this mystery, a team of scientists combined three different ways of looking at the lake: satellite images to see the surface, underwater acoustic instruments to measure the flow and particles, and a computer model to simulate how the water moves. They wanted to find out if the algae were being fueled by nutrients coming from the bottom sediments, stirred up by the movement of the water itself.
The team began by looking at the surface from space. Using images from a satellite that passes over the Earth every few days, they tracked the amount of green algae pigment, known as chlorophyll-a, across the entire lake from 2020 to 2025. They first checked that their satellite measurements matched the actual water samples taken by boats, confirming the technology was accurate. The results showed a clear pattern: the lake was consistently full of algae. Every summer during this period, the entire surface of the lake showed signs of eutrophication, a condition where water is rich in nutrients and supports excessive plant growth. The algae levels were high enough to be considered a bloom in every single year, with no part of the lake returning to a clean, low-nutrient state. This confirmed that the problem was not just a temporary spike but a chronic condition affecting the whole reservoir.
To understand why the algae were so persistent, the researchers needed to look beneath the surface. They deployed a specialized instrument called an acoustic Doppler current profiler, which uses sound waves to measure how fast the water is moving at different depths and to detect how many particles are floating in it. They sailed this device across ten different lines cutting through the lake. The data revealed a striking difference between the deep western part of the lake near the dam and the shallower middle sections. In the deep basin, the water moved very slowly, almost like a still pond. However, in the middle of the lake, where the water was shallower and the channel narrowed, the water moved much faster.
More importantly, the sound waves bouncing off the water revealed where the sediment was being stirred up. The instrument detected a significant increase in the amount of suspended particles right near the bottom of the lake in that middle zone. This "near-bed enhancement" meant that the water flowing through the middle of the lake was strong enough to lift the muddy sediment off the lakebed and keep it floating. In contrast, the deep western basin showed no such activity, with particles settling out and the water remaining clear. This suggested that the physical force of the water moving through the middle of the lake was constantly churning up the bottom, releasing the nutrients trapped in the mud.
To confirm this picture, the scientists used a computer model that simulates the physics of the lake, including how wind and water flow move sediment. The model produced a map of suspended solids that matched the real-world measurements perfectly. It showed high concentrations of sediment in the shallow eastern arms and the middle of the lake, tapering off to almost zero in the deep western basin near the dam. This simulation supported the idea that the lake acts like a trap for sediment coming from the rivers, but the middle section is a zone of constant disturbance where the sediment is picked up again and again.
When the researchers put all these pieces together, a clear mechanism emerged. The wind and the flow of water create a specific zone in the middle of Tappan Lake where the bottom sediment is constantly being resuspended. This process releases nutrients stored in the mud back into the water, providing a steady food source for algae. Because this internal recycling happens regardless of how clean the rivers are, the lake remains fertile and prone to blooms. The study suggests that the algae are not just a result of pollution coming from the land, but are being sustained by the lake's own physical dynamics. This finding indicates that managing the lake will require more than just reducing external pollution; strategies must also address the internal recycling of nutrients caused by sediment resuspension in the middle of the lake.
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