Why Timing Matters: Weather, Waves, and Oxygen in Long Island Sound

What a Hurricane Can Teach Us About Hypoxia

As powerful winds and waves churned the water, the storm disrupted the Sound’s normally layered water, often described as a “water column,” mixing oxygen-rich surface waters into deeper waters. While the waves subsided within a few days, the storm’s effects below the surface lingered much longer, helping bring an early end to hypoxic conditions during what became a historic year for improved water quality in Long Island Sound.

A success story that’s still evolving

Erin offered scientists a rare opportunity to observe one piece of a much larger puzzle: why hypoxia still varies from year to year despite improvements in water quality.

Over the past several decades, nitrogen loading to the Sound has been drastically reduced by upgrading wastewater treatment plants and implementing other pollution reduction measures. Those efforts are considered one of the biggest successes in water quality improvement for the Sound. The size and duration of the Sound’s hypoxic zone have declined significantly, with the maximum extent dropping to a historic low of only 18 square miles in 2025. The hypoxia forecast for 2026 predicts another year of limited hypoxia, potentially even less than last year.

While these long-term improvements have clearly reduced the overall extent of hypoxia, scientists have also observed substantial differences from one year to the next. Some summers experience more severe hypoxia than others, even when nitrogen inputs are relatively similar.

Researchers do, however, understand the fundamental processes that lead to hypoxia.

Nitrogen entering the Sound fuels the growth of micro algae. When those algae die, they sink to the bottom where bacteria decompose them, consuming oxygen in the process.

In the summer, the surface and bottom waters of Long Island Sound separate into layers, a process called stratification. Fresh water from rivers remains near the surface while saltier, denser water settles near the bottom. In between is a transitional zone called a pycnocline. Additionally, heat from the sun causes surface waters to warm, decreasing its density relative to the deeper waters which receive less solar radiation.

Because the top and bottom layers don’t mix easily, oxygen-rich surface water can’t readily replenish the oxygen being consumed near the bottom.

That’s when hypoxia develops.

Scientists have identified several factors in addition to nutrient loading that contribute to year-to-year variability, including waves, wind, and temperature, but the relative importance of these factors combined with nutrient loading remains an active area of research.

That’s where a new tool, called the Wirewalker, comes in.

Watching the whole water column

The waters of Long Island Sound are closely monitored. Research vessels regularly sample water quality throughout the Sound, while University of Connecticut’s (UConn) Long Island Sound Integrated Coastal Observing System (LISICOS) buoy network continuously measures conditions at several locations. Samples from the research vessels and the buoys are typically collected at two to three different depths: near the bottom, near the surface, and sometimes midway through the water column.

In the summer of 2025, researchers added a new tool.

Funded through the Long Island Sound Research Grant Program, UConn scientists Cara Manning, Leonel Romero, and Samantha Siedlecki deployed a Del Mar Wirewalker, an autonomous water quality profiling system that continuously moves up and down in the water column. Traveling at a speed of up to about 20 meters per minute, the round trip between the surface and bottom can take as little as two minutes.

Instead of collecting data at just three points in the water column, the Wirewalker measures the entire vertical profile at 300 different depths.

Imagine trying to understand a layer cake by sticking three toothpicks into it: one near the top, one in the middle, and one near the bottom. You’d know there were different layers, but not exactly where one ended and the next began. The Wirewalker slices through the entire cake over and over again, revealing every layer in between.

By profiling the entire water column hundreds of times each day, scientists can find the location of the boundary between surface and bottom waters and watch stratification form, weaken, and break down in remarkable detail.

“If you have measurements at three depths, you can see if there’s a gradient between the surface and the bottom, but you can’t necessarily resolve exactly where stratification is,” explained Manning. “You know the surface is different than the bottom, but you don’t know how thin is that really high oxygen layer of the surface. [With the Wirewalker] we’re able to see the stratification breakdown more. And so, using this combination of high-resolution measurements of oxygen, but also things like currents and turbulence, we get a better understanding of the physical drivers as well as biological.”

Connecting physics and biology

The Wirewalker was deployed from June to September during the summer of 2025 in the western Sound between two other coastal monitoring buoys. During this time, the Wirewalker completed nearly 300 profiles each day, recording changes in temperature, salinity, oxygen, chlorophyll, particles, and turbulence throughout the water column.

For the first time in Long Island Sound, scientists could simultaneously observe physical forces – wind, waves, and mixing – and oxygen distribution at very high vertical resolution throughout the water column.

That provided valuable data during two summer mixing events.

One event in late June was followed by an increase in dissolved oxygen near the bottom. Wirewalker was able to capture the mixing event, and the data showed increased oxygen near the bottom but not a full breakdown of the stratification.

Then came Hurricane Erin.

Although the storm never made landfall, its winds and waves completely disrupted the seasonal stratification, mixing oxygen-rich surface water throughout the water column and producing a strong reoxygenation event in late August.

The timing is what makes this event particularly interesting. Scientists monitor both the maximum extent of hypoxia and how long low-oxygen conditions persist in the Sound each season. Although the maximum extent of hypoxia was already declining when the hurricane passed through, no hypoxic conditions were detected at any sampling stations after Erin. The storm appears to have played a role in ending the season’s hypoxic conditions. As Manning explained, “this storm event in late August dramatically increased bottom water oxygen throughout Long Island Sound by mixing the full water column. If not for this event, then stratified conditions with low-oxygen bottom waters would have likely persisted in western Long Island Sound into September.”

Scientists are careful not to attribute that outcome to Hurricane Erin alone.

Any wind or storm events occur against a backdrop of nutrient loading, seasonal warming, and changing weather patterns. Rather than one storm determining the outcome, the evidence suggests that the sequence and timing of weather events throughout the summer helped shape how hypoxia developed.

From one summer to better predictions

The Wirewalker isn’t expected to become part of Long Island Sound’s permanent monitoring network. Deploying and maintaining the instrument requires regular boat trips, cleaning, and significant time to process the enormous amount of data it collects.

But the 2025 observations will contribute more than just a season’s worth of data.

“These observations will help us better understand the physical processes that shape oxygen conditions in Long Island Sound and improve the computer models used to simulate and predict those conditions,” said Leonel Romero.

The 2025 data will be used to improve a new model of physical processes in Long Island Sound that was built using the Regional Ocean Modeling System (ROMS). The new model simulates the physical, biological, and chemical processes that influence water quality in Long Island Sound. It will help scientists better understand how nutrient pollution, weather, and climate interact to shape conditions throughout the Sound. The Wirewalker’s detailed observations will allow scientists to compare the model’s predictions with what actually happened in the water.

Did the model capture the strength of the mixing?

Did it predict when stratification broke down?

Did it reproduce the changes in oxygen observed after storms?

Those comparisons help researchers improve the model, so it more accurately represents how Long Island Sound functions. Once refined, the model can help scientists predict how nutrient loading and other conditions expected in the future may affect water quality. This can help managers set new nitrogen reduction targets.

A clearer picture

The story of hypoxia isn’t simply about nutrient pollution. It isn’t simply about storms or wind, or rain. It’s about the interaction between the factors and when they happen.

Nutrients set the stage by creating the conditions for low oxygen to develop. Weather can shape how those conditions evolve, sometimes allowing hypoxia to intensify and other times interrupting the process through mixing.

By combining traditional monitoring, continuous buoy observations, intensive field studies like the Wirewalker, and increasingly sophisticated computer models, scientists are assembling a clearer picture of how Long Island Sound breathes through the summer. And with each new observation, they’re moving closer to answering one of the Sound’s most important questions: not just whether hypoxia will form, but why one summer can look so different from the next.