Nutrient pollution brings dramatic changes to Waquoit Bay ecosystem

Feb. 22, 2023
Research finds that eutrophic and warming conditions changed the makeup of Cape Cod’s Waquoit Bay, and is a harbinger for estuaries across the Northeastern U.S.

New research finds that the seagrass meadow ecosystem in Cape Cod’s Waquoit Bay has shifted from a benthic to a pelagically-dominated ecosystem due to human causes such as nutrient pollution, according to a press release by the Woods Hole Oceanographic Institution.

When Matthew Long, an employee of the Woods Hole Oceanographic Institution, looked for seagrass meadows in Waquoit Bay where he had seen them in previous years, there were only a few shoots of dying Zostera marina eelgrass, a type of seagrass.

This prompted Long and Jordan Mora, a restoration ecologist with the Association to Preserve Cape Cod, to analyze decades’ worth of local environmental monitoring data to find out what has happened to the estuary. What they determined is that Waquoit Bay has shifted from a benthic to a pelagically-dominated ecosystem due to human causes, including an excess influx of nutrient pollution along with climate change.

That disruption to Waquoit Bay’s ecosystem presents broad concerns about the fate of coastal estuaries worldwide, according to the researchers.

In addition, the researchers point to the importance of tapping into and analyzing long-term monitoring data to better understand the changes to Waquoit Bay and potentially to other estuaries as well.

A Harbinger for Northeastern Estuaries

“This shift toward pelagic dominance in Waquoit Bay may indicate that other eutrophic and warming estuaries may also shift toward pelagic dominance in the future, as the Northeastern U.S. is one of the fastest warming,” according to the paper co-authored by Long and Mora published in Estuaries and Coasts, the journal of the Coastal and Estuarine Research Federation. “The range of nitrogen loading across the Waquoit Bay sub-watersheds is comparable to the range of nitrogen loading across 90% of the world’s estuaries making it an ideal site for investigating eutrophication impacts.”

The water quality and overall health of estuaries continue to degrade due to excess nutrients from leaching septic systems, agricultural runoff, and other anthropogenic sources, the researchers note.

In addition, warming water temperatures from climate change, particularly in the northeastern United States, exacerbates the nitrogen loading problem by reducing dissolved oxygen levels and accelerating microbial metabolism which further reduces oxygen levels.

The Findings

The researchers’ analysis revealed recent and unexpectedly large increases in chlorophyll concentrations, an indicator of microalgal blooms, in the water column throughout the estuary, which coincided with ongoing decreases in macroalgal density on the bottom of the estuary. In addition, the analyzes showed an increase in temperature over the last 20 years and significant declines in oxygen and pH levels, among other changes.

The analysis relied on long-term monitoring data collected over decades from two monitoring programs coordinated by the Waquoit Bay National Estuarine Research Reserve, including the reserve’s System-Wide Monitoring Program and the Waquoit BayWatchers.

One of the main objectives of the current study was to apply time-series analysis techniques and substantial knowledge about the history of the monitoring programs to reveal long-term trends in water quality, according to the paper.

Long said eelgrass provides a number of ecosystem benefits including stabilizing sediments and offering habitat for a variety of organisms. In addition, eelgrass is a great indicator of good estuarine water quality and also serves as a carbon sink.

“Carbon storage is extremely important across the world, and we're actively trying to figure out ways to store and sequester carbon. Seagrass meadows represent a really significant and efficient carbon storage sink,” Long said. “Let's not lose the seagrass meadows and the carbon sequestration that we already have in place, and let's actively maintain and restore seagrass meadows. With the loss of seagrass meadows, such as what we've seen in Waquoit Bay, we're actively releasing that carbon back to the atmosphere.”

Long added that using environmental monitoring data helped to put together the story of the switch from a seagrass-dominated system to a macroalgal-dominated system from the 1980s to the present in Waquoit Bay. Without the long-term data, gradual changes to the system would be more difficult to detect, he said.

“This paper isn’t just significant because it demonstrates that the estuaries on southern Cape Cod, and more generally the northeastern US, are entering a new level of degradation where not even macroalgae or seaweeds can persist, but also because it provides clear evidence that long-term monitoring programs are extremely important and worth maintaining,” said Mora, who worked at the Waquoit Bay National Estuarine Research Reserve for 10 years collecting water quality and submerged vegetation data with visiting scientists, volunteers, and other staff, and witnessed the gradual decline in habitat quality firsthand.

“My hope is that by showing the impact of increasing temperatures on already degraded systems, this paper will help facilitate local and regional management discussions and accelerate the decision-making needed to mitigate the overload of nutrients in our estuaries,” Mora added.

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