Palaeoecological Study Goes Back in Time to Assess Water Quality Progress in the Sound

By Juanita Asapokhai

Something Is in the Water

At the turn of the 21^st century, resource managers were extremely concerned by the quality of the water flowing into Long Island Sound. For decades, the Sound had been absorbing a high influx of nitrogen from human sources—including through water discharges from wastewater treatment plants, fertilizer runoff, and stormwater runoff—and facing head-on the adverse effects of eutrophication, or pollution of a water body by the buildup of certain nutrients. These effects included hypoxia, or low levels of dissolved oxygen, and the overgrowth of algal blooms, both harmful to fish and the overall health of the Sound’s ecosystem. In 2001, the U.S. Environmental Protection Agency, through the Long Island Sound Study, partnered with the states of Connecticut and New York in developing a strategic plan to reduce the nitrogen pollution load in the Sound with the implementation of the Total Maximum Daily Load (TMDL). Since the institution of the TMDL, which includes billions of dollars in investments in upgrading wastewater treatment plants to reduce water discharges containing nitrogen, the Sound has seen significant improvements in its water quality metrics, including a decline in the area and volume of the Sound experiencing hypoxia. What these programs have not captured, however, are the impacts that the TMDL water quality management regulation has had on the marine life of the Sound over time.  

This gap is exactly where the research of paleoecologist Dr. Gregory Dietl of the Paleontological Research Institution fits in. Paleoecologists are scientists who study organisms and their environments across different geological timescales. With a $39,000 grant from the Long Island Sound Study’s research grant program, Dietl and his team will be conducting an analysis across recent timescales of the assemblages of dead mollusks left behind on the seafloor, comparing biological indicators of habitat condition using mollusks that lived pre-TMDL and mollusks that lived post-TMDL. The mollusks are a proxy for all invertebrates on the seafloor to:  1) assess how improved water quality in the Sound has affected benthic macroinvertebrate communities, and 2) how much farther nitrogen reduction efforts should go to promote their health and survival.

“The TMDL was developed to try and reduce the amount of nitrogen coming into the Sound, which would hopefully then reduce the extent of hypoxia. From that perspective, the management intervention seems to be working,” said Dietl, whose Ithaca-based organization is affiliated with Cornell University. “What we don’t know is if macroinvertebrate communities responded to the improved water quality in the Sound over the same time period.”

Comparing the past with the present, said Dietl, should help determine if areas of the Sound where water quality has improved has led to  “habitat conditions that could support diverse macroinvertebrate communities” or places that are only fit for organisms that  “are tolerant of stress.”

Using Snails, Clams, and other Mollusks to Study Biological Health of the Sound.

To assess the impact of water quality improvements on aquatic life, accomplished through the TMDL,  Dietl will be studying benthic macroinvertebrates, small animals without a backbone that live on the seafloor (the “benthos”). They include mollusks, which are soft-bodied animals with shells, such as mussels, clams, and snails. Macroinvertebrates are useful to study as indicators, according to EPA’s National Coastal Condition Assessment website,  because they are easy to collect and easy to identify in the laboratory, they are around because they have limited mobility, they respond to human disturbance in predictable ways, and they differ in their tolerance to pollution.  

The NCCA collects mollusks about every five years as part of a water quality monitoring program that tracks the nation’s coastal waters, and the Great Lakes.  The sampling includes Long Island Sound, a resource which Dietl is able to use for his research. In 2020 and 2021, sampling was conducted by EPA contractors and the Connecticut Department of Energy and Environmental Protection at 140 stations across the Sound. These are areas  near where CT DEEP and the Interstate Environmental Commission also conduct water quality monitoring for the Long Island Sound Study. The bottom-dwelling macroinvertebrates were collected from a boat using a clam-like shovel device called a Van Veen sediment grab. The live specimens were then separated from the sediment with a sieve and preserved and placed in jars to be sent to a lab for identification.

From the 2020-2021 data, Dietl will be creating a record of the post-TMDL period of the project with an M-AMBI score, a biological index based on three metrics – species richness (the number of species), species diversity (which includes  not only the number of species but the relative abundance of each  species in the  community), and AZTI’s Marine Biotic Index (AMBI), which measures how well the different species in a community tolerate environmental stressors such as eutrophication and hypoxia. Good sites have a wide variety of species, more diversity, and an increase in species that are intolerant of pollution because more of these species can thrive when water quality is good. M-AMBI scores range from zero to one, with scores <0.39 indicating poor condition.

“If you think of it this way,” said Dietl. “Each of the species that we identify can be classified by how sensitive it is to pollution in the environment. Some species are sensitive to pollution and other ones are going to be tolerant of it.”  

Dietl’s project focuses on 10 sampling stations in the western Sound and Narrows. These are locations that are close to New York City and its densely populated suburbs, areas that have historically experienced the most pollution in the Sound. Many of these sites, however, also have experienced reductions in hypoxia since the TMDL was adopted and nitrogen has been reduced through wastewater treatment upgrades. Of the 10 sites Dietl selected, nine have seen moderate to high water quality improvements from 1994-2021, while the tenth has seen a moderate decline in water quality according to the Long Island Sound Water Quality Monitoring Program, the monitoring program conducted by CT DEEP and IEC.   

Going Back In Time:  A Paleoecological Approach to Tracking Water Quality

Using the NCCA standards enables Dietl to build efficiency for the project. For example, he does not have to collect, identify, and assess mollusks from the post TMDL era because the data already exists. As it turns out the NCCA monitoring program also provides an important head start for paleoecologists such as Dietl to conduct the pre- TMDL assessment as well. The monitoring crews that collect live specimens with the sediment grab tool also are picking up shells from the surface of the seafloor and just below the surface. These shells are the remains of mollusks whose soft bodies decomposed at an undetermined time. Like the live specimens, the shell remains also are collected and stored in jars. But normally they would get discarded without being identified, said Dietl. For this project, however, Dietl received permission from EPA to study the shell remains so he could get a historical record of the 10 sites and conduct the comparison study. To do that, Dietl and his research team first needed to determine the age of the shells, which are referred to as death assemblages when sorted out and identified, to see if they corresponded to the period before the TMDL and when CT DEEP was collecting water quality monitoring data. He sent a sub-sample of 200 shells to partners at the Arizona Climate and Ecosystems (ACE) Isotope Laboratory at the Northern Arizona University to conduct radio-carbon dating to estimate the ages, and the results were what he was looking for.

“They’re mostly coming back from the 1980s and 1990s,” said Dietl. “So, we have a good pre-TMDL baseline to compare our dead shells to the data from the living community.”

Dietl and his team are now going through the process of identifying and counting thousands of the shells from the 10 sites, classifying each species found by its tolerance to pollution,  and estimating each sample’s M-AMBI score. He hypothesizes that biological condition as reflected by the change in M-AMBI scores from the pre-TMDL and post TMDL periods will correlate with improvement or decline in water quality from the 1990s to the 2020s.

“At sites where water quality has improved, we predict that ecological quality (estimated by our M-AMBI analyses using mollusks) will increase since the TMDL intervention,” said Dietl. “We expect a less-pronounced response at sites where water quality has not improved as much and even a decline at one site where water quality has decreased since the TMDL intervention.”

The Connection Between Paleoecology and Modern-Day Management of the Sound

This retrospective look and approach to create a historical record of biological changes to mollusks in the Long Island Sound comes from the field of conservation paleobiology, or the application of paleoecological insights and data derived from fossils, sediment cores, and other natural archives to modern-day ecosystem management and conservation work. As the Curator of Cenozoic Invertebrates at the Paleontological Research Institution Dietl’s work focuses on this emerging research area. Paleoecologists use death assemblages to study a variety of ecological changes.

“Reconstructing the past…gives us some sense of what’s possible, or how much something has changed,” Dietl said. “There’s a lot of value in establishing a narrative of how a place has changed over time and how humans have impacted the environment.”

As he and his research team work through analyses of mollusk shells using the M-AMBI metric, Dietl emphasizes the importance of paleoecology in the long-term management and conservation effects for the Sound. “There has been very little monitoring of the benthic macroinvertebrate communities in response to the TMDL intervention, because the resources weren’t available,” he said. “It’s hard to monitor everything, everywhere, all the time.”

The analysis of the dead “residue” of shells from benthic grab samples presents an untapped opportunity for the management community, to see into the past and fill a critical gap in knowing how macroinvertebrate communities responded to the TMDL intervention.

“While managers would have to maintain a monitoring program for years or decades to build a long-term dataset, conservation paleobiologists may be able to address this need retroactively by putting the dead to work,” said Dietl.

For more information about the Long Island Sound Study’s Nitrogen Reduction Strategy, a segment of our Comprehensive Conservation and Management Plan, click here.

Juanita Asapokhai was a Communications Intern for the Long Island Sound Study in summer 2023. She attends Tufts University and will be graduating with degrees in Community Health and Sociology in the spring of 2024.