Scientist Looks to Data from the Past to Gauge Restoration Expectations

A Bay inhabitant that most people love to hate,
the sea nettle plays a key role in the ecosystem.
Scientist Denise Breitburg is looking to data from
the past to see how nettle abundance has changed,
and what this may say about the Chesapeake.

Denise Breitburg’s journey into piles of old data stems from a bit of frustration.

The Smithsonian Environmental Research (SERC) scientist spent the summer of 2003 researching the role that Chesapeake Bay’s small inlets play in the distribution of sea nettles — those pesky stinging jellyfish that send many a Bay swimmer scrambling for shore.

While digging through an old file cabinet at the Chesapeake Biological Laboratory (CBL), Breitburg found unpublished reports showing that scientist Dave Cargo had done practically the same study over thirty years ago. Cargo worked at the University of Maryland Center for Environmental Science lab from the 1950s until the 1990s. Neither Breitburg nor any of her fellow scientists whom she consulted about her research had any idea that such work had been done before.

While her research that summer still brought valuable insights, Breitburg couldn’t help but wonder, “What more could have been done if we had known about Cargo’s earlier work rather than starting from scratch?”

The experience left her with a greater appreciation for the amount of data collected over decades of studying the Bay. Struck by the importance of making information from older studies more accessible, she’s spent the past two years reconstructing unpublished historical data on sea nettle abundance. Funded by Maryland Sea Grant, the project aims to reveal a new perspective on how the Bay has changed over time.

Counting sea nettles at the Chesapeake Biolog-
ical Laboratory's pier (above) coupled with
plankton tows (shown below) revealed that sea
nettles are not as abundant as they were in the
mid-20th century. This may be good for swimmers,
but it is not necessarily good for the Bay.

Most people know sea nettles as much-maligned blobs that disrupt summertime swims in the Bay and its tributaries. The animals — composed mostly of water and salt and known as gelatinous zooplankton — were once thought to be so troublesome that Congress enacted a bill to help find ways to control them along with other jellyfish in waters throughout the United States. Some of Dave Cargo’s prolific work on sea nettles was supported by funds allocated through this “Jellyfish Nuisance Act.”

Despite their reputation as pests, sea nettles serve as key components of the Bay’s food web. Breitburg explains that they exert a tremendous amount of control because they prey on comb jellies (ctenophores). Sea nettles and comb jellies both eat fish eggs and larvae and compete with fish for zooplankton prey. But comb jellies have the potential to decimate prey much more so than sea nettles. By keeping comb jellies in check, sea nettles have what Breitburg calls a “protective effect.” This includes protection of oyster larvae, a common food of comb jellies, but not a favorite of sea nettles.

Since the late 1980s the density of sea nettles has seen significant declines. Breitburg and others think one reason for this may be the loss of oysters. While sea nettles protect oysters from predation, oysters, in turn, have a role in nettle survival. Sea nettle polyps use oyster shell as substrate from which to grow. In a loop of interconnection, low numbers of sea nettles and high numbers of comb jellies may have limited oyster survival and reinforced a persistent state of low oysters, low sea nettles, and high comb jellies in today’s Bay (see Road to Restoration).

The mid 20th century — the time Cargo began his sea nettle studies — is often used as a benchmark for Bay restoration efforts. Because of this, Breitburg says, understanding both the historical abundance of sea nettles and the effect of that abundance on other species is critical to restoration. She thinks the unpublished data collected by Cargo and his colleagues from 1960 to the late 1990s may help give new insight on how the Bay has changed and how to gauge expectations for restoration.

Making this unpublished data accessible requires more than rifling through dusty boxes of lab notebooks. Like Breitburg today, Cargo relied on pier counts and plankton tows to measure sea nettle abundance. His sampling methods in the 1960s and 70s, however, differed markedly from modern techniques. Breitburg notes that Cargo used less efficient nets and smaller boats than those used today. He also conducted “timed tows,” deploying a net for a certain period of time, rather than for a certain volume of water as is done today. This means Cargo’s data and data collected today by Breitburg and others cannot simply be compared at face value.

To study how the abundance of sea nettles has changed over time,
Denise Breitburg's research team towed plankton nets to collect
samples at the same locations as Dr. Cargo did decades before.

To address these discrepancies and to help document how earlier sampling was done, Breitburg and her team turned to scientists familiar with Cargo’s work. CBL research specialist Bud Millsaps and former assistant director Mike Reber have assisted Breitburg in developing written descriptions of their earlier methods. Reber also spent a day on the Patuxent River with her, going over the former protocol and pinpointing exact sampling locations. Old reports often referenced study sites informally, such as “Younger’s pier.” With the Younger family long since moved from that home, such identifications no longer mean much. Through Reber’s help, Breitburg now has Global Positioning System (GPS) locations for all the former study sites.

Armed with this historical knowledge, Breitburg was able to begin the task of calibrating the data. She and her team visited the study sites and simultaneously collected data on sea nettle abundances using Cargo’s older methods and today’s modern ones. Mathematical computations then allowed them to convert old data into numbers that can be used today.

Early results have brought surprises. Not only were jellyfish densities recorded back in the 60s and 70s much higher than today’s counts, but they were underestimated by about half, according to Breitburg. She says that means that over the last fifty years sea nettle abundance may have declined even more than previously thought.

Though startling, Breitburg welcomes findings like this. “We really need this kind of historical perspective in order to manage the Bay, especially when we’re talking about ecosystem-based management, where we appreciate the importance of the interactions among the different species and stressors in the Bay.”

She notes that if efforts to restore the Chesapeake to mid-20th century conditions succeed, we shouldn’t expect that everything will be more to our liking. Recalling the loop of interconnection between species, we may have clearer water and more oysters, but we may also return to a Bay with more sea nettles.

-- Jessica Smits