A recently published study by researchers at the Virginia Institute of Marine Science suggests that filter feeding by Atlantic menhaden has little net effect on overall water quality in Chesapeake Bay.
The study is part of a larger effort to understand the role of menhaden in the Bay ecosystem. Driving the effort are public concerns about the status of the Atlantic menhaden stock in light of a major commercial fishery in Virginia's bay and coastal waters, as well as interest in the fish as a natural filterer for improving Bay water quality.
The study, published in the February 22nd issue of Marine Ecology Progress Series, was co-authored by VIMS graduate student Patrick Lynch, professors Mark Brush and Rob Latour, and marine technician Beth Condon.
Atlantic menhaden, small oily fish that inhabit coastal waters along the U.S. East Coast, feed by filtering plankton and organic matter from the water through a sieve-like structure on their gills. In the spring, young and larval menhaden enter Chesapeake Bay, which serves as an important nursery ground for the species.
Lynch, a Ph.D student and the study's lead author, says "Menhaden play an ecological role as filter feeders that can potentially help clear nutrient-polluted waters. We wanted to quantify their influence on Chesapeake Bay water quality by measuring their ability to eat phytoplankton and thus remove nutrients like nitrogen from the water column."
Excess nitrogen—from fertilizers, sewage, tailpipes, and smokestacks—fuels blooms of phytoplankton and other algae. These blooms cloud Bay waters and encourage the growth of low-oxygen dead zones when the tiny plants die, sink, and decay.
The researchers did their study in the laboratory, measuring the amount of phytoplankton and nitrogen consumed and excreted by small groups of juvenile and adult menhaden during 6-hour periods. The experiments took place during summer within 90-gallon tanks filled with water from the York River.
"What really stands out from these experiments," says Lynch, "is the complexity of menhaden feeding. They aren't just vacuum cleaners that remove everything. They are more like sieves that have to match up with the right particle sizes."
In fact, the team's experiments showed that older menhaden hardly fed on phytoplankton at all, likely because many of the species of tiny plant cells available in York River water were too small for them to filter. The experiments also showed that, for juveniles, filtering increased with increasing phytoplankton concentrations.
Extrapolating the results of these lab experiments to population-level estimates for Chesapeake Bay suggests that the Bay's menhaden population probably plays little role in removing nitrogen from Bay waters, and may actually provide additional nitrogen to Bay phytoplankton.
"Menhaden only have a net uptake of nitrogen from phytoplankton ingestion when phytoplankton concentrations are high," says Lynch. "So much nitrogen is being put into the Bay that they are likely only able to remove a small fraction of the total."
"Based on our results as well as ecosystem modeling simulations, menhaden do not appear to represent a significant mechanism for removing nutrient inputs to the Bay," says Brush, "at least not at the scale of the entire Chesapeake."
"Far more nitrogen is coming into the Bay than what is possible for menhaden to remove," adds Latour—"even if all they ate was phytoplankton."
The researchers caution that their calculations of Baywide nitrogen removal rest on estimates of the Bay's total menhaden population—a number that is difficult to gauge due to a lack of monitoring programs and the schooling nature of the fish.
"We face a big scaling issue," says Latour. "We take what we find in the lab on a per-fish basis, and scale it to the population level by multiplying by total numbers. But since we have little idea what the total number is, we have a lot of uncertainty."
The researchers handled this uncertainty by presenting different population and feeding scenarios. One scenario—which puts half of the 9.4 billion juvenile menhaden thought to live between Maine and Florida into the Bay and has them feed continuously on a rich broth of phytoplankton—suggests consumption of up to 202 tons of nitrogen per day. If the same number of fish are assumed to ingest no phytoplankton, juvenile menhaden could actually provide an additional 12 tons of nitrogen per day to Bay waters.
For comparison, estimates of the daily amount of nitrogen added to the Bay—from agriculture, wastewater treatment plants, and vehicles—range from 247 to 585 tons. Thus, under the scenarios described above, juvenile menhaden could either remove up to 80% of the daily nitrogen load (202 of 247 tons), or contribute 5% more nitrogen to the Bay (247 plus 12 tons). However, these estimates are unlikely extremes, and may only occur over very short time scales—if at all. For average Baywide phytoplankton concentrations, it is most likely that menhaden have little influence on water quality with a net removal of nitrogen close to 0 tons per day.
While the investigators stress the uncertainty of their population-level estimates, they also note that their laboratory findings are crucial for future calculations, when additional research provides more accurate measures of the Bay's menhaden population. "Aerial surveys and a dedicated monitoring program would give us a much better idea of the number of menhaden in the Bay," says Latour.
"A better understanding of spatial variations in menhaden abundance within the Bay would also give us a better idea of where menhaden may play a larger role," adds Brush. "While our calculations show little impact at the Baywide scale, menhaden may still be locally important if large numbers overlap with large phytoplankton blooms."
The study was funded by the Chesapeake Bay Program and the Keith Campbell Foundation for the Environment.