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Timing is Everything:
The Importance of Salinity to Estuaries

by Tracey Crago, WHOI Sea Grant

Cape Cod draws many to its shores come spring, but the melting snow beckons Woods Hole scientist Anne Giblin north, to the marshes of the Parker River­Plum Island Sound region on Massachusetts' north shore.

Project co-investigator Chuck Hopkinson, Jr., takes water samples in the Parker River.
photo courtesy of Debbie Scanlon, MBL Ecosystems Center

Giblin, an associate scientist at the Marine Biological Laboratory's Ecosystems Center, is co-investigator in a Woods Hole Sea Grant-supported project examining the effects of freshwater discharge on the nitrogen dynamics of estuarine systems. Along with Chuck Hopkinson, Jr., a senior scientist at MBL's Ecosystems Center, Jane Tucker, a senior research assistant at MBL, professor Gary Banta and two of his graduate students from Roskilde University in Denmark, Giblin is hoping to add key pieces to the complex puzzle of estuarine productivity. Previous Sea Grant support for the research team has contributed to a greater understanding of the effects of salinity on nitrogen cycling in the subtidal portions of the oligohaline zone, or low salinity portion, of the estuary.

The oligohaline zone in many estuaries is a region of high biodiversity and productivity. It is also the area where high inputs of riverine nutrients can cause nuisance algal blooms. Surprisingly, says Giblin, it is not as well studied as the saline regions of estuaries.

In a previous Sea Grant study, the team sampled the upper reaches of Plum Island Sound in the Parker River, an area that experiences significant temporal salinity changes: a gradual increase in average salinity from late spring to late summer as river discharge decreases; a monthly oscillation in the maximum salinity that corresponds to neap and spring tidal cycles; and twice-daily oscillations in the salinity caused by the semi-diurnal tidal cycle.

"Salinity changes in the overlying water can have a profound effect on both the timing and magnitude of benthic nitrogen release," says Giblin. The researchers' work has shown that an increase in salinity causes a decrease in ammonium adsorption (adherence to sediments or organic matter) and a decrease in the rate of denitrification.

While this is important in subtidal areas, it may be even more significant in the surrounding fresh and brackish tidal marshes. In the Plum Island study site, for example, intertidal marshes occupy more than 10 times the area of subtidal sediments, and they experience greater changes in salinity.

Could intertidal marshes—considered a net sink for nitrogen due to denitrification and burial—also act as a source of nitrogen in the summer, when salinity is higher? Giblin and her colleagues are midway through their project, with promising results.

Giblen and her colleagues collect salinity and exchangeable ammonium samples at several subtidal and intertidal stations within the Plum Island Sound estuary.
graphic courtesy of Anne Giblin, MBL Ecosystems Center

Sampling the intertidal region is "not nearly as easy" as measuring in subtidal areas, says Giblin. For starters, she says, measuring denitrification is a little more problematic because of the high nitrogen concentrations in air. And then there are the smaller-scale interfaces that characterize marsh systems. "There is a whole family of places where the interaction between the marsh and open water represent varying degrees of potential exchange, making quantifying such interactions difficult," says Giblin.

Having enthusiastic and hard-working graduate students to study these interactions helps, says Giblin. One, Ketil Koop-Jakobsen, focused much of his master's thesis project on exchange processes in the marsh, focusing on the importance of adsorbed ammonium in the tidal flushing of ammonium from intertidal salt marsh sediments.

"In a freshwater environment, ammonium tends to stickbetter to sediment and organic matter—up to 100 times more than you would see in a saline environment," says Giblin. But in summer, when nitrogen inputs are at their lowest and the freshwater input decreases dramatically, investigators see an ammonium pulse. "The high nitrogen fluxes we see in the summer suggest a physio­chemical effect. How important is that? Why are there plankton blooms in the summer when there are very few external inputs of nitrogen to the system?"

Early results, including Koop-Jakobsen's work, have provided some clues. In summer, the salinity causes adsorbed ammonium to be released from the sediment and, through diffusion and convection, it becomes available in the water column where it could stimulate a phytoplankton bloom.

Koop-Jakobsen found that the amount of adsorbed ammonium released from the sediment was substantial during inundation at high tides.

While these results contribute to the broader goal of modeling the nitrogen cycle of the estuary—a goal Giblin shares with many others working in the Plum Island Sound region—the project also speaks to the importance of hydrological manipulation within estuarine systems.

"A dammed system, like the Parker River, has a greatly accentuated hydrographic flow," says Giblin. "By controlling the timing and/or flow of freshwater inputs, the nitrogen cycling and residence time in an estuary could be altered."

A model of the nitrogen cycle could, for example, be used to assess the impact of water withdrawal or addition on the watershed and to look at storm and drought events. What's more, says Giblin, the hydrologic conditions of an estuary could play a role in enhancing or retarding eutrophication. "This model could be a powerful management tool for estuaries like the Parker River, where there is some control over the freshwater inputs on a seasonal basis, to help better manage estuaries to minimize eutrophication problems."

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