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Coastal & Estuarine Science News (CESN)

Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.

January 2006


Drainage Networks in Restored Wetlands: Should Nature or a Shovel Introduce Tidal Creeks to Restoration Sites?
Investigation of Nitrogen Dynamics Goes Underground in New England
Salt Marsh Loss in New England: Beyond Aerial Photography
Functional Trajectory Models Transplanted to Eelgrass Bed

Drainage Networks in Restored Wetlands: Should Nature or a Shovel Introduce Tidal Creeks to Restoration Sites?

Restoration scientists have long known that just because a restored marsh looks like a natural marsh does not mean it acts like one. While vegetative growth and fish use are usually considered in evaluating the function of restored areas, geomorphological factors, such as the existence of drainage networks of tidal creeks, are not. These tidal creek networks are critical to overall marsh function, as they provide conduits for organisms, water, sediment and nutrients to move in and out of the marsh. A study of a marsh restoration in the Tijuana Estuary, CA, used an experimental approach to determine if excavating the downstream end of a tidal network during the restoration process would start the marsh on the road to establishing a steady state network.

Six experimental marsh cells were defined in this high-sedimentation environment; creek networks were excavated in three of the cells and all six were compared to reference sites. After five years, the constructed creeks had stabilized to resemble, or nearly resemble, natural reference creeks, and drainage density (length of creek per unit marsh area) exceeded that of the natural marsh. In the cells without excavated networks, "volunteer creeks" had developed on their own during that five-year time horizon, but drainage density of these cells remained about half that of the natural sites. Excavated creeks elongated into vegetated zones, while volunteer creeks did not. The study outlines many other differences between the sets of experimental cells, most indicating that excavating at least the lower reaches of salt marsh drainage networks "jump starts" the establishment of critical creek systems. Depending on the geomorphological character of the site to be restored (which should be better characterized than it is in most projects), only a little encouragement might be needed for equilibrium drainage networks to become established. This effort will likely result in a more naturally-functioning marsh in a shorter time frame.

Source: Wallace, K. J., J. C. Callaway and J. B. Zedler. 2005. Evolution of tidal creek networks in a high sedimentation environment: A 5-year experiment at Tijuana Estuary, California. Estuaries 28(6): 795-811. (View Abstract)

Investigation of Nitrogen Dynamics Goes Underground in New England

Now that coastal managers and policy makers understand (or at least recognize the need to understand) estuarine nitrogen dynamics, it might be important to dig deeper, literally. The parallel world of the subterranean estuary, the mixing zone of fresh groundwater and salt water in some coastal aquifers, may hold important pieces of the N dynamics puzzle. While groundwater is usually considered an important source of N to coastal waters, it is also possible that significant denitrification occurs as the groundwater makes its way to the estuary, meaning the groundwater serves as an N sink rather than a source.

Groundwater flow and denitrification capacity were recently measured in a subterranean estuary below a Rhode Island salt marsh in order to determine which of these N dynamics scenarios dominates there. Denitrification was measured by "injecting" the subterranean estuary with labeled nitrogen and observing its rate of disappearance. Results varied by season and location within the marsh but indicated that in this marsh groundwater denitrification could potentially remove as much as 30-60 mg N/L. Especially during warmer times of year, groundwater could be a sink for N at this site rather than a source. Interestingly, while one product of denitrification is usually N2O, a greenhouse gas, here the primary product of denitrification was N2.

Of course this study was conducted at a single small site, but the results underscore the need to go underground to evaluate potential groundwater denitrification in and flowpaths toward other estuaries.

Source: Addy, K., A. Gold, B. Nowicki, J. McKenna, M. Stolt and P. Groffman. 2005. Denitrification capacity in a subterranean estuary below a Rhode Island (USA) fringing salt marsh. Estuaries 28(6): 896-908. (View Abstract)

Salt Marsh Loss in New England: Beyond Aerial Photography

The devastation of last year's Gulf Coast hurricanes served as grim reminders of the importance of coastal wetlands. Protection of the coast from storm surges is only one of the critical ecosystem services these wetlands provide. In order to conserve and restore wetlands, so many of which have already been altered or destroyed, it is important to know how much remains compared to historical acreages. Estimating changes in salt marshes has traditionally relied on comparing "past" and "present" aerial photographs, but what happened to these systems before the advent of aerial photography? The history of human impact likely dates to the time of European settlement in the New World, while aerial photography can only take us back about 75 years.

A New England-based project demonstrates that older data sources may provide a more complete overview of salt marsh loss. Investigators established criteria for use of historical maps, and then digitized accurate maps dating from the 1700s and 1832-1854. These maps were compared to recent GIS maps of salt marsh coverage in the same areas; the analysis included about 20% of the coastlines of RI, MA, NH and ME. Results indicated that the New England coast has lost about 37% of its original salt marsh. RI experienced the highest loss (53%), and Maine the least (<1%). Much of the destruction was probably related to urban growth rather than conversion to other uses. The area around Boston, for example, has lost 81% of its original salt marsh. As expected, this analysis identified a greater amount of salt marsh loss than observed in shorter-term estimates, sometimes by an order of magnitude. This approach could be valuable for setting restoration monitoring goals, monitoring environmental health, and evaluating management success.

While accurate historical maps are hard to come by for some areas, a little digging into local and national archives might prove fruitful. These investigators found their historical maps in university library map collections, the U.S. Library of Congress, the U.S. Naval Archives and the U.S. Office of Coast Survey.

Source: Bromberg, K. D. and M. D. Bertness. Reconstructing New England salt marsh losses using historical maps. Estuaries 28(6): 823-832. (View Abstract)

Functional Trajectory Models Transplanted to Eelgrass Bed

The critical role of eelgrass beds in many estuaries - habitat for fish and invertebrates, filter for nutrients and particulates, etc. - means that increasing attention is being paid to their restoration. Two recent Estuaries papers describe innovative tools for seagrass bed restoration. In a recent New Hampshire eelgrass restoration project more than just eelgrass was transplanted: Functional Trajectory Models, statistical tools commonly used in salt marsh restorations, were employed to assess the restoration, representing the first application of this statistical technique in a seagrass system. Trajectory models assess the time course of various ecosystem functions as compared to a baseline measured in reference sites, providing an estimate of when functional equivalence has been, or will be, achieved.

The functions modeled included primary productivity, three-dimensional habitat structure, use of new beds by invertebrates and fish, and sediment trapping. All functions were compared to nearby reference sites. Data were collected over a 9-year period, the longest monitoring of transplanted eelgrass to date. Measured proxies for primary productivity and habitat structure reached functional equivalence after three years, while faunal use of the new meadow achieved parity 2-4 years after transplant. After reaching equivalence, the functional trajectory curves revealed that monitored parameters in the restored site tracked those in the reference sites and even rebounded from disturbances as well as the reference beds did. This study suggests that Functional Trajectory Models might be useful tools for planning and evaluating seagrass restorations, and the more they are used, the more predictive power they will have in a given system.

Meanwhile, to the south in the Indian River Lagoon, FL, managers are using metrics of areal coverage, depth limit, and light requirements to set targets for seagrass protection. The target-setting protocols used, along with the use of retrospective analysis of aerial photography to monitor progress, could be instructive in other systems.

Sources: Evans, N. T. and F. T. Short. Functional trajectory models for assessment of transplanted eelgrass, Zostera marina L., in the Great Bay Estuary, New Hampshire, USA. Estuaries 28(6): 936-947. (View Abstract)

Steward, J.S., R. W. Virnstein, L. J. Morris, and E. F. Lowe. Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida. Estuaries 28(6): 923-935. (View Abstract)