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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.

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2014 May

Contents

Shoreline Armoring in Puget Sound Walls off Connections between Marine and Terrestrial Ecosystems
Plant Patterns in the Tidal Freshwater Hudson River: Ten Years of Change
Defining “Restoration”: Can Ecosystems be Fixed and How Do We Know When they Are?
Engineering Approach to Estuarine Restoration Focuses on Physical Aspects of the Ecosystem

 Shoreline Armoring in Puget Sound Walls off Connections between Marine and Terrestrial Ecosystems

As the threat of sea level rise becomes a reality for coastal communities, it is likely that shoreline armoring such as construction of seawalls and rip-rap will increase in an attempt to guard against erosion. While these structures may protect what’s behind them from the rising sea, they may also have unintended effects. Because these structures are often located in the ecotone between marine and terrestrial habitats, they can alter the structure and function of both systems, as well as the connectivity between them.

The effects of shoreline armoring have been studied fairly extensively on exposed sandy beaches, but not on lower-energy mixed-sediment beaches of the type common in the Pacific Northwest. A recent study surveyed amount and composition of wrack and other parameters at paired armored/unarmored shorelines in Puget Sound. Results indicated that armored beaches had substantially less wrack than unarmored areas, and a much higher proportion of marine-derived algae in the wrack line; wrack at unarmored beaches contained a higher proportion of terrestrial materials, including logs. Although beach slope and sediment grain size were similar between treatments, beaches at armored sites were lower in elevation and narrower than unarmored beaches, thus reducing the amount of space available for wrack and logs to accumulate.

Reduced wrack and logs on armored beaches means reduced subsidy of both habitat and organic material to these sites. Although edge habitats like shorelines are generally highly productive and diverse, “walling off” the marine from the terrestrial environments has consequences for those interested in conservation and restoration.

Source: Heerhartz, S. M., M. N. Dethier, J. D. Toft, J. R. Cordell, and A. S. Ogston. 2014. Effects of shoreline armoring on beach wrack subsidies to the nearshore ecotone in an estuarine fjord. Estuaries and Coasts 37 (December 2013). DOI: 10.1007/s12237/013-9754-5.

Plant Patterns in the Tidal Freshwater Hudson River: Ten Years of Change

Sometimes a bird’s eye view is the best way to look at certain ecosystem components. One example is aquatic vegetation, the distribution of which can often be examined using aerial photography. Investigators recently used ten years of aerial photos of the tidal freshwater reaches of the Hudson River to determine changes in distribution and patterns of the dominant species of submerged aquatic vegetation (SAV; Vallisneria americana or water celery) and an alien invasive floating plant, Trapa natans (water chestnut). Environmental factors such as water clarity and shoreline type were also recorded. A number of interesting patterns emerged.

Overall, SAV coverage declined by about 30% between 1997 and 2007. The decline in Vallisneria was not associated with a concomitant increase in water chestnut, which remained fairly stable over the ten-year period of the study. Although overall SAV coverage declined, the number of patches increased, indicating that larger SAV beds are splitting into smaller beds, possibly leading to a reduction in the habitat quality of the patches. For the first five years (1997-2002), armored shorelines were associated with lower SAV coverage and more likelihood of loss. However, this effect was overshadowed by declining water clarity during the second half of the study (2002-2007), which is when most of the system-wide SAV losses occurred.

Water clarity is a crucial variable in the Hudson: SAV was rarely found at depths greater than 1 m below low water, and there was only a 0.5 m difference in depth between areas that supported SAV and those that did not. Light limitation in this system is controlled more by suspended sediment than nutrient loadings so monitoring in this system should focus more on suspended sediments than nutrients. In addition, because SAV coverage was found to be highly dynamic, the authors recommend that protective efforts should include areas where SAV might occur in the future as well as areas where it is presently documented.

Source: Findlay, S. E. G., D. L. Strayer, S. D. Smith, and N. Curri. 2014. Magnitude and patterns of change in submerged aquatic vegetation of the tidal freshwater Hudson River. Estuaries and Coasts 37 (January 2014). DOI: 10.1007/s12237-013-9758-1.

Defining “Restoration”: Can Ecosystems be Fixed and How Do We Know When they Are?

Over the past few decades, investment of time, dollars, and other resources in ecosystem restoration has been significant. But too often, vague definitions of recovery and untested recovery paradigms complicate our best efforts. When is an ecosystem “recovered?” How should recovery be assessed? What does a recovery trajectory look like?

A team of investigators recently examined some of these issues, including reviewing the range of definitions of coastal ecosystem recovery. A suite of case studies in the literature was reviewed to examine the assumptions of six common ecosystem recovery paradigms. Some were substantiated by case studies but most were less clearly supported:

  1. Marine ecosystem recovery is highly idiosyncratic: The authors found examples that both supported and opposed this paradigm.
  2. Ecosystem function is easier to recover than ecosystem structure: Again, evidence was found both in support of and in opposition to this idea.
  3. Degradation is fully reversible: Evidence shows that partial recovery is much more common than “full” recovery.
  4. Degradation and recovery follow similar, but opposite, trajectories: Most case studies indicate that this assumption is false. Recovery trajectories usually depend on the type, magnitude, frequency, and timing of the pressures placed on that system, and interim stable states are often achieved.
  5. Recovery depends on the characteristic of the pressures: This paradigm is strongly supported in the literature.
  6. Connectivity accelerates recovery: This paradigm is also strongly supported in the literature.

The authors also develop a conceptual model of the degradation and recovery process that could be useful in assessing and planning recovery and restoration projects. Often, the authors caution, stressors need to be reduced below pre-degradation levels in order for recovery to take place.

Source: Duarte, C. M., A. Borja, J. Carstensen, M. Elliott, D. Krause-Jensen, and N. Marbà. 2014. Paradigms in the recovery of estuarine and coastal ecosystems. Estuaries and Coasts 37 (December 2013). DOI: 10.1007/s12237-013-9750-9.

Engineering Approach to Estuarine Restoration Focuses on Physical Aspects of the Ecosystem

One way to approach estuarine restoration is by restoring the physical aspects of the system using engineering approaches. Most engineering solutions rely on dike breaching or removal, which often lead simply to flooding. A new approach, which seems to be more effective, is installation of a controlled reduced tide system (CRT), which restricts the tidal regime in low-lying areas by building high inlet culverts and low outlet valves.  This type of structure is hypothesized to work better than dike breaching or removal, which often simply lead to flooding.

A CRT was installed in 2006 in the freshwater zone of the Schelde estuary in Belgium in a formerly agricultural area. Investigators examined a range of chemical and physical variables in the sediments to examine the efficacy of the CRT at restoring estuarine functions. They found that sediment accumulated and transformed rapidly in the CRT, switching from agricultural soils in spring 2006 to silty, wet estuarine sediments by spring 2009. The most nutrient-rich sediments were found in the most frequently-flooded zones. Although biological processes and parameters were not measured in this study, the appropriate functioning of some biological processes can be inferred, such as the microbial processes that could have led to some of the observed changes in nutrient cycles in the CRT. The authors conclude that CRTs are a potential solution for restoration and could be implemented in other similar environments, particularly where lands behind dikes lie a few meters below mean high water.

Source: Beauchard, O., J. Teuchies, S. Jacobs, E. Struyf, T. Van der Spiet, and P. Meire. 2014. Sediment abiotic patterns in current and newly created intertidal habitats from an impacted estuary. Estuaries and Coasts 37 (November 2013). DOI: 10.1007/s12237-013-9743-8.