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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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2015 March

Contents

Mussels as Mitigation
How High is the Water?
Urban Living, Estuarine Style

Mussels as Mitigation

Farmed mussels could help address eutrophication problems, says Danish study, if timing of harvest is right

One eutrophication management tool that has been used and studied in European estuaries is long-line mussel farming, in which the bivalves are grown along hanging ropes in order to filter the water and remove nutrients. The mussels are later harvested, thereby transferring nutrients back to land. However, it’s also possible that the mussel farms could cause more harm than good if they contribute to nutrient regeneration and sedimentation of biodeposits. What impact do the mussel farms actually have on eutrophication?

A study in Skive Fjord, Denmark, sought to determine whether the mussels helped mitigate or contributed to eutrophication. Investigators collected data on nutrients, oxygen, chlorophyll a, organic matter, and other parameters in small-scale incubations in a mussel farm and at a control site, and extrapolated their results from the microcosms to the entire farm. Mussel biomass for the farm as a whole yielded 1,100 tons of mussels in one year; the mussels contributed primarily ammonium to the dissolved inorganic nitrogen released from the farm. Sedimentation due to biodeposits was only slightly higher at the farm than at the reference site, and was restricted to the immediate vicinity of the farm. While organisms (tunicates, sea stars) did accumulate on the mussel lines, they did not contribute significantly to respiration or nutrient regeneration. During most of the production cycle, the farm served as a net sink for N due to uptake of N in the sediments, but after a year of production the farm became a net source of N to the fjord, with mussel biodeposits accounting for most of the N released. Investigators concluded that harvest of mussels before winter (i.e., within the first year of the production cycle) will ensure N removal before they become a source of N to the system. 

Source: Holmer, M., S. W. Thorsen, M. S. Carlsson, and P. J. Kjerulf. 2014. Pelagic and benthic nutrient regeneration processes in mussel cultures (Mytilus edulis) in a eutrophic coastal area (Skive Fjord, Denmark). Estuaries and Coasts (December 2014). DOI: 10.1007/s12237-14-9864-8.

How High is the Water?

A solution to the challenge of measuring marsh submergence and sea level rise

Obtaining accurate information on sea level rise and submergence potential in coastal wetlands, both of which are critical for forecasting whether these critical habitats will be able to survive sea level rise, can be difficult because multiple relevant parameters are varying simultaneously. Tide gauges record changes in water level, but the elevation of the land to which the gauge is affixed is also changing. Therefore, measurements of sea level rise using tide gauge data are referred to as relative sea level rise (RSLR) because they include changes in land elevation. Wetland surface elevation is usually changing too, as a result of a range of forces that cause surface accretion and erosion or subsurface soil subsidence and expansion, sometimes on short time scales. These wetland elevation changes are not recorded by a tide gauge. Wetland surface elevation is evaluated using surface elevation table (SET) measurements. A recent paper suggests that SET values should be subtracted from RSLR values, resulting in a new, more accurate measure of sea level rise in coastal wetlands termed “wetland RSRL” (RSLR_wet).

In an extensive literature review, the author calculated RSLR_wet for 18 previous studies, and found that the elevation trend for 80% of the wetlands studied was not accurately reflected by local tide gauge measures. Sea level was dropping relative to the wetland surface for 39% of sites and rising for 58% of sites. These proportions were consistent across wetland types.

The study recommends co-locating SET and tide gauge stations to obtain continuous records of local relative sea level change, vertical accretion, surface elevation change, and subsidence across a spatial gradient of wetland types. These measurements will become more critical as sea level rise continues to rise, threatening coastal wetlands in many areas.

Source: Cahoon, D. R. 2014. Estimating relative sea-level rise and submergence potential at a coastal wetland. Estuaries and Coasts (December 2014). DOI: 10.1007/s12237-014-9872-8.

Urban Living, Estuarine Style

Urbanization correlated with differences in body condition of fish and crabs in Mississippi estuaries

Urbanization of salt marsh landscapes changes the value of these critical habitats for the range of nekton species that use them, as evidenced by the many studies that have found differences in biotic communities between pristine and impacted sites. What are the mechanisms that lead to these differences? One suspect is differences in food sources or food quality, leading to shifts in foraging patterns and, ultimately, fitness of nekton species. A recent study took aim at this issue, examining body condition of two fish and two crustaceans along a gradient of urbanization/fragmentation. The authors examined gut contents for the two fish species, spot and Gulf killifish, and body condition for the fish and crustaceans (brown shrimp and blue crab) at intact, partially urbanized, and completely urbanized sites in two Mississippi estuaries.

Significant differences among landscape types were observed. In the highly urbanized marshes, the body condition of shrimp and crabs collected were comparable to those from other habitats, but fewer shrimp were found. Perhaps, the authors suggest, urbanized marshes can still support brown shrimp but at comparably lower densities. The body condition of both fish species was significantly reduced in the urbanized habitats, and fewer individuals of each species were found at these sites. Spot were found to have fewer full stomachs and to have consumed smaller prey at the urbanized sites, indicating that prey availability might be a problem for this species at these sites.

Because a previous study by the same authors found fewer and smaller benthic fauna, the main prey for all four species studied here, at urbanized sites, they suggest that urbanization of marsh habitats could be intensifying competition for food among these four species. They conclude that the habitat value of small, fragmented salt marsh landscapes nestled within urban areas varies depending on the species in question, but would generally be diminished compared to healthy, natural marshes.

Source: Lowe, M. R. and M. S. Peterson. 2014. Body condition and foraging patterns of nekton from salt marsh habitats arrayed along a gradient of urbanization. Estuaries and Coasts (December 2014). DOI: 10.1007/s12237-014-9865-7.