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2018 July

Contents

Mitigations by Wetlands Worldwide
Fish Forays Into Seagrass
Do Freshwater Diversions Benefit Wetlands?
Tracking the Wild Horseshoe Crab

 Mitigation by Wetlands Worldwide

New model of storm surge mitigation by coastal wetlands in 11 major river deltas

Low-lying river deltas worldwide are vulnerable to coastal flooding, and the risk is growing as coastal population density and projected storm surge intensity increase. However, tidal wetlands such as salt marshes and mangroves can help—they reduce wave strength and slow storms down, potentially lessening storms’ effects. Drawing on global datasets, the researchers behind a new study created a simple GIS model to evaluate tidal wetlands’ ability to mitigate storm surge risk in 11 of the world’s largest river deltas.

They used the model to calculate the inland area and number of people in the path of a storm with and without wetlands present in order to come up with a measure of flood risk mitigation provided by wetlands. The results show large-scale differences in the amount of protection afforded by existing tidal wetlands worldwide. The three deltas with the highest degree of protection in terms of both land (>80%) and people (>70%) were the Mahakam (Indonesia), the Chao Praya (Thailand), and the Niger (Nigeria), followed by the Mississippi (U.S.). Although total land area and population protected were not related to total wetland area, the magnitude of flood risk mitigation was—larger swaths of intact wetland bestowed a greater reduction in risk.

This type of model can’t predict local-level storm surge effects, but it provides large-scale insights into how mangroves and salt marshes contribute to “nature-based” storm surge mitigation. In deltas where historic conversion for human land use has been limited and large tidal wetlands still exist, such as in the Mississippi, Niger, and Ganges-Brahmaputra deltas, the ongoing conservation of existing wetlands should be a priority. Deltas where large areas of wetland have already been lost, on the other hand, would likely benefit from a focus on restoring native ecosystems instead of relying only on flood defense structures.

Source: Van Coppenolle, R., C. Schwartz, and S. Temmerman. 2018. Contribution of mangroves and salt marshes to nature-based mitigation of coastal flood risks in major deltas of the world. Estuaries and Coasts. DOI: 10.1007/s12237-018-0394-7

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Fish Forays Into Seagrass

Bream living on artificial reefs need other habitat types nearby for foraging

One popular strategy for enhancing fish habitat is installing artificial reefs, but there is a gap in our understanding of how these reefs fit into the ecology of the estuaries where they’re used—we know very little about how they function in relation to other, adjacent habitats. By combining accelerometer tags that measured and transmitted relative fish activity with an array of receivers to track their movements and locations, the authors of a recent study provided new insights into how Yellowfin Bream used one system of artificial reefs in a coastal saltwater lagoon in Australia.

Tracking eight fish for six months, the research team found that fish generally remained close to the reefs on which they were tagged, only rarely moving between reefs. However, they regularly made forays into the surrounding seagrass habitat around sunset, presumably to forage—the stomach contents of bream from a previous study were mostly made up of organisms associated with seagrass and open sediment, confirming the importance of seagrass habitat to their diet. The largest fish spread out and moved farther away from the reefs, whereas smaller fish remained closer to the reef and around the edge of the seagrass bed.

Bream’s reliance on seagrass for foraging shows that managers need to consider an artificial reef’s proximity to other habitat types in order to maximize its colonization and use by fish. Traditional fish surveys don’t capture information on fish behavior across the estuary seascape, and though this study was limited in scope, its results suggest that new tools like acoustic telemetry can be a useful supplement to more familiar methods.

Source: Taylor, M.D., A. Becker, and M.B. Lowry. 2018. Investigating the functional role of an artificial reef within an estuarine seascape: A case study of Yellowfin Bream (Acanthopagrus australis). Estuaries and Coasts. DOI: 10.1007/s12237-018-0395-6

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Do Freshwater Diversions Benefit Wetlands?

Optimizing for sediment and monitoring nutrients are keys

The Mississippi River delta is losing its coastal wetlands at a rate of almost 30 square kilometers each year, jeopardizing many important ecosystem services that are vital to human wellbeing. One approach for reducing the rate of land loss and restoring these wetlands is to use diversions to reconnect the river channel to its floodplain in areas that have long been separated by levees. However, a number of questions remain unanswered regarding how increased nutrient and sediment loads associated with diverted Mississippi River water will affect the structure and function of adjacent marsh ecosystems.

In a new study, researchers collected 60 sections of sod (vegetation and intact soil) from an oligohaline wetland and, under greenhouse conditions, tested the effects of several nutrients (ammonium, nitrate, phosphate, and sulfate), alone and in combination, as well as with and without the addition of sediment at rates consistent with those of the existing Davis Pond freshwater diversion.. Their findings show that nitrogen, regardless of form, boosted plant growth, while sulfate was detrimental. However, the negative effects of sulfate were ameliorated by the simultaneous application of nitrogen and phosphate. Although sediment had positive effects on plant growth, the negative effects of sulfate on the marsh community were not altered at application rates based on an existing freshwater diversion.

In order for diversions to have a meaningful effect, the authors of this study recommend that coastal resource managers optimize them to carry more sediment while monitoring the effects of excess nutrient loading. Reconnecting these isolated systems has the potential to promote wetland recovery, but only if it’s done right.

Source: Poormahdi, S., S.A. Graham, and I.A. Mendelssohn. 2018. Wetland plant community responses to the interactive effects of simulated nutrient and sediment loading: implications for coastal restoration using Mississippi River diversions. Estuaries and Coasts. DOI: 10.1007/s12237-018-0390-y

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Tracking the Wild Horseshoe Crab

High connectivity throughout the Delaware Bay region’s horseshoe crab population

Horseshoe crabs are ecologically and commercially important in the northeastern U.S. Although managers closely monitor the total size of the Delaware Bay region’s horseshoe crab population, little is known about the travels of individual crabs. Understanding horseshoe crabs’ movements, including determining the boundaries of regional populations and the degree of mixing between different spawning bays, could enhance management efforts.

To learn more, the author of a new study used data from a USFWS program that tagged 5,568 crabs tagged in Delaware Inland Bays over the course of 15 years. Of the 1,123 re-sights reported by members of the public, 81.5% were in Delaware Inland Bays, 14.3% were in Delaware Bay itself, 3.5% were in Virginia and Maryland coastal waters, and 0.6% were reported from New Jersey coastal bays and Connecticut. Though crabs stuck close to their spawning beaches for around five days, they regularly moved from one embayment to another within a single year. The results show that a large amount of interchange occurs between the Inland Bays and Delaware Bay and, to a lesser extent, to other areas. The regional population is primarily distributed between Barnegat Bay, New Jersey, to the north, and Chincoteague Inlet, Virginia, to the south.

The spatial boundaries of the population shown in this study are similar to those found by previous tagging efforts and support the current strategy of splitting the Delaware Bay horseshoe crab harvest among New Jersey, Delaware, Maryland, and Virginia. However, the high degree of connectivity between embayments highlights the importance of smaller inland bays to the population as a whole, and the author recommends protecting spawning habitat in these smaller embayments and incorporating spawning surveys from these other systems into regional stock assessments.

Source: McGowan, A. 2018. Horseshoe Crab (Limulus polyphemus) movements following tagging in the Delaware Inland Bays, USA. Estuaries and Coasts. DOI: 10.1007/s12237-018-0406-7

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