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2019 September

Table of Contents

The Other Side of Dam Removal
Hurricane Harvey Tests Coastal Ecosystem Protection
Did Hurricane Sandy Change the Ecosystem of a Long Island Lagoon?
Resilience and Recovery in Biscayne Bay

Machine learning suggests suspended sediment drives macroalgal loss

Between 2011 and 2014, two large dams were removed in Washington’s Elwha River as part of an effort to open up waterways to historical fish passage. Such dam removals have become a popular and widespread effort nationwide, and their positive effects are well understood, yet little research has focused on the magnitude of coastal impacts caused by the removal process.

During the removal of the Elwha dams, macroalgae — the primary habitat type in the nearshore — virtually disappeared from the river’s ecosystem. Scientists hypothesized that lack of benthic light, limited by the roughly 19 megatons of sediment released by dam removal, was the primary cause. To examine this hypothesis, between 2016 and 2017 a research team deployed benthic light-monitoring platforms east and west of the Elwha River mouth, as well as a bottom-boundary layer tripod to measure sediment transport in the upper and lower parts of the water column. This collected data was combined with information collected by the U.S. Geological Survey and used to develop a machine learning tool called a Regression Tree, which was able to hindcast light availability and potential macroalgal losses during and immediately after dam removal. The results allowed the researchers to largely reject other potential causes for the loss of macroalgae, suggesting that severe light attenuation from suspended sediment did indeed drive this ecosystem change.

The detailed level of analysis allowed by this machine learning tool could provide essential data for managers contemplating dam removals, especially those tackling dams with critical habitats downstream and in the nearshore region. Examining the potential effects of dam removal could help mitigate negative impacts, as managers could time dam removals to biological cycles of local flora and fauna, seasonal river discharge patterns, and marine conditions. With dam removal becoming more common, this research also highlights the importance of studying the hydrological, geophysical, and biological patterns of both the affected river and coastal system.

Source: Glover, H., A.S. Ogston, I.M. Miller et al. 2019. Impacts of suspended sediment in nearshore benthic light availability following dam removal in a small mountainous river: in-situ observations and statistical modeling. DOI: 10.1007/s12237-019-00602-5

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Hurricane Harvey Tests Coastal Ecosystem Protection
Mangroves provide better erosion protection, while marshes are more resilient

Along the coasts of the southern United States, climate change is already influencing intertidal vegetation, with mangroves migrating north and replacing salt marshes in some areas. At the same time, climate change is also likely increasing the intensity of tropical storms and hurricanes — raising the question of how these changing coasts will respond to future storms. A study of mangrove expansion into salt marshes in Texas compared the response of the two habitats to  Hurricane Harvey, a category 4 storm that passed directly over their experimental site in 2017. This paper was part of a special issue about hurricanes in Estuaries and Coasts.

Following the hurricane, the researchers compared changes in plant cover, shoreline erosion, and accreted soil depth at sites with measurements made prior to the storm. They found that mangroves were generally less resilient than marshes, showing substantial loss of foliage and broken limbs due to wind damage. However,  this did not result in extensive mortality, and mangroves showed evidence of regrowth on damaged branches within two months of the storm. In contrast, marsh plants showed little to no damage, likely because they were covered by storm surge that protected them from the winds. Yet marsh plants, for all their resilience, were less effective in preventing erosion; shoreline erosion was more than 5 meters in places with marsh cover, as compared to less than 0.5 meters in places with 11 to 100% mangrove cover.

This research presents an interesting paradox: Mangroves are more effective than marshes in preventing erosion, yet their susceptibility to damage suggest they may be more vulnerable to successive hurricane landfalls. It is too soon to say how long it will take mangroves to recover to their pre-hurricane health, which may potentially make them less effective at protecting shorelines over the long term.

Source: Armitage, A.R., C.A. Weaver, J.S. Kominoski, S.C. Pennings. 2019. Resistance to Hurricane Effects Varies Among Wetland Vegetation Types in the Marsh–Mangrove Ecotone. Estuaries and Coasts. DOI: 10.1007/s12237-019-00577-3

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Did Hurricane Sandy Change the Ecosystem of a Long Island Lagoon?
Research shows post-storm shifts in salinity and community structure

In 2012, Hurricane Sandy passed over Long Island, New York, breaching the barrier beach along Fire Island and increasing the connectivity of the Great South Bay to the Atlantic Ocean. In the aftermath of this breach, scientists from Stony Brook University conducted a series of trawling cruises in the Bay during spring, summer, and fall of three post-breach years — 2013, 2014, and 2015 — and compared them to trawls conducted in the same seasons in 2007. What they found was intriguing: the breach provided a new entrance for salt water, as shown by both increased salinities and lower temperatures in the bottom water of the Bay.

So, did these physical changes correspond with community changes in the Great South Bay? There is often a lot of internal variability in community structure, and the results from the 2013 and 2014 trawls were not significantly different than the pre-breach observations. However, 2015 was higher in species richness, diversity and biomass than 2007. There had also been a shift from estuarine to marine species, with decreases in the populations of species like blue crabs and silverside, alongside increases in bluefish and spot.

This paper, which was included in the Estuaries and Coasts special issue on hurricanes, not only highlights an ecosystem change that will be fascinating to study into the future, but also the challenges of working in a naturally fluctuating system populated by long-lived species — one where it may take many years for change to become apparent. For managers, this paper also shows the importance of general monitoring and consistent data collection, which can be vital to illuminate future ecosystem change.

Source: Olin, J.A, R.M Cerrato, J,A. Nye et al. 2019. Evidence for Ecosystem Changes Within a Temperate Lagoon Following a Hurricane-Induced Barrier Island Breach. DOI: 10.1007/s12237-019-00593-3

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Resilience and Recovery in Biscayne Bay
Water quality and phytoplankton communities recovered quickly after Hurricane Irma

Biscayne Bay is a shallow estuary partially enclosed by barrier islands. In 2017 Hurricane Irma, a strong category 4 storm, passed over the Florida Keys and the southern tip of Florida, including Biscayne Bay. In another paper within the Estuaries and Coasts special issue about hurricanes, researchers documented the changes in Bay-wide salinity, nutrient levels, and phytoplankton community dynamics as a measure of Biscayne Bay’s resilience to such episodic, large-scale events.

Using data collected by federal, state, and county agencies, the researchers constructed a picture of Biscayne Bay before and after the hurricane. They found that inshore regions of Biscayne Bay experienced the largest freshwater inflows in a decade in 2017, due to a combination of record rainfall, the hurricane, and flooding control measures that led to unrestricted water exchange during and after the storm. This temporarily created fresher conditions in the Bay. High nutrient levels, caused by land-based inflows from the storm, changed the regional phytoplankton community and led to algal blooms.

Despite these large-scale changes in water quality and phytoplankton biomass, they found that the overall effects of the hurricane in Biscayne Bay were short-lasting. In less than three months, the Bay returned to normal seasonal patterns of temperature, salinity, and nutrient levels, and diatom communities once again out-competed opportunistic phytoplankton. These results present an interesting data point for managers facing water quality issues during large storms. They also suggest that, despite a degree of urban development, Biscayne Bay remains resilient to pulse disturbances like hurricanes.

Source: Wachnicka, A., J. Browder, T. Jackson et al. 2019. Hurricane Irma’s Impact on Water Quality and Phytoplankton Communities in Biscayne Bay (Florida, USA). DOI: 10.1007/s12237-019-00592-4

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