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2018 May

Contents

182 Years of Change in Narragansett Bay
Regime Shifts Prevent Seagrass Recovery
Defining Nearshore Nurseries
Modeling Coastal Flood Hazards

 182 Years of Change in Narragansett Bay

Benthic biodiversity reflects changes in environmental stressors over time

Benthic organisms contribute to crucial ecosystem services ranging from seafood production to the burial of pollutants, but the biodiversity of marine species is declining worldwide due to stressors such as pollution, habitat destruction, and over-harvesting. Historical records can provide a reminder of how conditions have been altered and help set goals for management and recovery. The authors of a new paper present a summary of how the benthic community of New England’s Narragansett Bay has changed since 1834.

The project drew data from 104 studies conducted in the Bay over 182 years, which collectively found 1,214 unique taxa from 21 phyla—the majority of all animal phyla on earth. The researchers found a decline in benthic biodiversity since 1855, reflecting a gradual deterioration in the face of increasing anthropogenic stressors. Compared with the 1800s, the bay today is more polluted, eutrophic, hypoxic, and warmer, with more altered habitat and invasive species. However, biodiversity appeared to show a partial recovery as some stressors waned following the passage of environmental legislation in the 1970s.

This link between the benthic invertebrate community’s decline and the bay’s urbanization and development underscores the value of benthic communities as indicators to track trends in ecosystem health over time. Although the positive effect of seminal environmental regulations should be celebrated, this study’s 182 years of data also reminds us that today’s Bay is different from its pre-Industrial Revolution condition, and that modern-day stressors such as climate change could threaten the recent potential recovery.

Source: Hale, S.S., M.M. Hughes, and H.W. Buffum. 2018. Historical Trends of Benthic Invertebrate Biodiversity Spanning 182 Years in a Southern New England Estuary. Estuaries and Coasts. DOI: 10.1007/s12237-018-0378-7

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Regime Shifts Prevent Seagrass Recovery

Restoration of Europe’s seagrass meadows hindered by sediment and algae

EWorldwide declines in seagrass meadows have spurred interest in how to restore this critical habitat. However, although there are examples of successful seagrass restoration projects, most restoration and replanting efforts fail, especially in Western Europe and Scandinavia. According to new research, the cause may lie in regime shifts that alter local conditions in ways that make seagrass recovery very difficult.

The researchers behind the study combined observational data and experimental field studies from the west coast of Sweden, where more than 60% of seagrass meadows have been lost since the 1980s despite efforts to reduce nutrient loading and improve water quality. They found that the loss of the binding roots and water-slowing canopies associated with seagrass destabilized bottom sediments, resulting in wind-driven sediment resuspension and decreased light availability. Declines in seagrass coverage also allowed drifting mats of algae to occupy former seagrass meadows and physically disrupt the recolonization of seagrass shoots, such that shoots transplanted to these areas showed poor growth and failed to survive. In contrast, shoots transplanted to nearby areas that still had natural seagrass coverage showed positive growth and long-term survival.

The study shows that the loss of seagrass beds causes a local “regime shift,” where increased sediment suspension and the proliferation of drifting algae mats create a negative feedback mechanism that prevents the recovery of seagrass once it has disappeared. In areas where this is the case, seagrass restoration is only likely to succeed if these altered conditions are addressed first, through means such as artificially stabilizing sediment and harvesting or fencing off algal mats.

Source: Moksnes, P-O., L. Eriander, E. Infantes, and M. Holmer. 2018. Local regime shifts prevent natural recovery and restoration of lost eelgrass beds along the Swedish west coast. Estuaries and Coasts. DOI: 10.1007/s12237-018-0382-y

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Defining Nearshore Nurseries

New perspective paper argues for considering nursery habitat in broader seascape context

The importance of estuaries as nurseries for the juveniles of many marine species is widely accepted, but scientists and managers have struggled to adequately define what makes a specific area a nursery. A new perspective paper makes the case for considering nursery habitat in the context of linkages across the broader “seascape.

Although the term nursery may refer to any area with a high density of juveniles, it can be refined to incorporate estimates of the relative contribution of specific habitat types to the production of adults. However, nursery habitats aren’t static or isolated, and few studies have focused on the underlying drivers of the spatial and temporal patterns of habitat use by juveniles. The idea of the nursery seascape concept is to describe estuarine nursery habitats as dynamic, functionally connected mosaics of habitats in which ecological processes and organismal behavior are also considered—not just whether or not individual habitat patches harbor juvenile fish. Using this approach, determining whether a particular habitat serves as a nursery requires a more complete evaluation of the particular role a habitat serves for the fish population. 

The authors of this paper argue that taking a more integrative, systems-based approach for evaluating nursery habitats will result in improved restoration and conservation goals and lead to increased recruitment success for the many marine species that spend the early part of their lives in estuaries.

Source: Litvin, S.Y., M.P. Weinstein, M. Sheaves, and I. Nagelkerken. 2018. What Makes Nearshore Habitats Nurseries for Nekton? An Emerging View of the Nursery Role Hypothesis. Estuaries and Coasts. DOI: 10.1007/s12237-018-0383-x

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Modeling Coastal Flood Hazards

How storm severity, timing, and other factors affect water levels in a hypertidal estuary

Predicting the severity of flood hazards from storms is becoming increasingly important in the era of climate change, and hypertidal estuaries—those with very large tidal ranges—are particularly prone to flooding during storm events. A recent study used a two-dimensional hydrodynamic model to examine the factors that affect flood severity during storms in England’s hypertidal Severn Estuary.

Researchers used long-term tide gauge records from four extreme weather events in 2012–2015 as input for the model. They found that event severity had the greatest effect on extreme water levels, but that the timing of the peak storm surge relative to high tide was also important, particularly in upper estuary, as was the asymmetry of the storm surge shaped, defined using “skewness.” Positive skewness meant that the time it took the water level to rise to its peak value was shorter than its decline afterward, causing water to remain high for a longer period of time after the peak.

The researchers point out that our ability to predict how a particular estuary would be affected by a storm event is often limited by the availability of accurate information on its bathymetry. Other factors such as local wind-waves generated by offshore storms could also increase flood hazard and should be taken into consideration in future modeling work. However, the authors of this paper hope that their methodology can be applied to understand past extreme water level events and, in turn, help identify future flood hazards in hypertidal estuaries worldwide.

Source: Lyddon C., J.M. Brown, N. Leonardi, and A.J. Plater. 2018. Flood Hazard Assessment for a Hyper-Tidal Estuary as a Function of Tide-Surge-Morphology Interaction. Estuaries and Coasts. DOI: 10.1007/s12237-018-0384-9

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