Title: Estuarine salinity extremes: Using the Coastal Salinity Index to quantify the role of droughts, floods, hurricanes, and freshwater flow alteration
This is a highlight for this paper: https://doi.org/10.1016/j.ecss.2023.108445
Estuaries are highly productive coastal environments that provide critical ecosystem services, such as carbon storage, coastal storm protection, nutrient cycling, fishery habitat, and recreational opportunities. However, as transition zones between marine, freshwater, and terrestrial environments, estuaries are highly vulnerable to changes in the frequency and intensity of extreme events. Extreme events such as droughts, hurricanes, floods, and freshwater flow alterations can drive diverse and rapid changes in estuarine salinities that have significant effects on plants and animals found in estuaries. As the frequency and intensity of extreme events are expected to increase in the future due to climate change, ecologists and resource managers will need standardized indicators for characterizing and monitoring changing salinity conditions in coastal ecosystems. In response to this need, the Coastal Salinity Index (CSI; Conrads and Darby 2017) was developed as a tool for characterizing salinity conditions in coastal surface-water bodies.
In a new NIDIS-funded study in Estuarine, Coastal and Shelf Science, U.S. Geological Survey researchers applied the CSI to 22 years (1998-2020) of long-term salinity data from four hydrography stations distributed throughout an estuarine salinity gradient to understand the impacts of extreme events on salinity regimes in the Barataria Estuary in southeast Louisiana.
The results of this study demonstrate how the CSI can be used to place salinity levels within a historical context and quantitatively compare the impacts of extreme events between areas with varying salinity regimes. The authors identified 10 events between 1998-2020 that produced extreme salinities, including two droughts, five hurricanes, three floods, and one freshwater diversion. The droughts of 2000 and 2006 caused surface water salinities to increase substantially throughout the estuary. The effects of hurricanes were highly variable, with some storms leading to elevated salinities throughout the entire estuary (e.g., Hurricanes Katrina and Rita in 2005), whereas other storms led to elevated salinities for some but not all stations (e.g., Hurricanes Gustav and Ike in 2008 and Hurricane Isaac in 2012). The opening of a freshwater river diversion in 2010 contributed to fresher conditions throughout the estuary and appeared to reduce or eliminate the increases in salinity that normally occur during the summer, although these effects were short-lived. Mississippi River floods in 2008, 2011, and 2019 reduced salinities throughout the estuary, but the effects were most pronounced in the lower estuary compared to the upper estuary. Collectively, the results of this study advance understanding of the influence of extreme events on estuarine salinity regimes. These analyses also highlight the value of the CSI for identifying periods with extreme salinities (i.e., extreme high or low salinities) via calculations that place salinity levels within and across estuaries within a historical context.
Figure caption: Map of the Barataria Estuary in the Mississippi River Deltaic Plain in southeastern Louisiana (USA). Stars denote the location of the four salinity stations included in this study: Salvador, Lafitte, Little Lake, and Barataria Pass. The two letter labels in italics represent major waterbodies in the basin: LD = Lac des Allemands, LC = Lake Cataouache, LS = Lake Salvador, LL = Little Lake, and BB = Barataria Bay.
Figure caption: Long-term mean monthly salinity (practical salinity scale) and 2-month Coastal Salinity Index (CSI) values at each salinity station: Salvador (a, b), Lafitte (c, d), Little Lake (e, f), and Barataria Pass (g, h). The background colors in the CSI panels reflect the CSI classes noted in the legend. HS indicates high salinity, LS indicates low salinity, and N indicates normal salinity. Horizontal dashed lines on the salinity panels (a, c, g, and h) represent the mean monthly salinity for each site, whereas horizontal dashed lines on the CSI panels (b, d, f, and h) represent a CSI value of 0. The time periods for extreme events are shown with the vertical grey boxes. The type of extreme event (i.e., drought, flood, hurricane, or freshwater diversion) is denoted with symbols on top of the figure and the accompanying symbol legend at the bottom of the figure.