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1. Groundwater–surface water interactions in a river estuary and the importance of geomorphology: Insights from hydraulic, thermal and geophysical observations

   https://doi.org/10.1002/hyp.14372  

   Zamora, Peter B. ; Cardenas, M. Bayani ; Cook, Perran L. M. ( October 2021 , Hydrological Processes)

   This study presents the groundwater flow and salinity dynamics along a river estuary, the Werribee River in Victoria, Australia, at local and regional scales. Along a single reach, salinity across a transverse section of the channel (~80 m long) with a point bar was monitored using time‐lapse electrical resistivity (ER) through a tidal cycle. Groundwater fluxes were concurrently estimated by monitoring groundwater levels and temperature profiles. Regional porewater salinity distribution was mapped using 6‐km long longitudinal ER surveys during summer and winter. The time‐lapse ER across the channel revealed a static electrically resistive zone on the side of the channel with a pronounced cut bank. Upward groundwater flux and steep vertical temperature gradients with colder temperatures deeper within the sediment suggested a stable zone of fresh groundwater discharge along this cut bank area. Generally, less resistive zones were observed at the shallow portion of the inner meander bank and at the channel center. Subsurface temperatures close to surface water values, vertical head gradients indicating both upward and downward groundwater flux, and higher porewater salinity closer to that of estuary water suggest strong hyporheic circulation in these zones. The longitudinal surveys revealed higher ER values along deep and sinuous segments and low ER values in shallow and straighter reaches in both summer and winter; these patterns are consistent with the local channel‐scale observations. This study highlights the interacting effects of channel morphology, broad groundwater–surface water interaction, and hyporheic exchange on porewater salinity dynamics underneath and adjacent to a river estuary.

    

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2. Storm-Driven Fresh Submarine Groundwater Discharge and Nutrient Fluxes From a Barrier Island

   https://doi.org/10.3389/fmars.2021.679010  

   Adyasari, Dini ; Montiel, Daniel ; Mortazavi, Behzad ; Dimova, Natasha ( July 2021 , Frontiers in Marine Science)

   null (Ed.)

   Quantifying and characterizing groundwater flow and discharge from barrier islands to coastal waters is crucial for assessing freshwater resources and contaminant transport to the ocean. In this study, we examined the groundwater hydrological response, discharge, and associated nutrient fluxes in Dauphin Island, a barrier island located in the northeastern Gulf of Mexico. We employed radon ( 222 Rn) and radium (Ra) isotopes as tracers to evaluate the temporal and spatial variability of fresh and recirculated submarine groundwater discharge (SGD) in the nearshore waters. The results from a 40-day continuous 222 Rn time series conducted during a rainy season suggest that the coastal area surrounding Dauphin Island was river-dominated in the days after storm events. Groundwater response was detected about 1 week after the precipitation and peak river discharge. During the period when SGD was a factor in the nutrient budget of the coastal area, the total SGD rates were as high as 1.36 m day –1 , or almost three times higher than detected fluxes during the river-dominated period. We found from a three-endmember Ra mixing model that most of the SGD from the barrier island was composed of fresh groundwater. SGD was driven by marine and terrestrial forces, and focused on the southeastern part of the island. We observed spatial variability of nutrients in the subterranean estuary across this part of the island. Reduced nitrogen (i.e., NH 4 + and dissolved organic nitrogen) fluxes dominated the eastern shore with average rates of 4.88 and 5.20 mmol m –2 day –1 , respectively. In contrast, NO 3 – was prevalent along the south-central shore, which has significant tourism developments. The contrasting nutrient dynamics resulted in N- and P-limited coastal water in the different parts of the island. This study emphasizes the importance of understanding groundwater flow and dynamics in barrier islands, particularly those urbanized, prone to storm events, or located near large estuaries. 

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3. Implications of high rates of sexual recruitment in driving rapid reef recovery in Mo’orea, French Polynesia

   https://doi.org/10.1038/s41598-018-34686-z  

   Edmunds, Peter J. ( November 2018 , Scientific Reports)

   Coral abundance continues to decline on tropical reefs around the world, and this trend suggests that coral reefs may not persist beyond the current century. In contrast, this study describes the near-complete mortality of corals on the outer reef (10 m and 17 m depth) of the north shore of Mo’orea, French Polynesia, from 2005 to 2010, followed by unprecedented recovery from 2011 to 2017. Intense corallivory and a cyclone drove coral cover from 33–48% to <3% by 2010, but over the following seven years, recovery occurred through rapid population growth (up to 12% cover y⁻¹) to 25–74% cover by 2017. The thirteen-year, U-shape trajectory of coral cover over time created by the loss and replacement of millions of corals through sexual reproduction underscores the potential for beneficial genetic responses to environmental conditions for at least one genus,Pocillopora. The high ecological resilience of this coral community appears to have been enhanced by variation among genera in the susceptibility to declining cover, and the capacity for population growth (i.e., response diversity). These results suggest that the outer coral communities of Mo’orea may be poised for genetic changes that could affect their capacity to persistence.

    

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4. Nutrient-rich submarine groundwater discharge increases algal carbon uptake in a tropical reef ecosystem

   https://doi.org/10.3389/fmars.2023.1178550  

   La Valle, Florybeth Flores ; Jacobs, Julian M. ; Thomas, Florence I. ; Nelson, Craig E. ( June 2023 , Frontiers in Marine Science)

   Submarine groundwater discharge (SGD) in high volcanic islands can be an important source of freshwater and nutrients to coral reefs. High inorganic nutrient content is generally thought to augment primary production in coastal systems but when this is delivered via a freshwater vector as is the case with SGD in this study, the effects on productivity are unclear. In the current literature, there is limited evidence for a direct association between SGD and primary productivity of reefs. To elucidate the response of primary productivity to SGD, we conducted spatially and temporally explicit in situ benthic chamber experiments on a reef flat along a gradient of SGD. We found significant quadratic relationships between C-uptake and SGD for both phytoplankton and the most abundant macroalga, Gracilaria salicornia , with uptake maxima at SGD-derived salinities of ~21−22 (24.5−26.6 μmol NO 3 -L −1 ). These results suggest a physiological tradeoff between salinity tolerance and nutrient availability for reef primary producers. Spatially explicit modeling of reefs with SGD and without SGD indicate reef-scale G. salicornia and phytoplankton C-uptake decreased by 82% and 36% in the absence of SGD, respectively. Thus, nutrient-rich and low salinity SGD has significant effects on algal C-uptake in reef systems. 

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5. Transport of Radium and Nutrients Through Eastern South African Beaches

   https://doi.org/10.1029/2018JC014772  

   Moore, Willard S. ; Humphries, Marc S. ; Benitez‐Nelson, Claudia R. ; Pillay, Letitia ; Higgs, Caldin ( March 2019 , Journal of Geophysical Research: Oceans)

   Submarine groundwater discharge (SGD) plays a critical role in coastal and ocean biogeochemistry. Elucidating spatially and temporally heterogeneous SGD fluxes is difficult. Here we use radium isotopes to explore the external sources and mixing regime along the eastern coast of South Africa. We demonstrate that the long‐lived radium isotope compositions are controlled by low inputs of low‐ and high‐salinity terrestrial groundwater. While activities of²²⁸Ra and²²⁶Ra in beach porewaters are similar to coastal waters,²²⁴Ra is enriched by inputs of²²⁸Th from coastal seawater. Porewater ages, based on the production of²²⁴Ra from²²⁸Th, range from 0.3 to 2.3 days, indicating rapid flushing of the beach system. Unlike radium, however, nutrients follow a more complex pathway. We hypothesize that high total dissolved nitrogen (TDN) and phosphorus concentrations in beach porewaters (TDN ranges from 1 to >700 μM) and the coastal ocean (TDN ranges from 1 to >40 μM) are derived from a source not enriched in radium. We speculate that this source is terrestrial water flowing below the dune barrier at depths exceeding our beach sampling depths. This water likely flows upward through breaches in the confining layer into the beach or enters the ocean directly through paleochannels. The presence of high nutrient concentrations in the coastal ocean unaccompanied by high²²⁸Ra activities leads to the hypothesis of this additional nutrient source. These combined inputs may be of considerable importance to the coastal ecology of southeastern Africa, an oligotrophic ecosystem dominated by the nutrient‐poor Agulhas Current.

    

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