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1. Patterns and Drivers of Carbon Dioxide Concentrations in Aquatic Ecosystems of the Arctic Coastal Tundra

   https://doi.org/10.1029/2020GB006552  

   Lougheed, V. L. ; Tweedie, C. E. ; Andresen, C. G. ; Armendariz, A. M. ; Escarzaga, S. M. ; Tarin, G. ( March 2020 , Global Biogeochemical Cycles)

   Multiple aquatic ecosystems (pond, lake, river, lagoon, and ocean) on the Arctic Coastal Plain near Utqiaġvik, Alaska, USA, were visited to determine their relative atmospheric CO₂flux and how this may have changed over time. The nearshore coastal waters and large freshwater lakes were small sinks of atmospheric CO₂, whereas smaller waterbodies were substantial sources.pCO₂was linked to dissolved organic carbon concentrations across broad spatial and temporal scales, with greater concentrations found in smaller freshwater systems (i.e., ponds and rivers). On a day‐to‐day basis, water temperatures appeared to be the strongest driver ofpCO₂levels in tundra ponds, where warmer temperatures likely stimulated microbial mineralization of carbon in both aquatic and hydrologically linked terrestrial environments. Large rainfall events, which may lead to inflow of carbon‐rich groundwater into these ponds, also were associated with increased daily averagepCO₂. Based on comparison to historical data, we estimate that CO₂concentrations in tundra ponds have increased more than 1.8 times over the past 40 years. Quantifying CO₂flux from these abundant aquatic ecosystems on the Arctic Coastal Plain and elsewhere in the high northern latitudes will likely have important implications for furthering understanding of landscape‐level and nearshore carbon dynamics in the Arctic.
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)

   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, andmore »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.« less
3. Extreme rainfall events pulse substantial nutrients and sediments from terrestrial to nearshore coastal communities: a case study from French Polynesia

   https://doi.org/10.1038/s41598-020-59807-5  

   Fong, Caitlin R. ; Gaynus, Camille J. ; Carpenter, Robert C. ( February 2020 , Scientific Reports)

   Rainfall mobilizes and transports anthropogenic sources of sediments and nutrients from terrestrial to coastal marine ecosystems, and episodic but extreme rainfall may drive high fluxes to marine communities. Between January 13^thand January 22^nd, 2017, the South Pacific Island of Moorea, French Polynesia experienced an extreme rainfall event. ~57 cm of rain was delivered over a 10-day storm. We quantified pulsed sediments and nutrients transported to nearshore reefs. We determined the spatial and temporal extent of the sediment pulse with estimates of water transparency. We quantified pulsed nutrients at multiple spatial and temporal scales. To determine if terrestrial nutrients were incorporated into the benthic community, we collected macroalgae over 10 days following the storm and measured tissue nutrient concentrations and δN¹⁵. Pulsed sediments impacted water clarity for 6 days following the storm, with greatest impacts closest to the river mouth. Nitrite +nitrate concentrations were >100 times the average while phosphate was >25 times average. Macroalgal tissue nutrients were elevated, and δN¹⁵implicates sewage as the source, demonstrating transported nutrients were transferred to producer communities. Future climate change predictions suggest extreme rainfall will become more common in this system, necessitating research on these pulses and their ramifications on marine communities.
4. Demonstrating a High‐Resolution Gulf of Alaska Ocean Circulation Model Forced Across the Coastal Interface by High‐Resolution Terrestrial Hydrological Models

   https://doi.org/10.1029/2019JC015724  

   Danielson, Seth L. ; Hill, David F. ; Hedstrom, Katherine S. ; Beamer, Jordan ; Curchitser, Enrique ( August 2020 , Journal of Geophysical Research: Oceans)

   We demonstrate a linking of moderately high resolution (1 km) terrestrial hydrological models to a 3‐D ocean circulation model having similar resolution in the northern Gulf of Alaska, where a distributed line source of freshwater runoff exerts strong influence over the shelf's hydrographic structure and flow dynamics. The model interfacing is accomplished via mass flux boundary conditions through the ocean model coastal wall at all land‐ocean adjoining grid cells. Despite the high runoff volume and lack of a coastal mixing estuary, the implementation maintains numerical stability by prescribing depth invariant and surface‐intensified inflows at fast and slow discharge grid cells, respectively. Based on comparisons against in situ hydrographic data, the coastal sidewall mass flux boundary condition results in more realistic hindcast surface salinity and salinity gradient fields than models that distribute coastal runoff in the form of spatially distributed precipitation. Correlations with observed thermal and haline monthly anomalies reveal statistically significant hindcast temporal variability during the freshet season when the signal‐to‐noise ratio is large. Comparisons of ocean models forced by high‐ and low‐resolution hydrological models reveal differences in salinity, surface elevation, and velocity fields, highlighting the value and importance of accurate coastal runoff fields. The model results improve our understandingmore »of the regional influence of runoff on sea level elevations and the distribution and fate of fresh water. Our approach has potential applications to biogeochemical modeling in regions where distributed line source freshwater coastal discharges deliver heat, momentum, and chemical constituents that may influence the marine carbon pump.

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5. The role of inputs of marine wrack and carrion in sandy‐beach ecosystems: a global review

   https://doi.org/10.1111/brv.12886  

   Hyndes, Glenn A. ; Berdan, Emma L. ; Duarte, Cristian ; Dugan, Jenifer E. ; Emery, Kyle A. ; Hambäck, Peter A. ; Henderson, Christopher J. ; Hubbard, David M. ; Lastra, Mariano ; Mateo, Miguel A. ; et al ( August 2022 , Biological Reviews)

   Sandy beaches are iconic interfaces that functionally link the ocean with the landviathe flow of organic matter from the sea. These cross‐ecosystem fluxes often comprise uprooted seagrass and dislodged macroalgae that can form substantial accumulations of detritus, termed ‘wrack’, on sandy beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on beaches form a rich food source (‘carrion’) for a diversity of scavenging animals. Here, we provide a global review of how wrack and carrion provide spatial subsidies that shape the structure and functioning of sandy‐beach ecosystems (sandy beaches and adjacent surf zones), which typically have littlein situprimary production. We also examine the spatial scaling of the influence of these processes across the broader land‐ and seascape, and identify key gaps in our knowledge to guide future research directions and priorities. Large quantities of detrital kelp and seagrass can flow into sandy‐beach ecosystems, where microbial decomposers and animals process it. The rates of wrack supply and its retention are influenced by the oceanographic processes that transport it, the geomorphology and landscape context of the recipient beaches, and the condition, life history and morphological characteristics of the macrophyte taxa that are the ultimate sourcemore »of wrack. When retained in beach ecosystems, wrack often creates hotspots of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. Nutrients are produced during wrack breakdown, and these can return to coastal waters in surface flows (swash) and aquifers discharging into the subtidal surf. Beach‐cast kelp often plays a key trophic role, being an abundant and preferred food source for mobile, semi‐aquatic invertebrates that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach‐cast marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst deposits of organic matter on sandy‐beach ecosystems underpin a range of ecosystem functions and services, they can be at variance with aesthetic perceptions resulting in widespread activities, such as ‘beach cleaning and grooming’. This practice diminishes the energetic base of food webs, intertidal fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) are predicted to cause substantial reductions in the amounts of marine organic matter reaching many beach ecosystems, likely causing flow‐on effects for food webs and biodiversity. Similarly, future sea‐level rise and increased storm frequency are likely to alter profoundly the physical attributes of beaches, which in turn can change the rates at which beaches retain and process the influxes of wrack and animal carcasses. Conservation of the multi‐faceted ecosystem services that sandy beaches provide will increasingly need to encompass a greater societal appreciation and the safeguarding of ecological functions reliant on beach‐cast organic matter on innumerable ocean shores worldwide.

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