Permeable Sediments

who while sharing the pleasures and burdens of life have continually affirmed the pursuit of science and given me the support and courage to go on with mine. have continually inspired me over the course of my studies. It has been a tremendous pleasure to be a part of his research group, where I have benefited from the assistance and friendship of Deborah Jahnke and Mary Richards. I thank Drs. Ellery Ingall, Philip Froelich, Markus Huettel, Jay Brandes and Martial Taillefert for serving on my thesis committee and providing helpful comments. I thank Dr. Carolyn Ruppel, for valuable training in numerical modeling techniques and insightful advice along the way. Dr. Ruppel gave me the opportunity to participate in a NOAA expedition to the Blake Ridge aboard the R/V Atlantis and a dive on DSV Alvin at the Cape Fear slide. I am particularly indebted to Dr.

Continental shelves are predominately (∼70%) covered with permeable, sandy sediments. While identified as critical sites for intense oxygen, carbon, and nutrient turnover, constituent exchange across permeable sediments remains poorly quantified. The central North Sea largely consists of permeable sediments and has been identified as increasingly at risk for developing hypoxia. Therefore, we investigate the benthic O2 exchange across the permeable North Sea sediments using a combination of in situ microprofiles, a benthic chamber, and aquatic eddy correlation. Tidal bottom currents drive the variable sediment O2 penetration depth (from ∼3 to 8 mm) and the concurrent turbulence‐driven 25‐fold variation in the benthic sediment O2 uptake. The O2 flux and variability were reproduced using a simple 1‐D model linking the benthic turbulence to the sediment pore water exchange. The high O2 flux variability results from deeper sediment O2 penetration depths and increased O2 storage during hi...

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We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 mm in diameter, surrounded by a hard transparent wall) were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. Specimens of these algae that were buried below the photic zone exhibited an unexpected stimulation of respiration by light, resulting in an increase of local oxygen concentrations upon darkening. Net photosynthesis of the sediments varied between 1.9 and 8.5 mmol O 2 m À2 h À1 and was strongly correlated with Chl a content, which lay between 31 and 84 mg m À2. An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals. Photosynthesis stimulated calcification at all investigated sites (0.2e1.0 mmol Ca 2þ m À2 h À1). The sediments of at least three stations were net calcifying. Sedimentary N 2-fixation rates (measured by acetylene reduction assays at two sites) ranged between 0.9 to 3.9 mmol N 2 m À2 h À1 and were highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N 2-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.

Although mudflats seem relatively planar, closer inspection reveals a succession of meso-topographical features, including consecutive convex and concave meso-and micro-topographical features. The objective of this study was to determine the influence of meso-scale surface sediment morphology on the dynamics of the macroalgae Ulvales (Chlorophyta) and associated macroepifauna in the Ria Formosa tidal lagoon (Southern coast of Portugal). Sampling took place at four sites in the Ria Formosa with monthly periodicity. Two were located on convex sections (mounds) of the mudflat and the other two on concave sections (depressions)., Macroalgae and related macroepifauna were sampled at each station. Biomass was quantified by determination of the ash-free dry weight (AFDW). Data were analysed using the software package "PRIMER" (Plymouth Routines In Multivariate Ecological Research). Results show a clear distinction between convex and concave areas. In † deceased

The role of submarine groundwater discharge (SGD) in releasing fluorescent dissolved organic matter (FDOM) to the coastal ocean and the possibility of using FDOM as a proxy for dissolved organic carbon (DOC) was investigated in a subterranean estuary in the northeastern Gulf of Mexico (Turkey Point, Florida). FDOM was continuously monitored for three weeks in shallow beach groundwater and in the adjacent coastal ocean. Radon (222 Rn) was used as a natural groundwater tracer. FDOM and DOC correlated in groundwater and seawater samples, implying that FDOM may be a proxy of DOC in waters influenced by SGD. A mixing model using salinity as a seawater tracer revealed FDOM production in the high salinity region of the subterranean estuary. This production was probably a result of infiltration and transformation of labile marine organic matter in the beach sediments. The non-conservative FDOM behavior in this subterranean estuary differs from most surface estuaries where FDOM typically behaves conservatively. At the study site, fresh and saline SGD delivered about 1800 mg d-1 of FDOM (quinine equivalents) to the coastal ocean per meter of shoreline. About 11% of this input was related to fresh SGD, while 89% were related to saline SGD resulting from FDOM production within the shallow aquifer. If these fluxes are representative of the Florida Gulf Coast, SGD-derived FDOM fluxes would be equivalent to at least 18% of the potential regional riverine FDOM inputs. To reduce uncertainties related to the scarcity of FDOM data, further investigations of river and groundwater FDOM inputs in Florida and elsewhere are necessary.

Quantifying groundwater discharge remains a challenge due to its large temporal and spatial variability. Here, we quantify groundwater discharge into a small estuary using radon (222 Rn) and radium isotopes (223 Ra and 224 Ra). High temporal resolution (30 min time steps) radon observations at 4 time series stations were used to determine where groundwater discharge is prevalent in the estuary, and to reduce mass balance model uncertainties. A three-endmember mixing model was developed based on short-lived radium isotopes (sampled at a single location) to separate the shallow saline and deep fresh sources of the discharging groundwater. The results show that using multiple 222 Rn time series stations decreased the overall uncertainty of groundwater discharge estimates from about 41% to 23%. The radon derived groundwater flux was 56±13 and 35±12 cm d-1 in wet and dry conditions, respectively. The spatially distributed stations detected a well-defined small area located four kilometers upstream from the mouth of the estuary as a groundwater discharging hotspot. Estimates based on a 223 Ra and 224 Ra mass balance resulted in groundwater discharge estimates of 65±18 and 84±48 cm d-1 in the wet and 18±5 and 20±6 cm d-1 in the dry. The mixing model revealed contrasting results for deep vs. fresh groundwater contribution in wet and dry conditions. In wet conditions, deep fresh groundwater discharging into the estuary contributed 65% compared to the shallow saline groundwater (35%), while during dry conditions a larger contribution (80%) was related to shallow groundwater. A comprehensive spatial and temporal sampling strategy can produce groundwater discharge estimates with lower uncertainty and provides additional insight on where groundwater enters surface waters.

Submarine Groundwater Discharge (SGD) has been recognized as an important component of the ocean-continent interface. The few previous studies in Brazil have focused on nearshore areas. This paper explores SGD on the Southern Brazilian Continental Shelf using multiple lines of evidence that include radium isotopes, dissolved nutrients, and water mass observations. The results indicated that SGD may be occurring on the Continental Shelf in the Albardão region, near a paleochannel located 50 km offshore. This paleochannel may thus be a preferential pathway for the delivery of nutrient- and metal-enriched groundwater and porewater into continental shelf waters.

We use concomitant radon (222 Rn, a natural groundwater tracer) and nutrient time series observations upstream and downstream of two New Zealand estuarine intertidal flats to assess porewater exchange rates and/or submarine groundwater discharge (SGD) and their influence on surface water nutrient dynamics. A detailed radon mass balance model and an uncertainty analysis revealed porewater exchange rates of 27 ± 7 and 14 ± 6 cm d −1 (or cm 3 cm −2 d −1) in Waikareao and Te Puna, respectively. The upscaled porewater exchange rates were slightly higher than the creek input upstream of both intertidal flats. A water and salt balance compared to the radon balance indicated that about 16% (Waikareao) and 49% (Te Puna) of the total volume of porewater exchange consisted of fresh SGD and the remaining was related to seawater recirculation in intertidal sediments. Porewater exchange in Waikareao released about 1.9-and 1.6-fold more total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) than creek inputs at the upstream end of the intertidal flat even though observations followed a 140 mm rain event. Dissolved organic nitrogen (DON) accounted for about 25% of TDN in shallow porewater. Nitrate dominated the nitrogen pool in Waikareao and ammonium was the main form of nitrogen in Te Puna porewaters. These dominant porewater N species were reflected in the surface waters that showed a relative enrichment of nitrate in Waikareao and ammonium in Te Puna as upstream waters travel to the downstream station and collect seeping porewater along the way. We suggest that porewater exchange may act as a buffered nutrient source to the estuary, continually releasing nutrients to surface waters and potentially sustaining primary production when other nutrient inputs cease. This study illustrates that combining bottom up (i.e., porewater exchange flux estimates) and top down (i.e., nutrient response and transformations in the water column) evidence may provide deeper insight when assessing the contribution of porewater to nutrient dynamics in estuaries.

We report the results of an experiment in which we measured 222 Rn (15,000 observations), CH 4 (40,000 observations), and associated variables in seawater nearly continuously at a coastal site in the Gulf of Mexico for almost two years. Significant correlations between 222 Rn and CH 4 imply that they are derived from a common source, most likely groundwater. However, we were unable to explain the overall tracer variability as a single function of groundwater table height, temperature, tidal range, and wind speed, indicating multiple, overlapping controls on SGD dynamics at this site. Methane and radon concentrations may vary 2fold in a given well in the subterranean estuary over tidal time scales, demonstrating the complexity of determining SGD endmember concentrations and suggesting that unaccounted for temporal changes in groundwater may explain some of the patterns observed in seawater. Surprisingly, the variability of 222 Rn and CH 4 in seawater over short (e.g., hourly) time scales was generally comparable to or even more pronounced than fluctuations over much longer (e.g., monthly) scales. While high tracer concentrations usually occurred during low tide and low tracer concentrations during high tide, this pattern was occasionally inverted or absent indicating that no single model can be used to describe the entire data set. We also describe a sequence of events in which SGD tracers were depleted in coastal waters during storms and regenerated afterwards. We found no increase in radon activities immediately after the largest storm (75 mm rainfall) perhaps because of the short residence times of groundwater in contrast to the ingrowth time of radon. Marine controls appeared to be the most important SGD drivers with only minor influence relating to the shallow and deep aquifers. This implies that seasonal investigations of SGD tracers in the coastal ocean may be masked by short-term variability.

We examined the residence time, seepage rate, and submarine groundwater discharge (SGD)-driven dissolved nutrients and organic matter in Hwasun Bay, Jeju Island, Korea during the occurrence of a typhoon, Kong-rey, using a humic fluorescent dissolved organic matter (FDOM H)-Si mass balance model. The study period spanned October 4-10, 2018. One day after the typhoon, the residence time and seepage rate were calculated to be 1 day and 0.51 m day −1 , respectively, and the highest SGD-driven fluxes of chemical constituents were estimated (1.7 × 10 6 mol day −1 for dissolved inorganic nitrogen, 0.1 × 10 6 mol day −1 for dissolved inorganic phosphorus (DIP), 1.1 × 10 6 mol day −1 for dissolved silicon, 0.5 × 10 6 mol day −1 for dissolved organic carbon, 1.6 × 10 6 mol day −1 for dissolved organic nitrogen, 0.4 × 10 6 mol day −1 for particulate organic carbon, and 38 × 10 6 g QS day −1 for FDOM H). SGD-driven fluxes of dissolved nutrient and organic matter were over 90% of the total input fluxes in Hwasun Bay. Our results highlight the potential of using the FDOM H-Si mass balance model to effectively measure SGD within a specific area (i.e., volcanic islands) under specific weather conditions (i.e., typhoon/storm). In oligotrophic oceanic regions, SGD-driven chemical fluxes from highly permeable islands considerably contribute to coastal nutrient budgets and coastal biological production. Submarine groundwater discharge (SGD) comprises terrestrially derived fresh groundwater and re-circulated seawater 1-3. SGD can be affected by tidal pumping, wave setup , currents, and density gradients 4-6. In particular, under strong winds of 10 m s −1 (i.e., storms and typhoons), wave pumping rates can increase by orders of magnitude exceeding the rates of fresh water inputs from runoff and SGD 7. In volcanic islands, such as Hawaii (USA), Jeju Island (Korea), and Mauritius, high rates of SGD occur owing to a high relief and permeability in addition to poorly developed river drainage systems 8-10. SGD is an important pathway for transporting chemical constituents, such as dissolved organic matter (DOM), nutrients, radionuclides, and trace elements to the coastal ocean on a regional scale 11-15 as well as the basin 2,16 and global scales 17. The chemical constituent fluxes via SGD are similar to or often much higher than those through river discharge into the coastal ocean. SGD may play an especially important role in tropical islands (e.g., Jeju Island, Hawaii, Mauritius, Balearic Islands) that are dominated by substantial precipitation and highly permeable rocks 13,18-21. For example, in Jeju island, SGD-driven fluxes of dissolved inorganic nitrogen (DIN) and organic nitrogen (DON) in Hwasun Bay are larger than those through large rivers around the world, such as

Dissolved carbon dioxide (CO 2) may be highly enriched in groundwater. However, the contribution of groundwater discharge as a source of CO 2 to rivers, estuaries and coastal waters is poorly understood. We performed high resolution measurements of radon (222 Rn, a natural groundwater tracer) and the partial pressure of CO 2 (pCO 2) in a highly modified tidal creek and estuary (North Creek, Richmond River, New South Wales, Australia) to assess whether CO 2 in surface waters was driven by groundwater discharge. A spatial survey revealed increasing 222 Rn activities (up to 17.3 dpm L À1) and pCO 2 (up to 11,151 latm) in the upstream direction. The enrichment occurred in a drained coastal acid sulphate soil wetland upstream of a mangrove forest. Time series experiments (24-h) were performed at two stations upstream and downstream of the pCO 2 enrichment area. Upstream measurements demonstrated a significant correlation between pCO 2 and 222 Rn while downstream values resulted in a significant inverse relationship between pCO 2 and dissolved oxygen apparently as a result of respiration in nearby mangroves. Measurements taken 2 days after a 245 mm precipitation event revealed the highest recorded 222 Rn activities (up to 86.1 dpm L À1) and high pCO 2 (up to 11,217 latm), showing a strong groundwater influence after flooding. These observations imply that groundwater discharge drove CO 2 dynamics at the upstream station while multiple complex processes drove CO 2 at the downstream station. A 222 Rn mass balance model demonstrated that groundwater discharge accounted for about 76% of surface water in this floodplain creek. The CO 2 evasion rates (799 ± 225 mmol m À2 d À1) were driven primarily by currents rather than wind. Groundwater-derived CO 2 fluxes into the creek averaged 1622 mmol m À2 d À1 , a value twice as high as atmospheric CO 2 evasion and consistent with carbon uptake within the creek and downstream exports. These results demonstrate that groundwater seepage was a major factor driving CO 2 evasion to the atmosphere from the creek. Groundwater discharge should be accounted for in CO 2 budgets in coastal systems.

Some predictions of how ocean acidification (OA) will affect coral reefs assume a linear functional relationship between the ambient seawater aragonite saturation state (Ω a) and net ecosystem calcification (NEC). We quantified NEC in a healthy coral reef lagoon in the Great Barrier Reef during different times of the day. Our observations revealed a diel hysteresis pattern in the NEC versus Ω a relationship, with peak NEC rates occurring before the Ω a peak and relatively steady nighttime NEC in spite of variable Ω a. Net ecosystem production had stronger correlations with NEC than light, temperature, nutrients, pH, and Ω a. The observed hysteresis may represent an overlooked challenge for predicting the effects of OA on coral reefs. If widespread, the hysteresis could prevent the use of a linear extrapolation to determine critical Ω a threshold levels required to shift coral reefs from a net calcifying to a net dissolving state.

Groundwater is often overlooked as a source of nutrients to estuaries and most previous groundwatersurface water exchange studies did not consider the input of dissolved organic nutrients. Here, we hypothesize that groundwater is contributing to high dissolved inorganic and organic nutrient concentrations in an eutrophic subtropical tidal river and estuary (Caboolture River, Queensland, Australia). Several spatial radon (222 Rn, a natural groundwater tracer) surveys indicated that the majority of groundwater discharge occurred in the tidal river just upstream of the estuary, and that the radon hotspot did not necessarily coincide with the nutrient hotspot. A radon mass balance revealed that groundwater discharge into the tidal river was equivalent to about 50% of the gauged river flow in February 2012. Groundwater discharge apparently contributed 85% of ammonium and 35% of phosphate entering the estuary. In spite of significant correlations between radon and nitrate and dissolved organic nitrogen (DON) during spatial surveys, groundwater could account for only 7% of nitrate and 9% of DON inputs due to low groundwater concentrations and other sources (i.e., apparently a sewage treatment plant for nitrate and floodplain tributaries for DON). Because total dissolved nitrogen (TDN) was dominated by DON (69%) and nitrate (23%), the groundwater ammonium inputs were a minor source to the TDN pool within the tidal river and estuary. This study demonstrated that correlations between a groundwater tracer and nutrient concentrations do not necessarily illustrate causation. To assess how groundwater drives nutrient dynamics in estuaries, it may be important to include the tidal river (not only the estuarine salinity gradient) in field investigations, consider DON (not only ammonium and nitrate), and perform detailed budgets that include minor tributaries.

The natural flux of groundwater into coastal water bodies has recently been shown to contribute significant quantities of nutrients and trace metals to the coastal ocean. Groundwater discharge and hyporheic exchange to estuaries and rivers, however, is frequently overlooked though it often carries a distinctly different chemical signature than surface waters. Most studies that attempt to quantify this input to rivers use multiple geochemical tracers. However, these studies are often limited in their spatial and temporal extents because of the labor-intensive nature of integrating multiple measurement techniques. We describe here a method of using a single tracer, 222 Rn, to rapidly characterize groundwater discharge into tidally-influenced rivers and streams. In less than one week of fieldwork, we determined that of six streams that empty into the Indian River Lagoon (IRL), Florida, three (Eau Gallie River, Turkey Creek, and Main Canal) did not receive substantial groundwater inputs, one canal (C-25 Canal) was dominated by groundwater exchange, and the remaining two (Sebastian River system and Crane Creek) fell somewhere in between. For more detailed discharge assessments, we focused on the Sebastian River system, a stratified tidal river estuary, during a relatively dry period (June) and a wet period (July) in 2008. Using time-series 222 Rn and current velocity measurements we found that groundwater discharge into all three branches of the Sebastian River increased by 1-2 orders of magnitude during the wetter period. The estimated groundwater flow rates were higher than those reported into the adjacent IRL, suggesting that discharge into these rivers can be more important than direct discharge into the IRL. The techniques employed here should work equally well in other river/stream systems that experience significant groundwater discharge. Such assessments would allow area managers to quickly assess the distribution and magnitude of groundwater discharge nature into rivers over large spatial ranges.

We report the results of an experiment in which we measured 222 Rn (15,000 observations), CH 4 (40,000 observations), and associated variables in seawater nearly continuously at a coastal site in the Gulf of Mexico for almost two years. Significant correlations between 222 Rn and CH 4 imply that they are derived from a common source, most likely groundwater. However, we were unable to explain the overall tracer variability as a single function of groundwater table height, temperature, tidal range, and wind speed, indicating multiple, overlapping controls on SGD dynamics at this site. Methane and radon concentrations may vary 2fold in a given well in the subterranean estuary over tidal time scales, demonstrating the complexity of determining SGD endmember concentrations and suggesting that unaccounted for temporal changes in groundwater may explain some of the patterns observed in seawater. Surprisingly, the variability of 222 Rn and CH 4 in seawater over short (e.g., hourly) time scales was generally comparable to or even more pronounced than fluctuations over much longer (e.g., monthly) scales. While high tracer concentrations usually occurred during low tide and low tracer concentrations during high tide, this pattern was occasionally inverted or absent indicating that no single model can be used to describe the entire data set. We also describe a sequence of events in which SGD tracers were depleted in coastal waters during storms and regenerated afterwards. We found no increase in radon activities immediately after the largest storm (75 mm rainfall) perhaps because of the short residence times of groundwater in contrast to the ingrowth time of radon. Marine controls appeared to be the most important SGD drivers with only minor influence relating to the shallow and deep aquifers. This implies that seasonal investigations of SGD tracers in the coastal ocean may be masked by short-term variability.