Tidal Exchange Research Articles

Zooplankton Dynamics in the Cache Slough Complex of the Upper San Francisco Estuary

Author(s): Kimmerer, Wim; Ignoffo, Toni R.; Bemowski, Brooke; Moderan, Julien; Holmes, Ann; Bergamaschi, Brian | Abstract: We studied abundance and dynamics of zooplankton in the tidal freshwater Cache Slough Complex (CSC) in the northern Delta of the San Francisco Estuary during June, July, and October 2015. We asked whether the CSC was an area of high zooplankton production that could act as a source region for open waters of the estuary. Abundance of the copepod Pseudodiaptomus forbesi was similar to that in freshwater reaches of the central and eastern Delta and higher than that in the adjacent Sacramento River. Growth rate of P. forbesi was higher than previously measured in large estuarine channels because of higher temperature and phytoplankton biomass in the CSC. Samples of P. forbesi examined with molecular techniques contained an unexpectedly high proportion of DNA from cyanobacteria and little DNA from more nutritious phytoplankton. We also examined tidal exchanges of phytoplankton biomass and copepods between Liberty Island, a shallow tidal lake within the CSC, and the adjacent southern Cache Slough, which links the CSC to the Sacramento River. We calculated zero net flux of phytoplankton over 127 days between June and October. The tidal flux of copepods, calculated using tidal flow from an in situ flow station and half-hourly sampling over three 24.8-hr tidal cycles, varied a great deal because of temporal patchiness and day/night variation in abundance. Overall, the tidal flux was indistinguishable from zero, while the tidally-averaged water flow (and therefore the net copepod flux) was always into the wetland. Our results show some promise for the CSC as a productive habitat for planktivorous fishes and as a laboratory for learning how to design future wetland restoration. However, we remain cautious about whether wetlands such as the CSC may export large quantities of food organisms that can support fishes in other regions of the estuary.

Deciphering the dynamics of inorganic carbon export from intertidal salt marshes using high-frequency measurements

The lateral export of carbon from coastal marshes via tidal exchange is a key component of the marsh carbon budget and coastal carbon cycles. However, the magnitude of this export has been difficult to accurately quantify due to complex tidal dynamics and seasonal cycling of carbon. In this study, we use in situ, high-frequency measurements of dissolved inorganic carbon (DIC) and water fluxes to estimate lateral DIC fluxes from a U.S. northeastern salt marsh. DIC was measured by a CHANnelized Optical Sensor (CHANOS) that provided an in situ concentration measurement at 15-min intervals, during periods in summer (July – August) and late fall (December). Seasonal changes in the marsh had strong effects on DIC concentrations, while tidally-driven water fluxes were the fundamental vehicle of marsh carbon export. Episodic events, such as groundwater discharge and mean sea water level changes, can impact DIC flux through altered DIC concentrations and water flow. Variability between individual tides within each season was comparable to mean variability between the two seasons. Estimated mean DIC fluxes based on a multiple linear regression (MLR) model of DIC concentrations and high-frequency water fluxes agreed reasonably well with those derived from CHANOS DIC measurements for both study periods, indicating that high-frequency, modeled DIC concentrations, coupled with continuous water flux measurements and a hydrodynamic model, provide a robust estimate of DIC flux. Additionally, an analysis of sampling strategies revealed that DIC fluxes calculated using conventional sampling frequencies (hourly to two-hourly) of a single tidal cycle are unlikely to capture a representative mean DIC flux compared to longer-term measurements across multiple tidal cycles with sampling frequency on the order of tens of minutes. This results from a disproportionately large amount of the net DIC flux occurring over a small number of tidal cycles, while most tides have a near-zero DIC export. Thus, high-frequency measurements (on the order of tens of minutes or better) over the time period of interest are necessary to accurately quantify tidal exports of carbon species from salt marshes.

Spatial Subsidies and Mortality of an Estuarine Copepod Revealed Using a Box Model

Mortality of planktonic populations is difficult to determine because assumptions of the methods are rarely met, more so in estuaries where tidal exchange ensures violation of the assumption of a closed or spatially uniform population. Estuarine plankton populations undergo losses through movement from productive regions, creating a corresponding subsidy to regions that are less productive. We estimated mortality rates of the copepod Pseudodiaptomus forbesi in the San Francisco Estuary using a vertical-life-table approach with a Bayesian estimation method, combined with estimates of spatial subsidies and losses using a spatial box model with salinity-based boundaries. Data came from a long-term monitoring program and from three sample sets for 1991–2007 and 2010–2012. A hydrodynamic model coupled with a particle-tracking model supplied exchange rates between boxes and from each box to several sinks. In situ mortality, i.e., mortality corrected for movement, was highly variable. In situ mortality of adults was high (means by box and sampling program 0.1–0.9 day−1) and appeared invariant with salinity or year. In situ mortality of nauplii and copepodites increased from fresh (~ 0) to brackish water (means 0.4–0.8 day−1), probably because of consumption by clams and predatory copepods in brackish water. High mortality in the low-salinity box was offset by a subsidy which increased after 1993, indicating an increase in mortality. Our results emphasize the importance of mortality and spatial subsidies in structuring populations. Mortality estimates of estuarine plankton are feasible with sufficient sampling to overcome high variability, provided adjustments are made to account for movement.

Storm-driven particulate organic matter flux connects a tidal tributary floodplain wetland, mainstem river, and estuary.

The transport of terrestrial plant matter into coastal waters is important to regional and global biogeochemical cycles, and methods for assessing and predicting fluxes in such dynamic environments are needed. We investigated the hypothesis that upon reconnection of a floodplain wetland to its mainstem river, organic matter produced in the wetland would reach other parts of the ecosystem. If so, we can infer that the organic matter would ultimately become a source for the food web in the mainstem river and estuary. To accomplish this, we adapted numerical hydrodynamic and transport modeling methods to estimate the mass of particulate organic matter (POM) derived from the annually senescent aboveground parts of herbaceous marsh plants (H-POM). The Finite-Volume Community Ocean Model (FVCOM), parameterized with flow, tide, and aboveground biomass data, simulated H-POM mobilization from fluid shear stress during tidal exchange, flooding, and variable river flow; entrainment into the water column; transport via channel and overland flow; and entrapment when wetted surfaces dry. We examined export from a recently reconnected, restoring tidal emergent marsh on the Grays River, a tributary to the Columbia River estuary. Modeling indicated that hydrologically reconnecting 65ha at the site resulted in export of about 96×103 kg of H-POM, primarily during pulsed storm flooding events in autumn and early winter. This exported mass amounted to about 19% of the summer peak aboveground biomass measured at the site. Of that 19%, about 48% (47×103 kg) was deposited downstream in the Grays River and floodplain wetlands, and the remaining 52% (50×103 kg) passed the confluence of the Grays River and the mainstem estuary located about 7km from the study site. The colonization of the restoring study site largely by nonnative Phalaris arundinacea (reed canarygrass) may have resulted in 18-28% lower H-POM mobilization than typical marsh plant communities on this floodplain, based on estimates from regional studies of marshes dominated by less recalcitrant species. We concluded that restored floodplain wetlands can contribute significant amounts of organic matter to the estuarine ecosystem and thereby contribute to the restoration of historical trophic structure.

Observations of episodic breaching and closure at an ephemeral river

Carmel River, near Carmel, CA, is a seasonally open, ephemeral river that has a basin of 660 km2. During the dry summer months, the barrier beach is built across the river mouth, limiting water exchange. Precipitation during the winter months increases the discharge within the river until water levels are sufficiently high to breach the barrier beach. Observations during the 2016–2017 winter allow for a dynamic balance between discharge, wave forcing, and tidal exchange that led to several distinct breaching and closure events. The initial breach that occurred after the first major precipitation event was insufficient to keep the river open, owing to wave forcing at the mouth and a relatively low river discharge. Wave penetration into the estuary through overtopping and upstream propagation is routinely observed during this stage. In order to evaluate the conditions responsible for the intermittent opening and closing of the inlet, water level, wave, tidal and discharge data were collected. With this data, a momentum balance was developed in order to estimate the ocean forcing (tides and waves) as compared to discharge. It is hypothesized that a critical discharge is required to maintain an open river mouth, which depends on offshore wave forcing and tidal stage.

Assessing rehabilitation of managed mangrove ecosystems using high resolution remote sensing

Mangroves are valuable ecosystems for coastal protection, carbon sequestration and storage, and they provide habitat, refuge and rearing areas for many important marine species. To control mosquito outbreaks in coastal regions, mangroves were often impounded and managed using a variety of techniques that ranged from the application of insecticides to water level manipulation. Since continuous impounding had been shown to have negative effects on mangrove vegetation, other techniques have been used to manage hydrology in impoundments. A recent technique is called rotational impoundment management (RIM) and it involves flooding impoundments in summer and spring, the reproductive season of the mosquitos. In this study, we assessed the effects of 5 years of RIM management on mangrove vegetation in an impoundment on the east coast of Florida. We compared mangrove vegetation in the RIM impoundment with an adjacent impoundment that was not managed. We created a map of leaf area index (LAI) to assess vegetation productivity and its change in the two impoundments. We classified color-infrared aerial photographs from 2008 to 2010 and a WorldView-2 satellite image from 2014 to measure the extent of mangrove vegetation types and temporal changes in the two impoundments. We found a 38% increase in cover of dense mangrove vegetation after five years for the RIM-impounded area. Classification accuracy was around 80% for all imagery. The increased growth of plants and cover of dense mangroves in the RIM impoundment was corroborated by observed leaf area index values. Overall, the study demonstrates that vegetation in the RIM impoundment is becoming denser and in the near future will probably become similar to an impoundment that is open to tidal exchange or mangrove dominated areas that are not impounded.

Assessing the contribution of porewater discharge in carbon export and CO2 evasion in a mangrove tidal creek (Can Gio, Vietnam)

Although mangrove forests are efficient natural carbon sinks, most of the atmospheric carbon dioxide (CO2) fixed by its vegetation is believed to be exported via tidal exchange, rather than stored in the vegetative biomass and sediment. However, the magnitude of tidal export is largely unknown because direct measurements are scarce. We deployed a novel experimental design that combined automated high-resolution measurements of hydrodynamic, hydrogeochemical and biogeochemical parameters during the dry season in a mangrove tidal creek in the Can Gio Mangrove Forest in Vietnam. The objective was to quantify the tide-controlled water, porewater, DIC and DOC exchange, and estimate the CO2 evasion throughout tidal cycles contrasted by amplitude. Data from three 25-h time series showed a clear peak of DIC, DOC, pCO2, and 222Rn at low tide, particularly during tidal cycles of large amplitude, which directly relate to porewater discharge. Our mass balance models revealed that the tidal creek was a net exporter of dissolved carbon to coastal waters, with an important contribution (38%) coming from DIC in porewater discharge. Porewater exchange varied from 3.1 ± 1.6 to 7.1 ± 2.4 cm day−1. DIC exchange ranged from 352 ± 34 to 678 ± 79 mmolC m−2 day−1; DOC exchange, 20.6 ± 1.9 to 67.7 ± 7.9 mmol C m−2 day−1; and CO2 evasion, 69.9 ± 10.5 to 173.7 ± 26.1 mmolC m−2 day−1. These estimates were in the high range of previous carbon assessments and were explained by (i) the monitoring station being located at equal distance from the head and the mouth of the creek, which minimized carbon degradation and losses associated to transport in water; and (ii) the site being a highly productive mangrove within South East Asia.

Surge dynamics across a complex bay coastline, Galveston Bay, TX

While tidal exchange across bay coastlines has been well studied, research to date has not provided a comprehensive analysis of the volume of surge that can flow across bay coastlines during tropical cyclone events. Such insight can particularly be useful for understanding coastal storm surge dynamics that influence regional inundation and can help guide surge mitigation strategies for semi-enclosed bay systems. In this study, a suite of 80 synthetic tropical cyclones were simulated to characterize the volume of surge that flows across Galveston Bay's complex 98 km coastline, which is made up of tidal inlets, intermittent barrier islands, and existing surge defenses along the Upper Texas Gulf Coast. Tropical cyclone characteristics analyzed in this study include the wide variety of tropical cyclone intensity (Vmax) and size (Rmax) combinations possible for the region, with the storm surge response of each tropical cyclone simulated using the ADCIRC + SWAN model. In-depth analysis of coastal and in-bay storm surge responses was performed for baseline or existing conditions as well as scenarios representing severe barrier island erosion and future conditions based on intermediate sea level projections for the year 2050 and 2 100. Findings demonstrate that while the majority of surge may flow across tidal inlets for smaller, weaker storms, a storm intensity and size threshold exists that results in greater flow across barrier islands, having important implications for surge mitigation strategies. Analysis demonstrates that while this threshold may occur for extreme tropical cyclones under baseline conditions, this transition can occur for relatively, significantly weaker and smaller storms under the erosion and sea level rise scenarios evaluated.

Nutrient flux estimates in a tidal basin: A case study of Magdalena lagoon, Mexican Pacific coast

Bahia Magdalena (BM), known for its high primary productivity, is one of the largest tidal lagoons on the Mexican Pacific coast of the Baja California Peninsula. BM is located in an area of active coastal upwelling and significant tides with a maximum range of about 2.4 m. Dissolved inorganic nutrients upwelled from the depths are transported by tidal water exchange into the lagoon, contributing to its fertilisation. To estimate the magnitude and mechanisms driving the tidal exchange of water and nutrients, field observations of the nutrient content were made in the inlet area and on the adjacent shelf during March 2003, December 2004 and June, August and November 2005. In March 2003, the research vessel El Puma carried out a complete hydrological study of the area using Seabird-19 CTD profiler. At the same time, a current meter with a tide gauge was installed in the BM inlet, and multiple measurements of currents were made on a section across the inlet with a SonTek hull-mounted Acoustic Doppler Current Profiler (ADCP). Field studies were complemented by numerical experiments with the hydrodynamic model ECOM 3D. Analysis of the currents in the inlet area, deriving from both the ADCP data and the numerical simulation, indicates that the water volume transported during a semidiurnal tidal cycle through the inlet varied from 0.3 km3 for neap tide to 0.82 km3 for spring tide. Net nitrate mass intakes to the lagoon deriving from currents in the mouth can be estimated as 7.0 × 103 kg for neap tides and 20.0 × 103 kg for spring tides, and the maximum phosphate contribution was estimated at 2.5 × 103 kg and 8.5 × 103 kg, respectively. Taking into account that fluvial contribution in the lagoon is practically absent, unexpanded mangroves are distributed mainly at its northern part, and organic sediment decomposition is potentially evaluated as low, we can thus conclude that the coupled effect of upwelling and tidal currents play an important role in fertilising the BM. Naturally, this mechanism works only in periods of upwelling activity, namely, from March to June, and to a lesser extent from September to October.

Constraining Marsh Carbon Budgets Using Long‐Term C Burial and Contemporary Atmospheric CO2 Fluxes

AbstractSalt marshes are sinks for atmospheric carbon dioxide that respond to environmental changes related to sea level rise and climate. Here we assess how climatic variations affect marsh‐atmosphere exchange of carbon dioxide in the short term and compare it to long‐term burial rates based on radiometric dating. The 5 years of atmospheric measurements show a strong interannual variation in atmospheric carbon exchange, varying from −104 to −233 g C m−2 a−1 with a mean of −179 ± 32 g C m−2 a−1. Variation in these annual sums was best explained by differences in rainfall early in the growing season. In the two years with below average rainfall in June, both net uptake and Normalized Difference Vegetation Index were less than in the other three years. Measurements in 2016 and 2017 suggest that the mechanism behind this variability may be rainfall decreasing soil salinity which has been shown to strongly control productivity. The net ecosystem carbon balance was determined as burial rate from four sediment cores using radiometric dating and was lower than the net uptake measured by eddy covariance (mean: 110 ± 13 g C m−2 a−1). The difference between these estimates was significant and may be because the atmospheric measurements do not capture lateral carbon fluxes due to tidal exchange. Overall, it was smaller than values reported in the literature for lateral fluxes and highlights the importance of investigating lateral C fluxes in future studies.

On the Hydrodynamic Geometry of Flow-Through versus Restricted Lagoons

The classification of a lagoon as a restricted lagoon is shown to depend not solely on its geometry but also on the tidal hydraulics. By numerically simulating the tidal exchange of two lagoons of similar geometrical dimensions, the Nidova lagoon and the Papas lagoon, in Western Greece, subject to very similar tidal forcing, applied to the two tidal inlets in the first case and three in the second, very different residence times are found, namely 2.5 days for the Nidova and 25–30 days for the Papas lagoon. This large difference is attributed to the fact that whereas the Papas lagoon functions as a typical restricted lagoon, in which the water renewal is achieved by mixing in the lagoon of the tidal prism water exchanged within a tidal cycle, the Nidova lagoon functions as a flow-through system because of the differential arrival of the tide at its two tidal inlets. It is suggested that this way of enhancing the flushing rate of a lagoon be considered, whenever possible, when creating a new tidal inlet to the lagoon.

Tropical land-sea couplings: Role of watershed deforestation, mangrove estuary processing, and marine inputs on N fluxes in coastal Pacific Panama

We review data from coastal Pacific Panama and other tropical coasts with two aims. First, we defined inputs and losses of nitrogen (N) mediating connectivity of watersheds, mangrove estuaries, and coastal sea. N entering watersheds—mainly via N fixation (79-86%)—was largely intercepted; N discharges to mangrove estuaries (3-6%), small compared to N inputs to watersheds, nonetheless significantly supplied N to mangrove estuaries. Inputs to mangrove estuaries (including watershed discharges, and marine inputs during flood tides) were matched by losses (mainly denitrification and export during ebb tides). Mangrove estuary subsidies of coastal marine food webs take place by export of forms of N [DON (62.5%), PN (9.1%), and litter N (12.9%)] that provide dissimilative and assimilative subsidies. N fixation, denitrification, and tidal exchanges were major processes, and DON was major form of N involved in connecting fluxes in and out of mangrove estuaries. Second, we assessed effects of watershed forest cover on connectivity. Decreased watershed forest cover lowered N inputs, interception, and discharge into receiving mangrove estuaries. These imprints of forest cover were erased during transit of N through estuaries, owing to internal N cycle transformations, and differences in relative area of watersheds and estuaries. Largest losses of N consisted of water transport of energy-rich compounds, particularly DON. N losses were similar in magnitude to N inputs from sea, calculated without considering contribution by intermittent coastal upwelling, and hence likely under-estimated. Pacific Panama mangrove estuaries are exposed to major inputs of N from land and sea, which emphasizes the high degree of bi-directional connectivity in these coupled ecosystems. Pacific Panama is still lightly affected by human or global changes. Increased deforestation can be expected, as well as changes in ENSO, which will surely raise watershed-derived loads of N, as well as significantly change marine N inputs affecting coastal coupled ecosystems.

Intra-annual variations in phytoplankton biomass and its composition in the tropical estuary: Influence of river discharge

To examine the influence of river discharge on phytoplankton composition, time-series observations were conducted at upper, middle and lower Godavari estuary during 2009. The salinity variations in the estuary were depended on freshwater discharge and tidal exchange. River discharge brought a significant amount of nutrients, however it did not induce phytoplankton bloom due to severe light limitation driven by high turbidity. Enhanced phytoplankton biomass and abundance were associated with water column stratification during moderate discharge period, suggesting that water column stability is more important than nutrients to promote phytoplankton blooms. The contribution of diatoms to the total phytoplankton abundance increased with decrease in salinity and vice versa for blue-green algae. The relationship of phytoplankton abundance with salinity and nutrients suggests that low salinity and high N:P ratio favored growth of blue-green algae, whereas high salinity and low N:P/N:Si favored diatoms. This study suggested that discharge brought modification in phytoplankton composition.

The role of catchment land use and tidal exchange in structuring estuarine fish assemblages

Changes in land use often increase nutrient loading to aquatic ecosystems, affecting primary productivity and water quality, with flow-on effects to consumers. We explored whether fish-assemblage composition, species diversity, and the representation and richness of ecological guilds were associated with catchment land use in 31 estuaries in Victoria, Australia. Fish assemblages were surveyed using fyke and gill nets. Species were assigned to ecological guilds based on salinity associations and their use of estuaries, water column position and trophic characteristics. The Shannon diversity index, representation and richness of several marine-associated species and trophic guilds dominated by marine-associated species were positively related to tidal exchange, indicating the widespread influence of marine connectivity on estuarine fish assemblage and trophic structure. These patterns were driven by adult life stages. The richness and the proportion of demersal species in juvenile assemblages were negatively associated with the proportion of the catchment with land uses expected to elevate nutrient loads (e.g. animal production, horticulture, industry and urbanization). This relationship may reflect shifts in vegetated habitat, resource availability or changes in water quality induced by nutrient enrichment. Juvenile demersal fish show promise as an indicator of the effects of catchment land use on the structure of estuarine fish assemblages.

Characterization of Salinity and Temperature Patterns in a Large River Delta to Support Tidal Wetland Habitat Restoration

Although the Snohomish River estuary remains the second largest tidal wetland complex in Puget Sound, approximately 90% of pre-settlement habitat has been disconnected from tidal exchange. This estuary is currently the focus of the largest restoration effort in Puget Sound, with opportunity to restore tidal exchange to over 50% of pre-settlement levels. The Snohomish River also currently supports populations of all anadromous Pacific salmon species, including Endangered Species Act listed Chinook (Oncorhynchus tshawytscha), steelhead (O. mykiss), and bull trout (Salvelinus confluentus). The combination of extant anadromous Pacific salmon populations, large existing tidal wetland complexes, and large restoration potential make the Snohomish River estuary a great opportunity to benefit salmon population recovery and conservation efforts. To support restoration planning and effectiveness monitoring, we developed baseline characterizations of key physical attributes (salinity and temperature). Our results indicated that brackish (0.5–30 ppt) conditions extended farther upriver than previously described, with distributary channels downstream of the middle mainstem and lower Ebey Slough remaining brackish throughout most of the year. During extreme low flows ( 0.5 ppt) can at times intrude throughout the distributaries and up to river kilometer 15.9 above the first bifurcation. We also observed temperatures exceeding stress thresholds for juvenile salmonids throughout the estuary from July through September, a period that overlaps with juvenile rearing. This research is timely with several large restoration projects scheduled for construction by 2020, and these baseline characterizations can be used to evaluate restoration responses, as well as to inform project prioritization and monitoring.

Tidal exchange in a choked coastal lagoon: A study of Mundaú Lagoon in northeastern Brazil

Changes in the inlet morphology of choked coastal lagoons often restrict the water exchange with the sea, making them vulnerable to pollution events and eutrophication processes. In this study, the importance of tides for the water exchange was investigated in Mundaú Lagoon, which is a choked lagoon located in northeastern Brazil that has a very dynamic inlet and channel system. The analysis was carried out for critical scenarios during the dry season when the river flow to the lagoon decreases markedly. The concepts of integrated flushing time and spatially distributed residence time scales were applied using a Lagrangian particle tracking approach coupled with a hydrodynamic model. The flushing time of Mundaú Lagoon was estimated to be 12.6 days during neap tide and 5.7 days during spring tide, including a return flow factor. Employing an e-folding form, the corresponding value was calculated to be 64 and 54 days for particles allowed and not allowed to return to the lagoon, respectively. The spring tides were responsible for the most pronounced water exchange in the Mundaú Lagoon. Different particles release times displayed the effect of the initial tidal conditions on the flushing time; lower exchange times were estimated for releases during ebb tides and spring tides. The integrated estimation of the water exchange when imposing a scenario of most frequent wind conditions indicated a considerable delay caused by this forcing, increasing the flushing time up to 23 days. Another observed effect of the wind was a spatial redistribution of the tidal exchange in the lagoon into different zones with similar residence time.

Hydrographic maintenance of deep anoxia in a tidally influenced saline lagoon

Low dissolved oxygen concentrations are of increasing concern in aquatic ecosystems, particularly at the interface between freshwater and marine environments. Oxygen depletion occurs naturally in many perennially stratified systems and it remains to be seen how climate change will affect these habitats. This is due, in part, to a lack of high-resolution, long-term data describing interannual variability in dissolved oxygen concentrations within stratified basins. Physicochemical parameters for Lough Furnace, an ecologically important tidal lagoon, were assessed using daily measurements (2009–14) from an undulating CTD (conductivity, temperature and depth) profiler and observations of tidal exchange flow. Continuous vertical saline stratification existed, with anoxia (<0.1mgL–1) typically persisting below 6m. Tidal inflows were generally restricted, with deep-water renewal events by intrusions of denser spring tidal water occurring episodically (three times in 6 years), following prolonged periods of low freshwater input. Although wind forcing alone was not sufficient to generate basin-scale mixing, the conditions that led to deep-water renewals may also be conducive to wind-driven upwelling events in nearshore areas. These findings have wider application to larger-scale two-layered stratified systems with deep anoxia because the ability to forecast such dynamic events is important for assessing the ecological implications of dissolved oxygen depletion.

Remote sensing retrievals of colored dissolved organic matter and dissolved organic carbon dynamics in North American estuaries and their margins

Dissolved organic carbon, DOC, and the colored component of dissolved organic matter, CDOM, are key indicators of coastal water quality and biogeochemical state. Yet applications of space-based remote sensing to monitoring of CDOM variability across estuarine ecosystems and assessment of DOC exchanges along highly dynamic terrestrial-aquatic interfaces have been scarce, in part due to the coarse spatial resolution of most existing ocean color sensors and the seasonal and regional dependence of most existing algorithms. Here, we used a rich dataset of field observations to develop and validate new CDOM and DOC algorithms that are broadly applicable to different estuarine and coastal regions, over different seasons and a wide range of in-water conditions. Algorithms were applied to satellite imagery from MERIS-Envisat at a spatial resolution (300m) that can resolve much of the spatial variability that characterizes estuaries and their margins. Multi-spectral remote sensing reflectance (Rrs) was used to retrieve CDOM absorption at various wavelengths and CDOM absorption spectral slope in the 275–295nm spectral range (S275–295). DOC concentrations were obtained from a tight relationship between the DOC-specific CDOM absorption and S275–295, two optical quantities that depend only on the quality of CDOM and strongly covary across spatial and temporal scales. Algorithm evaluation using MERIS satellite data across different estuarine and coastal environments (i.e., the northern Gulf of Mexico, the Delaware Bay, the Chesapeake Bay estuary, and the Middle Atlantic Bight coastal waters) and across different seasons over multiple years resulted in relative errors (mean absolute percent difference; MAPD) of 29% (N=17), 9.5% (N=14), and 18% (N=32), for aCDOM(300), S275–295, and DOC, respectively. These relative errors are comparable to those previously reported for satellite retrievals of CDOM and DOC products in less optically complex offshore waters. Application of these algorithms to multi-year MERIS satellite imagery over the Chesapeake Bay estuary allowed, for the first time, to capture the impact of tidal exchanges on carbon dynamics along wetland-estuary interfaces, and resolved spatial gradients, seasonal variability, and year-to-year changes in estuarine carbon amount and quality associated with marsh carbon export, riverine inputs, and extreme precipitation events.

A study of fluid flow in sediments and the effect of tidal pumping

Offshore drilling and production operations can result in spills or leaks of hydrocarbons into seabed sediments, which can potentially contaminate these sediments with oil. If this oil later migrates to the water surface it has the potential for negative environmental impacts. For proper contingency planning and to avoid larger consequences in the environment, it is essential to understand mechanisms and rates for hydrocarbon migration from oil containing sediments to the water surface as well as how much will remain trapped in the sediments. It is believed that the amount of oil transported out of the sediment can be affected by tidal pumping, a common form of subterranean groundwater discharge (SGD). However, we could find no study experimentally investigating the phenomenon of fluid flow in subsea sediments containing oil and the effects of tidal pumping. This study presents an experimental investigation of tidal pumping to determine if it is a possible mechanism that may contribute to the appearance of an oil sheen on the ocean surface above a sediment bed containing oil. An experimental apparatus was constructed of clear PVC pipe allowing for oil migration to be monitored as it flowed out of a sand pack containing oil, while tidal pressure oscillations were applied in three different manners. The effect of tidal pumping was simulated via compression of air above the water (which simulated the increasing static head from tidal exchange). Experimental results show that sustained oil release occurred from all tests, and tests with oscillating pressure produced for longer periods of time. Furthermore, the experimental results showed that the oil migration rate was affected by grain size, oil saturation, and oscillation wave type. In all oscillating experiments the rate and ultimate recovery was less than the comparable static experiments. For the conditions studied, the experimental results indicate that with an oscillating pressure on top of a sand pack, movement of a non-replenishing source of oil is suppressed by pressure oscillation.

Tidal Marsh Restoration Design Affects Feedbacks Between Inundation and Elevation Change

Tidal marsh (re)creation on formerly embanked land is increasingly executed along estuaries and coasts in Europe and the USA, either by restoring complete or by reduced tidal exchange. Ecosystem functioning and services are largely affected by the hydro-geomorphologic development of these areas. For natural marshes, the latter is known to be steered by feedbacks between tidal inundation and sediment accretion, allowing marshes to reach and maintain an equilibrium elevation relative to the mean sea level. However, for marsh restoration sites, these feedbacks may be disturbed depending on the restoration design. This was investigated by comparing the inundation-elevation change feedbacks in a natural versus restoration site with reduced tidal exchange in the Scheldt estuary (Belgium). This study analyzes long-term (9 years) datasets on elevation change and tidal inundation properties to disentangle the different mechanisms behind this elevation-inundation feedback. Moreover, subsequent changes in sediment properties that may affect this feedback were explored. In the restoration area with reduced tidal exchange, we found a different elevation-inundation feedback than on natural marshes, which is a positive feedback on initially high sites (i.e., sediment accretion leads to increasing inundation, hence causing accelerating sediment accretion rates) and a gradual silting up of the whole area. Furthermore, there is evidence for the presence of a relict consolidated sediment layer. Consequently, shallow subsidence is less likely to occur. Although short-term ecological development of the tidal marsh was not impeded, long-term habitat development may be affected by the differences in hydro-geomorphological interactions. An increase of inundation frequency on the initially high sites may cause inhibition of habitat succession or even reversed succession. Over time, the climax state of the restoration area may be different compared to natural marshes. Moreover, sediment-related ecosystem services, such as nutrient and carbon burial, may be positively influenced because of continuing sedimentation, although flood water storage potential will decrease with increasing elevation. Depending on the restoration goals, ecosystem trajectories and delivery of ecosystem services can be controlled by adaptive management of the tidal volume entering the restoration area.