Biogeochemistry Commons^™

Phytoplankton And Nutrient Dynamics In A Tidally Dominated Eutrophic Estuary: Daily Variability And Controls On Bloom Formation, Ryan E. Morse, Margaret R. Mulholland, Todd A. Egerton, Harold G. Marshall

OES Faculty Publications

To better understand nutrient dynamics and factors that promote the initiation of algal blooms, the Lafayette River, a tidal subestuary of Chesapeake Bay that experiences seasonal algal blooms, was sampled daily for a period of 54 d in the fall of 2005. Three phytoplankton blooms (chl a concentrations exceeding twice the average of monthly measurements from 2000 to 2009) occurred during this period: a mixed bloom of Akashiwo sanguinea and Gymnodinium sp., a monospecific Skeletonema costatum bloom, and a monospecific Gymnodinium sp. bloom. Over the sampling period, nutrient concentrations increased following precipitation events and were elevated between bloom periods but …

The Great 2012 Arctic Ocean Summer Cyclone Enhanced Biological Productivity On The Shelves, Jinlun Zhang, Carin Ashjian, Robert Campbell, Victoria Hill, Yvette H. Spitz, Michael Steele

OES Faculty Publications

A coupled biophysical model is used to examine the impact of the great Arctic cyclone of early August 2012 on the marine planktonic ecosystem in the Pacific sector of the Arctic Ocean (PSA). Model results indicate that the cyclone influences the marine planktonic ecosystem by enhancing productivity on the shelves of the Chukchi, East Siberian, and Laptev seas during the storm. Although the cyclone's passage in the PSA lasted only a few days, the simulated biological effects on the shelves last 1 month or longer. At some locations on the shelves, primary productivity (PP) increases by up to 90% and …

The Impact Of Changing Surface Ocean Conditions On The Dissolution Of Aerosol Iron, Matthew P. Fishwick, Peter N. Sedwick, Maeve C. Lohan, Paul J. Worsfold, Kristen N. Buck, Thomas M. Church, Simon J. Ussher

OES Faculty Publications

The proportion of aerosol iron (Fe) that dissolves in seawater varies greatly and is dependent on aerosol composition and the physicochemical conditions of seawater, which may change depending on location or be altered by global environmental change. Aerosol and surface seawater samples were collected in the Sargasso Sea and used to investigate the impact of these changing conditions on aerosol Fe dissolution in seawater. Our data show that seawater temperature, pH, and oxygen concentration, within the range of current and projected future values, had no significant effect on the dissolution of aerosol Fe. However, the source and composition of aerosols …

Direct Stratospheric Injection Of Biomass Burning Emissions: A Case Study Of The 2009 Australian Bushfires Using The Nasa Giss Modele2 Composition-Climate Model, Robert Field, Mike From, Apostolos Voulgarakis, Drew Shindell, Mike Flannigan, Peter Bernath

Chemistry & Biochemistry Faculty Publications

Direct stratospheric injection (DSI) of forest fire smoke represents a direct biogeochemical link between the land surface and stratosphere. DSI events occur regularly in the northern and southern extratropics, and have been observed across a wide range of measurements, but their fate and effects are not well understood.

DSIs result from explosive, short-lived fires, and their plumes stand out from the background concentrations immediately. This makes it easier to associate detected DSIs to individual fires and their estimated emissions. Because the emissions pulses are brief, chemical decay can be more clearly assessed, and because the emissions pulses are so large, …

A Coupled Geochemical And Biogeochemical Approach To Characterize The Bioreactivity Of Dissolved Organic Matter From A Headwater Stream, Rachel L. Sleighter, Rose M. Cory, Louis A. Kaplan, Hussain A.N. Abdulla, Patrick G. Hatcher

Chemistry & Biochemistry Faculty Publications

The bioreactivity or susceptibility of dissolved organic matter (DOM) to microbial degradation in streams and rivers is of critical importance to global change studies, but a comprehensive understanding of DOM bioreactivity has been elusive due, in part, to the stunningly diverse assemblages of organic molecules within DOM. We approach this problem by employing a range of techniques to characterize DOM as it flows through biofilm reactors: dissolved organic carbon (DOC) concentrations, excitation emission matrix spectroscopy (EEMs), and ultrahigh resolution mass spectrometry. The EEMs and mass spectral data were analyzed using a combination of multivariate statistical approaches. We found that 45% …

Compositions And Constituents Of Freshwater Dissolved Organic Matter Isolated By Reverse Osmosis, Yulong Zhang, Wen Huang, Jingdong Mao

Chemistry & Biochemistry Faculty Publications

Dissolved organic matter (DOM) from riverine and lacustrine water was isolated using a reverse osmosis (RO) system. Solid-state ¹³C nuclear magnetic resonance (¹³C NMR) was used to quantitatively evaluate the compositions and constituents of DOM, which are compared with previous investigations on marine DOM. Results indicated that concentration factor (CF) was a key metric controlling yield and sorption of DOM on the RO system. The sorption was likely non-selective, based on the ¹³C NMR and δ¹³C analyses. Carbohydrates and lipids accounted for 25.0–41.5% and 30.2–46.3% of the identifiable DOM, followed by proteins (18.2–19.8%) and …

Wavelength And Temperature-Dependent Apparent Quantum Yields For Photochemical Formation Of Hydrogen Peroxide In Seawater, David J. Kieber, Gary W. Miller, Patrick J. Neale, Kenneth Mopper

Chemistry & Biochemistry Faculty Publications

Wavelength and temperature-dependent apparent quantum yields (AQYs) were determined for the photochemical production of hydrogen peroxide using seawater obtained from coastal and oligotrophic stations in Antarctica, the Pacific Ocean at Station ALOHA, the Gulf of Mexico, and at several sites along the East Coast of the United States. For all samples, AQYs decreased exponentially with increasing wavelength at 25 °C, ranging from 4.6 × 10−4 to 10.4 × 10−4 at 290 nm to 0.17 × 10−4 to 0.97 × 10−4 at 400 nm. AQYs for different seawater samples were remarkably similar irrespective of expected differences in the composition and concentrations …

Estimating Hydroxyl Radical Photochemical Formation Rates In Natural Waters During Long-Term Laboratory Irradiation Experiments, Luni Sun, Hongmei Chen, Hussain A. Abdulla, Kenneth Mopper

Chemistry & Biochemistry Faculty Publications

In this study it was observed that, during long-term irradiations (>1 day) of natural waters, the methods for measuring hydroxyl radical (˙OH) formation rates based upon sequentially determined cumulative concentrations of photoproducts from probes significantly underestimate actual ˙OH formation rates. Performing a correction using the photodegradation rates of the probe products improves the ˙OH estimation for short term irradiations (<1 day), but not long term irradiations. Only the ‘instantaneous’ formation rates, which were obtained by adding probes to aliquots at each time point and irradiating these sub-samples for a short time (≤2 h), were found appropriate for accurately estimating ˙OH photochemical formation rates during long-term laboratory irradiation experiments. Our results also showed that in iron- and dissolved organic matter (DOM)-rich water samples, ˙OH appears to be mainly produced from the Fenton reaction initially, but subsequently from other sources possibly from DOM photoreactions. Pathways of ˙OH formation in long-term irradiations in relation to H2O2 and iron concentrations are discussed.

Phosphorus Cycling In Tropical Carbonate Sediment-Seagrass Systems, Zachary Howerton

OES Theses and Dissertations

This thesis presents two studies focusing on phosphorus cycling in calcium carbonate sediments inhabited by seagrasses. Phosphorus is a major limiting nutrient for primary productivity in these sediments as well as in the overlying waters. In large part this is due to removal of phosphate from solution by adsorption and precipitation reactions.

In chapter II, the relationship between the size of the sedimentary phosphorus pool and the abundance of seagrass on the Great Bahama Bank, as well as the relationship between phosphorus content and grain size in the sediments, were examined to better understand the biogeochemistry of carbonate sediments and …

Springtime Contribution Of Dinitrogen Fixation To Primary Production Across The Mediterranean Sea, E. Rahav, B. Herut, A. Levi, Margaret Mulholland, I. Berman-Frank

OES Faculty Publications

Dinitrogen (N-2) fixation rates were measured during early spring across the different provinces of Mediterranean Sea surface waters. N-2 fixation rates, measured using N-15(2) enriched seawater, were lowest in the eastern basin and increased westward with a maximum at the Strait of Gibraltar (0.10 to 2.35 nmol NL-1 d(-1), respectively). These rates were 3-7 fold higher than N-2 fixation rates measured previously in the Mediterranean Sea during summertime and we estimated that methodological differences alone did not account for the seasonal changes we observed. Higher contribution of N-2 fixation to primary production (4-8 %) was measured in the western basin …

Uncoupling Between Dinitrogen Fixation And Primary Productivity In The Eastern Mediterranean Sea, Eyal Rahav, Barak Herut, Noga Stambler, Edo Bar-Zeev, Margaret R. Mulholland

OES Faculty Publications

In the nitrogen (N)-impoverished photic zones of many oceanic regions, prokaryotic organisms fixing atmospheric dinitrogen (N₂; diazotrophs) supply an essential source of new nitrogen and fuel primary production. We measured dinitrogen fixation and primary productivity (PP) during the thermally stratified summer period in different water regimes of the oligotrophic eastern Mediterranean Sea, including the Cyprus Eddy and the Rhodes Gyre. Low N₂ fixation rates were measured (0.8-3.2μmol N m-2 d-1) excluding 10-fold higher rates in the Rhodes Gyre and Cyprus Eddy (~20μmol N m^-2 d^-1). The corresponding PP increased from east to west (200-2500μmol …

Arsenic And Phosphorus Biogeochemistry In The Ocean: Arsenic Species As Proxies For P-Limitation, Oliver Wurl, Louise Zimmer, Gregory A. Cutter

OES Faculty Publications

Arsenic and phosphorus are biochemically very similar, and hence arsenate (As⁵⁺) is toxic by interfering with the energy metabolism, in particular during P limitation. However, many phytoplankton detoxify As by reducing arsenate to arsenite (As³⁺), and/or methylating it to mono and dimethyl As. Such As detoxification becomes operative in oligotrophic waters when phosphate concentrations are below those for As; therefore, we evaluated the potential use of these detoxification products as indicators of P-limitation by measuring As speciation during the US GEOTRACES North Atlantic transect. The distribution of As³⁺ concentrations in surface waters is similar to …

Phosphorus Cycling In The Sargasso Sea: Investigation Using The Oxygen Isotopic Composition Of Phosphate, Enzyme-Labeled Fluorescence, And Turnover Times, Karen Mclaughlin, Jill A. Sohm, Gregory A. Cutter, Michael W. Lomas, Adina Paytan

OES Faculty Publications

Dissolved inorganic phosphorus (DIP) concentrations in surface water of vast areas of the ocean are extremely low (<10 nM) and phosphorus (P) availability could limit primary productivity in these regions. We explore the use of oxygen isotopic signature of dissolved phosphate (δ¹⁸O_PO4) to investigate biogeochemical cycling of P in the Sargasso Sea, Atlantic Ocean. Additional techniques for studying P dynamics including ³³P-based DIP turnover time estimates and percent of cells expressing alkaline phosphatase (AP) activity as measured by enzyme-labeling fluorescence are also used. In surface waters, δ¹⁸O_PO4 values were lower than equilibrium by 3–6%, indicative of dissolved organic phosphorous (DOP) remineralization by extracellular enzymes. An isotope mass balance model using a variety of possible combinations of …

Using Ammonium Pore Water Profiles To Assess Stoichiometry Of Deep Remineralization Processes In Methanogenic Continental Margin Sediments, David J. Burdige, Tomoko Komada

OES Faculty Publications

In many continental margin Sediments, a deep reaction zone exists which is separated from remineralization processes near the sediment surface. Here, methane diffuses upward to a depth where it is oxidized by downwardly diffusing sulfate. However, the methane sources that drive this anaerobic oxidation of methane (AOM) in the sulfate-methane transition zone (SMT) may vary among sites. In particular, these sources can be thought of as either (i) "internal" sources from in situ methanogenesis (regardless of where it occurs in the sediment column) that are ultimately coupled to organic matter deposition and burial, or (ii) "external" sources such as hydrocarbon …

Surface Production Fuels Deep Heterotrophic Respiration In Northern Peatlands, J. Elizabeth Corbett, David J. Burdige, Malak M. Tfaily, Angela R. Dial, William T. Cooper, Paul H. Glaser, Jeffrey P. Chanton

OES Faculty Publications

Multiple analyses of dissolved organic carbon (DOC) from pore waters were conducted to define the processes that govern carbon balance in peatlands: (1) source, reactivity, and transport of DOC with respect to vegetation, peat, and age of carbon substrate, (2) reactivity of DOC with respect to molecular size, and (3) lability to photoxidation of surficial DOC. We found that surface organic production fuels heterotrophic respiration at depth in advection-dominated peatlands, especially in fens. Fen DOC was Δ¹⁴Cenriched relative to the surrounding fen peat, and fen respiration products were similar to this enriched DOC indicating that DOC was the …

Improved Method For Quantifying The Air-Sea Flux Of Volatile And Semi-Volatile Organic Carbon, Emily Hauser, Rebecca M. Dickhut, Renee Falconer, Andrew S. Wozniak

Chemistry & Biochemistry Faculty Publications

A method for quantifying the diffusive air-sea exchange of gaseous organic carbon (OC) was developed. OC compounds were separated into two operational pools-those that were kinetically air limited in diffusion across the air-sea interface and those that were water limited-during simultaneous air/water sampling. The method separates OC compounds into low Henry's law constant (low-H) semivolatile OC (SOC) and high Henry's law constant (high-H) volatile OC (VOC) pools that can be categorized by relating diffusion kinetic parameters to Henry's Law constant. Air limited (low-H; H << similar to 0.1 L atm mol(^-1)) compounds were collected in pure water traps and were quantified as dissolved OC, whereas …

The Ross Sea: In A Sea Of Change, Walker O. Smith, Peter N. Sedwick, Kevin R. Arrigo, David G. Ainley, Alejandro H. Orsi

OES Faculty Publications

The Ross Sea, the most productive region in the Antarctic, reaches farther south than any body of water in the world. While its food web is relatively intact, its oceanography, biogeochemistry, and sea ice coverage have been changing dramatically, and likely will continue to do so in the future. Sea ice cover and persistence have been increasing, in contrast to the Amundsen-Bellingshausen sector, which has resulted in reduced open water duration for its biota. Models predict that as the ozone hole recovers, ice cover will begin to diminish. Currents on the continental shelf will likely change in the coming century, …

Spectroscopic Characterization Of Dissolved Organic Matter: Insights Into Composition, Photochemical Transformation And Carbon Cycling, John Robert Helms

Chemistry & Biochemistry Theses & Dissertations

This dissertation explores processes affecting the composition of dissolved organic matter (DOM) and how DOM composition changes in sunlit surface waters and in the dark interior ocean. Simulated solar irradiations were used to investigate the impact of photochemistry on terrestrial waters and deep ocean DOM. The photochemically mediated processes observed in Dismal Swamp samples included (i) light induced flocculation of up to 12% of the organic matter and 84% of the dissolved iron originally present; (ii) 74-88% mineralization of dissolved organic carbon (DOC) and 95-99% bleaching of chromophoric DOM (CDOM) during 110 days of irradiation; and (iii) nearly complete loss …

Evaluation Of Electrophoretic Protein Extraction And Database-Driven Protein Identification From Marine Sediments, Eli K. Moore, Brook L. Nunn, Jessica F. Faux, David R. Goodlett, H. Rodger Harvey

OES Faculty Publications

Intact proteins comprise a major component of organic carbon and nitrogen produced globally and are likely an important fraction of organic matter in sediments and soils. Extracting the protein component from sediments and soils for mass spectral characterization and identification represents a substantial challenge given the range of products and functionalities present in the complex matrix. Multiple forms of gel electrophoresis were evaluated as a means of enhancing recovery of sedimentary protein before proteomic characterization and compared with a direct enzymatic digestion of proteins in sediments. Resulting tryptic peptides were analyzed using shotgun proteomics and tandem mass spectra were evaluated …

Diversity, Distribution, And Expression Of Diazotroph Nifh Genes In Oxygen-Deficient Waters Of The Arabian Sea, Amal Jayakumar, Mamoon M. D. Al-Rshadat, Bess B. Ward, Margaret R. Mulholland

OES Faculty Publications

The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is responsible for up to half of the global mesopelagic fixed nitrogen ( N ) loss from the ocean via denitrification and anammox. Dinitrogen (N₂) fixation is usually attributed to cyanobacteria in the surface ocean. Model prediction and physiological inhibition of N₂ fixation by oxygen, however, suggest that N₂ fixation should be enhanced near the oxygen-deficient zone (ODZ) of the Arabian Sea. N₂ fixation and cyanobacterial nifH genes (the gene encoding dinitrogenase reductase) have been reported in surface waters overlying …

Controls On Dissolved Cobalt In Surface Waters Of The Sargasso Sea: Comparisons With Iron And Aluminum, R. U. Shelley, Peter N. Sedwick, T. S. Bibby, P. Cabedo-Sanz, T. M. Church, R. J. Johnson, A. I. Macey, C. M. Marsay, E. R. Sholkovitz, S. J. Ussher, P. J. Worsfold, M. C. Lohan

OES Faculty Publications

Dissolved cobalt (dCo), iron (dFe) and aluminum (dAl) were determined in water column samples along a meridional transect (∼31°N to 24°N) south of Bermuda in June 2008. A general north-to-south increase in surface concentrations of dFe (0.3-1.6 nM) and dAl (14-42 nM) was observed, suggesting that aerosol deposition is a significant source of dFe and dAl, whereas no clear trend was observed. for near-surface dCo concentrations. Shipboard aerosol samples indicate fractional solubility values of 8-100% for aerosol Co, which are significantly higher than corresponding estimates of the solubility of aerosol Fe (0.44-45%). Hydrographic observations and analysis of time series rain …

Effect Of Reworking And Bioturbation On Sedimentary Reactive Iron Within A Microtidal Estuary, Amy Kathleen Pitts

OES Theses and Dissertations

Mixed redox conditions in sediments due to physical reworking may allow for enhanced remineralization of refractory organic matter due to Fe(III) redox cycling. In part this may occur because easily reducible iron oxides can be used by heterotrophic bacteria to remineralize the organic carbon. This phenomena has been observed in bioturbated sediments and in areas where physical factors (such as strong bottom currents) constantly rework the sediments. To specifically determine the effects of physical reworking and bioturbation on concentrations of easily reducible iron oxides, reactive iron concentrations were measured in surface sediments taken from two contrasting sites in the York …

Model-Based Analyses Of Nitrogen On The Mid-Atlantic Bight Continental Shelf, Tian Tian

OES Theses and Dissertations

The biogeochemistry of continental shelf systems is an important, but poorly quantified, component of the global cycling of nitrogen and carbon. In this study, simulations obtained from a one-dimensional (ID) biogeochemical model developed for the Middle Atlantic Bight (MAB) portion of the U.S. eastern continental shelf were analyzed to investigate nitrogen cycling processes. The 1D model included lower trophic level interactions and was forced by advective fields obtained from a corresponding three-dimensional biogeochemical model. Taylor and target diagram analyses, which compared in situ measurements of sea surface temperature (SST) and salinity, and chlorophyll and particulate organic carbon (POC) derived from …

Early Season Depletion Of Dissolved Iron In The Ross Sea Polynya: Implications For Iron Dynamics On The Antarctic Continental Shelf, Peter N. Sedwick, C. M. Marsay, B. M. Sohst, A. M. Aguilar-Islas, M. C. Lohan, M. C. Long, K. R. Arrigo, R. B. Dunbar, M. A. Saito, W. O. Smith, G. R. Ditullio

OES Faculty Publications

The Ross Sea polynya is among the most productive regions in the Southern Ocean and may constitute a significant oceanic CO₂sink. Based on results from several field studies, this region has been considered seasonally iron limited, whereby a "winter reserve" of dissolved iron (dFe) is progressively depleted during the growing season to low concentrations (~ 0.1 nM) that limit phytoplankton growth in the austral summer (December-February). Here we report new iron data for the Ross Sea polynya during austral summer 2005-2006 (27 December-22 January) and the following austral spring 2006 (16 November-3 December). The summer 2005-2006 data show …

Fossil And Contemporary Aerosol Particulate Organic Carbon In The Eastern United States: Implications For Deposition And Inputs To Watersheds, Andrew S. Wozniak, James E. Bauer, Rebecca M. Dickhut

Chemistry & Biochemistry Faculty Publications

Atmospheric particulate matter samples were collected from mid-Atlantic and northeastern U. S. (Virginia and New York, respectively) sites to assess the fossil versus contemporary sources contributing to aerosol organic carbon (OC) and the implications for its deposition to watersheds. Mean particulate matter total OC (TOC) deposition rates (wet + dry deposition) were calculated to be 1.6 and 2.4 mg C m^-2 d^-1 for the Virginia and New York sites, respectively. Wet deposition of particulate TOC was determined to be the dominant depositional mode, accounting for >65% (Virginia) and >80% (New York) of total aerosol TOC deposition. Isotopic mass …

Phosphorus Uptake In Rhodomonas Salina (Wislouch) And Its Effect On Allocation And Elimination In Acartia Tonsa (Dana), Danna Palladino

OES Theses and Dissertations

Phosphorus is a key element in important biochemical compounds, such as RNA and phospholipids, and can become limiting in a variety of marine systems. The uptake of phosphorus into biochemical fractions (protein, low molecular weight (LMW) compounds, lipid, polysaccharide and nucleic acid) in Acartia tonsa fed ³³P -labeled Rhodomonas salina was examined. R. salina was cultured on two variations of one media that in one case contained phosphorus in balance and the other out of balance with relation to other standard f/2 components. The P-balanced (PB) media had a N:P ratio of 24.5, which is higher than that found …

Trace Metal Biogeochemistry In The Western North Pacific, Peter Lynn Morton

OES Theses and Dissertations

Dissolved and suspended particulate samples collected during the Intergovernmental Oceanographic Commission 2002 Contaminant Baseline Survey were analyzed to determine the surface and vertical distributions of Cd, Co, Cu, Mn, Ni Zn and Pb across the western and central North Pacific.

Dissolved trace metal concentrations were measured using a novel isotope dilution-ICP-MS method after extraction over an 8-hydroxyquinoline resin column and validated through the use of the SAFe consensus reference materials. Filtered suspended particulate matter was digested using an HCl/HNO3/HF mixture and analyzed by HR-ICP-MS to obtain total particulate concentrations for the trace metals and other tracer elements such as Al, …

Concentrations, Distributions And Chemical Speciation Of Zinc And Cadmium In The Equatorial And South Atlantic Ocean, Gonzalo G. Carrasco

OES Theses and Dissertations

Certain trace metals are important cofactors in enzymatic systems and are thus, essential for life in the world's oceans. Two of these metals, Zn and Cd, are required by phytoplankton for enzymes that facilitate carbon uptake (Morel and Price, 2003). In seawater the total dissolved concentration of a metal (MTD) is distributed among different chemical species and this chemical speciation dictates a metal's bioavailability. Strong organic metal-binding ligands greatly affect the metal's chemical speciation in the ocean, potentially limiting phytoplankton growth by reducing the concentrations of Zn²⁺ and Cd²⁺, the bioavailable forms of these two metals to …

On The Fractional Solubillity Of Copper In Marine Aerosols: Toxicity Of Aeolian Copper Revisited, Edward R. Sholkovitz, Peter N. Sedwick, Thomas M. Church

OES Faculty Publications

Paytan et al. (2009) argue that the atmospheric deposition of aerosols lead to copper concentrations that are potentially toxic to marine phytoplankton in a large area of tropical and subtropical North Atlantic Ocean. A key assumption in their model is that all marine aerosols (mineral dust and anthropogenic particles) have a high (40%) fractional solubility of copper. Our data show that the fractional solubility of copper for Saharan dust over the Sargasso Sea and Bermuda is significantly lower (1-7%). In contrast, anthropogenic aerosols with non-Saharan sources have significantly higher values (10-100%). Hence, the potential Cu toxicity in the tropical and …

The Widespread Occurrence Of Coupled Carbonate Dissolution/Reprecipitation In Surface Sediments On The Bahamas Bank, David J. Burdige, Xinping Hu, Richard C. Zimmerman

OES Faculty Publications

Using two complimentary approaches (pore water advection/diffusion/reaction modeling and stable isotope mass balance calculations) we show that carbonate dissolution/reprecipitation occurs on early diagenetic time scales across a broad range of sediments on the Great Bahamas Bank. The input of oxygen into the sediments, which strongly controls sediment carbonate dissolution, has two major sources belowground input by seagrasses (that is, seagrass O₂ pumping), and permeability-driven advective pore water exchange. The relative importance of these O₂ delivery mechanisms depends on both seagrass density, and on how bottom water flow interacts with the seagrass canopy and leads to this advective exchange. …