DOC cycling in a temperate estuary: A mass balance approach using natural ¹⁴C and ¹³C isotopes

Abstract

We measured dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and their corresponding ⌬ 14 C and ␦ 13C values in order to study the sources and fates of DOC in the York River Estuary (Virginia, U.S.A.). The ⌬ 14 C and ␦ 13C values of DOC and DIC at the freshwater end-member indicate that during periods of moderate to high flow, riverine DOC entering the York was composed of decadal-aged terrestrially organic matter. In nearly all cases, DOC concentrations exceeded conservative mixing lines and were therefore indicative of a net DOC input flux from within the estuary that averaged 1.. The nonconservative behavior of DOC in the York River Estuary was also apparent in carbon isotopic mixing curves and the application of an isotopic mixing model. The model predicted that 20-38% of the DOC at the mouth of the estuary was of riverine (terrestrial ϩ freshwater) origin, while 38-56% was added internally, depending on the isotopic values assigned to the internally added DOC. Measurements of ⌬ 14 C and ␦ 13 C of DOC and DIC and marsh organic matter suggest that the internal sources originated from estuarine phytoplankton and marshes. The isotopic mixing model also indicates a significant concomitant loss (27-45%) of riverine DOC within the estuary.Changes in DOC concentration, ⌬ 14 C-DOC, and ␦ 13 C-DOC were also measured during incubation experiments designed to quantify the amounts, sources, and ages of DOC supporting the carbon demands of estuarine bacteria. Results of these experiments were consistent with an estuarine source of phytoplankton and marsh DOC and the preferential utilization of young ( 14 C-enriched) DOC in the low-salinity reaches of the York. However, the average removal of riverine DOC by bacteria accounts for only ϳ4-19% of the riverine pool; therefore, other significant sinks for DOC exist within the estuary.

Modeling the seasonal autochthonous sources of dissolved organic carbon and nitrogen in the upper Chesapeake Bay

“…Most of the variability in the modeled DOM concentrations was due to changes in the semi-labile pools of DOC and DON. While we do not have measurements of seasonal bioavailability at this station for comparison, this result is consistent with observations from various systems around that world, including Chesapeake Bay, which show that DOC, and sometimes DON, concentrations vary seasonally as a result of changes in the labile and/or semi-labile DOM pool (Bronk, 2002;Bronk et al, 1998;Carlson, 2002;Cauwet, 2002;Raymond and Bauer, 2001). The empirical light model that we used was able to reproduce the observed pattern of K d values reasonably well (Fig.…”

“…Modeled bacterial production fluctuated throughout the year between 0.4 and 0.7 mol C d −1 (average 0.53 mol C d −1 ) which compares well with the 0.5 mol C d −1 bacterial production measurements made by Raymond and Bauer (2001) during DOC consumption experiments in the nearby York River estuary, a subestuary of the Chesapeake Bay. The CBP does not collect bacterial production data so we were unable to make any direct comparisons to station data.…”

“…In many mid-Atlantic estuaries, the biochemical composition and isotopic signature of DOM changes across the salinity gradient with biomarkers and carbon isotopes indicating that a significant amount of DOM production and modification occurs within the estuary (Loh et al, 2006;Mannino and Harvey, 2000;Raymond and Bauer, 2001). In the largest of these estuaries, Chesapeake Bay, DOC exhibits a non-conservative distribution at certain times of the year, suggesting that seasonal autochthonous DOM production can be significant (Fisher et al, 1998;Rochelle-Newall and Fisher, 2002).…”

Characterization of Protein-like Fluorophores Released from Lake Phytoplankton on the Basis of Fractionation and Electrophoresis

“…A mixture of a 16-μl concentrated sample solution and 4 μl of a loading buffer, consisting of 62.5 mM Tris-HCl (pH 6.8), 25% glycerol, 2% SDS, 5% 2-mercaptoethanol and 0.01% bromophenol blue, was heated at 80 C for 5 min. Fifteen microliters of each sample solution and 7 μl of a molecular weight marker (Bio-Rad, Precision Plus Protein standards) as standards (10,15,20,25,37,50,75, 100, 150, and 250 kDa) were loaded onto each lane of a 1-mm gel in 15% acrylamide gel (Atto PAGEL) and subjected to 300 V (20 mA) for 75 min. Subsequently, the gel was stained using the Coomassie Brilliant Blue (CBB).…”

“…[5][6][7][8][9] The natural carbon stable isotope ratio of DOC has been used to identify DOC sources in aquatic systems. 10,11 This approach was used to examine relative contributions of autochthonous and allochthonous sources to dissolved organic matter (DOM) in Lake Biwa. 12,13 In our previous study, DOM in Lake Biwa and rivers was investigated so as to elucidate its origins and behavior.…”