A study of dissolved organic (DOC) and inorganic carbon (DIC) sources and sinks were undertaken in the York River estuary, Virginia using measurements of DOC, DIC, and CO2 along with isotopic (13C and 14C) measurements. Experimental incubations on DOC were also conducted in order to determine the controls and importance of DOC utilization by bacteria. Finally, 14C measurements were made on organic matter exiting 3 other rivers draining into the North Atlantic Ocean in order to estimate the age of organic matter transported to the North Atlantic Ocean from the continents.
Direct measurements of the partial pressure of CO2 (pCO2), and dissolved inorganic carbon (DIC) were made over a two-year period in surface waters of the York River estuary. The pCO2 in surface waters exceeded that in the overlying atmosphere indicating that the estuary was a net source of CO2 to the atmosphere at most times and locations. Salinity-based DIC mixing curves indicate there was also an internal source of both DIC and alkalinity implying net alkalinity generation within the estuary. The DIC and alkalinity source displayed seasonal patterns similar to that of pCO2 and was reproducible over a two-year study period.
Bacterial utilization of natural levels of dissolved organic carbon (DOC) was measured in the York River estuary. This study was undertaken in order to elucidate spatial and temporal changes in bacterial carbon utilization and to evaluate its importance as a pathway for organic matter transformation in estuaries. The data indicate that although the total amount of DOC utilized was low, there was a continual supply of both the G1 and G2 fractions within the estuary. Bacterial growth efficiency on the G1 pool averaged 28%. The production of CO2 through bacterial respiration of the G1 pool could not balance CO2 evasion for the majority of the estuary.
Dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and their corresponding 14C and 13C values were measured in order to study the sources and fates of DOC in the York River estuary. The 14C 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 terrestrial 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 1.2 micro-M/L/d. The non-conservative 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 + freshwater) origin, while 38-56% was added internally, depending on the isotopic values assigned to the internally added DOC. Measurements of 14C and 13C 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. However, the average removal of riverine DOC by bacteria accounts for only ~4-19% of the riverine pool and therefore other significant sinks for DOC exist within the estuary.
Measurements of natural 14C and 13C data from a number of rivers indicate that they are variable sources of both old (14C-depleted) and young (14C-enriched) terrestrial DOC, and of very old terrestrial POC to the North Atlantic Ocean. These findings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers in general might be enriched in bomb 14C, and hence newly produced. I also find that much of the young DOC can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, “pre-aging” and degradation may alter significantly the structure, distributions and quantities of terrestrial organic matter prior to its delivery to the oceans.