The interconnected estuarine complex of the Altamaha River and adjacent sounds located in Georgia (USA) functions as a hotspot for organic matter transformation as it is transported to the Atlantic Ocean. Here, we investigated how dissolved organic matter (DOM) composition changes both spatially and seasonally along the estuary and how it influences bacterial processing. Surface samples were collected during high tide at fifteen stations throughout the estuary in April, July, October 2017, and January 2018. Bulk, optical, and molecular analyses were conducted on samples before and after dark incubations to assess DOM sources and transformation patterns in the system. The dominant driver of change in DOM composition was found to be the terrigenous‐marine gradient in organic matter sources. Six distinct clusters were identified based on the terrigenous signature of the DOM pool, explaining 45% of the variance in DOM composition in the system. Bacterial consumption of dissolved organic carbon (DOC) was strongly influenced by DOM composition, with increased degradation rates for DOM with a larger terrigenous character. However, changes in optical properties suggested that less aromatic DOM that co‐varied with the terrigenous material was preferentially degraded. The passage of Hurricane Irma in September 2017 resulted in a 27% ± 7% increase in DOC content, likely due to inundation associated with storm surge and increased local precipitation, and DOC biodegradation was 17% ± 8% higher than during summer. These effects lasted for at least one month after the storm, revealing that hurricanes can have a large impact on DOM composition and cycling in coastal systems.
Dissolved organic matter (DOM) is a large and complex mixture of compounds with source inputs that differ with location, season, and environmental conditions. Here, we investigated drivers of DOM composition changes in a marsh‐dominated estuary off the southeastern United States. Monthly water samples were collected at a riverine and estuarine site from September 2015 to September 2016, and bulk, optical, and molecular analyses were conducted on samples before and after dark incubations. Results showed that river discharge was the primary driver changing the DOM composition at the mouth of the Altamaha River. For discharge higher than ~150 m3/s, dissolved organic carbon (DOC) concentrations and the terrigenous character of the DOM increased approximately linearly with river flow. For low discharge conditions, a clear signature of salt marsh‐derived compounds was observed in the river. At the head of Sapelo Sound, changes in DOM composition were primarily driven by river discharge and possibly by summer algae blooms. Microbial consumption of DOC was larger during periods of high discharge at both sites, potentially due to the higher mobilization and influx of fresh material to the system. The Georgia coast was hit by Hurricane Matthew in October 2016, which resulted in a large input of carbon to the estuary. The DOC concentration was ~2 times higher and DOM composition was more aromatic with a stronger terrigenous signature compared to the seasonal maximum observed earlier in the year during peak river discharge conditions. This suggests that extreme events notably impact DOM quantity and quality in estuarine regions.
more » « less- NSF-PAR ID:
- 10459626
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 124
- Issue:
- 6
- ISSN:
- 2169-8953
- Page Range / eLocation ID:
- p. 1545-1559
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
Extreme events such as hurricanes and tropical storms often result in large fluxes of dissolved organic carbon (DOC) to estuaries. Precipitation associated with tropical storms may be increasing in the southeastern U.S., which can potentially impact dissolved organic matter (DOM) dynamics and cycling in coastal systems. Here, DOM composition at the Altamaha River and Estuary (Georgia, U.S.A.) was investigated over multiple years capturing seasonal variations in river discharge, high precipitation events, and the passage of two hurricanes which resulted in substantial storm surges. Optical measurements of DOM indicate that the terrigenous signature in the estuary is linearly related to freshwater content and is similar after extreme events with or without a storm surge and during peak river flow. Molecular level analysis revealed significant differences, however, with a large increase of highly aromatic compounds after extreme events exceeding what would be expected by freshwater content alone. Although extreme events are often followed by increased DOC biodegradation, the terrigenous material added during those events does not appear to be more labile than the remainder of the DOM pool that was captured by ultrahigh-resolution mass spectrometry analysis. This suggests that the added terrigenous organic matter may be exported to the coastal ocean, while a fraction of the organic matter that co-varied with the terrigenous DOM may contribute to the increased biomineralization in the estuary, with implications to carbon processing in coastal areas.more » « less
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Uncovering which biogeochemical processes have a critical role controlling dissolved organic matter (DOM) compositional changes in complex estuarine environments remains a challenge. In this context, the aim of this study is to characterize the dominant patterns of variability modifying the DOM composition in an estuary off the Southeastern U.S. We collected water samples during three seasons (July and October 2014 and April 2015) at both high and low tides and conducted short- (1 day) and long-term (60 days) dark incubations. Samples were analyzed for bulk DOC concentration, and optical (CDOM) and molecular (FT-ICR MS) compositions and bacterial cells were collected for metatranscriptomics. Results show that the dominant pattern of variability in DOM composition occurs at seasonal scales, likely associated with the seasonality of river discharge. After seasonal variations, long-term biodegradation was found to be comparatively more important in the fall, while tidal variability was the second most important factor correlated to DOM composition in spring, when the freshwater content in the estuary was high. Over shorter time scales, however, the influence of microbial processing was small. Microbial data revealed a similar pattern, with variability in gene expression occurring primarily at the seasonal scale and tidal influence being of secondary importance. Our analyses suggest that future changes in the seasonal delivery of freshwater to this system have the potential to significantly impact DOM composition. Changes in residence time may also be important, helping control the relative contribution of tides and long-term biodegradation to DOM compositional changes in the estuary.more » « less
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Streams in the southeastern United States Coastal Plains serve as an essential source of energy and nutrients for important estuarine ecosystems, and dissolved organic matter (DOM) exported from these streams can have profound impacts on the biogeochemical and ecological functions of fluvial networks. Here, we examined hydrological and temperature controls of DOM during low-flow periods from a forested stream located within the Coastal Plain physiographic region of Alabama, USA. We analyzed DOM via combining dissolved organic carbon (DOC) analysis, fluorescence excitation–emission matrix combined with parallel factor analysis (EEM-PARAFAC), and microbial degradation experiments. Four fluorescence components were identified: terrestrial humic-like DOM, microbial humic-like DOM, tyrosine-like DOM, and tryptophan-like DOM. Humic-like DOM accounted for ~70% of total fluorescence, and biodegradation experiments showed that it was less bioreactive than protein-like DOM that accounted for ~30% of total fluorescence. This observation indicates fluorescent DOM (FDOM) was controlled primarily by soil inputs and not substantially influenced by instream production and processing, suggesting that the bulk of FDOM in these streams is transported to downstream environments with limited in situ modification. Linear regression and redundancy analysis models identified that the seasonal variations in DOM were dictated primarily by hydrology and temperature. Overall, high discharge and shallow flow paths led to the enrichment of less-degraded DOM with higher percentages of microbial humic-like and tyrosine-like compounds, whereas high temperatures favored the accumulation of high-aromaticity, high-molecular-weight, terrestrial, humic-like compounds in stream water. The flux of DOC and four fluorescence components was driven primarily by water discharge. Thus, the instantaneous exports of both refractory humic-like DOM and reactive protein-like DOM were higher in wetter seasons (winter and spring). As high temperatures and severe precipitation are projected to become more prominent in the southeastern U.S. due to climate change, our findings have important implications for future changes in the amount, source, and composition of DOM in Coastal Plain streams and the associated impacts on downstream carbon and nutrient supplies and water quality.more » « less
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Abstract The capacity for coastal river networks to transport and transform dissolved organic matter (DOM) is widely accepted. However, climate‐induced shifts in stormwater runoff and tidal extension alter fresh and marine water source contributions, associated DOM, and processing rates of nutrients entering coastal canals. We investigate how time‐variable interactions among coastal water source contributions influence the concentrations of dissolved organic carbon (DOC), nutrients, and DOM composition in urban canals. We quantified the spatiotemporal variability of DOM quality and nutrient concentrations to determine contributions of tidal marine water, rainwater, stormwater runoff, and groundwater to three coastal urban canals of Miami, Florida (USA). We created a Bayesian Monte Carlo mixing model using measurements of fluorescent DOM (fDOM), DOC concentrations, δ18O and δ2H isotopic signatures, and chloride (Cl−). Fractional contributions of groundwater averaged 17% in the dry season and 26% at peak high tide during the subtropical wet season (September–November). The canal‐to‐marine head difference (CMHD) was a primary driver of groundwater contributions to coastal urban canals and monthly patterns of fDOM/DOC. High tide (>1 m) and discharge events were found to connect canals to upstream sources of terrestrial DOM. Loading of terrestrially sourced DOC and DOM is pulsed to urban canals, shunted downstream and supplemented by microbially sourced DOM during the wet season at high tide. Overall, we demonstrate that a combined tracer approach with isotopes and fDOM can help identify groundwater contributions to coastal waterways and that autochthonous fDOM may prime the degradation of carbon or nutrients as the CMHD pushes inland.
