Article pubs.acs.org/est
Fluorescence Tracking of Dissolved and Particulate Organic Matter Quality in a River-Dominated Estuary Christopher L. Osburn,*,† Lauren T. Handsel,∥ Molly P. Mikan,† Hans W. Paerl,‡ and Michael T. Montgomery§ †
Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina, United States Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, North Carolina, United States ‡ Institute of Marine Sciences, University of North Carolina, Morehead City, North Carolina, United States § Chemistry Division, U.S. Naval Research Laboratory, Washington, DC, United States ∥
S Supporting Information *
ABSTRACT: Excitation−emission matrix (EEM) fluorescence was combined with parallel factor analysis (PARAFAC) to model base-extracted particulate (POM) and dissolved (DOM) organic matter quality in the Neuse River Estuary (NRE), North Carolina, before and after passage of Hurricane Irene in August 2011. Principle components analysis was used to determine that four of the PARAFAC components (C1−C3 and C6) were terrestrial sources to the NRE. One component (C4), prevalent in DOM of nutrient-impacted streams and estuaries and produced in phytoplankton cultures, was enriched in the POM and in surface sediment pore water DOM. One component (C5) was related to recent autochthonous production. Photoexposure of unfiltered Neuse River water caused an increase in slope ratio values (SR) which corresponded to an increase in the ratio C2:C3 for DOM, and the production of C4 fluorescence in both POM and DOM. Changes to the relative abundance of C4 in POM and DOM indicated that advection of pore water DOM from surface sediments into overlying waters could increase the autochthonous quality of DOM in shallow microtidal estuaries. Modeling POM and DOM simultaneously with PARAFAC is an informative technique that is applicable to assessments of estuarine water quality.
■
INTRODUCTION Coastal rivers and estuaries transport and transform dissolved and particulate materials from terrestrial to marine environments. Recent work indicates that organic matter (OM) cycling in rivers and estuaries responds to regional changes in land use and climate.1,2 The characteristics of dissolved (DOM) and particulate (POM) organic matter are often quite different from one another in river-dominated estuary systems. The distinction between these OM pools typically is by filtration. Often, fresher material dominates the POM pool and more degraded, recalcitrant material comprises the DOM pool, with both pools containing large fractions of humic substances.3,4 Transfer of OM between these pools occurs in the water column and sediments, and possibly through photoexposure of particles to sunlight, but these process are not well-known in estuaries.5−9 Simultaneous study of both POM and DOM composition in estuaries is rare in the literature but can reveal very dynamic biogeochemical cycling of terrestrial and autochthonous sources within and between these carbon pools.10 Fluorescence spectroscopy has been used to characterize DOM and the base-extractable organic matter (BEOM) from soil/sedimentary organic matter pools from most aquatic environments.11−14 By measuring and concatenating the © 2012 American Chemical Society
emission spectra of OM at several excitation wavelengths, an excitation−emission matrix (EEM) of fluorescence can be created, which contains qualitative information about the OM in a sample. Early comparisons of extracted humic substances to whole river, estuary, and seawater revealed several EEM peaks, originally designated A and C (terrestrial humic), T (protein resembling the amino acid tryptophan), and M (microbial humic).12 In the past 10 years, EEMs have been combined with parallel factor analysis (PARAFAC) to characterize OM sources.15,16 This methodology provides a statistical means to analyze the spectral components changing in a sample and use this information to understand the sources and transformations of OM in many environments. These extraction and fluorescence techniques have not been used to examine water column POM within a river−estuary continuum, even though POM likely contains fluorescent components.17 Moreover, POM photodissolution produces chromophoric DOM (CDOM), indicating that fluorescent molecules in the DOM pool partly have their origin in the Received: Revised: Accepted: Published: 8628
February 25, 2012 July 16, 2012 July 17, 2012 July 17, 2012 dx.doi.org/10.1021/es3007723 | Environ. Sci. Technol. 2012, 46, 8628−8636
Environmental Science & Technology
Article
Figure 1. Comparison of changes to POM (top) and DOM (bottom) EEM fluorescence (QSU) in the river (NR0) and estuary (NR180) stations in the NRE on Aug 24, 2012. Major peak regions of EEMs are indicated and identified below the figure.12
POM pool.8,9 River POM tends to be enriched in labile substrates such as amino acids relative to DOM, a signal which is maintained during mixing in estuaries as lingering terrestrial signals are diluted and commingle with signals from in situ primary production.6,18,19 Thus, compositional differences between POM and DOM could be assayed by fluorescence, and this information should be useful to understand phase transfer between POM and DOM pools as well as estuarine water quality.20,21 The aim of this study was to examine POM and DOM quality in the Neuse River Estuary (NRE), a river-dominated estuarine system located in the coastal plain of North Carolina, USA, through a combined PARAFAC model. As a proof of concept for using the combined POM−DOM PARAFAC model, we determined the change to OM quality in the NRE after passage of a major tropical cyclone, Hurricane Irene (August, 2011), which was expected to increase the loading of terrestrial OM throughout the estuary. The POM−DOM PARAFAC model demonstrated this overall change in OM quality in the NRE. Also, the model provided insight into the watershed contribution of organic nitrogen to the NRE, showed the relative importance of phase transfer of organic
matter from POM to DOM pools in the water column and sediments, and indicated OM degradation by sunlight exposure.
■
MATERIALS AND METHODS Surface water samples (N = 94) from the Neuse River Basin (NRB, N = 39) and the NRE (N = 55) were collected during August 24−September 17, 2011, before and after passage of Hurricane Irene on August 27, 2011, which made landfall at Cape Lookout, NC, and tracked northerly across Pamlico Sound. Between 17 and 30 cm of total rainfall fell across the basin, with higher amounts localized on the upper part of the NRE near New Bern, NC.22 Only results from the estuary proper before and after passage of Hurricane Irene are reported in this study (Supporting Information Table S1), though all 188 samples were used in fluorescence modeling. Samples (no replicates) were collected midchannel in the estuary as part of the Neuse River Estuary Modeling and Monitoring Program, ModMon (Figure S1), and included measurements of temperature, salinity, turbidity, and in situ chlorophyll a (Chl a) fluorescence, using a calibrated Yellow Springs Instrument 6600 multiparameter sonde. In situ Chl a fluorescence was normalized to actual Chl a concentrations by calibration against a certified Chl a standard (DHI, Hørsholm, Den8629
dx.doi.org/10.1021/es3007723 | Environ. Sci. Technol. 2012, 46, 8628−8636
Environmental Science & Technology
Article
and DOM fluorescence properties of Neuse River water, and (2) the application of the POM−DOM PARAFAC approach to modeling EEMs in separate data sets. Details of the model application are provided in the Supporting Information.
mark).23,24 POM was collected separately on 2 0.7-μm porosity, precombusted glass fiber filters (25-mm diameter; Whatman GF/F), and the filtrate was used for DOM. On one filter, particulate organic carbon (POC) and particulate nitrogen (PN) were measured using a Perkin-Elmer 2400 Series II elemental analyzer, after vapor acidification to remove carbonates. On a separate filter, POM fluorescent material was extracted into 10 mL of 0.1 N NaOH for 24 h in the dark at 4 °C following a procedure for estuarine soil and sediment.14 After neutralization with HCl of the BEOM to a pH near that of the original sample (which always was >6 thus avoiding precipitation of humic acids), the resultant solution was filtered through 0.2-μm polyethersulfone filters (Millipore Sterivex), and then absorbance and fluorescence were measured. In May 2012, pore water DOM from surface sediment (0−5 cm) at station (Sta.) NR70 was isolated by centrifugation (3000 rpm, 10 min) and filtered to 0.7 μm. BEOM was extracted from the remaining wet sediment using methods described for POM. Dissolved organic carbon (DOC) was quantified by wet chemical oxidation on 2-mL sample volumes.25 Dissolved organic nitrogen (DON) was calculated as the difference between total dissolved N (TDN) and total dissolved inorganic N (DIN: sum of nitrite, nitrate, and ammonium) measured on a Lachat Quick-chem 8000 auto analyzer (Lachat, Milwaukee, WI, USA). Absorbance spectra of DOM and POM solutions were measured from 200 to 800 nm on a Varian Cary 300UV spectrophotometer in 1-cm quartz cells against air. Milli-Q water or the 0.1 N NaOH extractant solution for POM (and sediment BEOM) were measured separately and used as a blanks. Blank-corrected absorbance values were converted to Napierian absorption coefficients, a(λ), in units of m−1, and the amount of CDOM was quantified at 250 (POM: a250p; DOM: a250d). Slope ratios of POM and DOM absorption (SRp, SRd, respectively) were calculated to assess molecular weight changes of OM.26 Fluorescence spectra were acquired on a Varian Eclipse spectrofluorometer. Excitation (ex) wavelengths were sampled from 240 to 450 at 5-nm intervals; emission (em) wavelengths were sampled every 2 nm from 300 to 600 nm. For POM samples, corrections were made for volume of water filtered, volume of extractant used, and for any dilution that was performed (see Supporting Information). For DOM, corrections were made for dilution if necessary. Appropriate instrument excitation and emission corrections were applied and all fluorescence measurements were corrected for innerfiltering effects before being calibrated against the Raman signal of the instrument and standardized in quinine sulfate units (ppb QSU).27 A total of 188 EEMs (NRB = 78, NRE = 110, each equally split between POM and DOM samples collected before and after the storm) were modeled with PARAFAC using the DOMFluor toolbox for Matlab.27 Due to the wide range in fluorescence intensity of POM and DOM samples, all EEMs were normalized to their individual maxima prior to PARAFAC modeling.28 Photoexposure Experiment. Triplicate samples of unfiltered Neuse River water (Sta. NR0 from March 2012) that was not used in the POM−DOM PARAFAC modeling were placed in quartz bottles and exposed to solar radiation for 3 days.29 The bottles were shaken periodically to resuspend particles. Afterward, the samples were filtered and processed as described above. The POM−DOM PARAFAC model was then applied to these data to test (1) the effect of sunlight on POM
■
RESULTS AND DISCUSSION EEM Characteristics of POM and DOM. Sample EEM plots for stations NR0 (river) and NR180 (estuary) prior to Hurricane Irene exhibited characteristics common to many coastal rivers and estuaries (Figure 1). Overall, the fluorescence intensity in the neutralized base-extracts of POM was much less than the corresponding DOM after correction for the volume of water filtered. Similarly, OC concentrations of the extracted POM were 7−24% of bulk POC concentrations (Table S1). Before passage of Hurricane Irene, the mean (±1σ) a250 of the POM solutions (a250p in Table S1), when corrected for volume filtered, was 0.96 ± 0.81 m−1, roughly 40 times less than the mean a250d (for DOM), 42.38 ± 14.42 m−1. Nine days after the storm, a250d was 205.75 ± 13.46 m−1, about 9 times greater than a250p (22.93 ± 1.28 m−1) in the regions of the NRE that were freshwater (Stas. NR0-NR50, unpaired t test, Welchcorrected: p < 0.001, N = 3, df = 4). Previously identified peak regions of EEM fluorescence are indicated in Figure 1 which includes their probable origins.12,13 The DOM EEMs show a common feature in which the major fluorescence in the peak A region elongates into a plateau situated between the peaks M and C. The DOM EEMs exhibited little peak T fluorescence, which generally is assigned to tryptophan fluorescence, and indicative of recent biological activity. Other than intensity, little change was apparent in the DOM between the freshwater head of the NRE (Sta. NR0, salinity = 0.08) and its more saline mouth at the entrance of Pamlico Sound (Sta. NR180, salinity = 25.63; Figure 1). By contrast, the POM EEMs showed similar intensities of fluorescence between Stas. NR0 and NR180, yet slightly different patterns. Similar to DOM, the POM EEM for Sta. NR0 showed an elongation of the peak A to peak C region and weak peak T fluorescence. However, this elongation was lacking in the POM EEM from Sta. 180, which instead showed three discrete peaks nearly centered on the ranges shown for peaks A, C, and T (Figure 1). Relative to NR0 POM, the fluorescence intensity of NR180 POM was greater at each of these peak regions. The EEM pattern exhibited by the POM at NR180 strongly resembled the distinct peaks in EEMs produced in DOM from phytoplankton cultures subjected to microbial degradation.30 EEM peak ratios in the POM (T:A, M:C) tracked closely with Chl a concentrations in the NRE (Figure S3). The mean carbon to nitrogen (C:N) ratio of POM in the NRE before Irene, 7.6 ± 0.3, was not significantly different after Irene, 7.7 ± 0.3 (paired t test on log transformed values: p = 0.584, N = 11, df = 10; Table S1). POM carbon stable isotope values (δ13C) in the NRE increase with salinity, and have indicated a shift from allochthonous to autochthonous sources of OM in this poorly flushed estuary.31 Together, these results suggest a distinct change in POM quality from the riverine to saline portion of the NRE and not just the dilution of a riverine signal. Results from widely different systems (culture work in seawater, wastewater) have showed the production of fluorescent molecules (amino sugars, exopolymeric substances) as components of cellular material from aquatic microbes.30,32,33 Our results agree with the observation from those studies that autochthonous OM can exhibit peaks (A and C) that have 8630
dx.doi.org/10.1021/es3007723 | Environ. Sci. Technol. 2012, 46, 8628−8636
Environmental Science & Technology
Article
Table 1. Comparison of Excitation/Emission Peak Maxima in POM and DOM Fluorescence of NRE Surface Waters along with Peak Maxima from Combined POM−DOM PARAFAC Model Componentsa component
λex (nm)
λem (nm)
%Fmax range (POM)
%Fmax range (DOM)
C1 C2 C3
