US Long-Term Ecological Research Network

Long-term trends and synchrony in dissolved organic matter characteristics in Wisconsin, USA lakes: quality, not quantity, is highly sensitive to climate

Abstract
Dissolved organic matter (DOM) is a fundamental driver of many lake processes. In the past several decades, many lakes have exhibited a substantial increase in DOM quantity, measured as dissolved organic carbon (DOC) concentration. While increasing DOC is now widely recognized, fewer studies have sought to understand how characteristics of DOM (DOM quality) change over time. Quality can be measured in several ways, including the optical characteristics spectral slope (S275-295), spectral ratio (SR), absorbance at 254 nm (a254), and DOC-specific absorbance (SUVA; a254:DOC). However, long-term measurements of quality are not nearly as common as long-term measurements of DOC concentration. We used 24 years of DOC and absorbance data for seven lakes in the North Temperate Lakes Long Term Ecological Research site in northern Wisconsin, USA to examine temporal trends and synchrony in both DOC concentration and quality. We predicted lower SR and S275-295 and higher a254 and SUVA trends, consistent with increasing DOC and greater allochthony. DOC concentration exhibited both significant positive and negative trends among lakes. In contrast, DOC quality exhibited trends suggesting reduced allochthony or increased degradation, with significant long-term increases in SR in three lakes. Patterns and synchrony of DOM quality parameters suggest they are more responsive to climatic variations than DOC concentration. SUVA in particular tended to increase with greater moisture and decrease with drier conditions. These results demonstrate that DOC quantity and quality can exhibit different complex long-term trends and responses to climate components, with important implications for aquatic ecosystems.
Contact
Core Areas
Dataset ID
329
Date Range
-
Methods
Data contained within is derived from data publicly available through the North Temperature Lakes Long Term Ecological Research (NTL-LTER) website at the following url: https://lter.limnology.wisc.edu/. All data is for the upper meter of the water column, collected in the deepest point of the lake. For DOC concentration data and ancillary data such as pH, iron, and total N, please see the NTL-LTER website.
The file dat.w.blank.fin 7_29_16.csv contains the absorbance scans that were used to calculate the spectral metrics (NTL LTER 2012). Samples for absorbance scans were collected approximately quarterly. Absorbance scans were run on a spectrophotometer over the wavelengths from 200-800 nm. In this file, the column value is the raw value for absorbance read directly off the instrument for the corresponding wavelength. The column blank.value is the value of the DI blank taken nearest in time for the corresponding wavelength and cuvette width. The column cor.value is the blank.value column subtracted from the value column. The cor.value column was used in all subsequent analyses and calculations.
The file metrics for_analysis.csv contains the spectral metrics calculated from the absorbance scans as well as select meteorological metrics. ab.254 is raw absorbance measured by the instrument (after correcting for DI blanks), corrected for a 1 m pathlength. lin.slope.275.295 is the slope for the log transformed absorbance scan over the wavelengths 275-295 nm, determined by linear regression. nlin.slope.275.295 is the spectral slope (S275-295) over the wavelengths 275-295 nm, calculated using non-linear regression in R version 3.2.3 (R Core Team 2015), fitting the following equation:
alambda = alambdarefe-S(lambda-lambdaref)
In this equation a is the Naperian absorption coefficient (see below), lambda is the wavelength, lambdaref is the reference wavelength, and S is the spectral slope (nm-1) (Twardowski et al. 2004; Helms et al. 2008). The initial estimate supplied to the non-linear regression procedure was supplied by the value for lin.slope.275.295. lin.slope.350.400 and nlin.slope.350.400 were calculated in a similar fashion, but over the wavelengths 350-400 nm. This calculation yields the spectral slope over the wavelengths 350-400 nm (S350-400). slope.ratio is the slope ratio (SR), the ratio of S275-295 to S350-400 (Helms et al. 2008). ab.254.1cm is similar to ab.254, but is corrected for a 1 cm cuvette width. a254.nap is the Naperian absorption coefficient and is calculated from the equation:
a = 2.303A/l
In this equation, a is the Naperian absorption coefficient, A is raw absorbance measured by the spectrophotomer, and l is the path length (Green and Blough 1994). SUVA was calculated by dividing raw absorbance at 254 nm by the DOC concentration of the DOC sample collected nearest in time to the absorbance sample. SUVA is reported as Ltimesmg C-1timesm-1 (Weishaar et al. 2003).
wk.prcp, mth.prcp, and ninety prcp are precipitation totals for the 7, 30, and 90 days up to and including the sampling date (mm). These data come from the Minocqua, WI station in the Global Historical Climatology Network (GHCN) dataset (available at https://www.ncdc.noaa.gov/data-access/quick-links#ghcn) (Menne et al. 2010).
wk.tmp, mth.tmp, and ninety.tmp are mean values for the mid-daily temperature in the 7, 30, and 90 days up to and including the sampling date (°C). These are from the same data source as the precipitation data.
The l.lev column contains lake level from the NTL LTER website in meters above sea level. wk.insol, mth.insol, and ninety.insol are mean solar insolation incident on a horizontal surface in the 7, 30, and 90 days up to and including the sampling date (kWh/m2/day) (these data were obtained from the NASA Langley Research Center Atmospheric Sciences Data Center NASA/GEWEX SRB Project).
Monthly Palmer Drought Severity Index (PDSI) data corresponding to the study period can be accessed at: ftp://ftp.ncdc.noaa.gov/pub/data/cirs/climdiv/ using StateCode=47 and Division=2. These data were retrieved from the US National Oceanic and Atmospheric Administration (NOAA) National Center for Environmental Information (NCEI). These data were used to relate trends and synchrony in spectral metrics to moisture conditions.
The DOC column contains DOC concentration (mg L-1) for the corresponding sample date, obtained from the NTL LTER website.

Short Name
DOM trends data
Version Number
19

North Temperate Lakes LTER: Color - Trout Lake Area 1989 - current

Abstract
Color is measured four times per year in the seven northern study lakes (Allequash, Big Muskellunge, Crystal, Sparkling, and Trout lakes, unnamed lakes 27-02 [Crystal Bog] and 12-15 [Trout Bog]) on water samples that are filtered in the field through 0.45 micron capsule filters (0.45 um nuclepore membrane filters before 2015). A spectrophotometer is used to quantify color in the lab as absorbance units, at 1nm intervals between the wavelengths of 200 and 800 nm. All values are given as measurements at the path length of the employed cuvette and should be divided by the cuvette length for a comparable value at a pathlength of 1 cm. Sampling Frequency: 4 times annually. Number of sites: 7.
Dataset ID
87
Date Range
-
Maintenance
ongoing
Metadata Provider
Methods
Water samples for color are collected at the deepest part of the lake four times per year: February under ice, spring mixis, August stratification, and fall mixis. The samples are surface water collected with a peristaltic pump and tubing, filtered in the field through 0.45 micron capsule filters ( 0.45u polycarbonate membrane filters before 2015). A wavelength scan from 800 to 200nm is run, using a 5cm rectangular quartz cell in a Beckman Coulter Model DU800 single beam spectrophotometer. Any samples that display absorbance values above 2AU are run again from 400 to 200nm using a 1cm quartz cuvette, as values above 2AU are not considered valid. Blank values in the dataset are from a deionized water scan run as a first sample after instrument blanking, and are reported alongside the scans but are not subtracted from the scan values.
Protocol Log: 1990 -- Scans run using a 10cm cylindrical cell on a Kontron spectrophotometer. 2001 -- we added scans in a 1cm cell for samples with absorbance greater than 2AU. July 2008 -- changed to Beckman Coulter DU800 spectrophotometer, and changed from 10cm to 5cm cell.
Short Name
NTLPH07
Version Number
32
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