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Understanding Dissolved Organic Matter Biogeochemistry Through In Situ Nutrient Manipulations in Stream Ecosystems

Published: October 29th, 2016

DOI:

10.3791/54704

1Department of Natural Resources and the Environment, University of New Hampshire

Dissolved organic matter provides an important source of energy and nutrients to stream ecosystems. Here we demonstrate a field-based method to manipulate the ambient pool of dissolved organic matter in situ through easily replicable nutrient pulses.

Dissolved organic matter (DOM) is a highly diverse mixture of molecules providing one of the largest sources of energy and nutrients to stream ecosystems. Yet the in situ study of DOM is difficult as the molecular complexity of the DOM pool cannot be easily reproduced for experimental purposes. Nutrient additions to streams however, have been shown to repeatedly alter the in situ and ambient DOM pool. Here we demonstrate an easily replicable field-based method for manipulating the ambient pool of DOM at the ecosystem scale. During nutrient pulse experiments changes in the concentration of both dissolved organic carbon and dissolved organic nitrogen can be examined across a wide-range of nutrient concentrations. This method allows researchers to examine the controls on the DOM pool and make inferences regarding the role and function that certain fractions of the DOM pool play within ecosystems. We advocate the use of this method as a technique to help develop a deeper understanding of DOM biogeochemistry and how it interacts with nutrients. With further development this method may help elucidate the dynamics of DOM in other ecosystems.

Dissolved organic matter (DOM) provides an important energy and nutrient source to freshwater ecosystems and is defined as organic matter that passes through a 0.7 µm filter. Within aquatic ecosystems, DOM can also influence light attenuation and metal complexation. DOM is a highly diverse and heterogeneous mixture of organic compounds with various functional groups, as well as essential nutrients such as nitrogen (N) and phosphorous (P). While the term "DOM" describes the entire pool including its C, N and P components, its concentration is measured as dissolved organic carbon (DOC). The inherent molecular complexity of the DOM pool however, creates chal....

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1. Identifying and Characterizing the Ideal Experimental Stream Reach

  1. Ensure that experimental stream reaches are long enough to promote complete mixing of solutes11 and long enough where biological uptake can occur. Reach lengths can vary among streams and experiments. In small first-order headwater streams, reach length may vary from 20-150 m (or longer if the system requires it) depending on discharge and other physical properties of the stream.
    1. Exclude large pools from experimental .......

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Figure 3
Figure 3: Example Results from Nitrate (NO3-) Additions with Dissolved Organic Nitrogen (DON) as the Response Variable. Analyses are linear regressions. Asterisks represent statistical significance at α = 0.05. Note the dynamic range in NO3- concentration that was achieved with the nutrient pulse method. Different pane.......

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The objective of the nutrient pulse method, as presented here, is to characterize and quantify the response of the highly diverse pool of ambient stream water DOM across a dynamic range of an added inorganic nutrient. If the added solute sufficiently increases the concentration of the reactive solute, a large inferential space can be created to understand how the biogeochemical cycling of DOM is linked to nutrient concentrations. This nutrient pulse approach is ideal as it involves none of the machinery associated with p.......

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The authors acknowledge the Water Quality Analysis Laboratory at the University of New Hampshire for assistance with sample analysis. The authors also thank two anonymous reviewers whose comments have helped to improve the manuscript. This work is funded by the National Science Foundation (DEB-1556603). Partial funding was also provided by the EPSCoR Ecosystems and Society Project (NSF EPS-1101245), New Hampshire Agricultural Experiment Station (Scientific Contribution #2662, USDA National Institute of Food and Agriculture (McIntire-Stennis) Project (1006760), the University of New Hampshire Graduate School, and the New Hampshire Water Resources Research Center.

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NameCompanyCatalog NumberComments
Sodium NitrateAnyAny
Sodium ChlorideAnyAnyStore purchased table salt can be used as well, however, it does contain trace levels of impurities
Whatman GFF glass-fiber filtersAnyAny
BD Filtering SyringeAnyAny
EMD Millipore Swinnex Filter HoldersAnyAny
Syringe stop-cockAnyAny
YSI Multi-parameter probeYellow Springs International556-01
Wide mouth HDPE 125 ml bottlesAnyAny
60 ml HDPE bottlesAnyAny
20 L bucketAnyAny
Field measuring tapeAnyAny
Lab labeling tapeAnyAny
Stir stickAnyAny
CoolerAnyAny
Sharpie penAnyAny
Field notebookAnyAny
TweezersAnyAny
Zip-lock bagsAnyAny

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