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

Summary

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.

Abstract

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.

Introduction

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....

Protocol

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 reaches, as they retard the downstream movement of solutes, minimal flow sections, and tributaries that dilute the added solution. Times of low discha....

Results

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.......

Discussion

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.......

Materials

Name	Company	Catalog Number	Comments
Sodium Nitrate	Any	Any
Sodium Chloride	Any	Any	Store purchased table salt can be used as well, however, it does contain trace levels of impurities
Whatman GFF glass-fiber filters	Any	Any
BD Filtering Syringe	Any	Any
EMD Millipore Swinnex Filter Holders	Any	Any
Syringe stop-cock	Any	Any
YSI Multi-parameter probe	Yellow Springs International	556-01
Wide mouth HDPE 125 ml bottles	Any	Any
60 ml HDPE bottles	Any	Any
20 L bucket	Any	Any
Field measuring tape	Any	Any
Lab labeling tape	Any	Any
Stir stick	Any	Any
Cooler	Any	Any
Sharpie pen	Any	Any
Field notebook	Any	Any
Tweezers	Any	Any
Zip-lock bags	Any	Any