Coarse Particulate Organic Matter

Chapter 26

Coarse Particulate Organic Matter: Storage, Transport, and Retention Gary A. Lamberti1, Sally A. Entrekin2, Natalie A. Griffiths3 and Scott D. Tiegs4 1

Department of Biological Sciences, University of Notre Dame; 2Department of Biology, University of Central Arkansas; 3Environmental Sciences

Division, Oak Ridge National Laboratory; 4Department of Biological Sciences, Oakland University

26.1 INTRODUCTION Coarse particulate organic matter, or CPOM, in streams is functionally defined as any organic particle larger than 1 mm in size (Cummins, 1974). CPOM can be further divided into nonwoody and wood material (Cummins and Klug, 1979), the former of which will be considered in this chapter. The nonwoody fraction includes allochthonous materials produced and donated by riparian organisms (e.g., leaves, needles, fruits, flowers, seeds, insects, frass) and autochthonous materials produced within the stream (e.g., algae, aquatic plants, dead aquatic animals). Smaller materials, including dissolved organic matter (DOM < 0.45 mm) and fine particulate organic matter (1 mm > FPOM > 0.45 mm), are primarily considered in Chapters 24 and 25, respectively, but are also discussed in the context of organic carbon spiraling as an advanced method in this chapter. Wood includes a broad range of size classes from branches to entire trees that fall into stream channels and perform important ecological functions (Bilby and Likens, 1980) that are considered in Chapter 29. Allochthonous CPOM is a major energetic resource for most stream ecosystems, providing a large proportion of carbon in small streams of both deciduous and coniferous forests and a significant input to larger streams and rivers (Vannote et al., 1980; Cummins et al., 1983; Tank et al., 2010). Autochthonous CPOM can be an important resource in open-canopy streams or in forested streams prior to leaf-out in the spring (Minshall, 1978; Roberts et al., 2007). Regardless of source, this CPOM is broken down by stream biota during an activity known as organic matter processing. CPOM is transported downstream by the unidirectional flow of water, with very few mechanisms for upstream movement. The process of deposition and trapping of this material is termed “retention,” which provides the critical link between input and the longterm storage and processing of CPOM. Retained CPOM can be measured as the areal amount in a particular habitat or an entire stream, often referred to as the standing crop. Standing crop suggests the amount available to the “stock” of microand macrodetritivores. Retention is therefore essential for subsequent microbial colonization and hydrolysis that precedes storage and consumption of CPOM (and associated microbes) by detritivores (Cummins and Klug, 1979; Graça, 2001). CPOM retentive capacity of streams is a function of hydrologic, substrate-related, and riparian features (Speaker et al., 1984; Cordova et al., 2008). High roughness levels of the channel (sensu Chow, 1959) (e.g., large substrate size, streambed heterogeneity, abundant wood), combined with certain hydraulic conditions (e.g., presence of backwaters, interstitial flow), tend to increase channel CPOM retention efficiency. Large wood, especially in accumulations, provides particularly important retention structures (Bilby, 1981; Smock et al., 1989; Jones and Smock, 1991; Entrekin et al., 2008). Young et al. (1978) noted that the probability that a particle in transport will be retained is a function of the “active” entrainment efficiency of that particle size by a channel obstacle (e.g., rock, log, root, etc.) and the density of those obstacles within the channel. A particle also will be retained “passively” when current velocity is insufficient to transport it (Jones and Smock, 1991), and thus the particle “settles.” Retention (R) can thus be expressed as a probability function: PðRÞ ¼ f ðE; N; VÞ

Methods in Stream Ecology. http://dx.doi.org/10.1016/B978-0-12-813047-6.00004-8 Copyright © 2017 Elsevier Inc. All rights reserved.

(26.1)

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where E ¼ entrainment efficiency by channel obstacles, N ¼ obstacle density in the channel, and V ¼ critical velocity required to transport a particle. If an organic particle is retained, it subsequently can be respired by microorganisms, consumed by detritivores, or, if flow conditions change, be dislodged and transported further downstream (Speaker et al., 1984). This combined process of organic carbon transport, retention, and processing in streams is termed “organic carbon spiraling” (Newbold et al., 1982). In this chapter, we describe basic methods to quantify CPOM (except for wood; see Chapter 29) stored in the stream channel. We also present a simple, quantitative field method to assess the CPOM retention efficiency of a specific stream reach. These methods are most easily used in small streams (orders 1e4), but can be adapted for larger streams and rivers. These approaches are also intended to relate storage and retention to hydraulics, streambed roughness, channel geomorphology, and riparian zone structure. We then present an advanced method to experimentally augment the retention capacity of a stream reach so that researchers can evaluate the effects that increased CPOM quantity may have on stream ecosystem processes (e.g., nutrient uptake, secondary production, invertebrate abundance). In a second advanced method, we demonstrate how to measure the transport and processing of organic carbon in stream ecosystems using a carbon spiraling approach. Our specific objectives are to (1) introduce the concepts of organic matter storage, retention, and dynamics; (2) demonstrate how to measure CPOM standing crop and retention, analyze data, and calculate indices of retention; (3) illustrate the utility of storage and retention measurements for assessing stream channel condition; and (4) describe field techniques to experimentally manipulate the retention capacity of a stream and to measure the longitudinal dynamics of organic carbon.

26.2 GENERAL DESIGN Organic matter in streams provides short- and long-term resources for benthic biota including detritivorous micro- and macroinvertebrates, and even some vertebrates. Therefore, the measurement of CPOM benthic standing crop and its size fractions is important for assessing resource availability and potential energy flow among trophic levels of aquatic food webs (Wallace et al., 1997; Rosi-Marshall and Wallace, 2002). In fact, CPOM standing crops can predict aquatic community secondary production (Chadwick and Huryn, 2007; Entrekin et al., 2009). Standing crop is typically quantified as organic matter size fractions >1 mm in a particular habitat type (e.g., pools or riffles) or across all habitat types, as sampled with a benthic corer and expressed as dry mass (DM) or ash-free dry mass (AFDM) per unit area (see Chapter 12). Small, high-gradient streams in coniferous, deciduous, and boreal forests tend to store the most CPOM per unit area (cf. mid- to high-order streams and streams in arid/semiarid biomes), although standing crops vary with season, precipitation, channel gradient, and large wood storage (Jones, 1997). At the local scale, CPOM storage also varies among and within habitats; therefore, habitat-specific sampling may be informative. For example, in low-gradient, sandy-bottom streams, CPOM storage will be greatest along stream margins, in pools, and around wood accumulations (Jones and Smock, 1991). Habitatspecific CPOM sampling will thus provide information on how differences in channel form affect CPOM storage and availability. Lotic retention can be quantified as the difference between the number of particles that enter a length of stream and the number transported through that same length (Speaker et al., 1984). Retention is most easily measured by releasing known numbers of readily distinguishable but representative particles into the channel. To compare different streams or stream reaches within a study, the experimental approach must be standardized for type and number of particles released, length of experimental reach, and duration of the retention measurement. Many types of CPOM have been released into streams, including leaves (Speaker et al., 1984), paper shapes (Webster et al., 1994), plastic strips (Bilby and Likens, 1980; Speaker et al., 1988), wood dowels (Ehrman and Lamberti, 1992), and even fish carcasses (Cederholm et al., 1989). In general, we believe that it is preferable to release natural (decomposable) materials into streams because analogs such as plastic items or paper strips (1) may not behave the same as natural materials and (2) will generally not be 100% retrievable, thereby leaving trash in the stream. In this chapter, we will demonstrate retention of leaves, but other materials significant to the specific stream can be substituted. For example, fruits and seeds are significant CPOM inputs in many tropical streams (Larned et al., 2001).

26.2.1 Site Selection The selection of a study stream in which to conduct these exercises may be influenced by logistical considerations. Storage and carbon spiraling measurements can be made in any stream where the equipment can be safely operated. For the retention experiments, wadeable second- to fourth-order streams are ideal. Very small streams at low flow will have low transport capacity, and these methods can be challenging (and also dangerous) to conduct in large rivers. In general,

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however, these methods can be scaled to a wide variety of stream sizes. For CPOM release experiments, at least two reaches in the same stream (or two different streams) with contrasting channel features should be selected by the research coordinator. Ideally, one reach would have a relatively simple channel (straight, low roughness, limited hydraulic diversity, sparse wood), whereas the other reach should have a complex channel (sinuous, high roughness, diverse hydraulic conditions, abundant wood). Alternatively, the same stream reach can be studied at different discharges or before and after some event, such as a treefall or a channel modification associated with an experiment or stream restoration. The length of the experimental reach should be scaled to stream size, with length increasing with stream order. As a rule of thumb, start with a stream length that is w10 times the wetted channel width. For example, 50 m may be an appropriate length for a 5-m wide, second-order stream, 100 m for a third-order stream, and 200 m for a fourth-order stream. If possible, use a pilot study to adjust reach length such that retention is not less than 10% nor greater than 90% of released particles.

26.2.2 Basic Methods We describe below two basic exercises that quantify CPOM storage and retention that can be conducted by small groups of students in the field or that can be used as components of more sophisticated research or monitoring programs. CPOM storage typically increases with greater riparian inputs and under the same conditions as retention; however, storm flow, detritivore consumption, and microbial respiration result in depletion. Within a stream, CPOM storage can be measured at the microsite (e.g., sediment type), habitat (e.g., riffle, pool), or stream-reach scale. Multiple, haphazard samples or stratified random sampling may be required to adequately characterize this often-patchy resource. In many cases, benthic corers (see Chapter 8) can be used to delineate a known area of streambed from which CPOM is collected and weighed to estimate standing crop. Studies that aim to quantify the importance of a given CPOM resource in a stream should at least sample weekly or biweekly during the period of greatest CPOM input (e.g., during autumnal leaf fall in deciduous forests, during algal senescence in open-canopy streams, or after major storms in intermittent streams). Monthly or biweekly sampling throughout the year may be required in other biomes to provide the most complete quantification of CPOM storage in regions without distinct seasonality in CPOM inputs. Leaves are the major form of nonwoody CPOM input to most streams, and their retention is an important ecological process (Webster et al., 1999). In retention experiments, released leaves must be distinguishable from leaves found naturally in the channel and should be easy to obtain and manipulate. We have found that, for North American streams, abscised leaves of the exotic Asian ginkgo tree (Ginkgo biloba) meet these requirements. The leaves are tough even when wet, their size approximates that of many leaf types of riparian vegetation, and the bright yellow leaves are easily spotted in the channel. Ginkgo trees have been planted worldwide as ornamentals (and very often on college campuses). Other species of leaves can be substituted depending on their availability, visibility during experiments, and the composition of local riparian vegetation. Physical measurements of the stream channel should be taken according to the level of effort possible and available equipment. Useful parameters include discharge, slope, sinuosity, cross-sectional area, and planar wetted area (see Chapters 2e5), along with volume of large wood (see Chapter 29). Retention data for leaves (using batch releases) should be fit to a negative exponential decay model, from which indices of retention (e.g., the retention coefficient, k; average particle travel distance, 1/k) can be calculated. Metrics for individual particle releases can be generated using simple statistics. If desired, CPOM releases can be conducted over longer periods of time, or at different seasons and discharges, to develop relationships between retention and stream temporal dynamics (e.g., Jones and Smock, 1991; Webster et al., 1999).

26.2.3 Advanced Methods Two advanced exercises involving additional sophistication, time, and facilities are also presented in this chapter. These are “research-level” approaches suitable for incorporation into studies intended for theses and published papers. First, we describe an experiment to enhance stream retention via the installation of litter retention devices in the stream channel, after which other whole-reach responses such as metabolism (see Chapter 34) can be measured (Tiegs et al., 2008). We then describe an advanced method to assess organic carbon spiraling, which is an integrative measure of the transport, retention, and processing of organic carbon within a stream channel (Newbold et al., 1982). Carbon spiraling involves the measurement of organic carbon pools in transport, standing crops on the benthos, processing of organic carbon (via a measure or estimate of heterotrophic respiration), and physical measurements of stream depth, width, velocity, and discharge. From these measurements, three carbon spiraling metrics can be calculated: (1) VOC, which is the downstream transport velocity of organic carbon (m/day), (2) KOC, which is the processing rate of organic carbon (day1), and (3) SOC, which is the organic carbon spiraling length and a measure of the distance organic carbon travels downstream before being respired (m).

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Carbon spiraling metrics estimate all fates of organic carbon (transport, retention, and processing) and thus can be used to address hypotheses pertaining to whole-stream carbon dynamics. For instance, carbon spiraling metrics have been used to examine seasonal patterns in whole-stream carbon dynamics (Thomas et al., 2005) to compare the retentiveness and processing of carbon across a variety of streams (Webster and Meyer, 1997) and land use types (Griffiths et al., 2012), and to examine the effect of stream consumers on carbon processing (Taylor et al., 2006).

26.3 SPECIFIC METHODS 26.3.1 Basic Method 1: Coarse Particulate Organic Matter Storage and Measurement 26.3.1.1 Field Measurements 1. Select two stream reaches (e.g., 50e100 m long depending on stream width) with differing riparian canopy cover, slope, or other contrasting geomorphological features. Flag 5- to 10-m subreaches depending on chosen reach length, and haphazardly1 select sampling locations within each subreach. If habitat-specific samples will be taken, identify the dominant habitat types and then haphazardly sample replicate habitats within each reach. 2. Using a coring device of known area (e.g., bucket with bottom removed, PVC pipe section), form a tight seal with the streambed (Fig. 26.1). By hand, remove as much coarse material as possible and place in a labeled paper bag. Then, stir the remaining material by hand, skim smaller pieces with a 1-mm mesh sieve or hand net, and place in a labeled paper bag. 3. Carefully fold the paper bag, and place in a labeled plastic bag for transport to the laboratory.

26.3.1.2 Laboratory Processing 1. Place all labeled paper bags in a labeled paper box and dry at 60 C for at least 24 h. Drying times will vary with amount and the type of litter. Once dried, litter types can be sorted (e.g., small wood, moss, leaves, other) and weighed and compared individually or together, depending on the research question.

FIGURE 26.1 Photograph of benthic corer (open-bottom bucket) used for coarse particulate organic matter storage measurements in an Ozark Mountain, USA, stream. Photo: A. Bates. 1. It will be challenging to truly sample randomly within the stream, and so “haphazard” sampling that attempts to minimize bias in sampling site selection is generally acceptable.

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2. Remove litter from the drying oven, allow to cool to room temperature in a desiccator, and take a total weight. Then, with gloved hands, crush and homogenize all litter, place a subsample in a labeled, ashed, and weighed aluminum pan, and weigh the subsample. A grinder will be useful for tough material (e.g., seeds, wood fragments). 3. Place subsamples in a muffle furnace at 500 C for 2 h.2 Remove litter, allow to cool at 60 C for 12 h, place in a desiccator until room temperature is reached, and then reweigh (see also Chapters 12 and 27).

26.3.1.3 Data Analysis 1. CPOM standing crop can be expressed as grams of DM or AFDM (dry mass minus ash mass after burning at 500 C) (Table 26.1; see also Chapter 12). Once DM or AFDM are estimated, convert to grams per unit area by dividing mass by corer area. To calculate proportion AFDM, subtract ash mass from DM and then divide by DM. 2. Various statistical analyses can be conducted depending on study objective. To compare two stream reaches, average the cores from the same habitats (or all benthic cores for a reach) and compare mean standing crop with a t-test. For a more advanced analysis, sample multiple times within a season or throughout the year and use one-way repeated measures analysis of variance (rmANOVA) to identify differences between stream reaches, or habitats, over time.

26.3.2 Basic Method 2: Coarse Particulate Organic Matter Transport and Retention 26.3.2.1 Laboratory Preparation 1. In the autumn, collect several thousand abscised (fallen) leaves of an exotic tree, such as G. biloba, or other readily identifiable species. Air-dry the leaves by spreading them over screens, netting (seines work well), or even on newspaper on the floor. Leaves can be stored dry in black garbage bags for a considerable length of time. Alternatively, you can use fresh-fallen leaves if the releases will be performed soon after collection (within days). 2. Count out two batches of 1000 leaves, used to conduct two releases in a third-order stream. Smaller or larger streams may require fewer or more leaves, respectively. The actual number of leaves is less important than knowing exactly how many are released. 3. The day before the release, soak leaves overnight in buckets of water to impart neutral buoyancy during transport. A soil sieve placed gently over dried leaves will help to keep them submersed. Drain most of the water before departing into the field.

26.3.2.2 Field Physical Measurements 1. Measure and flag an appropriate length (e.g., 100 m for a third-order stream) of at least two stream reaches differing in channel complexity, large wood abundance, or some other geomorphological feature. Stretch a meter tape along the bank over the length of the reach, with 100 m at the downstream end. 2. Measure major channel features at a level of intensity appropriate to the research objectives. We recommend working in a research team of three people (two making measurements and one recording data). Useful measurements include slope, average width, depth, sinuosity, and substrate composition (see Chapters 2 and 5). Determine discharge using the cross-sectional approach (see Chapter 3). Repeat for each reach. 3. Measure the abundance of large wood in the channel (see Chapter 29), noting if the wood is part of a dam (i.e., wood accumulation blocking some portion of stream flow). 4. If this method is being used for a class exercise, prior to the releases briefly discuss channel and riparian features. Have students predict retention for each reach (e.g., percentage of leaves that will be retained).

26.3.2.3 Field Coarse Particulate Organic Matter Releases 1. Position several researchers at the downstream end of the study reach. Release the leaf batch (e.g., 1000 leaves) at the upstream end of the reach (i.e., 0 m mark) by dispersing leaves over the entire width of the stream channel over a span of about 1 min (Fig. 26.2A). 2. Collect nonretained leaves at the downstream end of the reach (e.g., 100 m). Either of two approaches can be used to collect leaves. A beach seine can be stretched across the width of the channel (Fig. 26.2B), with the bottom lead line anchored, without gaps, to the streambed with rocks (in sandy-bottom streams, tent stakes can be substituted for rocks).

2. Wear appropriate personal protective equipment (PPE) including lab coat, oven mitts, and eyewear.

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TABLE 26.1 Example laboratory processing spreadsheet for coarse particulate organic matter (CPOM) standing crop and size fractions (see also Online Worksheet 26.1 for detailed calculations). Date:

Corer Area:

Stream:

Team:

Location:

Notes:

Reach: Length: Location

Unit

CPOM

Sample

Meter Mark

Riffle or Pool

Fraction Type (e.g., Leaf, Moss, Small Wood)

Total Dry Weight (g)

Subsample Dry Weight (g)

Ash Weight (g)

100e95 95e90 90e85 . . . 5e0

FIGURE 26.2 Photographs of coarse particulate organic matter releases. (A) Ginkgo biloba leaf release into a Oregon, USA, stream; (B) beach seine stretched across the channel of a northern Alaska, USA, stream (anchored to the bottom with rocks) for capturing unretained leaves. Photos: G. Lamberti.

The top of the seine should be held out of the water by attaching it to a taut rope tied to trees on both banks. In strong flows, it may be further necessary to support the top and rear of the seine with wood pieces driven into the substrate. Alternatively, researchers can line up across the channel and collect leaves in transport with hand-held dip nets (e.g., Dframe or delta nets) if safe to do so. The seine method is more efficient, especially if the number of researchers is low. The individual netting approach results in greater involvement of researchers in actual leaf collection, but some leaves may be missed. 3. Continue collecting leaves for a period of time specified by the coordinator, usually at least 15 min and up to 1 h, or when leaf transport ceases. This period should be consistent for all reaches. Count all collected (i.e., nonretained) leaves. 4. Move upstream to the next reach (or contrasting stream) and repeat the procedure.

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26.3.2.4 Data Analysis 1. Calculate reach physical parameters, such as slope, planar surface, cross-sectional area, mean depth, current velocity, hydraulic radius, sinuosity, and discharge (see Chapters 2e5). These fundamental physical parameters can be related empirically or theoretically to observed retention values. 2. Determine the density (pieces per reach) and total volume (m3) of wood in each reach, according to methods presented in Chapter 29. 3. Fit the leaf retention data to a negative exponential decay model of the form: Pd ¼ P0 ekd

(26.2)

where P0 ¼ number of particles released into the reach and Pd ¼ number of particles still in transport (i.e., collected) at a known downstream distance d from the release point (m). Calculate the slope k (the instantaneous retention rate; 1/m) and its reciprocal 1/k (the average distance traveled by a particle before it is retained; m). If particles are not inventoried after the release, then the model will be based on two data points, P0 and Pd. See Advanced Method 1 for data analysis if particles are reinventoried in the channel.

26.3.2.5 Option to Basic Method 2: Single-Particle Releases 1. As an alternative to batch releases of particles described above, the single-particle release method can be used (Webster et al., 1994). Single particles (e.g., leaves, fruits) or artificial analogs (e.g., “Rite in the Rain” field paper, cut into standard shapes) are released into the channel and individual travel distances are recorded. 2. Release a known number (e.g., 25e50) of visible particles one-by-one into the channel and record the distance traveled and retention structure for each particle. Repeat this procedure in as many stream reaches, or subreaches, as desired. Mean travel distances can be compared statistically among reaches using ANOVA, or relationships with stream characteristics such as discharge can be explored with regression (e.g., Webster et al., 1994, 1999). 3. This approach is especially useful in highly retentive streams where few or no particles may travel the entire stream reach, thereby invalidating the exponential decay model. However, this method requires relatively high water clarity and shallow depths to follow individual particles for their entire travel path.

26.3.3 Advanced Method 1: Enhancement of Stream Retentive Capacity Retention of CPOM by the stream channel stems from two very different processes: deposition of CPOM due to insufficient transport capacity, and the forcing of CPOM by moving water against roughness elements (e.g., substrata, wood, tree roots, etc.) such that it is held in place (hereafter termed “active retention”). Below we describe an experimental procedure for enhancing the active retention of organic matter in a stream reach. Although experimental reduction of the retentive capacity of stream channels has been performed (e.g., Díez et al., 2000), enhancing retention through the installation of retention devices is more commonly done (e.g., Tiegs et al., 2008) and more readily standardized. Retention devices can be customized to retain different types of CPOM (e.g., salmon carcasses, Tiegs et al., 2011; Fig. 26.3A and B), but in most instances researchers have used them to retain leaf litter (e.g., Dobson et al., 1995). In some instances, large wood has been introduced into streams to replicate past (prelogging) conditions with the explicit goal of retaining in-channel CPOM (Fig. 26.3C; see Entrekin et al., 2008). By installing litter retention devices, the standing crop of litter in the stream channel can be significantly increased above background levels, and the response of the stream ecosystem to this enhanced CPOM evaluated. Below we present a method for installing leaf litter retention devices made from metal or wooden stakes and pieces of extruded plastic mesh (Fig. 26.3D). The method as described is for small, shallow streams, with a study reach of 50 m in length, and will yield a retention device density of w1/m2 (after Dobson, 2005), but it can be adjusted to accommodate larger or smaller streams.

26.3.3.1 Laboratory Preparation 1. Assemble materials for field deployment of retention devices (see Materials and Supplies).

26.3.3.2 Field MeasurementsdDeployment of Retention Devices 1. Identify the stream reach in which retention is to be enhanced and a paired upstream reference reach that will not be enhanced. Retention will be most readily enhanced in reaches that lack abundant retention structures (e.g., pools, wood accumulations, or coarse substrate) and have significant CPOM inputs.

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FIGURE 26.3 Photographs of coarse particulate organic matter (CPOM) retention devices. (A) Salmon carcass retention devices in a southeast Alaska, USA, stream; (B) close-up of single carcass retention device (1.5 m wide) with upright rebar stakes, capped for safety, and attached conduit pipe crossmember; (C) large wood (logs with cut ends) introduced into a Michigan, USA, stream; (D) CPOM retention devices (upright rebar stakes and green mesh fences) in a small stream in the Black Forest, Germany; each fence is w20 cm  20 cm. (A) Photo: S. Tiegs. (B) Photo: G. Lamberti. (C) Photo: S. Entrekin. (D) Photo: F. Peter.

2. Walk the length of the experimental reach, and distribute the materials for each retention device (two of the wood stakes, and one of the mesh squares) along the shoreline for every square meter of stream channel in the reach. For example, for a stream that is 1 m wide on average, distribute the materials for one retention device for every meter of stream length.3 3. Near the location where the materials for the retention devices were placed, begin installation by hammering one of the stakes into the stream substratum (Fig. 26.3C). Hammer until half the stake is in the stream substratum. 4. Attach one edge of the mesh square to the stake with binder clips or cable ties (three clips or ties along each edge, one at the top, one at the bottom, and the other near the middle should suffice). Do not tighten yetdwait until the other stake has been secured to the streambed. 5. Hammer the other stake into the substratum approximately 20 cm (i.e., width of mesh square) away from the first stake and aligned perpendicular to stream flow. Attach the clips or ties to the second stake, and tighten all the attachments securely. 6. Repeat this procedure for each of the retention devices. The CPOM retention of each trap will be maximized by locating them in areas of relatively fast water and by “staggering” the locations such that each trap is not located immediately downstream from the one above it. 7. After installation and following a suitable period for CPOM trapping (typically on the order of weeks to months), randomly select a subset of the retention devices, and collect all the CPOM trapped on each device. When removing CPOM from a retention device, make sure to position a 1-mm sieve or screen immediately downstream to capture detached particles. 8. Procedures described in Basic Method 1 can then be used to estimate the DM and AFDM (see Chapter 12) of the retained material. To estimate the total mass of CPOM experimentally captured, extrapolate the subset to the total number of devices installed in the reach.

3. Install the devices during low flow conditions for ease and safety, such as before autumn rains and the onset of leaf fall.

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26.3.3.3 Field MeasurementsdRelated Ecosystem Measurements Many ecosystem attributes can be measured and evaluated after the experimental augmentation of CPOM, such as nutrient cycling, leaf decomposition, invertebrate responses, and ecosystem metabolism in the paired reference and experimental reaches (see relevant chapters in this book). The timing of measurements should be adjusted to the length of the experiment and the period required for the response. Below, as an example, we describe an enhanced method for measuring leaf retention in the context of this experiment. 1. Prior to the installation of the retention devices, perform a leaf release in both reaches (i.e., an upstream reference reach, and the reach where the retention devices will be installed) as in Basic Method 2. After device installation, and under similar flow conditions, repeat the releases to estimate the enhanced retentiveness provided by the retention devices. 2. Inventory the location, number, and retention structure (including installed devices) for retained leaves. This is best accomplished by dividing the reach into longitudinal increments of 5 m using a bankside meter tape. 3. Researchers should move up the channel as a single line of observers, perpendicular to flow. Locate and count released leaves within each increment, noting also the retention structure (e.g., rock, wood, bank, retention device, etc.; see Table 26.2). 4. These inventory data can be used to refine the exponential model described in Basic Method 2 and produce a more accurate estimate of k, or to fit retention data to an alternate regression model (e.g., linear, power) more appropriate for the specific reach. The inventory most likely will not turn up all of the retained leaves; therefore, it is necessary to normalize inventory data to a percent of total leaves found. Graph the particle transport data for each release, using distance downstream from the release point as the x-axis and percent of particles still in transport as the y-axis (see Ehrman and Lamberti, 1992 for examples). Using a bar diagram, plot the percentage of leaves retained by specific channel structures in each reach. Describe the longitudinal pattern of retention, and identify important retention structures within the channel. 5. Data from the paired reference and experimental reaches can be analyzed by various means depending on study objectives and the frequency of sampling. If sufficient samples are taken in both reaches before and after the construction of retention devices, then a before-after-control-intervention (BACI) analytical approach can be used (Stewart-Oaten et al., 1986).

TABLE 26.2 Sample data sheet for inventory of retained coarse particulate organic matter (CPOM) particles in a 100-m reach; add a column for retention devices (Advanced Method 1) if employed (see also Online Worksheet 26.2). Date:

CPOM Type:

Stream:

Duration:

Location:

Total Released:

Reach: Length:

Total Captured:

Team: Notes:

Total Retained:

Location

Unit

Meter Mark

Riffle or Pool

100e95 95e90 90e85 . . . 5e0

Number of Particles Retained on Structure Rock

Root

Backwater

Bank

Wood

Debris Dam

Other

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26.3.4 Advanced Method 2: Measurement of Organic Carbon Spiraling Organic carbon spiraling is a measure of the transport and turnover of organic carbon within a stream (Newbold et al., 1982). To estimate carbon spiraling in a given stream, carbon pools (in transport and as standing crops), carbon turnover rates (i.e., heterotrophic respiration), and physical characteristics (stream depth, width, velocity, and discharge) are measured. From these measurements, three carbon spiraling metrics are calculated: (1) the downstream transport velocity of organic carbon, (2) the processing rate of organic carbon, and (3) organic carbon spiraling length (Newbold et al., 1982; Thomas et al., 2005; Griffiths et al., 2012). All size fractions of organic carbon (coarse, fine, ultrafine, dissolved) are measured and described in the method below, but this method can also be modified to focus on only the CPOM fraction.

26.3.4.1 Site Selection 1. Select a study reach that is at least 100e200 m in length for a first- to second-order stream, or 500e1000 m in length for a wadeable mid-order stream. Ensure that there are minimal lateral inputs of water along the reach (i.e., tributaries).

26.3.4.2 Field MeasurementsdTransported Organic Carbon Transported organic carbon (TOC) is the sum of CPOM (>1 mm), FPOM (52 mme1 mm), ultrafine particulate organic matter (UPOM, 0.45e52 mm), and DOM (<0.45 mm). 1. To measure CPOM in transport, position a CPOM net (1-mm mesh size; drift net or similar, see Chapter 21) in an area with representative flow, with the bottom of the net slightly off the streambed. Allow CPOM to collect in the net, and remove the net before it begins to clog (usually 1e2 h). Record the length of time the net was in the stream collecting CPOM. Measure water velocity (see Chapter 3) and depth of the net at multiple (3) locations across the width of the net after the sample has been collected but before the net is removed from the water (Table 26.3). The net width and depth and water velocity are used to calculate discharge at the net, and the total volume of water that passed through the net during the CPOM collection is calculated based on the length of time the net was in the water. Rinse all organic matter caught in the net into a labeled sample cup or plastic bag, and keep the sample on ice until returning to the laboratory. If using a drift net, the net will likely not be wide enough to span the entire width of the stream. Therefore, sample multiple times at various locations along the reach to account for spatial and temporal variation in CPOM transport. Make sure to start downstream and work upstream to collect additional samples.

TABLE 26.3 Sample data sheet for total organic carbon collection in the field as part of the carbon spiraling analysis (see also Online Worksheet 26.3 for detailed calculations). Date: Stream: Team: Notes: Location

Net Type

Sample ID

Time Net In

Time Net Out

Net Depth (m)

Water Velocity (m/s)

Meter Mark

CPOM, FPOMa

##

(hh:ss)

(hh:ss)

(3 Locations)

(3 Locations)

Net Width (m)

a Make sure to collect an ultrafine particulate organic matter and dissolved organic matter sample with every coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) sample.

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2. To measure FPOM in transport, repeat the procedure for CPOM with an FPOM net (52 mm mesh size). Note that the FPOM net will clog more quickly (e.g., <5 min, depending on concentration and water velocity). A nested set of CPOM and FPOM nets could also be used (i.e., FPOM net downstream of CPOM net). 3. To measure UPOM, collect an unfiltered water sample (2 L or greater) along with each CPOM and FPOM sample (see also Chapter 25). 4. To measure DOM, collect a filtered (0.45 mm) 50-mL water sample into an amber glass vial, and acidify the sample with two drops of 6N hydrochloric acid4 (see also Chapter 24).

26.3.4.3 Field MeasurementsdBenthic Organic Carbon All organic matter fractions except for DOM are also measured on the streambed. Collect benthic organic carbon (BOC) at five locations along the study reach, and select more locations to sample if BOC in the reach is heterogeneous. Collect BOC samples 1e2 days after the TOC and heterotrophic respiration measurements as BOC sampling disrupts the streambed. The method to measure BOC described below does not distinguish among different organic matter substrata (e.g., leaves, algae). If this is desired, a habitat-weighted transect approach can be used (see Hoellein et al., 2007; Griffiths et al., 2012). 1. Measure CPOM on the benthos by placing a corer on the streambed and removing all CPOM contained within the core (see Basic Method 1 above). 2. After all CPOM is removed from the core, use your hand to swirl the top 3e5 cm of sediments, causing the particles to be suspended in water. Collect all other particulate fractions by quickly sampling the suspension with a plastic cup, and then place the sample on ice.

26.3.4.4 Field MeasurementsdOrganic Carbon Turnover Multiple methods can be used to estimate heterotrophic respiration, which is a measure of organic carbon processing in stream ecosystems. Below, we describe a method for estimating heterotrophic respiration from whole-stream metabolism measurements. Heterotrophic respiration can also be measured using benthic chambers (see Chapter 27; Bott et al., 1978) or can be measured on different types of benthic organic matter (e.g., leaves, fine benthic organic matter) using small closed chambers (Hoellein et al., 2009; Griffiths et al., 2012). 1. Use the diel oxygen change method described in Chapter 34 to estimate gross primary production (GPP) and ecosystem respiration (ER). Measure GPP and ER on a day when no other measurements are occurring in the stream. If possible, use the two-station metabolism method (see Chapter 34) to measure GPP and ER in the study reach only.

26.3.4.5 Field MeasurementsdPhysical Characteristics of the Stream Measurements of stream discharge (Q), width (w), velocity (v), and depth (z) are needed to calculate metrics of organic carbon spiraling. 1. Measure stream discharge using the velocityearea protocol at the bottom and top of the study reach and calculate mean discharge for the reach from these measurements (see Chapter 3). 2. Measure stream width every w5 m along the study reach using a tape measure. 3. Measure average reach water velocity using the salt (NaCl) release method (see Chapter 30). 4. Calculate mean stream depth z from Q, w, and v as: z ¼ Q=v  w

(26.3)

26.3.4.6 Laboratory Processing 1. Each fraction of TOC should be processed separately in the laboratory. For the CPOM samples, rinse the sample over a 1-mm sieve and place all detritus caught on the sieve into a precombusted and preweighed aluminum pan. For the FPOM samples, first pass the sample through a 1-mm sieve to remove the CPOM fraction (if the nets were not nested 4. Wear appropriate PPE including lab coat, eyewear, and gloves when handling HCl.

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SECTION j D Organic Matter Dynamics

in the field), and then collect the filtrate on a precombusted and preweighed 0.7-mm glass fiber filter. For the UPOM samples, first pass the sample through the 52-mm mesh net to remove any CPOM and FPOM fractions, and then collect the filtrate on a precombusted, preweighed 0.45-mm filter. All pans and filters should then be dried at 60 C for 48 h, weighed, and then ashed in a muffle furnace at 500 C for 2 h. Calculate AFDM as described in Basic Method 1 (see also Chapter 12). Convert mass from g AFDM to g organic carbon assuming that 48.4% of AFDM is organic carbon (Thomas et al., 2005). Measure DOC on the filtered (0.45 mm) water sample using the method described in Chapter 24. 2. The same processing steps are used to analyze the BOC samples. For the CPOM samples, pass the detritus through a 1-mm sieve and place the material captured by the sieve in a precombusted and preweighed aluminum pan. For the FPOM/UPOM sample, pass the sample through nested 1-mm and 52-mm sieves, collect the material on the 52-mm sieve for FPOM, and collect the filtrate on a 0.45-mm filter for UPOM. Measure g AFDM and convert to g C as described in step 1.

26.3.4 7 Data Analysis The following describes the steps to calculate the three metrics of organic carbon spiraling: VOC, the downstream transport velocity of organic carbon; KOC, the processing rate of organic carbon, and SOC, the organic carbon spiraling length: 1. Downstream transport velocity of organic carbon: VOC ¼ TOC  Q=BOC  w

(26.4)

TOC is the total organic carbon concentration in transport (g C/m3) and is calculated as the sum of CPOC þ FPOC þ UPOC þ DOC in transport. CPOC and FPOC in transport are calculated as the g organic carbon in the sample divided by the volume of water passing through the nets when the sample was being collected. UPOC in transport is calculated as the g organic carbon in the sample divided by the total volume of water in the sample. Q is stream discharge measured in the field (converted from L/s to m3/day). BOC is the total benthic organic carbon standing stock (g C/m2) and is calculated as the sum of CPOC þ FPOC þ UPOC on the streambed. CPOC, FPOC, and UPOC on the streambed are calculated as the g organic carbon in the sample divided by the surface area of the benthic corer/sampler. Mean stream width, w (m) is measured as described previously. 2. Processing rate of organic carbon: KOC ¼ Rhet =BOC þ ðTOCxzÞ

(26.5)

BOC and TOC are benthic organic carbon (g C/m2) and transported organic carbon (g C/m3), as described above, and z is mean stream depth (m). Heterotrophic respiration (Rhet) is estimated from measurements of GPP and ER using the following equation: Rhet ¼ ER  aGPP

(26.6)

where a is the fraction of GPP that is respired by autotrophs; a has been estimated at 0.2 (Young and Huryn, 1999) to 0.5 (Webster and Meyer, 1997). The autotrophic respiration fraction may also be estimated from the relationship between GPP and ER, but only if continuous metabolism data are available (Hall and Beaulieu, 2013). Rates of heterotrophic respiration are then converted to g Cm-2 d-1 by multiplying Rhet (g O2 m2 d1) by a respiratory quotient of 0.85 and the molar ratio of atomic C to O2 (12/32) (see Chapter 34). 3. Organic carbon spiraling length: SOC ¼ VOC =KOC

(26.7)

The organic carbon spiraling length, SOC (m), is defined as the distance organic carbon travels downstream before being respired and is calculated as the downstream velocity of organic carbon, VOC (m/day), divided by the processing rate of organic carbon, KOC (day1).

26.4 QUESTIONS 1. How did CPOM storage differ between the stream reaches? Describe the conditions under which there was greater or lesser carbon storage.

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2. Compare CPOM storage among stream habitats. What might explain the differences among habitats? Consider hydraulics, geomorphology, sediments, and other features. 3. In the leaf release experiment, to what features do you attribute any differences in retention between the two study reaches? Did retention exceed your expectations? 4. If leaves were reinventoried, what were the most important retention structures in the two reaches? Were more leaves retained in pools or in riffles? What were the mechanisms responsible for retention in these two habitat types? 5. Did the exponential model adequately describe the leaf retention patterns? What exactly do the parameters of this model describe? Under what conditions would alternate models be appropriate? 6. In the retention enhancement experiment, how would you expect detritivorous macroinvertebrates to respond in experimentally enhanced CPOM retention in the short term (i.e., scales of days to weeks) and long term (scales of months to years)? How about predators that may respond to invertebrate prey? 7. Describe how higher CPOM standing crops might influence carbon and nutrient (N, P) spiraling lengths. 8. How does carbon spiraling change seasonally in a temperature deciduous forested stream? In what season is spiraling length shortest? Longest? How would this differ in a tropical deciduous forest versus a prairie grassland stream? 9. In light of your findings, discuss the implications of stream and riparian management practices that tend to reduce the amount of wood in streams, to simplify stream channels, or to modify the hydrograph. What restoration approaches would you suggest to increase CPOM storage, retention, and consumption by stream biota?

26.5 MATERIALS AND SUPPLIES Materials for CPOM storage measurement Benthic corer (e.g., bottomless bucket, PVC pipe section, or stovepipe corer) Meter stick to measure water depth 1-mm sieve or hand net Labeled brown paper lunch bags to collect and dry leaves Labeled plastic bags to transport paper bags back to the laboratory Data sheets Small aluminum pans Top-loading balance Desiccator Drying oven Muffle furnace Materials for CPOM release Dried or fresh-fallen leaves (e.g., 2000 abscised G. biloba leaves) Garbage bags (to store leaf batches until released) Buckets [two 20-L (5-gallon), to soak leaves] and brass sieves (if available) Current velocity meter (optional) Field notebook with data sheets Flagging tape Meter tape (100 m, 50 m) Dip (D-frame) nets (1 per investigator) Seine (1 cm mesh) with lead line (at least as long as channel width) Materials for retention enhancement (50-m long, 1-m wide reach with a trap density of 1/m2) Drilling hammer 60 cm L  5 cm W  5 cm D wooden stakes (100) Plastic mesh (w1 cm pore size) Cable ties or binder clips (300) Materials for carbon spiraling estimate 1-mm and 52-mm mesh nets Sample collection cups or plastic bags 2-L and 60-mL HDPE bottles 6N HCl (for acidifying DOC sample) and appropriate PPE Current velocity meter NaCl solution, conductivity meter

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SECTION j D Organic Matter Dynamics

Meter stick and meter tape Benthic corer (e.g., bottomless bucket or stovepipe core) Logging dissolved oxygen sensor Data sheets Small aluminum pans 0.7- and 0.45-mm glass fiber filters Filtration system Top-loading balance Drying oven Muffle furnace Total organic carbon analyzer

ACKNOWLEDGMENTS We thank all those individuals who have inspired and instructed us in the wonders of organic matter dynamics, especially Mike Dobson, Mark Gessner, Stephen Golladay, Stanley Gregory, Patrick Mulholland, J. Denis Newbold, Jennifer Tank, J. Bruce Wallace, and Jack Webster.

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