Robert O. Hall

Can't See the Forest for the Stream? In-stream Processing and Terrestrial Nitrogen Exports

Abstract There has been a long-term decline in nitrate (NO3−) concentration and export from several long-term monitoring watersheds in New England that cannot be explained by current terrestrial ecosystem models. A number of potential causes for this nitrogen (N) decline have been suggested, including changes in atmospheric chemistry, insect outbreaks, soil frost, and interannual climate fluctuations. In-stream removal of NO3− has not been included in current attempts to explain this regional decline in watershed NO3− export, yet streams may have high removal rates of NO3−. We make use of 40 years of data on watershed N export and stream N biogeochemistry from the Hubbard Brook Experimental Forest (HBEF) to determine (a) whether there have been changes in HBEF stream N cycling over the last four decades and (b) whether these changes are of sufficient magnitude to help explain a substantial proportion of the unexplained regional decline in NO3− export. Examining how the tempos and modes of change are distinct for upland forest and stream ecosystems is a necessary step for improving predictions of watershed exports.

Improving the fluorometric ammonium method: matrix effects, background fluorescence, and standard additions

Abstract Our understanding of the N cycle is affected by how accurately we can measure NH4+ in natural waters. Measuring NH4+ concentrations requires accounting for matrix effects (ME) that are caused by substances in the sample that attenuate or intensify the signal of the samples relative to the standards. We show that the ME calculation in the recently published fluorometric NH4+ method is mathematically incorrect, producing results that consistently underestimate NH4+ concentration as a nonlinear function of the ME. We provide the correct equation and offer an alternative approach that accounts for ME by using sample water rather than deionized water to make the standards, thereby producing a standard curve that contains the same background chemical properties as the samples. In addition, we show that the previous method for measuring a samples background fluorescence does not include the background signal of the reagent or its interaction with the matrix constituents of the sample. We provide a new method for measuring a samples background fluorescence that includes the background fluorescence of the sample, reagent, and their interaction. The simple changes we suggest produce more accurate and precise NH4+ measurements.

Particle transport and transient storage along a stream-size gradient in the Hubbard Brook Experimental Forest

The transport and deposition of fine particulate organic matter (FPOM) is an important flux linking upstream and downstream reaches of stream ecosystems. However, few studies have attempted to identify physical controls on particle transport. One reason has been the lack of relatively simple, inexpensive methods. We describe a new technique for measuring fine particle transport in streams using fluoresently labeled yeast as FPOM analogs. We used steady state injections of yeast and a conservative tracer (Cl) in 6 reaches along a stream continuum at the Hubbard Brook Experimental Forest to explore the relationship between hydrologic properties of stream reaches and particle transport. The yeast technique is relatively easy and inexpensive, and measures of fine particle transport derived from this approach were comparable to those obtained for natural seston and other seston analogs in similarly sized streams. The transport distance of yeast particles (Sp) increased along the stream continuum. Sp was negatively correlated with relative transient storage (k1/k2) and positively correlated with hydrologic exchange rates of the main channel (k1) and transient storage (k2). The depositional velocity of yeast (vdep), which normalizes average transport distances for stream velocity and depth, showed no trend along the continuum and was not related to k1, k2, or k1/k2. Together, these results suggest that velocity and depth were the most important factors in determining differences in particle transport along the continuum.

Hyporheic invertebrates affect N cycling and respiration in stream sediment microcosms

Abstract The region of surface water–groundwater interaction in streams, the hyporheic zone, is important for biogeochemical processes and provides habitat for specialized microbial and invertebrate assemblages. Although hyporheic invertebrates contribute little biomass and respiration relative to microbes in stream sediments, invertebrate effects on biogeochemical processes may be disproportionately large. We tested how various interstitial invertebrate assemblages affected N cycling and respiration in flow-through microcosms filled with alluvial sediment in the laboratory. Average invertebrate biomasses in low and high invertebrate treatments were 0.20 and 19 mg dry mass/L sediment, respectively. Average net NO3− regeneration/uptake rate increased with increasing invertebrate biomass, showing invertebrates suppressed NO3− uptake or stimulated in situ NO3− production. Average respiration (normalized for sediment organic matter) and particulate organic matter (POM) increased 51% and 33%, respectively, with increasing invertebrate biomass, suggesting direct contribution to hyporheic metabolism and/or stimulation of microbial activity and an accumulation of POM driven by invertebrates. We suggest that interstitial invertebrates can substantially alter biogeochemical processes in hyporheic zones.

Introduced Lake Trout Produced a Four-Level Trophic Cascade in Yellowstone Lake

Abstract Introduction of lake trout Salvelinus namaycush into a system can add a trophic level, potentially affecting organisms at lower trophic levels. Similar to many lakes and reservoirs in the western United States, lake trout were introduced into Yellowstone Lake, Wyoming. Previous studies showed that lake trout reduced the population and altered the size structure of native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, but we sought to determine the degree to which lake trout predation changed lower trophic levels. We predicted that the structure of lower trophic levels would change in conformance with trophic cascade theory because Yellowstone cutthroat trout consume zooplankton. We compared zooplankton and phytoplankton assemblages between the period when Yellowstone cutthroat trout were abundant and the period after they declined. As predicted by trophic cascade theory, zooplankton biomass shifted from being dominated by copepods before lake trout introduction to ...

The influence of floodplain restoration on whole-stream metabolism in an agricultural stream: insights from a 5-year continuous data set

Abstract: Channelized streams are common in North American agricultural regions, where they minimize water residence time and biological nutrient processing. Floodplain restoration done via the 2-stage-ditch management strategy can improve channel stability and nutrient retention during storms. We examined the influence of floodplain restoration on whole-stream metabolism by measuring gross primary production (GPP) and ecosystem respiration (ER) for 1 y before and 4 y after restoration of an upstream, unaltered control reach and a downstream, restored reach. Both reaches were biologically active and dynamic. GPP ranged from 0.1 to 22.1 g O2 m-2 d-1, and ecosystem respiration (ER) rates ranged from -0.1 to -38.7 g O2 m-2 d-1. We used time-series analysis and found that GPP increased postrestoration during floodplain inundation when expressed per unit length, but not per unit area, of stream. GPP was more resilient post- than prerestoration and returned to prestorm levels more quickly after than before floodplain construction. In contrast, the floodplain restoration had no effect on ER or on any metric of metabolism during base flow. Overall, we showed that floodplain—stream linkages can be important regulators of metabolism in restored agricultural streams.

Metabolism of Streams and Rivers: Estimation, Controls, and Application

Abstract Ecosystem metabolism is a fundamental property of streams and rivers that comprises carbon fixation as gross primary production (GPP) and mineralization as respiration by all organisms in the ecosystem (ER). Ecologists estimate GPP and ER at a stream reach scale by measuring dissolved oxygen throughout a day and converting these data to metabolism given an estimate of the air-water gas exchange flux. Use of this method has increased greatly in the past decade due to ease of data collection and statistical modeling methods, in particular estimates of the difficult to measure gas exchange. Here, I review recent improvements in methods to estimate GPP and ER, examine physical and biotic controls on rates, and address future applications of this technique. Nearly, all streams are heterotrophic (i.e., GPP

Ammonium uptake kinetics and nitrification in mountain streams

Human activities have increased the availability and distribution of dissolved inorganic N (DIN) in the biosphere. Streams can remove some of this excess DIN, but in-stream uptake pathways of NO3− and NH4+ can be sensitive to the concentration of DIN. DIN uptake kinetics (i.e., changes in uptake in response to changes in concentration) are used to predict how streams will respond to increased DIN. This concentration–uptake relationship is unclear for NH4+ because of complex interactions governing uptake, including the influence of dissimilatory processes (i.e., nitrification) on total uptake. We used sequential pulse additions of NO3− and NH4+ to investigate DIN uptake in 3 subalpine streams in Rocky Mountain National Park, Colorado, USA. Mass removal rates (U; µg m–2 min–1) of NO3− were higher than NH4+, but NH4+ had greater uptake rate relative to concentration (vf; mm/min). We used a new method of estimating nitrification that accounts for concomitant NH4+ uptake and found that 7 to 19% of the NH4+ added was nitrified immediately. We used the Tracer Additions for Spiraling Curve Characterization (TASCC) method to quantify ambient spiraling parameters and to parameterize kinetic models for NH4+ by following changes in whole-reach uptake in response to short-term NH4+ additions. Relationships between NH4+ concentration and vf were consistent within, but not among, streams. The relationship between vf and NH4+ concentration followed an efficiency-loss model for 2 streams, in which vf decreased exponentially with NH4+ concentration. vf increased with NH4+ concentration in 1 stream on 2 separate occasions. Different NH4+ uptake kinetics across streams indicate site-specific variation in processes controlling uptake and that some streams may accommodate increasing NH4+ concentrations by increasing uptake.

High Diet Overlap between Native Small-Bodied Fishes and Nonnative Fathead Minnow in the Colorado River, Grand Canyon, Arizona

AbstractRiver regulation may mediate the interactions among native and nonnative species, potentially favoring nonnative species and contributing to the decline of native populations. We examined food resource use and diet overlap among small-bodied fishes in the Grand Canyon section of the Colorado River as a first step in evaluating potential resource competition. We compared the diets of the predominant small-bodied fishes (native Speckled Dace Rhinichthys osculus, juvenile Flannelmouth Sucker Catostomus latipinnis, and juvenile Bluehead Sucker C. discobolus, and nonnative Fathead Minnow Pimephales promelas) across seasons at four sites downstream of Glen Canyon Dam using nonmetric multidimensional scaling and Schoeners similarity index. The diets of these fishes included diatoms, amorphous detritus, aquatic invertebrates (especially simuliid and chironomid larvae), terrestrial invertebrates, and terrestrial vegetation. Diets varied with season and were affected by high turbidity. Fish consumed more a...

Chapter 34 – Stream Metabolism

Stream metabolism, or gross primary production and ecosystem respiration, is a fundamental property of stream ecosystems and is simple to measure in an instructional setting. In this chapter, we present theory on measuring stream metabolism based on diel oxygen curves, describe field methods to collect oxygen and associated data, and detail analytical methods for converting oxygen data to metabolism. The difficult aspect of estimating metabolism is quantifying air-water gas exchange and we present field and modeling methods to estimate this parameter. We also provide supplemental R code and spreadsheets to simplify the calculations.