Jeremy B. Jones

Material Spiraling in Stream Corridors: A Telescoping Ecosystem Model

ABSTRACT Stream ecosystems consist of several subsystems that are spatially distributed concentrically, analogous to the elements of a simple telescope. Subsystems include the central surface stream, vertically and laterally arrayed saturated sediments (hyporheic and parafluvial zones), and the most distal element, the riparian zone. These zones are hydrologically connected; thus water and its dissolved and suspended load move through all of these subsystems as it flows downstream. In any given subsystem, chemical transformations result in a change in the quantity of materials in transport. Processing length is the length of subsystem required to “process” an amount of substrate equal to advective input. Long processing lengths reflect low rates of material cycling. Processing length provides the length dimension of each cylindrical element of the telescope and is specific to subsystem (for example, the surface stream), substrate (for instance, nitrate), and process (denitrification, for example). Disturbance causes processing length to increase. Processing length decreases during succession following disturbance. The whole stream-corridor ecosystem consists of several nested cylindrical elements that extend and retract, much as would a telescope, in response to disturbance regime. This telescoping ecosystem model (TEM) can improve understanding of material retention in running water systems; that is, their “nutrient filtration” capacity. We hypothesize that disturbance by flooding alters this capacity in proportion to both intensity of disturbance and to the relative effect of disturbance on each subsystem. We would expect more distal subsystems (for example, the riparian zone) to show the highest resistance to floods. In contrast, we predict that postflood recovery of functions such as material processing (that is, resilience) will be highest in central elements and decrease laterally. Resistance and resilience of subsystems are thus both inversely correlated and spatially separated. We further hypothesize that cross-linkages between adjacent subsystems will enhance resilience of the system as a whole. Whole-ecosystem retention, transformation, and transport are thus viewed as a function of subsystem extent, lateral and vertical linkage, and disturbance regime.

Denitrification in a nitrogen-limited stream ecosystem

Denitrification was measured in hyporheic, parafluvial, and bank sediments of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. We used three variations of the acetylene block technique to estimate denitrification rates, and compared these estimates to rates of nitrate production through nitrification. Subsurface sediments of Sycamore Creek are typically well-oxygenated, relatively low in nitrate, and low in organic carbon, and therefore are seemingly unlikely sites of denitrification. However, we found that denitrification potential (C & N amended, anaerobic incubations) was substantial, and even by our conservative estimates (unamended, oxic incubations and field chamber nitrous oxide accumulation), denitrification consumed 5–40% of nitrate produced by nitrification. We expected that denitrification would increase along hyporheic and parafluvial flowpaths as dissolved oxygen declined and nitrate increased. To the contrary, we found that denitrification was generally highest at the upstream ends of subsurface flowpaths where surface water had just entered the subsurface zone. This suggests that denitrifiers may be dependent on the import of surface-derived organic matter, resulting in highest denitrification rate at locations of surface-subsurface hydrologic exchange. Laboratory experiments showed that denitrification in Sycamore Creek sediments was primarily nitrogen limited and secondarily carbon limited, and was temperature dependent. Overall, the quantity of nitrate removed from the Sycamore Creek ecosystem via denitrification is significant given the nitrogen-limited status of this stream.

Surface-subsurface interactions in stream ecosystems

Stream ecologists have recently recognized that sediments below streams play an important role in lotic ecosystems. Water flows not only across the surface of stream channels, but also through sediment interstices; consequently, surface and subsurface biogeochemical processes are linked. Recent attempts to understand the influence of subsurface processes on stream ecosystems have tried to resolve the surface-subsurface hydrologic interactions, and to gain knowledge of the ecology of subsurface organisms.

Nitrification in the hyporheic zone of a desert stream ecosystem

Nitrification in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream, was examined, focusing on the association between respiration and nitrate production. Subsurface respiration in Sycamore Creek is highest in regions of hydrologic downwelling where organic matter derived from the stream surface is transported into the hyporheic zone. Similarly, nitrification was closely related to hydrologic exchange between the surface and hyporheic zone. Nitrification in downwelling regions averaged 13.1 μ gNO3-N· L sediments-1· h-1 compared with 1.7 μ gNO3-N· L sediments-1· h-1 in upwelling regions. Hyporheic respiration also varies temporally as a result of flash floods which scour and remove algae from the stream and thus reduce the pool of organic matter to support subsurface metabolism. Nitrification was also significantly affected by flooding; nitrification increased from an average of only 3.0 μ gNO3-N· L sediments-1· h-1 immediately following floods to 38.5 μ gNO3-N· L sediments-1· h-1 late in succession. Nitrification was significantly correlated with hyporheic respiration, supporting the hypothesis that nitrification is fueled by mineralization of organic nitrogen to ammonium. The coupling between subsurface respiration and nitrification is one step in a cyclic interaction between surface and hyporheic zones and serves to transform nitrogen from an organic to inorganic form.

Seasonal export of carbon, nitrogen, and major solutes from Alaskan catchments with discontinuous permafrost

[1] Frequent measurements of stream chemistry during snowmelt and summer storms were used in three watersheds that differ in permafrost coverage (high, 53%; medium, 18%; and low, 4%) to determine the role of water flow paths on the fluxes of carbon, nitrogen, and major solutes from Alaskan catchments. Permafrost was important in the seasonal pattern of stream chemistry as there was a distinct shift in chemistry and flow from winter through snowmelt and into summer in the permafrost-dominated catchment. Furthermore, the active layer above the permafrost was important for the late summer release of NO3 and DOC, suggesting a deeper active layer may increase N and C loss in permafrost-dominated areas. Overall, permafrost constrained water flow to the active layer, resulting in higher DOC but lower dissolved mineral fluxes (Ca 2+ Mg 2+ K + Na + )i n the high-permafrost watershed than in the watersheds with less permafrost coverage. However, the decline in dissolved mineral fluxes was not linearly related to permafrost coverage across watersheds. The flux of weathering ions may also be explained by total water runoff, since the medium-permafrost watershed, which had the greatest runoff on an areal basis, yielded the greatest loss of all major elements (Ca 2+ Mg 2+ K + Na + SO4 NO3 NH4 Cl) except DOC. Despite differences among watersheds in permafrost coverage, hydrologic flow paths, area, and total runoff, all watersheds were net sources of every individual ions or elements (Cl ,P O4 ,S O4 , DOC, DON, NO3 ,N a + ,K + Mg 2+ , Ca 2+ ) except NH4 , which was a small fraction of the total N concentration in streams.

Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole Catchment Soil Respiration

ABSTRACT The concentration of CO2 in stream water is a product of not only instream metabolism but also upland, riparian, and groundwater processes and as such can provide an integrative measure of whole catchment soil respiration. Using a 5-year dataset of pH, alkalinity, Ca2+, and Mg2+ in surface water of the West Fork of Walker Branch in eastern Tennessee in conjunction with a hydrological flowpath chemistry model, we investigated how CO2 concentrations and respiration rates in stream, bedrock, and soil environments vary seasonally and interannually. Dissolved inorganic carbon concentration was highest in summer and autumn (P < 0.05) although the proportion as free CO2 (pCO2) did not vary seasonally (P > 0.05). Over the 5 years, pCO2 was always supersaturated with respect to the atmosphere ranging from 374 to 3626 ppmv (1.0- to 10.1-fold greater than atmospheric equilibrium), and CO2 evasion from the stream to the atmosphere ranged from 146 to 353 mmol m−2 d−1. Whereas pCO2 in surface water exhibited little intra-annual or interannual variation, distinct seasonal patterns in soil and bedrock pCO2 were revealed by the catchment CO2 model. Seasonally, soil pCO2 increased from a winter low of 8167 ppmv to a summer high of 27,068 ppmv. Driven by the seasonal variation in gas levels, evasion of CO2 from soils to the atmosphere ranged from 83 mmol m−2 d−1 in winter to 287 mmol m−2 d−1 in summer. The seasonal variation in soil CO2 tracked soil temperature (r2= 0.46, P < 0.001) and model-derived estimates of CO2 evasion rate from soils agreed with previously reported fluxes measured using chambers (Pearson correlation coefficient = 0.62, P < 0.05) supporting the model assumptions. Although rates of CO2 evasion were similar between the stream and soils, the overall rate of evasion from the channel was only 0.4% of the 70,752 mol/d that evaded from soils due to the vastly different areas of the two subsystems. Our model provides a means to assess whole catchment CO2 dynamics from easily collected and measured stream-water samples and an approach to study catchment scale variation in soil ecosystem respiration.

Transport and retention of particulate organic matter in two low-gradient headwater streams

Transport and retention of particulate organic matter (POM) were examined in the channels and on the floodplains of two low-gradient headwater streams on the Coastal Plain of southeastern Virginia. During base discharge, POM was primarily retained as it settled onto the sediment surface, but during high discharge, debris dams became primary retainers. During overbank flooding much of the coarse particulate organic matter (CPOM) moved from the channels onto the floodplains. The mean distance that wood moved over a year at Colliers Creek, which had low current velocity and a broad, frequently inundated floodplain, was 23 m in the channel and 46 m on the floodplain; at Buzzards Branch, with higher current velocity and a smaller, less-frequently inundated floodplain, wood moved a mean distance of 136 m in the channel and only 2 m on the floodplain. Mean leaf transport distances in the channels ranged from 1.6 m in Colliers Creek during summer base discharge to 156 m at Buzzards Branch during a winter spate; mean leaf transport distances on the floodplains were 0.5-1.7 m. Fine particulate organic matter (FPOM) transport, studied only during base discharge, was farther than that of wood or leaves and varied from 1.8 m during summer base discharge in the Colliers Creek channel to 84.0 m during winter base discharge in the Buzzards Branch channel. Over one year, 97% and 27% of the marked wood placed in the channels was transported into the Colliers Creek and Buzzards Branch floodplains, respectively; only 4-9% of the marked wood placed in the floodplains moved into the channels. Base flow flux of POM was 24,000 kg/yr and 1700 kg/yr on the floodplains, and 7000 kg/yr and 13,000 kg/yr in the channels, at Colliers Creek and Buzzards Branch, respectively. Retention, transport distance, and magnitude of POM exchange between the channels and floodplains were dependent on the timing, frequency and extent of spates, and floodplain inundation, which thus were critical determinants of POM dynamics and hence system structure and function.

Influence of drainage basin topography and elevation on carbon dioxide and methane supersaturation of stream water

The partial pressures of CO2 (pCO2) andCH4 (pCH4) in streams are not only governed byinstream processes, but also by transformations occurring in soil andgroundwater ecosystems. As such, stream water pCO2 andpCH4 can provide a tool to assess ecosystem respiration andanaerobic metabolism throughout drainage basins. We conducted three surveyssampling the gas content of streams in eastern Tennessee and western NorthCarolina to assess factors regulating ecosystem metabolism in catchmentswith contrasting geomorphologies, elevations and soil organic matterstorage. In our first survey, the influence of drainage basin geomorphologyon ecosystem respiration was examined by sampling streams drainingcatchments underlain by either shale or dolomite. Geomorphology isinfluenced by geology with shale catchments having shallower soils, broader,unconstrained valley floors compared with dolomite catchments.pCO2 varied little between catchment types but increased froman average of 3340 ppmv in spring to 9927 ppmv in summer or 9.3 and 28 timesatmospheric equilibrium (pCO2(equilib)), respectively. Incontrast, pCH4 was over twice as high in streams drainingshale catchments (306 ppmv; pCH4(equilib) = 116) compared withmore steeply incised dolomite basins (130 ppmv; pCH4(equilib)= 51). Using the ratio of pCH4:pCO2 as an indexof anaerobic metabolism, shale catchments had nearly twice as muchanaerobiosis (pCH4:pCO2 = 0.046) than dolomitedrainages (pCH4:pCO2 = 0.024). In our secondsurvey, streams were sampled along an elevational gradient (525 to 1700 m)in the Great Smoky Mountains National Park, USA where soil organic matterstorage increases with elevation. pCO2 did not vary betweenstreams but increased from 5340 ppmv (pCO2(equilib) = 15) to8565 ppmv (pCO2(equilib) = 24) from spring to summer,respectively. During spring pCH4 was low and constant acrossstreams, but during summer increased with elevation ranging from 17 to 2068ppmv (pCH4(equilib) = 10 to 1216). The contribution ofanaerobiosis to total respiration was constant during spring(pCH4:pCO2 = 0.017) but during summer increasedwith elevation from 0.002 at 524 m to 0.289 at 1286 m. In our last survey,we examined how pCO2 and pCH4 changed withcatchment size along two rivers (ca. 60 km stretches in both riverscorresponding to increases in basin size from 1.7–477km2 and 2.5–275 km2). pCO2and pCH4 showed opposite trends, with pCO2decreasing ca. 50% along the rivers, whereas pCH4roughly doubled in concentration downstream. These opposing shifts resultedin a nearly five-fold increase of pCH4:pCO2along the rivers from a low of 0.012 in headwaters to a high of 0.266 65-kmdownstream. pCO2 likely declines moving downstream asgroundwater influences on stream chemistry decreases, whereaspCH4 may increase as the prevalence of anoxia in riversexpands due to finer-grained sediments and reduced hydrologic exchange withoxygenated surface water.

Resilience of Alaska's Boreal Forest to Climatic Change

This paper assesses the resilience of Alaska’s boreal forest system to rapid climatic change. Recent warming is associated with reduced growth of dominant tree species, plant disease and insect outbreaks, warming and thawing of permafrost, drying of lakes, increased wildfire extent, increased postfire recruitment of deciduous trees, and reduced safety of hunters traveling on river ice. These changes have modified key structural features, feedbacks, and interactions in the boreal forest, including reduced effects of upland permafrost on regional hydrology, expansion of boreal forest into tundra, and amplification of climate warming because of reduced albedo (shorter winter season) and carbon release from wildfires. Other temperature-sensitive processes for which no trends have been detected include composition of plant and microbial communities, long-term landscape-scale change in carbon stocks, stream discharge, mammalian population dynamics, and river access and subsistence opportunities for rural indige...

Benthic Organic Matter Storage in Streams: Influence of Detrital Import and Export, Retention Mechanisms, and Climate

In lotic ecosystems, BOM is a major energy source for secondary production (Minshall 1967, Benke et al. 1984), influences nutrient cycles (Mulholland et al. 1985), and affects export of DOM and POM (Bilby and Likens 1980, Bilby 1981, Smock et al. 1989). Benthic detritus also influences channel stability and retention characteristics (Keller and Swanson 1979, Mosley 1981, Webster et al. 1994) and provides habitat for stream microorganisms, macroinvertebrates (Benke et al. 1984, Huryn and Wallace 1987), and fish (Angermeirer and Karr 1984, Elliott 1986). However, in spite of the great importance of BOM to stream ecosystem function, benthic detrital storage is one of the most poorly understood components of stream organic matter budgets (Cummins et al. 1983). Storage of BOM is governed by a broad spectrum of processes. At the scale of the river continuum, BOM tends to decrease downstream as channels become larger and riparian influences decline (Naiman and Sedell 1979, Minshall et al. 1983, Conners and Naiman 1984, Naiman et al. 1987). Individual reaches, however, are influenced not only by their position along a river continuum but also by local variations such as the character of riparian vegetation (Minshall et al. 1983, Gurtz et al. 1988) and floodplain size. Within reaches, BOM distribution is further affected by retention mechanisms, including debris dams and pools (Huryn and Wallace 1987, Smock et al. 1989, Trotter 1990, Jones and Smock 1991), channel characteristics like gradient, and interactions between main-channels and flood-