Natalie A. Griffiths

A review of allochthonous organic matter dynamics and metabolism in streams

Abstract The role of allochthonous organic matter in lotic ecosystems has been an important research topic among aquatic ecologists since the seminal work by Lindeman was published in 1942. Since 1986, studies on organic matter budgets, ecosystem metabolism, and decomposition published in J-NABS have made significant contributions to the overall understanding of organic matter dynamics in streams. In this review, we summarize the utility of organic matter budgets, cover the major advances in research on ecosystem metabolism, and describe the intrinsic and extrinsic factors influencing organic matter decomposition. We also discuss future directions and current applications of research and highlight the need for additional studies on the role of land use and climate change, as well as continued use of organic matter processing as a functional metric in biomonitoring studies. We emphasize the need for continued data synthesis into comprehensive organic matter budgets. Such comparative studies can elucidate important drivers of organic matter dynamics and can assist in the understanding of large continental/global changes that might be occurring now and in the near future. In general, continued emphasis on synthesizing information into a larger framework for streams and rivers will improve our overall understanding of the importance of organic matter in lotic ecosystems.

Toxins in transgenic crop byproducts may affect headwater stream ecosystems

Corn (Zea mays L.) that has been genetically engineered to produce the Cry1Ab protein (Bt corn) is resistant to lepidopteran pests. Bt corn is widely planted in the midwestern United States, often adjacent to headwater streams. We show that corn byproducts, such as pollen and detritus, enter headwater streams and are subject to storage, consumption, and transport to downstream water bodies. Laboratory feeding trials showed that consumption of Bt corn byproducts reduced growth and increased mortality of nontarget stream insects. Stream insects are important prey for aquatic and riparian predators, and widespread planting of Bt crops has unexpected ecosystem-scale consequences.

Occurrence of maize detritus and a transgenic insecticidal protein (Cry1Ab) within the stream network of an agricultural landscape

Widespread planting of maize throughout the agricultural Midwest may result in detritus entering adjacent stream ecosystems, and 63% of the 2009 US maize crop was genetically modified to express insecticidal Cry proteins derived from Bacillus thuringiensis. Six months after harvest, we conducted a synoptic survey of 217 stream sites in Indiana to determine the extent of maize detritus and presence of Cry1Ab protein in the stream network. We found that 86% of stream sites contained maize leaves, cobs, husks, and/or stalks in the active stream channel. We also detected Cry1Ab protein in stream-channel maize at 13% of sites and in the water column at 23% of sites. We found that 82% of stream sites were adjacent to maize fields, and Geographical Information Systems analyses indicated that 100% of sites containing Cry1Ab-positive detritus in the active stream channel had maize planted within 500 m during the previous crop year. Maize detritus likely enters streams throughout the Corn Belt; using US Department of Agriculture land cover data, we estimate that 91% of the 256,446 km of streams/rivers in Iowa, Illinois, and Indiana are located within 500 m of a maize field. Maize detritus is common in low-gradient stream channels in northwestern Indiana, and Cry1Ab proteins persist in maize leaves and can be measured in the water column even 6 mo after harvest. Hence, maize detritus, and associated Cry1Ab proteins, are widely distributed and persistent in the headwater streams of a Corn Belt landscape.

Forecasting functional implications of global changes in riparian plant communities

Riparian ecosystems support mosaics of terrestrial and aquatic plant species that enhance regional biodiversity and provide important ecosystem services to humans. Species composition and the distribution of functional traits – traits that define species in terms of their ecological roles – within riparian plant communities are rapidly changing in response to various global change drivers. Here, we present a conceptual framework illustrating how changes in dependent wildlife communities and ecosystem processes can be predicted by examining shifts in riparian plant functional trait diversity and redundancy (overlap). Three widespread examples of altered riparian plant composition are: shifts in the dominance of deciduous and coniferous species; increases in drought-tolerant species; and the increasing global distribution of plantation and crop species. Changes in the diversity and distribution of critical plant functional traits influence terrestrial and aquatic food webs, organic matter production and pro...

Rapid decomposition of maize detritus in agricultural headwater streams

Headwater streams draining agricultural landscapes receive maize leaves (Zea mays L.) via wind and surface runoff, yet the contribution of maize detritus to organic-matter processing in agricultural streams is largely unknown. We quantified decomposition and microbial respiration rates on conventional (non-Bt) and genetically engineered (Bt) maize in three low-order agricultural streams in northwestern Indiana, USA. We also examined how substrate quality and in-stream nutrient concentrations influenced microbial respiration on maize by comparing respiration on maize and red maple leaves (Acer rubrum) in three nutrient-rich agricultural streams and three low-nutrient forested streams. We found significantly higher rates of microbial respiration on maize vs. red maple leaves and higher rates in agricultural vs. forested streams. Thus both the elevated nutrient status of agricultural streams and the lability of maize detritus (e.g., low carbon-to-nitrogen ratio and low lignin content) result in a rapid incorporation of maize leaves into the aquatic microbial food web. We found that Bt maize had a faster decomposition rate than non-Bt maize, while microbial respiration rates did not differ between Bt and non-Bt maize. Decomposition rates were not negatively affected by genetic engineering, perhaps because the Bt toxin does not adversely affect the aquatic microbial assemblage involved in maize decomposition. Additionally, shredding caddisflies, which are known to have suppressed growth rates when fed Bt maize, were depauperate in these agricultural streams, and likely did not play a major role in maize decomposition. Overall, the conversion of native vegetation to row-crop agriculture appears to have altered the quantity, quality, and predictability of allochthonous carbon inputs to headwater streams, with unexplored effects on stream ecosystem structure and function.

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.

You are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies

Analyses of 21 15 N stable isotope tracer experiments, designed to examine food web dynamics in streams around the world, indicated that the isotopic composition of food resources assimilated by primary consumers (mostly invertebrates) poorly reflected the presumed food sources. Modeling indicated that consumers assimilated only 33-50% of the N available in sampled food sources such as decomposing leaves, epilithon, and fine particulate detritus over feeding periods of weeks or more. Thus, common methods of sampling food sources consumed by animals in streams do not sufficiently reflect the pool of N they assimilate. Isotope tracer studies, combined with modeling and food separation techniques, can improve estimation of N pools in food sources that are assimilated by consumers. Food web studies that use putative food samples composed of actively cycling (more readily assimilable) and refractory (less assimilable) N fractions may draw erroneous conclusions about diets, N turnover, and trophic linkages of consumers. By extension, food web studies using stoichiometric or natural abundance approaches that rely on an accurate description of food-source composition could result in errors when an actively cycling pool that is only a fraction of the N pool in sampled food resources is not accounted for.

Global synthesis of the temperature sensitivity of leaf litter breakdown in streams and rivers

Abstract Streams and rivers are important conduits of terrestrially derived carbon (C) to atmospheric and marine reservoirs. Leaf litter breakdown rates are expected to increase as water temperatures rise in response to climate change. The magnitude of increase in breakdown rates is uncertain, given differences in litter quality and microbial and detritivore community responses to temperature, factors that can influence the apparent temperature sensitivity of breakdown and the relative proportion of C lost to the atmosphere vs. stored or transported downstream. Here, we synthesized 1025 records of litter breakdown in streams and rivers to quantify its temperature sensitivity, as measured by the activation energy (Ea, in eV). Temperature sensitivity of litter breakdown varied among twelve plant genera for which Ea could be calculated. Higher values of Ea were correlated with lower‐quality litter, but these correlations were influenced by a single, N‐fixing genus (Alnus). Ea values converged when genera were classified into three breakdown rate categories, potentially due to continual water availability in streams and rivers modulating the influence of leaf chemistry on breakdown. Across all data representing 85 plant genera, the Ea was 0.34 ± 0.04 eV, or approximately half the value (0.65 eV) predicted by metabolic theory. Our results indicate that average breakdown rates may increase by 5–21% with a 1–4 °C rise in water temperature, rather than a 10–45% increase expected, according to metabolic theory. Differential warming of tropical and temperate biomes could result in a similar proportional increase in breakdown rates, despite variation in Ea values for these regions (0.75 ± 0.13 eV and 0.27 ± 0.05 eV, respectively). The relative proportions of gaseous C loss and organic matter transport downstream should not change with rising temperature given that Ea values for breakdown mediated by microbes alone and microbes plus detritivores were similar at the global scale. &NA; Warmer water enhances decomposition of organic matter in streams and rivers, but it is unclear if climate change will result in more carbon emitted to the atmosphere or transported to the ocean. We assembled over 1000 published data points on leaf litter breakdown in streams and rivers globally to assess how rates of breakdown will change with elevated temperature. Across 85 plant genera, we found that rates may increase only half as much as expected should water temperature rise by 1–4 °C. Among 12 plant genera for which temperature sensitivity could be calculated individually, higher sensitivity was correlated with lower quality litter. Similarity in the temperature sensitivity of breakdown mediated by microbes alone or microbes plus detritivores suggests the relative proportions of carbon converted to gas or transported as smaller particles will not change with elevated temperature. Figure. No caption available.

Organic-matter decomposition along a temperature gradient in a forested headwater stream

We used a natural temperature gradient in Walker Branch, a spring-fed forested stream in eastern Tennessee, USA, to examine the influence of temperature on organic-matter decomposition. In this stream, upstream sites are warmer than downstream sites in winter and are cooler than downstream sites in summer. We used a cotton-strip assay to examine breakdown of a substrate of uniform quality (95% cellulose) along the temperature gradient monthly for 2 y. We also used litter bags to examine the interactive effects of leaf-litter quality (labile red maple [Acer rubrum] and tulip poplar [Liriodendron tulipifera] and less labile white oak [Quercus alba]), invertebrates, and temperature on breakdown rates along the downstream temperature gradient for 90 d in winter. Cotton-strip tensile loss and leaf-litter breakdown rates were highly variable. Tensile-loss rates probably were driven by a combination of daily and diel temperature, discharge, and streamwater nutrients that varied seasonally and spatially along the temperature gradient. Leaf-litter breakdown rates tended to be faster in warmer upstream sites (red maple = 0.0452/d, tulip poplar = 0.0376/d, white oak = 0.0142/d) and slower in cooler downstream sites (red maple = 0.0312/d, tulip poplar = 0.0236/d, white oak = 0.0107/d), and breakdown rates were positively correlated with total invertebrate density. Temperature sensitivity of decomposition was similar among the 3 litter types. These results highlight the high degree of spatial and temporal heterogeneity that can exist for ecosystem processes and their drivers. Quantifying this heterogeneity is important when scaling functional metrics to stream and watershed scales and for understanding how organic-matter processing will respond to the warmer streamwater temperatures expected as a result of global climate change.

Decomposition of maize leaves and grasses in restored agricultural streams

Abstract Headwater streams draining row-crop agriculture receive allochthonous inputs of maize detritus and grasses, but organic matter (OM) processing is not well studied in agricultural streams. Agricultural streams in the midwestern USA have incised, trapezoidal channels that retain less particulate OM than forested streams. The 2-stage ditch is a restoration strategy in which small floodplains are constructed and connected to stream channels to increase channel stability and decrease erosion. Microbial decomposition may be higher on restored floodplains because water residence times are longer than on the steep banks of trapezoidal streams. We examined decomposition of maize leaves (Zea mays), native rice cutgrass (Leersia oryzoides), and invasive reed canary grass (Phalaris arundinacea) in 4 restored streams. We measured breakdown rates in the main channel of upstream control reaches (incised, trapezoidal channel), the main channel of downstream treatment reaches (restored with constructed floodplains), steep control banks, and treatment floodplains. OM decomposed faster in channels than on banks and floodplains, and maize decomposed faster (stream k  =  0.0160/d, riparian k  =  0.0040/d) than rice cutgrass (stream k  =  0.0065/d, riparian k  =  0.0018/d) and reed canary grass (stream k  =  0.0036/d, riparian k  =  0.0014/d) probably because lignin and N content differed. Breakdown rates varied among streams because of differences in shredder density (primarily Isopoda: Lirceus and Caecidotea) and water temperature. Floodplain restoration did not affect breakdown rates. Floodplains of 3 streams were inundated longer than steep banks in upstream control reaches, but breakdown rate and inundation duration were not related. However, OM must be retained within the stream to be available for decomposition. Thus, the floodplains may promote the retention of OM, and ultimately, incorporation of maize and grasses into headwater-stream food webs.