Leaf-litter decomposition is a key ecosystem function that fuels aquatic food webs through the consumption of diverse leaf litter by invertebrates. During decomposition, coarse and fine particulate organic matter (CPOM and FPOM, respectively) are produced, which differentially support aquatic trophic systems. Much less is understood about FPOM, and there has been little study on the role of leaf-litter properties in the production of FPOM. In this study we asked how leaf species, leaf diversity, and microbial preconditioning of leaf litter influence FPOM production rates and FPOM particle-size distributions by macroinvertebrate shredders. We conducted microcosm experiments at the WasserCluster Lunz Biological Station (Austria) using leaves from alder (Alnus glutinosa [L.] Gaertn.), maple (Acer platanoides L.), and beech (Fagus sylvatica L.), as well as their pairwise mixtures. These leaves were subjected to oxic or anoxic microbial preconditioning and served as food sources for Sericostoma caddisflies. We predicted that leaf treatment (i.e., leaf species, leaf diversity, and preconditioning) would drive the production rate of FPOM and its particle-size distribution. Across all treatments we retrieved similar to 50%, on average, of the decomposed leaf mass as FPOM, with values ranging from 37 to 69%. We found the highest and lowest rates of FPOM production on alder and beech leaves, respectively, with little effect of microbial preconditioning on FPOM production rates. Leaf species strongly affected FPOM particle size, with smaller particle sizes being produced from beech leaves. Inclusion of alder in mixtures affected production rates and particle size of FPOM more strongly than expected, caused by an over-proportional use of alder leaves by shredders. We conclude that in addition to being locally relevant for shredder consumers, properties of leaves supplied to stream food webs can affect FPOM supply to downstream ecosystems.
Low flows due to global warming, smaller snowpack, early snowmelt timing, and water withdrawals for human use are increasingly affecting freshwater ecosystems. Low flows caused by drying events are expected to become more frequent and extreme in the future, necessitating an improved understanding of the effects of water temperature variation and low flow on freshwater organisms. One group of insects, the stoneflies, can be especially sensitive to stressors such as low flows. An iconic stonefly species, the giant salmonfly Pteronarcys californica Newport, 1848, is declining in several rivers in the western United States, but the stressors that are driving this decline are not well understood. To study the effects of temperature and low flow on the survival, growth, and behavior of salmonflies, we conducted a 45-d laboratory experiment. We simulated a drought with no surface water in flume channels and only a fraction of the hyporheic zone wetted across 3 temperature treatments (4, 12.5, and 20 degrees C; n = 3 flumes/temperature; 6 salmonflies/flume). We found that high water temperature decreased salmonfly survival by 6.5 & times; under these low-flow conditions. Lower survival in the medium- and high-temperature treatments was likely driven by temperature-induced increases in metabolic demand for O2 and energy resources, as evidenced by observed increases in salmonfly body movements related to respiration, removal of food resources, and salmonfly mass loss at higher temperatures. Although male and female salmonflies had differential growth responses to temperature, both sexes lost mass in the highest-temperature treatment. Overall, our results suggest that in high-temperature, low-flow conditions, acclimation responses, such as increased food consumption and ventilation, cannot sufficiently compensate for the effects of rising metabolism, leading to mass loss and eventually to mortality. Our findings provide insight into the ways that global change may affect aquatic invertebrates like stoneflies as low flows and warming become more prevalent.
The metamorphosis of aquatic emergent insects results in energetic subsidies that connect aquatic and terrestrial ecosystems, providing important resources to riparian consumers but also serving as a pathway for the transfer of bioaccumulative and biomagnifying contaminants. The transfer of contaminants can put riparian predators at risk of exposure and negative health effects; therefore, understanding the mechanisms of emergent-insect contaminant flux is important for ecological risk assessments. Metamorphosis can affect the concentration of some chemical contaminants in larvae relative to adult insects. However, ecological risk assessments often do not consider life stage in the use of aquatic insects. Additionally, foodweb tracers (e.g., stable isotopes) are commonly used in risk assessments, but the influence of metamorphosis on these tracers is not well studied. In this study, we investigated whether metamorphosis was associated with changes to commonly used foodweb tracers in mayflies. We compared stable isotopes, polyunsaturated fatty acid profiles, and body composition in multiple life stages of the model laboratory mayfly species Neocloeon triangulifer (McDunnough, 1931) and field-collected mayflies (order Ephemeroptera, family Heptageniidae) to evaluate life-stage effects under controlled and natural conditions and across species. In laboratory-reared mayflies, delta 15N, %N, and arachidonic acid increased with metamorphosis, but in field-collected mayflies, delta 15N decreased and polyunsaturated fatty acid profiles did not differ. Our results indicate that metamorphosis has an effect on foodweb tracers in both laboratory and field-collected mayflies, which should be considered when using mayflies, or potentially other emergent aquatic insects, in calculations connected to ecological risk assessments.
Past research has suggested that geology has an important influence on stream energetic functions. However, potential geological factors are confounded by other variables, such as drainage basin area, current velocity, and habitat. We approached this problem by identifying 3 similar basins in each of 3 ecoregions (Ridge and Valley [RV], Piedmont [PI], and Southwestern Appalachians [SA]) underlain by different lithologies in Alabama, USA. All 9 streams were sampled quarterly for 1 y to assess the influence of geology (lithology) and habitat (substratum) on primary and secondary production. All streams were similar in location (northern Alabama), discharge, basin area, habitat types, and forest composition, but different in underlying lithology (phyllite, sandstone, carbonate) and associated alkalinity (4-108 mg/L as CaCO3). Quantitative invertebrate samples were collected from major habitats (bedrock, cobble, gravel, sand) in all streams, and secondary production was estimated using empirical models. Algal net primary production was estimated using C-14 techniques from 3 habitats (bedrock, cobble, gravel) in all streams. Habitat-specific daily primary production was correlated with alkalinity and was higher in RV > PI > SA throughout the year and highest on bedrock, followed by cobble and gravel, in all ecoregions. Habitat-specific secondary production was similarly highest in RV and PI, but always higher on cobble than other habitats. Surface areas of habitats were quantified, and habitat-specific primary and secondary production were weighted by habitat surface area to obtain stream-wide estimates. Stream-wide analyses were consistent with habitat-specific results among ecoregions, but were not as strong because the highest habitat-specific production was sometimes in streams with the lowest amounts of highly productive surfaces. Stream-wide secondary production of only the scraper functional feeding group was related to primary production. Our synoptic study demonstrates that although primary and secondary production were both related to lithology, their responses to habitat type differed.
The Zebra Mussel Dreissena polymorpha (Pallas, 1771) has invaded surface waters throughout North America, and it alters benthic habitats by depositing dense layers of shells. Although these shell deposits can increase habitat complexity in some systems, they may also lead to habitat homogenization, particularly in streams with coarse substrates. We sought to understand how these shell deposits may affect macroinvertebrate assemblages in the Rouge and Huron watersheds (Michigan, USA; August 2017). We examined whether shell deposit density is related to macroinvertebrate community diversity and relative abundance and if those relationships might differ between rural and urban streams, noting changes in taxa sensitive to habitat structure, such as Ephemeroptera and Trichoptera (mayflies and caddisflies). In one rural stream, the % abundance of specific mayfly and caddisfly families decreased with increasing shell density, suggesting a localized negative response to shell-induced substrate changes. However, this pattern was not consistently observed across other rural streams, indicating that site-specific factors may mediate these effects. Regardless, shell deposits in urban streams, where baseline habitat complexity is often low, may offer structural refuge and benefit specific invertebrate taxa. Our findings highlight the context-dependent nature of Zebra Mussel relationships with stream invertebrate community metrics and suggest that the ecological consequences of shell deposits may vary with stream habitat characteristics.
Water-level fluctuations are typical and expected in hydrologic regimes, and aquatic organisms can often adapt to these fluctuations. However, because of water management for societal needs such as hydroelectric power, irrigation, drinking water, and flood control, water-level fluctuations can be extreme. These variations can be difficult for aquatic organisms to overcome, especially sedentary, benthic fauna like freshwater mussels (unionids). Because dam activities, such as drawdowns, affect both lotic and lentic ecosystems, our study attempts to understand differences in species' responses to dewatering. In temperature-controlled mesocosms, we investigated unionid response to dewatering events for 2 different species in the family Unionidae, Lampsilis cardium Rafinesque, 1820 (Lampsilini; a primarily lotic species) and Pyganodon grandis (Say, 1829) (Anodontini; a primarily lentic species), by completing trials of dewatering events. Using fine-scale measurements and observations, we found that under dewatering stress, L. cardium burrowed and moved longer distances laterally toward water than P. grandis, and in contrast, P. grandis tended to move upward away from water, perhaps related to their varied shell characteristics. The most common movement patterns of these unionids were burrowing, movements upward, and combination movements (i.e., burrowing and moving laterally as well as moving both upward and laterally). Overall, both unionid species moved less in treatment events compared with the controls, indicating a muted behavioral change in movements during dewatering. Understanding fine-scale movement patterns of 2 unionid species in a laboratory setting adds to the current, but minimal, empirical data available for decision makers. These data can then be used as an aid in making water management decisions, which directly influence unionids, in efforts to preserve unionid communities influenced by dam activity.
Headwater streams are abundant worldwide and important to global biogeochemical cycles, serving as critical processors and transporters of C. C spiraling is a useful way to understand the retention and mineralization of organic C (OC) in streams. However, analyses of seasonal and interannual variability in OC spiraling are currently limited. In this study, we aimed to understand the temporal patterns and driving mechanisms of OC spiraling, which will inform our understanding of future OC changes under climate change. We used 7 y of daily data in a small headwater stream (Walker Branch, Tennessee, USA) to assess seasonal and interannual variability in OC spiraling length (S-OC) and mineralization velocity (v(fOC)), as well as their potential related variables. On average, S-OC in Walker Branch was similar to 10x shorter than in previously studied small streams, indicating strong connections between the water column and the benthic environment where OC mineralization mostly takes place. OC spiraling was faster during the more biologically active periods of spring and autumn compared with more elongated OC spiraling in summer and winter, when OC retention was lower and downstream transport was higher. Gross primary production (GPP) was most strongly related to S-OC and v(fOC). Photosynthetically active radiation (PAR) and NO3- were also positively and negatively related to v(fOC), respectively. Trends toward earlier and longer canopy cover and reduced GPP and PAR may result in longer S-OC and slower v(fOC), reducing localized instream processing of OC and potentially shunting more OC downstream. However, long-term observations indicate reduced NO3- at Walker Branch, suggesting opposing effects to those of GPP and PAR, leading to faster v(fOC) and greater OC retention. Time-series analyses of OC spiraling in streams can enhance our understanding of current and future responses of OC processing and downstream transport to climate change, as well as implications for downstream OC dynamics.
Human influences, including climate change, have increased the occurrence of dry periods in rivers, likely disrupting the natural seasonal dynamics of ecosystems and altering aquatic insect assemblages. Japanese rivers, which are typically perennial with clear seasonal flow regimes driven by a monsoon climate, support aquatic insects with life cycles that exhibit synchronized growth and reproduction. These phenological traits may be particularly vulnerable to flow alteration, such as the occurrence of unprecedented dry periods. This study sought to understand how aquatic insect populations respond to short-term flow cessation and how insect phenology may mediate those responses. We sampled aquatic insects every week for 6 months in 2 experimental streams-one subjected to a 1-wk dry period and the other maintained as a control. We analyzed the temporal dynamics of insect population abundances following the dry period with state space models. The artificial dry period created extreme conditions, including watercourse fragmentation, water temperatures exceeding 40 degrees C, and nighttime dissolved O-2 approaching a detection limit (<0.1 mg/L). These conditions led to severe reductions, to near zero, in total insect abundance and taxa richness. Recovery to baseline levels comparable to the control occurred within similar to 4 wk after rewetting. Despite this resilience, the dry period altered the subsequent seasonal dynamics, depending on taxa phenological traits. Taxa with individuals that simultaneously spanned various size classes and life stages incrementally recovered their abundance to the same or higher level following rewetting, whereas taxa that exhibited seasonal abundance peaks due to synchronized growth of individuals did not recover until the next generation. In addition, desiccation-tolerant life stages influenced the taxon-specific responses to drying. Our findings emphasize that phenological traits, such as the degree of life cycle synchrony and the timing of life-history events, are key in shaping the recovery processes of aquatic insect assemblages following dry periods.
Living freshwater mussels of the order Unionoida are considered important ecosystem engineers for aquatic habitats, but the role of their dead shells in ecosystem dynamics is not yet fully understood. Therefore, we investigated the influence of decomposing dead mussel shells on freshwater chemistry, particularly with regard to their effects on pH and their potential buffering capacity in acidified water bodies through remineralization. To address this objective, we conducted standardized laboratory (28 d) and field (6 mo) experiments, considering different geology (siliceous and carbonate), pH, and conductivity. In the laboratory experiment, fresh, dead shells were exposed to 9 different hydrochemical settings, consisting of 3 different ambient media with varying ionic strengths and 3 different starting pH levels, to monitor the effects of the shells on the media. In parallel, shells were exposed in 3 streams with extant mussel populations in Bavaria, Germany, to monitor the degradation of the shells in a natural setting. In the laboratory experiment, mussel shells increased total hardness, pH, Ca2+ concentrations, and conductivity, which was attributed to the dissolution of CaCO3 from the shells. These effects were stronger in soft than hard water, and in hard water these effects were higher at lower pH values. In the field experiment, the mass loss of the shells was highest in siliceous streams. These findings indicate that dead shells can play a role in buffering pH fluctuations, especially if they occur in high numbers and in acidified siliceous regions with low ionic strength. The stabilizing effect of dead shells on water chemistry should be considered in restoration projects and in decision-making concerning their removal, for example, after mass die-off events.
Dramatic loss of submerged aquatic vegetation (SAV) and proliferation of nuisance algae in the karst springs of the Suwannee River basin, Florida, USA, have been principally attributed to press disturbances such as nutrient pollution. The role of pulse disturbances, specifically river intrusion events (RIEs), in driving this shift has been hypothesized but remains poorly documented. During RIEs, acidic floodwaters high in dissolved organic matter from adjacent blackwater rivers displace the clear, alkaline groundwater within a spring, reducing light availability and altering dissolved O2 (DO) concentrations, with potentially important consequences for autotroph communities. We asked, to what extent do RIEs, alone and in combination with other environmental gradients such as DO, influence autotrophic community structure in Florida springs? To address this question, we surveyed autotrophic community structure at 62 springs and quantified flood disturbance frequency using water-quality observations and geomorphic predictors. RIE frequency showed a relationship with both algae and SAV cover. The relationship between RIE frequency and algae cover was mediated by baseline DO concentrations, with higher RIE frequency associated with increased algae cover in high-DO springs but reduced algae cover in low-DO springs. High-DO springs were more likely to support SAV, suggesting that disturbances promote algal proliferation by reducing the competitive advantage of SAV. Variables that predicted flood disturbance frequency included the spring-pool elevation relative to the receiving river and local stage variance. These findings suggest that interacting effects of press (low DO) and pulse (RIE) disturbance regimes shape autotrophic communities, informing approaches for successful SAV restoration.
Chironomidae are ubiquitous in freshwater ecosystems, often dominating invertebrate assemblages and emergence fluxes from streams, rivers, ponds, and lakes. These fluxes can represent substantial transfers of materials from aquatic to terrestrial habitats, including biomass, C, fatty acids, and contaminants. Such aquatic-terrestrial linkages shape the function of these linked ecosystems, underscoring the importance of accurately estimating them. We and others have frequently used a published length-mass regression for adult Nematocera (all long-horned flies including Chironomidae) to estimate the biomass of adult Chironomidae emerging from freshwater ecosystems (i.e., flux); however, the broad taxonomic resolution of that regression may bias flux estimates. To help correct for this taxonomic mismatch, we present a new adult Chironomidae length-mass regression using the power function, total length (mm), and dry mass (DM; mg) derived from specimens (n = 288) that emerged from wetlands in North Dakota, USA. We compare our new regression with 3 other published regressions and datasets for adult Chironomidae or Nematocera. We then apply each of these regressions to estimate annual emergence flux (g DM m-2 y-1) from a stream in Yellowstone National Park, USA. We found that the 4 length-mass regressions used datasets with varying specimen characteristics, including taxonomic resolution, size distribution, habitat, and sex. In turn, the 4 length-mass regressions yielded emergence flux estimates that ranged from 0.78 to 6.57 g DM m-2 y-1, with one Nematocera regression yielding an estimate 3 to 8 & times; higher than the others (nonoverlapping 95% CI). Flux estimates from the adult chironomid regressions also varied 1- to 5-fold, with one regression having far lower R2 and a lower emergence flux estimate (nonoverlapping 95% CI). These findings suggest researchers should carefully consider the available regressions for adult chironomids, selecting a regression with a high R2 that is derived from similar specimen characteristics as their dataset.
Understanding biotic and abiotic drivers of predator changes is important for predicting spatial variation in stream community composition. Transitions from ephemeral to permanent streams are paired with concurrent shifts in vertebrate communities that seemingly support the application of the predator-permanence hypothesis for low-order streams. Semiaquatic amphibians tend to decline at this transition, and one hypothesis for this decline has been the presence of their fish predators, who cannot succeed in streams that dry for part of the year. Both prey avoidance and asymmetrical competition favor fish in previous experiments, but these studies have not facilitated prey switching or access to alternative habitats that could allow for coexistence. This study sought to identify nonconsumptive effects of fish predators on stream salamander adults provided with an alternative habitat to escape the interaction. We evaluated the performance and habitat selection of 2 species of stream salamander-Desmognathus amphileucus Bishop, 1941 and Desmognathus monticola Dunn, 1916-in the presence or absence of a caged fish-Cottus carolinae (Gill, 1861)-when provided terrestrial prey. We observed that in the presence C. carolinae, both salamander species had reduced body condition and selected more terrestrial habitat. This study supports conclusions that competitive and likely predatory interactions between headwater fish and semiaquatic salamanders cause salamanders to become more terrestrial, grow more slowly, and transform at smaller sizes. Thus, stream permanence and associated environmental conditions may interact to result in a shifting predator community where fish occur. If the stream-permanence transition moves upstream as a consequence of climate change, spatial constriction of stream communities that are most successful in fishless streams will be accelerated.
