Eric C. Merten

Effects of forest type and stream size on volume and distribution of stream wood: legacies of wildfire in a Euro-Mediterranean context

Abstract. Downed wood pieces are key links between terrestrial and aquatic ecosystems. They promote organic matter retention, create habitat, and potentially increase stream productivity. The stock of downed wood in a river system is a product of the interaction between wood supply, transport, in situ losses, and retention characteristics of the system. Fire and forest management are important disturbances that influence the amount and organization of stream wood with boom-and-bust periods of recruitment and fluvial transport processes. We examined 1st- through 3rd-order Portuguese streams flowing through 3 common silvicultural systems in southern Europe: forests of cork oak, eucalyptus, and maritime pine. Our data set included 1483 pieces of wood in 27 streams, all of which had experienced extensive wildfires within the previous 6 y. We used binned neighbor-k analysis to assess wood organization (segregated, random, or aggregated). We then used linear mixed-effects modeling to evaluate the effects of stream order, forest type, and their interaction on wood volume and organization. The best predictor of wood volume and organization was the interaction between forest type and stream order. Most wood pieces were burned and organization was low, suggesting that arrangement of wood was largely a product of input dynamics rather than transport processes at this time. Potential drivers of across-system variability included vegetation obstructions, wood length∶channel width ratios, management actions, and effects of fire. Climate models predict more droughts in the Euro-Mediterranean region in the future, with implications for wood volume, transport, and function as terrestrial vegetation invades intermittent stream channels and plant communities shift from managed forests to shrublands with few trees.

Effects of burn status and conditioning on colonization of wood by stream macroinvertebrates

Abstract The combination of changing climate and anthropogenic activities is increasing the probability of wildfire around the world. When fires occur in riparian zones, associated tree mortality can add wood directly to streams or wood may fall to the forest floor and remain there for some time before being transported into stream channels. Wood provides critical structure for aquatic macroinvertebrates, so our objectives were to assess the effects of wood burn status, conditioning, and their interaction on macroinvertebrate community composition, taxon and functional diversity, and trait affinities. We conducted a field experiment with pieces of freshly cut wood (length = 10 cm, diameter ≈ 7.5 cm) for which we first manipulated burn status (burned, unburned) and then, conditioned by placing burned and unburned wood directly into streams (no conditioning) or by leaving pieces in streams (water conditioning) or on the forest floor (soil conditioning) for a year before submergence. We used distance-based redundancy analysis to assess community structure by wood treatments and linear mixed-effects modeling to assess effects of wood treatments on taxon and functional diversity and trait affinity. Changes in wood quality resulting from fire may not alter macroinvertebrate community structure. Taxonomic and functional patterns of stream invertebrate colonization did not differ between burned and unburned wood, even after a year of incubation in the stream or on the forest floor. Conditioning status affected taxonomic composition, taxon and functional diversity, and trait affinities of wood invertebrate communities. The terrestrial legacy of soil conditioning was clearly important in structuring macroinvertebrate assemblages. Our results suggest that macroinvertebrate communities may be more sensitive to fire effects on the dynamics of wood input than to effects of fires on the wood itself.

Microhabitat influences on stream insect emergence

Understanding the factors controlling insect emergence from streams has applications to ecological theory regarding cross-boundary flux, along with practical value for monitoring stream function after restoration projects. We hypothesized that stream microhabitat would have effects on emergence that were independent of those mediated by the local stock of benthic macroinvertebrates. We set 50 emergence traps in a third-order stream in northern Minnesota, USA, during two study periods and used structural equation modeling to examine direct and indirect effects of benthic stock and microhabitat features on emergence. Emergence by biomass showed direct positive relationships to substrates of fines and detritus in the first sampling period, and to shallow depth and wood area in the second period. Emergence by abundance had direct positive relationships with benthic stock, CPOM, and fewer macrophytes in the first period, and with benthic stock and periphyton in the second period. Fine substrates may act to concentrate burrowing larvae, whereas CPOM and particularly wood may intercept drifting pre-emergent insects and provide exiting surfaces. Shallow depths may reduce the extent to which resident insects drift downstream (and leave the sample area) while emerging. Periphyton may be an indicator for patches with greater illumination, which itself attracts emergers. Our results suggest emergence is sensitive to environmental conditions at the microhabitat scale, and that stream restoration activities should consider habitat for emerging insects when designing projects.

Standing Stocks of Large Wood in Twelve Tributary Streams along the North Shore of Lake Superior

ABSTRACT We quantified standing stocks of large wood in 12 streams along the north shore of Lake Superior in northeastern Minnesota during June—September 2007. All large pieces of wood (> 0.1 m in diameter for a portion > 1 m in length, n = 963) were measured. Standing stocks of large wood ranged from 0.01 to 0.48 piece/m, lower than most published values from around the world. Wood inputs to streams in northeastern Minnesota may be less than elsewhere due to differences in tree species, environmental conditions, and past forest management practices. Regardless of wood input rates, the standing stocks of wood in northeastern Minnesota may have been reduced by intentional wood removal.

Current velocity as a factor in determining macroinvertebrate assemblages on wood surfaces

The velocity of moving water near submerged wood surfaces may be an important factor for shaping benthic communities in sandy-bottomed streams. As wood can be suspended above the streambed and be kept free of inundating sediments, wood substrates may provide the primary hard surface for macroinvertebrate colonization. To examine this, we sampled macroinvertebrate assemblages from the surface of 20 pieces of submerged wood in a sandy-bottomed stream in the west-central Wisconsin. Near-surface velocity was characterized at the wood surface and the streambed macroinvertebrate assemblage was sampled near each wood piece for comparison. We found that the density of macroinvertebrates was five times greater on wood than the sandy streambed and the average richness on wood was twice as great. Macroinvertebrate abundance and richness on wood surfaces increased with velocity; however, abundance increased as a consequence of adding taxa rather than adding individuals of the dominant taxa. These data suggest that near-surface current velocity on wood may be an important determinant of benthic community composition when the availability of other hard surfaces is limited.

Macroinvertebrate grazers, current velocity, and bedload transport rate influence periphytic accrual in a field-scale experimental stream

Periphyton plays an important role in stream ecology, and can be sensitive to macroinvertebrate grazers, near-bed current velocity, and bedload abrasion. We manipulated conditions to examine influences on periphytic accrual in the St. Anthony Falls Laboratory Outdoor StreamLab in Minneapolis, MN, USA. Macroinvertebrate grazers were excluded from 27 of 65 clay tiles using electric pulses. We examined periphytic biomass accrual as a function of grazer presence, sampling run, and near-bed current velocity using ANCOVA. We found significant temporal differences between sampling runs but no significant effect of grazer presence. Along with a strong association between bedload transport rates and mean periphytic biomass, our results suggest that grazers are relatively unimportant in stream systems with high levels of physical disturbance from floods and associated sand bedload. However, the interaction between grazer presence and velocity was marginally significant. Regression analyses showed no relation between velocity and periphyton in the absence of grazers but a negative relation when grazers were present, suggesting that mechanical dislodgement of periphyton by grazers may increase with velocity. We conclude that grazers can have subtle effects on periphyton, particularly in streams with high bedload transport rates.

Range expansion of the invasive rusty crayfish Orconectes rusticus (Girard, 1852) (Decapoda: Astacoidea) in northeastern Iowa (USA) rivers

The rusty crayfish Orconectes rusticus (Girard, 1852) is an invasive species that has been introduced to the Upper Midwest, USA from the Ohio River Valley, and is associated with ecological damage to aquatic systems. We examined the distributions of rusty and native crayfishes in two Iowa river systems. One hundred sites were surveyed in the upper Cedar and middle Wapsipinicon river drainages from 2013 to 2014. Four native crayfish species were collected, and contrary to a study conducted in 2002, the rusty crayfish was documented in the mainstream of both systems. Analysis of sampling methods indicates that crayfish capture rates were significantly higher in cylindrical minnow traps than in flat crayfish traps. Genetic analyses confirmed field identifications and support a single introduction of the rusty crayfish in northeastern Iowa. The pervasive distribution and life history characteristics of the rusty crayfish suggest that eradication is currently not feasible. Further monitoring is warranted in order to determine its distribution throughout Iowa’s river systems.

Fire meets inland water via burned wood: and then what?

Wood is a key structural element in aquatic ecosystems. Wood provides habitat complexity, alters geomorphology, retains organic and inorganic material, promotes hyporheic flow, and acts as substrate for biofilms and invertebrates. Fire is an important disturbance promoting wood recruitment into inland waters, but most studies have focused on streams in western North America. Less is known about fire-derived wood dynamics on other continents or in lake environments. Here, we review fire effects on the recruitment, distribution, and function of in-stream wood, with emphasis on a series of studies from the Euro-Mediterranean. The amount of large wood in these streams was low and is expected to decline in the future because of wildfire. Wildfire engendered inputs of wood with low structural complexity, probably reducing habitat heterogeneity for aquatic organisms. Fire also provided wood with greater diameter than wood recruited by other means, but its longevity may be shorter because burned wood was more decayed and less anchored in the channel than unburned wood. Wood delivery processes are important because macroinvertebrate colonization differed between fire-derived wood that fell directly into the river and wood conditioned first on the forest floor. We present a case study describing wood dynamics in a lake after a wildfire in northern Minnesota, USA. In this study, wildfire created an area of lake shoreline with disproportionately more wood than areas unaffected by wildfire. In contrast to streams, burned wood was more complex than unburned wood in the lake system. One of the explanations may be greater scouring, abrasive action by stream flows, which breaks down burned wood faster than in lakes. Given the expected increase in the probability and severity of fire around the world, information in our review can be used to help manage riparian zones of streams and lakes.

Effects of Six-Hour Suspended Sediment Treatments on White Sucker (Catostomus commersoni) and Smallmouth Bass (Micropterus dolomieu) in an Artificial Stream

ABSTRACT To examine effects of short-term exposure to two types of sediment on health of warmwater fishes, a controlled experiment was conducted with Micropterus dolomieu and Catostomus commersoni. Fishes were stocked into net pens at upstream and downstream ends of an artificial stream. Following an acclimation period of five to six days, discharge was increased throughout the stream to bankfull levels, and fishes in downstream pens (only) were exposed to sediment concentrations of 200 mg/L of fine sand or 500 mg/L of agricultural soil for six hours in two independent trials. Fishes were then immediately assigned a health assessment index value based on conditions of internal and external organs. There were no significant effects on health assessment indices for either species from the experimental treatments, although the indices may have been more sensitive if fish were afforded a post-treatment latent period to allow stress-induced changes to manifest. Our data agree with existing models that describe effects of smaller sediment particles on warmwater fishes and help address the lack of information on effects of larger, coarser suspended sediment.

Web-building Spider Response to a Logjam in a Northern Minnesota Stream

Abstract Logjams, or accumulations of wood in streams, can increase aquatic macroinvertebrate production through organic material retention and habitat diversification. Past studies showed a positive correlation between web-building spiders near streams and aquatic insect emergence. We hypothesized there would be an increase in terrestrial web-building spider density near logjams. To test this, we counted webs within 6 m of the stream bank along a 40 m reach in a northern Minnesota stream centered on a spanning logjam. We then estimated the availability of web-building substrate during late May and late Aug. of 2011. Webs in May heavily concentrated around the logjam, whereas webs in Aug. appeared dispersed throughout the reach. The web-building substrate did not show a significant correlation with web density in May, but it had a significant effect in Aug. These results cautiously suggest logjams have a positive effect on spider web density, but that effect varies through time. Further studies may explicitly link logjam-mediated prey to spider web distributions.