We measured nitrification rates in sediment samples collected from a variety of aquatic habitats in Navigation Pool 8 of the Upper Mississippi River (UMR) 7 times between May 2000 and October 2001. We also conducted nutrient-enrichment experiments and analyzed vertical profiles of sediment to determine factors regulating nitrification. Nitrification rates were relatively high compared to other ecosystems (ranging from 0–8.25 μg N cm⁻² h⁻¹) and exhibited significant temporal and spatial patterns. Nitrification rates were greatest during the summer and spring compared to autumn and winter (ANOVA, p < 0.05) and were greater in contiguous backwater and impounded habitats compared to main and side-channel habitats (p < 0.05). Regression analysis indicated that nitrification rates were weakly (r² = 0.18, p < 0.0001) related to temperature and exchangeable NH₄ of the sediment. However, nutrient-enrichment experiments showed that NH₄ availability did not limit nitrification in 3 sediment types with variable organic matter. Vertical profiles of sediment cores demonstrated that oxygen concentration and nitrification had similar patterns suggesting that nitrification may be limited by oxygen penetration into sediments. We conclude that temperature and sediment NH₄ can be useful for predicting broad-scale temporal and spatial nitrification patterns, respectively, but oxygen penetration into the sediments likely regulates nitrification rates in much of the UMR. Overall, we estimated that nitrification produces 6982 mt N/y of NO₃⁻ or 7% of the total annual NO₃⁻ budget.

Work in terrestrial ecosystems has suggested that the breakdown rate of leaf litter may not change predictably with increasing plant species richness. Indeed, it may be that certain combinations of leaf species demonstrate significant non-additive effects on breakdown rates, mediated by the presence of a single key species. Such effects have not been explored in running-water ecosystems despite the strong interest in the conservation and restoration of riparian systems. We documented the magnitude and species composition of leaf litter inputs, the species richness and composition of leaf litter on the streambed, and estimated the breakdown rate of mixed litter in both the summer and autumn in a warmwater stream. We found that leaf species richness of litter packs on the streambed varied from 1 to 11 species, and leaf species composition reflected the composition of litter inputs from May through November. We did not find a general relationship between breakdown rate and leaf litter species richness. However, we did find a strong effect of species composition of leaf packs on the breakdown rate during the summer. Overall, breakdown rates of mixed-species leaf packs were non-additive during the summer, but very predictable in the autumn. In particular, leaf mixtures containing American sycamore always exhibited slower breakdown rates than predicted in summer. One explanation for the discrepancy between summer and autumn results may be decreased temperature in autumn; reduced temperature may have slowed breakdown rates across treatments to the extent that any non-additive effects found in summer were masked by the effect of temperature. Given the importance of detritus to stream food webs, the simplification of plant assemblages along intact or restored streams may have important implications for stream ecosystems.

Successional changes in the animal community of a temporary pond on the Iberian Peninsula were analyzed based on taxon abundance and the biomass–size spectrum. Changes in the community were studied over 6 hydroperiods during 1996 and 1997. Community dynamics were analyzed using multivariate analysis and a new statistical approach based on the Pareto probability distribution. Three successional phases could be distinguished on the basis of changes in taxon abundance. This pattern is consistent with the 3-phase model reported in other temporary ponds in temperate latitudes. The 1^st and 3^rd phases corresponded to changes in hydrological factors (pond drying, water turnover) and to seasonality, whereas the 2^nd phase corresponded to more stable hydrological conditions. Five successional phases could be distinguished on the basis of biomass–size spectra with similar structure. These phases were consistent with variations over time of parameter c of Pareto Model I. The 3 middle phases (II, III, and IV) identified using the size-spectrum approach corresponded to the single middle phase identified using the taxonomic approach. Thus, community changes during the middle phase were better resolved by analysis of the biomass–size spectrum than by analysis of taxon abundance.

We related macroinvertebrate communities and environmental variables in 66 small seasonal woodland ponds of northern Minnesota, USA. These wetlands were relatively pristine, being embedded in 50- to 100-y-old 2^nd-growth forests. Macroinvertebrate taxon richness in ponds increased as hydroperiods lengthened, tree canopies opened, water pH declined, and litter input decreased. Eighteen macroinvertebrate taxa were widespread (occurred in >50% of ponds), and hydrology, water chemistry, geomorphology, vegetation, occurrence of other macroinvertebrate taxa, and presence of amphibian larvae each explained some variation in relative abundance of widespread macroinvertebrates. The first 4 axes of a canonical correspondence analysis explained 37% of total variation in relative abundance of widespread macroinvertebrate taxa. Overall, however, macroinvertebrates were remarkably unresponsive to environmental variables. Most relationships between macroinvertebrates and environmental variables were nonsignificant, and the few significant relationships observed were weak (<20% of variation). We suggest that this lack of response occurs because most macroinvertebrates in seasonal woodland ponds are habitat generalists. These species routinely endure pronounced and unpredictable environmental changes; hence, they possess a durability that makes them resistant to most natural variation in habitat conditions.

We used a repeated-measures factorial field experiment to ask whether the arrangement and stability of leafy debris affect the density of chironomids in leaf packs in dynamic, streambed landscapes. We replaced natural debris in 24 streambed landscapes with leaf packs suspended from wires that snagged organic debris as it moved downstream. We arranged 64 wires (each with 3 leaf packs) in each landscape so that they were distributed among 8 large (aggregated) or 64 small (fragmented) patches. We manipulated landscape stability by removing accumulated debris from 0% (stable), 50% (semi-stable), or 100% (unstable) of the patches in each landscape 3 d after a flood. We determined chironomid density in leaf packs 2 d before the flood, 3 d after the flood, and 5 d after manipulating stability (8 d after the flood). We mapped the debris in each landscape and used multiple regression to evaluate the effect of landscape characteristics on density. Mean patch area was significantly smaller, and the number of patches, landscape perimeter, and landscape perimeter to area (P:A) ratio were significantly greater in fragmented than in aggregated landscapes. Chironomid density was significantly higher in fragmented than in aggregated landscapes and significantly lower in all landscapes after the flood. Density was ∼30% higher in unstable landscapes than in stable or semi-stable landscapes after debris removal. Density was negatively correlated with patch area, and debris accumulation may have caused patches to lose their value as refugia by making patches too large for chironomids to enter. Thus, disturbances that maintain fragmented patch arrangements with small patches and high landscape P:A ratios may be critically important for retention of individuals and to population persistence in dynamic landscapes.

In forested streams, litter patches are important microhabitats for macroinvertebrates, and the nature of litter patches can affect structure and function of macroinvertebrate assemblages. We examined whether litter patch types with different characteristics could be predicted by their location within stream pools (pool middle, alcove, edge) and, if so, whether patch types had different macroinvertebrate assemblages. Mean mass of leaves per unit area of streambed was 2 to 3x higher in edge patches than in other patches, whereas mean mass of wood and small litter particles was 2 to 6x higher in middle patches. Densities of nemourid stonefly taxa were higher in edge patches than other patches, with density of Nemoura being highly correlated with leaf mass, whereas densities of lepidostomatid caddisfly taxa were higher in middle patches, with density of Goerodes complicatus being highly correlated with mass of small litter particles. Mean biomass and annual secondary production of shredders, collectors, and predators were 1.6 to 4x higher in middle patches than in other patches. Our results indicate that macroinvertebrate community structure and production may differ within and among forested streams according to relative composition of litter patch types, even if overall litter abundance is similar.

Symbiotic interactions can affect populations, communities, and ecosystems by creating novel flows of material and energy in food webs. However, there is considerable variability in host specificity among symbionts. We examined the host specificity of an ectosymbiotic annelid worm (Annelida:Clitellata:Branchiobdellidae) on 2 sympatric species of host crayfishes. In host-selection experiments, the branchiobdellid Cambarincola ingens colonized the crayfish Cambarus chasmodactylus 2.5x more frequently than similarly sized Orconectes cristavarius even though O. cristavarius was 2 to 4x more abundant in the field. Infestation intensity–body size regressions for Cambarincola in the New River reflected host-selection experiment results; the slopes for C. chasmodactylus were up to 8x steeper than those for O. cristavarius. In addition, 96% of C. chasmodactylus had at least one attached Cambarincola compared to only 36% for O. cristavarius. There are many possible explanations for the observed host preference, including ratio-dependent selection and differences in host behaviors. However, the most likely explanations for host selection in Cambarincola seem to be differences in cost-benefit relationships or evolved direct responses to host identity.

We determined the trophic positions of 2 species of freshwater mussels, Elliptio dilatata and Ptychobranchus fasciolaris, from 2 small streams in central Ohio by measuring stable C and N isotope ratios and digestive fluid enzyme activities. We also examined stable C and N isotopes, microbial biomass, microbial community structure, nutrient (i.e., C, N, and P) concentrations, and contribution of microbial C to total fine particulate organic C (FPOC). We hypothesized that 1) allochthonous inputs compose most of FPOC, 2) mussels use fine particulate organic material (FPOM) as a food source, and 3) mussels respond to the low-protein content of FPOM by showing high protease activity. Microbial C composed 35 to 86% of total FPOC during the autumn sampling period. FPOM stable isotope values varied seasonally, whereas δ¹³C and δ¹⁵N content in mussel tissue was spatially (i.e., among sites) and temporally similar. Mussels were 2 to 4‰ more depleted in δ¹³C than seasonal FPOM. Digestive fluid enzymes were spatially and temporally stable across species, with activity of esterase > protease > lipase > glucosidase. Lipase:protease of digestive fluids from mussels were <1, indicating a low-protein diet. Our results suggest that microbial biomass in FPOM constitutes a large portion of mussel diet and that mussels assimilate significant amounts of C from this source.

Freshwater mussels are among the most rapidly declining components of global biodiversity, but causes of local species disappearances are frequently unknown. We estimated decade-scale local extinction by resampling 118 stream reaches representing the best mussel habitat across a region that was once rich in species and is now mostly converted from prairies and riparian woodlands to intensive agriculture (Iowa, USA). Average species richness was reduced from >5 to <2 species, maximum richness was reduced from 22 to 15 species, and all mussel species were extirpated from 47% of the reaches since 1984 to 1985. More than half of the sites lost >75% of their species. Although 5 of the species were found at 20% to 140% more sites in 1998 than 1984 to 1985, 29 species (83%) decreased an average of 80% in geographic coverage, whereas 8 species were completely lost from these stream sites. Correlation analyses with reach and watershed characteristics determined using GIS and local sampling methods linked the greatest declines to rarity of streamside woodlands, high siltation, and most intensive agricultural land uses, i.e., where conditions had changed most from the historical land cover. The surveys indicated a very large extinction debt has been created by large-scale habitat modification over the last century and ongoing agricultural land uses.

Rapid bioassessments of stream health using macroinvertebrates are particularly useful when information is required quickly or when a large number of sites is to be investigated. The purpose of our study was to examine the effect of small-scale temporal variability (weeks) on rapid bioassessments using the New South Wales AUStralian RIVer Assessment System (AUSRIVAS) protocols. AUSRIVAS provides an assessment of stream condition based on the ratio of the number of taxa collected (observed) at a site, and the number predicted (expected) by a multivariate model. Repeated bioassessments were conducted in both riffle and edge habitats at 2 reference condition sites. Mean ratio of observed to expected number of taxa (O/E-Taxa value) was close to 1 for all sites and times, as expected for reference condition sites. O/E-Taxa values did not vary significantly over 4 to 6 wk, except for riffle samples from one site. Collecting 4 replicate samples increased the consistency of the allocation of sites to categories or bands of biological quality. However, the costs of further replication are likely to outweigh the benefits of using a rapid assessment method. Although tested using the AUSRIVAS rapid bioassessment protocol, these findings have application to other qualitative rapid bioassessment protocols.

The spawning sites, incubation success, and density of young-of-the-year (YOY) brook trout (Salvelinus fontinalis) were examined in stream populations experiencing varying levels of sediment stress. Our objective was to determine effects on embryos and YOY and examine cumulative effects on the populations. Early development was unaffected (88% survival). Mortality occurred at late, encapsulated embryo stages likely as a function of oxygen deprivation within redds. Survival to emergence from the substrate was significantly reduced (∼50%) in redds in which fine sediments accumulated. Groundwater reduced sediment accumulation in redds and enhanced survival. Sediment may have affected dispersal of YOY trout. All populations appeared successful in term of total density (average = 72/100 m²). Our findings emphasize the importance of targeting multiple life-history stages and understanding local adaptations when searching for true population-level effects in any stressed ecosystem.