Benthic organic carbon influences denitrification in streams with high nitrate concentration

1. Anthropogenic activities have increased reactive nitrogen availability, and now many streams carry large nitrate loads to coastal ecosystems. Denitrification is potentially an important nitrogen sink, but few studies have investigated the influence of benthic organic carbon on denitrification in nitrate‐rich streams. 2. Using the acetylene‐block assay, we measured denitrification rates associated with benthic substrata having different proportions of organic matter in agricultural streams in two states in the mid‐west of the U.S.A., Illinois and Michigan. 3. In Illinois, benthic organic matter varied little between seasons (5.9–7.0% of stream sediment), but nitrate concentrations were high in summer (>10 mg N L⁻¹) and low (<0.5 mg N L⁻¹) in autumn. Across all seasons and streams, the rate of denitrification ranged from 0.01 to 4.77 μg N g⁻¹ DM h⁻¹ and was positively related to stream‐water nitrate concentration. Within each stream, denitrification was positively related to benthic organic matter only when nitrate concentration exceeded published half‐saturation constants. 4. In Michigan, streams had high nitrate concentrations and diverse benthic substrata which varied from 0.7 to 72.7% organic matter. Denitrification rate ranged from 0.12 to 11.06 μg N g⁻¹ DM h⁻¹ and was positively related to the proportion of organic matter in each substratum. 5. Taken together, these results indicate that benthic organic carbon may play an important role in stream nitrogen cycling by stimulating denitrification when nitrate concentrations are high.     

Spatial and temporal patterns in fish assemblages of upper coastal plain streams, Mississippi, USA

We assessed spatial, seasonal, and annual variation in fish assemblages over 17 months in three small- to medium-sized, incised streams characteristic of northwestern Mississippi streams. We sampled 17 962 fish representing 52 species and compared assemblages within and among streams. Although annual and seasonal variability in assemblage structure was high, fish assemblages maintained characteristics unique to each stream. High variability in fish catch-per-unit-effort (CPUE) was exemplified in one site where total CPUE increased an order of magnitude from July 1993 to 1994. Species turnover and percent dissimilarity were often higher seasonally than annually, consistent with a period of change in spring to early summer and a return to similar species compositions between summers. Temporal variability was also high at the individual species level, and no species were classified as 'stable'. We found little evidence for correlation between changes in fish assemblage structure and measured habitat conditions. The fish characteristics fit the profile of 'colonizing assemblages', which probably resulted from both natural and anthropogenic causes. Flashy hydrographs , created in part by stream channelization and incision and watershed deforestation, may play a large role in structuring these fish assemblages. Extreme interannual variability in assemblages in the absence of detectable habitat change has important implications for the statistical power of fish monitoring programs designed to detect trends in fish assemblages over time.     

Data synthesis and bioindicator development for nontidal streams in the interstate Potomac River basin, USA

Water resource agencies are increasingly confronted with issues of methods and data comparability when assessing inter-jurisdictional waters. The Interstate Commission on the Potomac River Basin (ICPRB) is developing an assessment framework for the diverse nontidal monitoring data collected by Potomac basin jurisdictions. The ICPRB's goal is to augment the jurisdictions’ water quality assessments with uniform, basin-wide evaluations of habitat and biological integrity. Disparate datasets were integrated and used to select and calibrate a common suite of biological indicators of human disturbance. Aggregated physiographic regions were most effective at grouping streams into site classes with similar attributes. Common elements from states’ habitat and water quality assessments were used to define regional reference and impairment criteria. Seven macroinvertebrate metrics distinguished reference from impaired sites in most regions: EPT richness, Hilsenhoff Family Biotic Index, percent clingers, percent collectors, percent dominance, percent EPT, and taxonomic richness. Reference communities in the Piedmont region of the basin were dissimilar from those in Highland and Valley regions, suggesting a need for different reference conditions and metric thresholds in these regions. Results confirm that useful bioindicators of aquatic health can be generated from multiple datasets if the synthesis is done with care.     

Seasonal succession of the phytoplankton in the upper Mississippi River

Species composition and seasonal succession of the phytoplankton were investigated on the upper Mississippi River at Prairie Island, Minnesota, U.S.A. Both the numbers and volume of individual species were enumerated based on cell counts with an inverted microscope. A succession similar to algal succession in the local lakes occurred. The diatoms were dominant during the spring and fall and blue-green algae were dominant during the summer. The algal concentrations have increased up to 40 fold the concentrations of the 1920's, since the installation of locks and dams. The maximum freshweight standing crop was 4 mg · l^–1 in 1928 (Reinhard 1931), 13 mg · l^–1 in 1975 a wet year, and 47 mg · l^–1 in 1976, a relatively dry year with minimal current discharge. The diatoms varied from 36–99%, the blue-green algae from 0–44% and the cryptómonads from 0–50% of the total standing crop. The green algae were always present but never above 21% of the biomass. The dominant diatoms in recent years were centric -Stephanodiscus andCyclotella spp. (maximum 50,000 ml^–1). The dominant blue-green algae wereAphanizomenon flos-aquae (L.) Ralfsex Born.et Flahault andOscillatoria agardhii Gomont (maximum 800 ml^–1). These algal species are also present in local lakes. Shannon diversity values indicated greatest diversity of algae during the summer months.     

Extensive diversification of pebblesnails (Lithoglyphidae: Fluminicola) in the upper Sacramento River basin, northwestern USA

Mitochondrial DNA sequences from the cytochrome c oxidase subunit I (COI) and cytochrome b (cytb) genes were obtained from the nine extant, previously described species of the northwestern North American freshwater gastropod genus Fluminicola (commonly known as pebblesnails) and from a large number of taxonomically undescribed populations of these animals from the upper Sacramento River basin, California and Oregon, which is composed of the Sacramento River headwaters, and the McCloud and Pit Rivers. Phylogenetic analyses of separate and combined molecular datasets yielded well‐supported and largely congruent trees delineating 13 genetically divergent and morphologically distinctive upper Sacramento basin lineages, which we describe as new species. These include two groups of closely related and geographically proximal species that are further united by unique radular or shell features. Most of these novelties have narrow geographical distributions and are restricted to headspring areas, whereas several are more wide ranging and typically occupy larger, well‐integrated habitats. The highly endemic fauna of upper Sacramento River pebblesnails is not a single species flock, but instead a polyphyletic assemblage spread among four separate clades. Our phylogeny, together with the application of a COI molecular clock for Fluminicola, suggests that upper Sacramento River clades originated as a result of late Neogene separation of this basin from neighbouring regions (northwestern Great Basin, Klamath River basin), which is consistent with previous biogeographical hypotheses based on the distributions of fishes. The upper Sacramento River pebblesnails evolved in association with the complex late Cenozoic history of regional landscape and drainage and diversification was also facilitated by the invasion of and adaptation to insular spring habitats. Our findings are consistent with the generally limited dispersal ability and geologically ancient (mid‐Tertiary) age of this genus and imply that other portions of northwestern North America may also harbour a large number of undescribed pebblesnail species. Journal compilation © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 149 , 371–422. No claim to original US government works.     

Effects of temperature and current discharge on the concentration and photosynthetic activity of the phytoplankton in the upper Mississippi River

SUMMARY. Temperature and current discharge regulated phytoplanktonic concentration, chlorophyll-a concentration, the light-saturated rate of photosynthesis (P_max), and photosynthetic capacity (P_cap) in the Mississippi River at Prairie Island, Minnesota. The chlorophyll-a maximum was 48 mg m⁻³ in 1975, a wet year with a high current discharge, and 190 mg m⁻³ in 1976, a relatively dry year. The highest values of P_max were 0.37 (mgO₂ I⁻¹h⁻¹) in 1975 and 1.60 in 1976. P_cap varied from 3 to 21 (gO₂ per g chlorophyll-a h⁻¹) both years, and its value was highly correlated with temperature. The temperature optimum shifted from 16°C for P_cap in the spring, to greater than 28°C in the summer. Multiple regression analysis indicated a second-order relationship of P_cap in the spring to temperature. Other independent variables explained only negligible variation of P_cap.     

Effects of oxygen, pH and nitrate concentration on denitrification by Pseudomonas species

Abstract The production of nitrogen-containing gases by denitrification in three organisms was examined using membrane inlet mass spectrometry. The effects of O₂ (during both growth and maintenance) and of pH, nitrate concentration and carbon source were tested in non-proliferating cell suspensions. Two strains of Pseudomonas aeruginosa were capable of co-respiration of NO₃⁻ and O₂ and, under controlled O₂ supply, gave oscillatory denitrification. Variations in culture and assay conditions affected both the rate of denitrification and the ratio of end products (N₂O:N₂). Higher rates were seen following anaerobic growth. Optimum values of pH and nitrate concentration for denitrification are given. Generally, the optimum pH was 7.0–7.5, approximately that of the growth medium. Optimum nitrate concentration was generally 20 mM.     

Gulf sturgeon summer habitat use and fall migration in the Pascagoula River, Mississippi, USA

The locations and habitat features of freshwater holding areas and the timing of fall migration are undocumented for Gulf sturgeon in the Pascagoula River drainage, Mississippi. Our objectives were to identify and characterize holding areas for Gulf sturgeon (Acipenser oxyrinchus desotoi), document their summer and fall movement patterns, and determine migration cues. To do this we captured, radio‐tagged, and monitored movement of Gulf sturgeon in the Pascagoula River drainage and analyzed these data using geographic information systems. From May to November Gulf sturgeon congregate in a holding area in the lower portion of the Pascagoula River and Big Black Creek [river kilometers (rkm) 57–68] and near Cumbest Bluff (rkm 40), before they return to the Gulf of Mexico. While in the holding area, Gulf sturgeon were typically found in deep locations, either in or downstream from river bends. As found in other rivers, Gulf sturgeon in the Pascagoula River showed little movement within the holding area and often stayed within a single river bend; although we observed local movements by some individuals (under 10 rkm). In the Pascagoula River, Gulf sturgeon initiated their migration out of fresh water from late‐September to mid‐October, coincident with shorter day length, falling water temperature (mean = 23.7°C, range 21–26°C), and elevated river flow. Our work demonstrates that the lower Pascagoula River serves as a vital area for Gulf sturgeon.     

Effect of nitrate and thiosulfate concentration on the denitrification activity of bacterial isolates

The denitrification activity of five bacterial isolates was studied in samples of soil and freshwater sediments and from a wastewater treatment plant. The bacteria were exposed in a mineral medium supplied with Na₂S₂O₃·5H₂O (electron donor) and KNO₃ (electron acceptor). The effect of different concentrations of donor and/or acceptor of electrons was evaluated by the method of acetylene inhibition of N₂O-reductase. No positive influence on denitrification activity was observed with increased levels of individually added substrates. Three bacterial isolates exhibited a significant increase of denitrification activity when measured in the presence of both tested substances.     

Declining populations of the fingernail clamMusculium transversum in the upper Mississippi River

We examined recent temporal trends in the abundance of fingernial clamsMusculium transversum (formerlySphaerium transversum) in the upper Mississippi River. Historical data on densities of fingernail clams were obtained from regional scientists and published literature. We also sampled benthos in six navigation pools in summer 1991, finding very few fingernail clams. The combined data set, including historical data and sampling results, extended from 1973 to 1992 and was sufficient to statistically evaluate trends in densities of fingernail clams in eight pools. Populations of fingernail clams declined significantly in five of the eight pools examined (Pools 2, 5, 7, 9, and 19), which spanned a 700-km reach of river from St. Paul, Minnesota, to Keokuk, Iowa. Densities in Pool 19, which had the longest historical record on fingernail clam abundance, averaged 30 000 m⁻² in 1985 and progressively declined to zero in 1990. Combined data from all eight pools showed a significant decline in abundance of fingernail clams. An evaluation of potential causal factors led us to hypothesize that the population declines in Pools 2 to 9 were linked to point-source pollution rather than to dredging activity or commercial navigation traffic. In Pool 19, the declines of fingernail clams may have resulted from low-flow conditions during drought periods, but the causal mechanisms by which low flow influences fingernail clam abundance are unclear. The decrease in fingernail clam populations may adversely affect certain fish and wildlife, such as migrating lesser scaupAythya affinis, which feed heavily on the small mollusk. Moreover, the decreases in populations of this pollution-sensitive mollusk may signal a large-scale deterioration in the health of this riverine ecosystem. The U.S. Government right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.     

Red shiner invasion and hybridization with blacktail shiner in the upper Coosa River,USA

Human disturbance increases the invasibility of lotic ecosystems and the likelihood of hybridization between invasive and native species. We investigated whether disturbance contributed to the invasion of red shiner (Cyprinella lutrensis) and their hybridization with native blacktail shiner (C. venusta stigmatura) in the Upper Coosa River System (UCRS). Historical records indicated that red shiners and hybrids rapidly dispersed in the UCRS via large, mainstem rivers since the mid to late 1990s. We measured the occurrence and abundance of parental species and hybrids near tributary-mainstem confluences and characterized populations at these incipient contact zones by examining variation across morphological traits and molecular markers. Red shiners represented only 1.2% of total catch in tributaries yet introgression was widespread with hybrids accounting for 34% of total catch. Occurrence of red shiners and hybrids was highly correlated with occurrence of blacktail shiners, indicating that streams with native populations are preferentially colonized early in the invasion and that hybridization is a key process in the establishment of red shiners and their genome in new habitats. Tributary invasion was driven by post-F1 hybrids with proportionately greater genomic contributions from blacktail shiner. Occurrence of red shiners and hybrids and the relative abundance of hybrids significantly increased with measures of human disturbance including turbidity, catchment agricultural land use, and low dissolved oxygen concentration. Red shiners are a significant threat to Southeast Cyprinella diversity, given that 41% of these species hybridize with red shiner, that five southeastern drainages are invaded, and that these drainages are increasingly disturbed by urbanization.     

Habitat use and movements of shovelnose sturgeon in Pool 13 of the upper Mississippi River during extreme low flow conditions

We monitored habitat use and movement of 27 adult shovelnose sturgeon in Pool 13 of the upper Mississippi River, Iowa-Illinois, by radio-telemetry in April through August 1988. Our objective was to determine the response of this species to unusually low water conditions in the upper Mississippi River in 1988. Most (94%) telemetry contacts were made in 3 habitat types: main channel (50%), main channel border where wing dams were present (29%), and tailwaters of Lock and Dam 12 (15%). Habitat use in spring was affected by the extreme low flows. We often found tagged shovelnose sturgeon in the main channel and tailwaters during the spring period (11 March–20 May) where water velocities were highest. This was in contrast to other studies where shovelnose sturgeon did not occupy those areas during years with normal spring flows. Shovelnose sturgeon were typically found in areas with a sand bottom, mean water depth of 5.8 m, and mean bottom current velocity of 0.23 m sec^-1. They occupied areas of swifter current but were not always found in the fastest current in their immediate vicinity. Tagged shovelnose sturgeon tended to remain in the upper, more riverine portion of the pool, and we observed no emigration from the study pool. Linear total range of movement from the tagging site ranged from 1.9 to 54.6 km during the study period.     

Phytoplankton uptake of ammonium, nitrate and urea in the Neuse River Estuary, NC, USA

Uptake rates of ammonium, nitrate and urea were measured during the spring, summer and autumn (2001) in the eutrophic, nitrogen (N) limited Neuse River Estuary (NRE), North Carolina, USA. Ammonium was the dominant form of N taken up during the study, contributing approximately half of the total measured N uptake throughout the estuary. Nitrate uptake declined significantly with distance downstream comprising 33% of the total uptake in the upper estuary but only 11 and 16% in the middle and lower estuary, respectively. Urea uptake contributed least to the total pool in the upper estuary (16%), but increased in importance in the middle and lower estuary, comprising 45 and 37% of the total N taken up, respectively. The importance of regenerated N for fuelling phytoplankton productivity in the mesohaline sections of the NRE is demonstrated. The contribution of urea to the regenerated N pool suggests that internal regeneration of dissolved organic N may support a large proportion of the phytoplankton primary production and biomass accumulation in the middle and lower NRE. These results suggest that N-budgets based on dissolved inorganic N uptake rates alone will seriously under estimate phytoplankton N uptake.     

Nitrate sources and watershed denitrification inferred from nitrate dual isotopes in the Beijiang River,south China

The great spatial and temporal variability of nitrogen (N) processing introduces large uncertainties for quantifying N cycles in large scales, e.g. a watershed scale, and hence challenges the present techniques in measuring ecosystem N mass balance. The dual isotopes of nitrate (δ¹⁸O and δ¹⁵N) integrate signals for both nitrate sources and N processing, making them promising for studies on large scale N cycling. Here, the dual isotopes, as well as some ion tracers, from a subtropical river in south China were reported to identify the main nitrate sources and to assess the possible occurrence and degree of denitrification in the context of monsoon climate. Our results indicated that nitrification of reduced fertilizer N in soil zones was the main nitrate source, with sewage and manure as another important source in dry winter. Seasonal changes of denitrification was apparent by the ~1:2 enrichment of ¹⁸O and ¹⁵N from April to August, and suggested to occur over the watershed rather than in the river. The lowest denitrification (10%) occurred in April, when the fertilizer application was strongest and the monsoon rainfall abruptly increased, causing enhancement of leaching. The highest denitrification (48%) took place in August due to the high soil temperature and moisture. In December, denitrification was significant (26%) perhaps due to the high enough temperature for microbial activities, whereas the low soil moisture appeared to limit the degree of denitrification. This study suggests that the seasonal variations in denitrification should be taken into account when estimating regional N mass balance.     

Effects of Mississippi River water on phytoplankton growth and composition in the upper Barataria estuary,Louisiana

Diversion of river waters to adjacent estuaries may occur during wetland restoration, navigation channel development, or storms. We proposed that diversions of nitrogen- and phosphorus-enriched waters from the river to estuarine waters would result in increased phytoplankton biomass and shifts to noxious or harmful algal blooms. We tested this hypothesis by conducting four seasonal microcosm experiments in which Mississippi River water was mixed at different volume ratios with ambient estuarine waters of three lakes in the upper Barataria Basin, Louisiana, USA. These lakes included two brackish lakes that were in the path of diverted Mississippi River water, and a freshwater lake that was not. The results from the 3- to 8-day experiments yielded a predictable increase in phytoplankton biomass related to nutrient additions from Mississippi River water. The subsequent decreases in the dissolved nitrate + nitrite, soluble reactive phosphorus, and silicate concentrations explained 76 to 86% of the increase in chlorophyll a concentrations in the microcosms. Our experiments showed that cyanobacteria can successfully compete with diatoms for N and P resources even under non-limiting Si conditions and that toxic cyanobacteria densities can increase to bloom levels with increased Mississippi River water inputs to ambient waters in the microcosms. Diversions of Mississippi River into adjacent estuarine waters should be considered in relation to expected and, possibly, unexpected changes in phytoplankton communities to the receiving waters and coastal ecosystems.

     

Silica fluxes and trapping in two contrasting natural impoundments of the upper Mississippi River

Human activities have altered riverine silica cycling and diminished the supply of silica to the oceans, but few rivers have been intensively monitored to evaluate the magnitude of these changes. In this study we measured dissolved silica (DSi) and amorphous silica (ASi) fluxes into and out of two large, culturally-impacted natural impoundments of the upper Mississippi River, Lakes St. Croix and Pepin, USA. ASi sedimentation rates and sediment–water fluxes of DSi were calculated for each lake, and a mass-balance approach was used to determine in-lake ASi production. ASi from terrestrial phytoliths in the lake sediments was determined to be only partially available to biotic recycling, and in-lake ASi dissolution was small relative to the total silica budgets. The river reaches upstream of the two lakes were found to have abundant DSi, and riverine diatom production was found to contribute significant amounts of ASi to each lake. The average total phosphorus concentration in Lake Pepin is four times that in Lake St. Croix but ASi production in Lake Pepin is only 2.3 times higher than in Lake St. Croix, indicating that diatom growth in Pepin is limited by factors such as turbidity. Lake St. Croix currently traps about 10% of the inflowing total bioavailable silica (TSi_b = DSi + ASi) while Lake Pepin traps closer to 20% of its inflowing TSi_b, clearly demonstrating the importance of silica retention in lakes and reservoirs along the land–ocean continuum.     

Limnological studies of the lakes and streams of the upper Qu'Appelle River System,Saskatchewan, Canada

High phytoplankton productivity characterizes the eutrophic lakes of the upper Qu'Appelle River system. Annual primary production varied from 187 to 561 g C m^? while daily areal production varied from 290 to 8 575 mg C m^?2. The A_max range was 164 to 315 mg C m^?3h^?1 with the highest rates usually occurring in August or September. A_max values on any given day normally occurred in the top 0.5 m. Blue-green algae (Aphanizomenon flos-aquae, Microcystis aeruginosa, Oscillatoria prolifica) dominated the phytoplankton communities during the summer and fall. Diatoms (Asterionella formosa, Fragilaria capucina, Stephanodiscus niagarae) often dominated the spring communities but sometimes persisted as dominants through the summer. Fragilaria on occasion was present at very high concentrations in the late fall. Ceratium hirundinella was sometimes dominant but was usually an important part of the biomass. Green algae, although always present, rarely formed an important part of the biomass with Pediastrum duplex the only exception. A species list is appended.     

Predicting occurrence of the fungal symbiote Harpella colonizing black fly larvae in coastal streams of Alabama and Mississippi, USA

The environmental conditions governing symbioses are poorly known in aquatic systems. Stream conditions associated with the distribution of the black fly (Simuliidae) midgut symbiote Harpella were investigated in southern Alabama and Mississippi streams. Stream conditions that were most useful in predicting the distribution of Harpella spp. in the study area were dissolved oxygen and water temperature. Presence of Harpella species in streams was associated with higher dissolved oxygen and decreased water temperature compared to streams where Harpella spp. was absent. Stream conditions associated with the distribution of Harpella spp. in other regions of the world vary according to conditions other than those elucidated here, indicating that geography, host species, and stream conditions play important roles in the spatial distribution of Harpella species.