Factors controlling streambed coverage of <Emphasis Type="Italic">Didymosphenia geminata</Emphasis> in two regulated streams in the Colorado Front Range

Didymosphenia geminata is a stalk-forming freshwater diatom which was historically found primarily in oligotrophic lakes and streams, but has recently become a nuisance species in many lotic systems worldwide. In the last 5–8 years, D. geminata has become established in Boulder Creek and South Boulder Creek, two regulated montane streams in the Front Range of the Colorado Rocky Mountains. Factors that may influence the growth of D. geminata were monitored during the summer of 2006. D. geminata abundance decreased in Boulder Creek after an unusual flood event caused by 3 days of sustained rainfall in the headwaters of the watershed. However, within a week, coverage had been restored to pre-flood levels. Variations in D. geminata abundance among sites were found to be negatively correlated with total dissolved phosphorus concentrations and bed movement, as measured by Shields stress. In contrast, D. geminata abundance was not significantly correlated with temperature, conductivity, pH, total suspended solids, or dissolved inorganic nitrogen. Our results suggest that bed movement may be a dominant scouring mechanism that acts to control the growth and distribution of D. geminata. The potential role of total dissolved phosphorus and bed movement in decreasing D. geminata coverage adds to the limited base of knowledge regarding controls on the growth and distribution of this species, and could be investigated by researchers studying D. geminata blooms in other stream ecosystems. Handling editor: J. Saros     

Genetic structure of a disjunct peripheral population of mountain sucker <Emphasis Type="Italic">Pantosteus jordani</Emphasis> in the Black Hills,South Dakota,USA

A peripheral population of mountain sucker, Pantosteus jordani, located in the Black Hills of South Dakota, USA, represents the eastern-most range of the species and is completely isolated from other populations. Over the last 50 years, mountain sucker populations have declined in the Black Hills, and now only occur in 40 % of the historic local range, with densities decreasing by more than 84 %.We used microsatellite DNA markers to estimate genetic diversity and to assess population structure across five streams where mountain suckers persist. We evaluated results in the context of recent ecological surveys to inform decisions about mountain sucker conservation. Significant allele frequency differences existed among sample streams (Global F_ST = 0.041) but there was no evidence of isolation by distance. Regionally, genetic effective size, N_e, was estimated to be at least 338 breeding individuals, but N_e within streams was expected to be less. Despite almost complete demographic isolation and reduced population size, there appears to be little evidence of inbreeding, but genetic drift and local isolation due to fragmentation probably best explains genetic structure in this peripheral mountain sucker population. Recommended strategies for population enhancement include restoration of stream connectivity and habitat improvement. Moreover, repatriation and assisted movement (i.e., gene flow) of fishes should maximize genetic diversity in stream fragments in the Black Hills region.     

Didymosphenia geminata invasion in South America: Ecosystem impacts and potential biogeochemical state change in Patagonian rivers

The diatom Didymosphenia geminata has emerged as a major global concern, as both an aggressive invader of rivers and streams in the southern hemisphere, and for its ability to form nuisance blooms in oligotrophic systems in its native range. South American D. geminata blooms were first documented in Chilean Patagonia in May 2010, and have spread to over five regions and three provinces, in Chile and Argentina respectively. The Patagonian invasion represents a distinct challenge compared to other regions; not only are affected systems poorly characterized, but also a general synthesis of the nature and magnitude of ecosystem impacts is still lacking. The latter is essential in evaluating impacts to ecosystem services, forms the basis for a management response that is proportional to the potentially valid threats, or aids in the determination of whether action is warranted or feasible. Based on a revision of the recent literature, some of the most significant impacts may be mediated through physical changes: substantially increased algal biomass, trapping of fine sediment, altered hydrodynamics, and consequent effects on biogeochemical states and processes such as redox condition, pH and nutrient cycling in the benthic zone. Surveys conducted during the early invasion in Chile show a strong correlation between benthic biomass and associated fine sediments, both of which were one–two orders of magnitude higher within D. geminata blooms. Experimental phosphorous amendments showed significant abiotic uptake, while interstitial water in D. geminata mats had nearly 10–20 fold higher soluble reactive phosphorous and a pronounced pH cycle compared to the water column. A dominant and aggressive stalk-forming diatom with this combination of characteristics is in sharp contrast to the colonial cyanobacteria and bare gravel substrate that characterize many Patagonian streams. The potential displacement of native benthic algal communities with contrasting functional groups, increased primary producer biomass, and fine benthic sediment accumulation, all may have a significant effect on stream nutrient cycling and ecosystem function, in Patagonia and elsewhere where nuisance blooms have been reported.     

Assessing temporal genetic variation in a cougar population: influence of harvest and neighboring populations

The geography of the Black Hills region of South Dakota and Wyoming may limit connectivity for many species. For species with large energetic demands and large home ranges or species at low densities this can create viability concerns. Carnivores in this region, such as cougars (Puma concolor), have the additive effect of natural and human-induced mortality; this may act to decrease long-term viability. In this study we set out to explore genetic diversity among cougar populations in the Black Hills and surrounding areas. Specifically, our objectives were to first compare genetic variation and effective number of breeders of cougars in the Black Hills during three harvest regimes: pre (2003–2006), moderate (2007–2010), and heavy (2011–2013), to determine if harvest impacted genetic variation. Second, we compared genetic structure of the Black Hills cougar population with cougar populations in neighboring eastern Wyoming and North Dakota. Using 20 microsatellite loci, we conducted genetic analysis on DNA samples from cougars in the Black Hills (n = 675), North Dakota (n = 113), and eastern Wyoming (n = 62) collected from 2001–2013. Here we report that the Black Hills cougar population maintained genetic variation over the three time periods. Our substructure analysis suggests that the maintenance of genetic variation was due to immigration from eastern Wyoming and possibly North Dakota.     

Development and validation of a quantitative PCR assay for the early detection and monitoring of the invasive diatom Didymosphenia geminata

Didymosphenia geminata is a large, invasive, freshwater diatom that can produce distinctive and robust mucilaginous stalks. Over the last two decades, there has been a worldwide increase in the distribution and severity of D. geminata blooms. These dense, persistent blooms can have severe impacts on native species and ecosystem functioning. D. geminata is usually identified by microscopic methods that are time consuming, resource intensive, and dependent upon expert taxonomic identification, so the extent of surveillance programs has been limited. As an alternative, we have developed a TaqMan quantitative polymerase chain reaction (QPCR) assay for sensitive and rapid detection and enumeration of D. geminata in environmental samples. Species-specific QPCR primers and probe were designed by aligning the D. geminata 18S ribosomal DNA (rDNA) sequence with closely related diatoms. The QPCR assay was linear (R² = 1.00) over a detection range of eight orders of magnitude with a lower limit of approximately two D. geminata cells. QPCR analysis of environmental samples employed the comparative cycle threshold (C_T)-method with an exogenous plasmid used as an internal reference standard. The assay was evaluated using samples collected during a survey of D. geminata in three rivers in the South Island, New Zealand, and from 13 international locations where D. geminata is known to be present. Positive QPCR amplifications were confirmed as the correct amplification product through direct DNA sequencing. Phylogenetic analysis of 18S rDNA sequences suggests that D. geminata is more closely related to species in the family Cymbellaceae rather than Gomphonemataceae as currently classified.     

Exploring the legacy effects of surface coal mining on stream chemistry

Surface coal mining results in dramatic alterations of the landscape in central Appalachia, leading to a myriad of environmental problems. In this study, we explore the long-term effects of surface coal mining on stream chemistry and endeavor to gain a better understanding of the efficacy of reclamation. We examined 30 sites in the Raccoon Creek watershed in southeastern Ohio, where the majority of surface mine sites are in various stages of reclamation. Our results show that conductivity (r = 0.862; P = 0.000), sulfate (r = 0.619; P = 0.000), and aluminum (r = 0.469; P = 0.009) levels increase linearly as a function of the areal extent of reclaimed mines in each subwatershed, suggesting limited success of reclamation to restore natural stream chemistry. In contrast, pH was not significantly linearly correlated with the areal extent of surface mines. This suggests that local acid mine drainage remediation projects are able to regulate acidity levels in the watershed but not conductivity and certain heavy metal concentrations. Many sites had conductivity levels high enough to impair aquatic biota via ionic and osmoregulatory stress. In sum, surface coal mining appears to have a strong legacy effect on stream chemistry in the Raccoon Creek watershed.     

Responses of invertebrates and algae of a boreal coniferous forest stream to experimental manipulation of leaf litter inputs and shading

Forest harvesting alters leaf litter inputs and shading of small streams. Most of the previous studies of harvesting effects are limited to coastal or deciduous forests, so here we consider a sub-boreal forest stream. To test the hypothesis that changes in light and litter inputs would affect the benthic community in these streams, we experimentally manipulated these variables in stream mesocosms. We used a 2 × 2 factorial design with light (shaded or full light) and leaf litter inputs (equivalent to a forested stream or one quarter that rate) as factors. The high leaf litter treatment resulted in differences in macroinvertebrate community composition and higher densities of two shredders, Limnephilus sp. and Podmosta sp., suggesting food limitation. Algal filaments were longer in the high light treatments indicating a change in periphyton composition. There were no significant differences in chlorophyll a or ash-free dry mass, suggesting that light was not limiting to periphyton. The community structure clearly shifted in response to both resources, although primarily to detrital inputs. These results provide evidence that changes to shading and leaf inputs to small streams can affect the benthos and may limit secondary production.     

A rapid technique for assessing the suitability of areas for invasive species applied to New Zealand's rivers

Early responses to incursions of non‐indigenous species (NIS) into new areas include modelling and surveillance to define the organisms’ potential and actual distributions. For well‐studied invasive species, predictive models can be developed based on quantitative data describing environmental tolerances. In late 2004, an invasive freshwater diatom Didymosphenia geminata, an NIS for which we had no such quantitative data, was detected in a New Zealand river. We describe a procedure used to rapidly develop a classification of suitability for all New Zealand's rivers, based on two sources of information. First, from a review of the limited available literature and unpublished data, we determined that temperature, hydrological and substrate stability, light availability, and water pH were the most important environmental gradients determining D. geminata's broad‐scale distribution and capacity for establishing and forming blooms in rivers. The second information source was a GIS‐based river network developed for a national classification of New Zealand's rivers, with associated data describing environmental characteristics of each section of the network. We used six variables that were available for every section of the network as surrogates for the environmental gradients that determine suitability. We then determined the environmental distance of all the river sections in the network from our assessment of the optimal conditions conducive to D. geminata blooms. The analysis suggested that > 70% of New Zealand's river sections (stream order > 3) fell into the two highest suitability categories (on a five‐point scale). At the time of writing, D. geminata had spread to 12 catchments, all of which were within these two categories. The technique is applicable in initial responses to incursions of NIS where quantitative information is limited, and makes optimal use of available qualitative information. Our assessment contributed to evaluations of the potential ecological, social, and economic impacts of D. geminata and is currently being used to stratify site selection for ongoing surveillance.     

Macroinvertebrate Responses to Algal and Bacterial Manipulations in Streams

Our study was designed to assess the relative importance of algae and bacteria as sources of energy for stream macroinvertebrates. In one experiment, we manipulated algae by artificially shading six sections in each of two streams, one stream with an open canopy (clear-cut drainage basin) and the other with a closed canopy (forested drainage basin); both streams were in Hubbard Brook Experimental Forest, New Hampshire, USA. Chlorophyll a concentrations were reduced from 0.2 to 0.05 μg/cm² in artificially shaded sections of both streams. However, macroinvertebrates showed no response to these algal manipulations in either the clear-cut or forested stream. Nutrient concentrations (N and P) were low and limiting to primary production in both the clear-cut and forested streams. Additionally, both streams had relatively low macroinvertebrate densities suggesting bottom-up controls were important in macroinvertebrate abundance. However, the forested stream did have higher macroinvertebrate densities presumably because of higher inputs of coarse particulate organic matter from the riparian vegetation. In a second experiment, in Augusta Creek, Michigan, we manipulated both algae and bacteria. To reduce algae, we artificially shaded experimental stream channels so that chlorophyll a was reduced from natural levels of 3.0–5.6 to 0.4–0.7 μg/cm². Half of the shaded channels had dissolved organic carbon (DOC – sucrose) dripped into them to raise DOC levels by 2–3 mg/l and thus stimulate bacterial abundance. Open channels, with higher algal abundance, had higher densities of Ephemerella, but only in November when nymphs were larger. Channels with increased DOC had higher bacterial abundances, higher densities of Chironomidae and lower densities of Heptageniidae. Several other macroinvertebrate taxa that were at relatively low abundance in our samples showed no significant response to these manipulations. Our results suggest that early instar Ephemerella may not rely as heavily on algae as later instars. Also, certain taxa were able to use the heterotrophic microbial community, especially chironomids which increased in numbers when bacterial density increased; thus, the bacterial carbon source may be more important to some stream macroinvertebrates than previous studies have suggested.     

Effects of urban stream burial on nitrogen uptake and ecosystem metabolism: implications for watershed nitrogen and carbon fluxes

Urbanization has resulted in the extensive burial and channelization of headwater streams, yet little is known about the impacts of stream burial on ecosystem functions critical for reducing downstream nitrogen (N) and carbon (C) exports. In order to characterize the biogeochemical effects of stream burial on N and C, we measured NO₃ ^? uptake (using ¹⁵N-NO₃ ^? isotope tracer releases) and gross primary productivity (GPP) and ecosystem respiration (ER) (using whole stream metabolism measurements). Experiments were carried out during four seasons, in three paired buried and open stream reaches, within the Baltimore Ecosystem Study Long-term Ecological Research site. Stream burial increased NO₃ ^? uptake lengths by a factor of 7.5 (p < 0.01) and decreased NO₃ ^? uptake velocity and areal NO₃ ^? uptake rate by factors of 8.2 (p < 0.05) and 9.6 (p < 0.001), respectively. Stream burial decreased GPP by a factor of 11.0 (p < 0.01) and decreased ER by a factor of 5.0 (p < 0.05). From fluorescence Excitation Emissions Matrices analysis, buried streams were found to have significantly altered C quality, showing less labile dissolved organic matter. Furthermore, buried streams had significantly lower transient storage (TS) and water temperatures. Differences in NO₃ ^? uptake, GPP, and ER in buried streams, were primarily explained by decreased TS, light availability, and C quality, respectively. At the watershed scale, we estimate that stream burial decreases NO₃ ^? uptake by 39 % and C production by 194 %. Overall, our results suggest that stream burial significantly impacts NO₃ ^? uptake, stream metabolism, and the quality of organic C exported from watersheds. Given the large impacts of stream burial on stream ecosystem processes, daylighting or de-channelization of streams, through hydrologic floodplain reconnection, may have the potential to alter ecosystem functions in urban watersheds, when used appropriately.     

Conservation genetics of American Dipper (<Emphasis Type="Italic">Cinclus mexicanus</Emphasis>): the genetic status of a population in severe decline

American Dippers (Cinclus mexicanus) were once known to occur in streams throughout the Black Hills of South Dakota and Wyoming, but now dippers number about 50–75 individuals and reside almost exclusively in a single stream. The recent decline of the American Dipper in the Black Hills of South Dakota is thought to be due to local stream degradation. As a result of the decline of C. mexicanus in the Black Hills of South Dakota and Wyoming, the Black Hills population of American Dippers is a candidate for designation as a distinct population segement (DPS) and might warrant protection and special management. One criterion for DPS designation is genetic uniqueness. Here we present the results of a genetic assessment of the Black Hills population of C. mexicanus. Data presented here indicate that the dipper population in the Black Hills is genetically distinct from other sampled populations. Further population sampling will be needed to understand the genetic population structure of C. mexicanus throughout its range. Furthermore, the recent decline in the Black Hills dipper population should be a warning that other populations (and other species) may be experiencing similar declines and that such montane habitats are worthy of special management.     

Local effects of forest fragmentation on diversity of aquatic insects in tropical forest streams: implications for biological conservation

The influence of forest fragmentation (habitat isolation) on biological and ecological diversity of aquatic insects was investigated in streams of fragmented forests in Hulu Gombak (6 streams) and Gunung Angsi (5 streams) and un-fragmented forest of Berembun (6 streams) in peninsular Malaysia. Several environmental parameters including canopy cover, DO, temperature and pH differed significantly among the three catchments (P < 0.05). We found that taxonomic richness in Berembun forest was significantly different from Gunung Angsi (P < 0.05), but not with Hulu Gombak forests (P > 0.05). Nestedness pattern that measures the effect of habitat isolation on taxonomic assemblages showed that aquatic insect’s community in un-fragmented forest (Berembun) was less nested (T = 54.4), indicating high diversity compared to highly nested (less diverse) in the two fragmented forests (Hulu Gombak, T = 30.45 and Gunung Angsi, T = 35.45). Taxa similarity in Berembun streams was negatively correlated with the geographical distance among streams (Mantel test, r = ? 0.462, P < 0.05). Such correlation was absent in both Gunung Angsi and Hulu Gombak streams. Forest fragmentation in Hulu Gombak and Gunung Angsi measured as the distance of the forests from the nearest forested area had negative effect on aquatic insects diversity (r ² = ? 0.149, P < 0.05), but not on their abundances (r ² = 0.003, P > 0.05). We concluded that local habitat conditions were the most important in shaping the aquatic insects community among streams of both unfragmented and fragmented forests.     

Biogeochemical transformation of a nutrient subsidy: salmon, streams, and nitrification

Migratory animals can alter ecosystem function via the provision of nutrient subsidies. These subsidies are heterogeneous in space and time, which may create hot spots or hot moments in biogeochemical transformations, in turn altering the ecosystem effect of the subsidy by changing the form of the nutrients. Annual migrations of Pacific salmon (Oncorhynchus spp.) transport nutrients from the marine environment to their natal freshwater ecosystems. Salmon subsidies provide high quality nutrients (e.g., nitrogen, phosphorus, carbon) that may also be large in quantity where salmon migrations are near historic levels. We hypothesized that the nutrient subsidy provided via the excretion of ammonium (NH₄ ⁺) by live salmon would stimulate microbially mediated nitrification rates in stream sediments and increase streamwater nitrate (NO₃ ^?) concentrations. We quantified sediment nitrification in seven streams in Southeast Alaska before and during the salmon run in 2007 and 2008. Nitrification rates increased 3-fold from before to during the salmon run (mean ± SE = 0.07 ± 0.01 to 0.24 ± 0.02 mgN gAFDM^?1 d^?1, respectively). The variation in nitrification was explained by both streamwater and exchangeable NH₄ ⁺ concentrations (R ² = 0.50 and 0.71, respectively), which were low before salmon and increased relative to the size of the salmon run. To experimentally test the effect of salmon subsidies on nitrification rates, we staked senesced salmon carcasses on stream sediments for 3 weeks during the salmon run and then measured nitrification rates directly under the carcasses. Sediment nitrification was 2–5 times higher under the carcasses compared to nearby sediments without the direct carcass influence. Our results confirm that biogeochemical transformations alter the form of salmon-derived nitrogen, representing an overlooked aspect in the dynamics of this subsidy. Therefore, animal-derived nutrient subsidies are not passively retained or exported in recipient ecosystems, but also transformed, thereby influencing the form and incorporation of these nutrient subsidies.     

Seed dynamics of an endemic palm in a Northwestern Mexican tropical dry forest: implications for population spatial structure

Seed dynamics are an important part of the life history of plants and may have strong implications on abundance and spatial distribution of populations. In this study, we explored how seed dynamics (removal, predation, germination) interact with micro-environmental conditions to affect the spatial structure of populations of Brahea aculeata (Arecaceae) in a tropical dry forest. B. aculeata is distributed throughout arroyo basins and attains its highest densities near to arroyos/rivers. We hypothesized that: (i) seed removal, predation and germination vary across topographic positions resulting in greater palm abundances adjacent to arroyos and (ii) seed removers/predators respond to both a seed density-dependent effect and a microclimate effect. To test this, in six arroyos basins, seeds were sown across three topographic positions (stream, mid and top of basins) with two seed abundances (1 and 10), protected and non-protected from potential predators. Predation, removal and germination were then followed. After 107 days, 100 % of the exposed seeds were removed/predated and none germinated. For seed removal, we found differences among topographic positions and seed densities with higher removal (up to 80 %) and lower predation rates for grouped seeds. Germination was only observed for protected seeds with higher germination rates in single (17 % ± 9) than in grouped seeds (4 % ± 1). The highest germination and establishment rates were adjacent to the streams; areas which had the lowest light intensity (mean ± SE = 883 ± 160 lm/ft²) and temperatures (mean ± SE = 20.1 ± 0.6 °C), and highest humidity (mean ± SE = 50.8 ± 1.8 %), especially during the rainy season. Differential seedling establishment rates across the landscape due to spatial patterns of seed predation/removal as well as micro-environmental variables appear to have implications for shaping the spatial structure of B. aculeata population at Sierra de Álamos, Mexico.     

Distribution of detritivores in tropical forest streams of peninsular Malaysia: role of temperature,canopy cover and altitude variability

The diversity and abundance of macroinvertebrate shredders were investigated in 52 forested streams (local scale) from nine catchments (regional scale) covering a large area of peninsular Malaysia. A total of 10,642 individuals of aquatic macroinvertebrates were collected, of which 18.22 % were shredders. Biodiversity of shredders was described by alpha (α_average ), beta (β) and gamma diversity (γ) measures. We found high diversity and abundance of shredders in all catchments, represented by 1,939 individuals (range 6–115 and average per site of 37.29?±?3.48 SE) from 31 taxa with 2–13 taxa per site (α_average?=?6.98?±?0.33 SE) and 10–15 taxa per catchment (γ?=?13.33?±?0.55 SE). At the local scale, water temperature, stream width, depth and altitude were correlated significantly with diversity (Adj-R ²?=?0.205). Meanwhile, dissolved oxygen, stream velocity, water temperature, stream width and altitude were correlated to shredder abundance (Adj-R ²?=?0.242). At regional scale, however, water temperature was correlated negatively with β and γ diversity (r ²?=?0.161 and 0.237, respectively) as well as abundance of shredders (r ²?=?0.235). Canopy cover was correlated positively with β diversity (r ²?=?0.378) and abundance (r ²?=?0.266), meanwhile altitude was correlated positively with β (quadratic: r ²?=?0.175), γ diversity (quadratic: r ²?=?0.848) as well as abundance (quadratic: r ²?=?0.299). The present study is considered as the first report describing the biodiversity and abundance of shredders in forested headwater streams across a large spatial scale in peninsular Malaysia. We concluded that water temperature has a negative effect while altitude showed a positive relationship with diversity and abundance of shredders. However, it was difficult to detect an influence of canopy cover on shredder diversity.     

Habitat conditions at beaver settlement sites: implications for beaver restoration projects

Recognition that beavers are integral components of stream ecosystems has resulted in an increase in beaver‐mediated habitat restoration projects. Beaver restoration projects are frequently implemented in degraded stream systems with little or no beaver activity. However, selection of restoration sites is often based on habitat suitability research comparing well‐established beaver colonies to unoccupied stream sections or abandoned colonies. Because beavers dramatically alter areas they occupy, assessing habitat conditions at active colonies may over‐emphasize habitat characteristics that are modified by beaver activity. During 2015–2017, we conducted beaver activity surveys on streams in the upper Missouri River watershed in southwest Montana, United States, to investigate habitat selection by beavers starting new colonies in novel areas. We compared new colony locations in unmodified stream segments to unsettled segments to evaluate conditions that promoted colonization. Newly settled stream segments had relatively low gradients (β ± SE = ?0.72 ± 0.27), narrow channels (β = ?1.31 ± 0.46), high channel complexity (β = 0.76 ± 0.42), high canopy cover of woody riparian vegetation (β = 0.56 ± 0.21), and low‐lying areas directly adjacent to the stream (β = 0.36 ± 0.24), where β denotes covariate effect sizes. Habitat selection patterns differed between our new settlement site analysis and an analysis of occupied versus unoccupied stream segments, suggesting that assessing habitat suitability based on active colonies may result in misidentification of suitable site conditions for beaver restoration. Our research provides recommendations for beaver restoration practitioners to select restoration sites that will have the highest probability of successful colony establishment.     

Didymosphenia geminata growth rates and bloom formation in relation to ambient dissolved phosphorus concentration

1. The bloom‐forming freshwater stalked diatom Didymosphenia geminata is unusual among algae in that nuisance growths occur almost exclusively in oligotrophic waters. Current hypotheses to explain this phenomenon have assumed supplemental acquisition of phosphorus from novel sources within the stalk/mat matrix. 2. We carried out a synoptic survey of river sites in the South Island, New Zealand, to determine whether D. geminata cell division and stalk development (measured as mat coverage or standing crop) were related to ambient phosphorus concentrations in the overlying river water. 3. High coverage (>50%) by D. geminata was largely concentrated at sites with mean dissolved reactive phosphorus (DRP) <2 mg m^?3 in the overlying water. Didymosphenia geminata was present at only one site with DRP >4 mg m^?3, with very low coverage. Cell division rate (measured as the frequency of dividing cells, FDC) was positively correlated with mean DRP suggesting that division rates were controlled by the available phosphorus concentration in ambient river water. At the same time, FDC was negatively correlated with D. geminata standing crop (measured as an index incorporating percentage cover and mat thickness). 4. In a single river reach with a stable cross‐channel gradient of total dissolved phosphorus (TDP) caused by inflows from a high‐nutrient tributary, again we observed a negative correlation between percentage cover by D. geminata and concentrations of TDP. 5. Finally, we made a series of observations on D. geminata‐dominated communities that had been exposed to water enriched with ‐N and ‐P for 4 weeks, followed by exposure to unenriched water. After 2 weeks of nutrient deprivation, D. geminata cell division rates declined by 60%, mean stalk length increased by 250%, and total carbohydrate quadrupled relative to initial values. The appearance of the community changed from a dark brown mat to a thick pale mat typical of D. geminata blooms. 6. All these results indicate that D. geminata cell division rates are actively controlled by concentrations of available phosphorus in the overlying water and that stalk production (represented by mat thickness and extent, stalk length and total carbohydrate) is inversely related to D. geminata cell division rates. They thus support an explanation for D. geminata blooms in oligotrophic rivers tied to enhanced stalk production in nutrient‐poor waters, rather than through acquisition of additional phosphorus through recycling processes within the mat.     

The expansion of woody riparian vegetation,and subsequent stream restoration,influences the metabolism of prairie streams

1. Tallgrass prairies and their streams are highly endangered ecosystems, and many remaining streams are threatened by the encroachment of woody riparian vegetation. An increase in riparian vegetation converts the naturally open‐canopy prairie streams to closed‐canopy systems. The effects of a change in canopy cover on stream metabolism are unknown. 2. Our goal was to determine the effects of canopy cover on prairie stream metabolism during a 4‐year period in Kings Creek, KS, U.S.A. Metabolic rates from forested reaches were compared to rates in naturally open‐canopy reaches and restoration reaches, the latter having closed canopies in 2006 and 2007 and open canopies in 2008 and 2009. Whole‐stream metabolism was estimated using the two‐station diurnal method. Chlorophyll a concentrations and mass of filamentous algae were measured after riparian removal to assess potential differences in algal biomass between reaches with open or closed canopies. 3. Metabolic rates were spatially and temporally variable even though the sites were on very similar streams or adjacent to each other within streams. Before riparian vegetation removal, whole‐stream community respiration (CR) and net ecosystem production were greater with greater canopy cover. In the vegetation removal reaches, gross primary production was slightly greater after removal. 4. Chlorophyll a concentrations were marginally significantly greater in open (naturally open and removal reaches) than in closed canopy and differed significantly between seasons. Filamentous algal biomass was greater in open than in closed‐canopy reaches. 5. Overall, the restoration allowed recovery of some features of open‐canopy prairie streams. Woody expansion apparently increases CR and moves prairie stream metabolism towards a more net heterotrophic state. An increase in canopy cover decreases benthic chlorophyll, decreases dominance of filamentous algae and potentially alters resources available to the stream food web. The results of this study provide insights for land managers and conservationists interested in preserving prairie streams in their native open‐canopy state.     

Evidence for consumer regulation of biofilm–nutrient interactions among hardwater streams (Pennsylvania, USA)

Experimental studies evaluating the simultaneous effects of consumers, nutrients, and other biotic/abiotic factors on intact, natural food webs are rare, particularly among ecosystems of varying trophic conditions. We conducted a series of in situ studies that used nutrient-diffusing substrata with nitrogen (N) and phosphorus (P) concentrations in a full factorial design in three temperate, limestone streams in Pennsylvania across a trophic gradient (mesotrophic, eutrophic, and hypereutrophic streams). We assessed differences in algal and macroinvertebrate biomass, taxonomic composition, and functional groups relative to amended nutrients across the trophic gradient; as such, these results facilitated predictions about regulators of food web structure. All factors varied significantly among the streams (e.g., algal biomass P = 0.005, macroinvertebrate biomass P < 0.001, algal diversity P = 0.006, macroinvertebrate diversity P < 0.001, algal group P < 0.001, macroinvertebrate guilds P < 0.001); the streams, however, did not exhibit simple responses to nutrient amendment. Algal and macroinvertebrate biomass and diversity responded greatest in the mesotrophic stream while grazing seemed to be a strong factor preventing algal nutrient response in the eutrophic and hypereutrophic streams. Brillouin’s Evenness Index was most influenced by nutrient amendment (nutrient effect on algae and macroinvertebrates P = 0.021). As such, we concluded that biomass and diversity were mediated by complexity within intermediate trophic levels.     

Impact of seasonal changes in stream metabolism on nitrate concentrations in an urban stream

Nitrate (NO₃ ^?) dynamics in urban streams differ from many natural streams due to stormwater runoff, sewage inputs, decreased groundwater discharge, often limited hyporheic exchange, increased primary productivity, and limited carbon input. We investigated NO₃ ^? dynamics in a first-order urban stream in Syracuse, NY, which has urbanized headwaters and a geomorphologically natural downstream section. Twice-monthly water sampling, NO₃ ^? injection tests, NO₃ ^? isotopic analysis, filamentous algae mat density, and riparian shading were used to identify processes regulating NO₃ ^? dynamics in the stream over a 12-month period. The urban headwater reach had low NO₃ ^? (0.006–0.2 mg N/L) in the spring through fall, with a minimum uptake length of 900 m, no canopy cover, and high algae mat density. The downstream natural reach (100% canopy cover during the summer and low algae mat density) had nitrate concentrations between 0.6 and 1.2 mg N/L from winter to summer, which decreased during autumn leaf-off. In the urban reach, autotrophic uptake by filamentous green algae is a major NO₃ ^? sink in summer. In the natural reach, the addition of organic matter to the stream at leaf-off led to a decrease in NO₃ ^? concentration followed by an increase in NO₃ ^? concentration in winter as gross primary productivity decreased. This study shows that the balance between autotrophy and heterotrophy in urban streams is variable and depends on an interplay of drivers such as temperature, light, and carbon inputs that are mediated by the riparian ecosystem.