Hydrodynamic control of filamentous macroalgae in a sub-tropical spring-fed river in Florida,USA

Hydrodynamic properties of rivers and streams are primary controls on the abundance and structure of algal communities. In Florida, USA, filamentous macroalgal proliferation in spring-fed rivers has largely been attributed to nitrogen enrichment; however, changes in hydrodynamics and other potential mechanisms have not been thoroughly evaluated. This study tested whether macroalgal abundance (primarily Lyngbya wollei) was related to hydrodynamics in Gum Slough, a sub-tropical Florida spring-fed river. Flow velocities and macroalgal cover were measured at three transects bimonthly between June 2010 and January 2013, a period during which river discharge fluctuated in response to a drought and a tropical storm. Temporal variation in macroalgal cover was significantly inversely related to flow velocity, excluding fall/winter sampling events when light limitation was likely. The results indicate a velocity threshold below which macroalgal cover increased substantially, and a lower threshold below which complete cover frequently occurred. Local differences in channel morphology and substrate type (i.e., aquatic plant beds) seemed to affect the relationship between macroalgal cover and velocity at each transect. Nitrogen and phosphorus concentrations and other water chemistry parameters were not associated with variation in macroalgal cover. Overall, this study suggests that hydrodynamics can strongly control filamentous macroalgae in Florida spring-fed rivers.     

Community composition and tree population structure in a sub-tropical broad-leaved forest along a disturbance gradient

Floristic composition, diversity, dominance and distribution pattern of species and tree population structure were studied in three stands of a sub-tropical wet hill broad-leaved forest of Meghalaya, India, along a disturbance gradient. Tree species diversity declined with increasing disturbance. Disturbed stands showed low equitability or high dominance and the undisturbed stand exhibited high equitability or low dominance. Contagious distribution among the tree species increased with increasing intensity of disturbance. Species showing regular distribution were restricted only to the undisturbed stand. Effect of disturbance on tree population structure was analysed using density-diameter curves. In the disturbed stands tree species showed reverse J-shaped and/or negative exponential curves, while those in the undisturbed stand exhibited sigmoid to bimodal mound shaped curves.     

Zebra mussel (Dreissena polymorpha) seasonal colonization patterns in a sub-tropical floodplain river

Zebra mussel (Dreissena polymorpha) seasonal colonization patterns, growth and habitat preferences were determined in a sub-tropical floodplain river at the southern edge of its distribution in North America during 1995–96 (Atchafalaya River Basin, Louisiana). Zebra mussel movement into subtropical areas represents a major frontier for this species worldwide. The onset of adult zebra mussel colonies occurred when the minimum daily temperature dropped below 31 °C and dissolved oxygen levels rose above 6.5 mg l^-1 in the fall. By mid-winter, mussel populations were established at lateral distances >10 km from the main river channel. Mussel growth occurred throughout the winter with an increase in growth in April and May. Adult mortality occurred during May–August as dissolved oxygen levels declined and minimum daily temperature warmed above 29 °C in the floodplain and 32.5 °C in riverine sites. Limiting factors responsible for the seasonal pattern include temperature and dissolved oxygen tolerances experienced during summer months in the ARB. Summer water conditions apparently preclude establishment of resident zebra mussel populations in the Atchafalaya floodplain. Naturally occurring seasonal patterns in temperature and dissolved oxygen in floodplain rivers may have implications for the expansion of this exotic mollusk in warmwater systems with source colonies restricted to mainstem rivers and seasonal sinks in floodplain regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Siak, a tropical black water river in central Sumatra on the verge of anoxia

The Siak is a black water river in central Sumatra, Indonesia, which owes its brown color to dissolved organic matter (DOM) leached from surrounding, heavily disturbed peat soils. The dissolved organic carbon (DOC) concentrations measured during five expeditions in the Siak between 2004 and 2006 are among the highest reported world wide. The DOM decomposition appeared to be a main factor influencing the oxygen concentration in the Siak which showed values down to 12 μmol l⁻¹. Results derived from a box-diffusion model indicated that in addition to the DOC concentration and the associated DOM decomposition the water-depth also plays a crucial role in regulating the oxygen levels in the river because of its impact on the turbulence in the aquatic boundary layer and the surface/volume ratio of water in the river. Model results imply furthermore that a reduced water-depth could counteract an increased oxygen consumption caused by an enhanced DOM leaching during the transition from dry to wet periods. This buffer mechanism seems to be close to its limits as indicated by sensitivity studies which showed in line with measured data that an increase of the DOC concentrations by ~15% could already lead to anoxic conditions in the Siak. This emphasizes the sensitivity of the Siak against further peat soil degradation, which is assumed to increase DOC concentrations in the rivers.     

The impact of drainage maintenance strategies on the flora of a low gradient, drained Irish salmonid river

In 1990, the Central Fisheries Board initiated research on how drainage maintenance practices and strategies might be modified to enhance the salmonid carrying capacity of the affected water while maintaining an acceptable degree of conveyance. Much of the maintenance requirement was caused by dense in-channel weed beds impeding discharge and facilitating siltation. The impact of various maintenance regimes on the aquatic flora was examined in the course of pilot studies on channels of base width 3–9 m. The findings from one of these, the R. Tullamore Silver which chokes annually with Sparganium erectum L., are presented. Overdigging the centre of the channel and placement of spoil at the margins confined S. erectum to a narrow marginal zone and facilitated development of a submerged, open-water flora.     

Influence of atypical pluviosity on phytoplankton assemblages in a stretch of a large sub-tropical river (Brazil)

This study aimed to evaluate the influence of alterations in pluviosity and ecological variables on microphytoplankton (> 20 μm) structure (composition, richness, diversity, and abundance) and its biomass (chlorophyll-a), comparing different regions in a stretch of the low Iguaçu River and in some tributaries. Phytoplankton was sampled in 10 stations (5 in Iguaçu River and 5 in tributaries) during a dry period (April/2004) and an atypical rainy period (June/2004). The conductivity showed significant difference among the sampling points. Temperature, dissolved oxygen, pH, silicate, and nitrate showed significant differences between the dry and wet periods. Phytoplankton was composed of 149 taxa, and the most representative class was Chlorophyceae (71 taxa), followed by Bacillariophyceae (35 taxa), and Cyanophyceae (25 taxa). During the rainy period, stations of Iguaçu River showed higher taxa number and total density compared to the tributaries, but the values were similar in the dry period. Tributaries presented higher diversity and equitability in both periods. Except by two stations in Iguaçu River, the higher taxa number, densities and biomass occurred in the dry period, associated to low levels of suspended matter. The canonical correspondence analysis indicated the dominance of Bacillariophyceae and Chrysophyceae in the rainy period related to nitrate and suspended matter. Two other groups were observed in the dry period: one formed by Cyanophyceae, Dinophyceae, and Rhodophyceae, related to temperature and nitrite and other by Euglenophyceae and Chlorophyceae related to total phosphorus and silicate. The groups suggest adequate conditions of the physical, chemical and climatic factors to the establishment of the algae classes. Phytoplanktonic assemblages responded quickly to the environmental regional variations under strong influence of pluviosity, while in the dry period, homogeneity among stations and environmental variables was observed. The importance of climatic events is relevant in ecological studies in a temporal scale.     

Effect of low dissolved oxygen on simultaneous nitrification and denitrification in a membrane bioreactor treating black water

Effect of low dissolved oxygen on simultaneous nitrification and denitrification was evaluated in a membrane bioreactor treating black water. A fully aerobic membrane bioreactor was operated at a sludge age of 60 days under three low dissolved oxygen (DO) levels below 0.5mg/L. It sustained effective simultaneous nitrification/denitrification for the entire observation period. Nitrification was incomplete due to adverse effects of a number of factors such as low DO level, SMPs inhibition, alkalinity limitation, etc. DO impact was more significant on denitrification: Nitrate was fully removed at low DO level but the removal was gradually reduced as DO was increased to 0.5mg/L. Nitrogen removal remained optimal within the DO range of 0.15-0.35 mg/L. Experimental results were calibrated and simulated by model evaluation with the same model coefficients. The model defined improved mass transfer with lower affinity coefficients for oxygen and nitrate as compared to conventional activated sludge.     

Periphyton responses to a hydraulic gradient in a regulated river in New Zealand

1. Periphyton species composition, chlorophyll a concentration, organic matter biomass, and metabolic activity were analysed at a site in a regulated river with low nutrient concentrations to investigate population and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hydraulic range [0.1–0.5m depth, 0.1–1.5ms^?1 velocity, and 0.01–1.5 Froude number (Fr)] with Cymbella kappii, Synedra ulna, and Gomphoneis herculeana having the highest relative biovolumes. 2. Unexpectedly, Cymbella kappii and Synedra ulna were abundant or co-dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within the mid-velocity range (velocity = 0.8–1.2ms^?1). 3. The physiognomy of the communities changed with a change in hydraulic conditions. There was progressively more diatom mucilage as velocity and Fr increased which resulted in a macroscopic change from relatively thin films (1–2mm) at low velocities and Fr (in pool habitats), to thick (approximately 10mm) mats at higher velocities and Fr (in riffle habitats). Associated with this, ash-free dry mass (APDM) increased strongly, but chlorophyll a concentration did not, resulting in a decrease in % chlorophyll a over the gradient. 4. The results of experiments conducted in an in situ benthic chamber showed no significant differences in gross primary production between two communities of different biomass, but indicated a slightly decreasing trend with increasing velocity (0.14 to 0.38 ms^?1). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combined AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with variable amounts of mucilage being secreted below, depending on hydraulic conditions. 5. The diatom community had considerable physiognomic plasticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help ameliorate environmental stress. If so, present theory on velocity control of periphytic algal development in streams, which is essentially based on passive responses, needs expanding. It is concluded that the response of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydraulic habitat preference curves for periphyton communities in nutrient-poor streams.     

How much water does a river need?

1. This paper introduces a new approach for setting streamflow-based river ecosystem management targets and this method is called the 'Range of Variability Approach' (RVA). The proposed approach derives from aquatic ecology theory concerning the critical role of hydrological variability, and associated characteristics of timing, frequency, duration, and rates of change, in sustaining aquatic ecosystems. The method is intended for application on rivers wherein the conservation of native aquatic biodiversity and protection of natural ecosystem functions are primary river management objectives.
2. The RVA uses as its starting point either measured or synthesized daily streamflow values from a period during which human perturbations to the hydrological regime were negligible. This streamflow record is then characterized using thirty-two different hydrological parameters, using methods defined in Richter et al . (1996). Using the RVA, a range of variation in each of the thirty-two parameters, e.g. the values at ± 1 standard deviation from the mean or the twenty-fifth to seventy-fifth percentile range, are selected as initial flow management targets.
3. The RVA targets are intended to guide the design of river management strategies (e.g. reservoir operations rules, catchment restoration) that will lead to attainment of these targets on an annual basis. The RVA will enable river managers to define and adopt readily interim management targets before conclusive, long-term ecosystem research results are available. The RVA targets and management strategies should be adaptively refined as suggested by research results and as needed to sustain native aquatic ecosystem biodiversity and integrity.     

Changes in the zooperiphyton structure in a small river along the spatial gradient

A total of 165 taxa have been recorded in the zooperiphyton of the Sestra River, among which larvae of chironomids (40), nematodes (24), and oligochaetes (21) have the highest species richness. The increase in the taxonomic richness of zoocenoses from the river head to its mouth is weakly expressed. The highest diversity of zoocenoses is characteristic for the contact zones of the river water with its tributaries and receiving waters. The discreteness of spatial characteristic of the taxonomic composition of zooperiphyton reflects the heterogeneity and diversity of habitat conditions of invertebrates. In regards to its effect on zoocenoses, anthropogenic pollution can exceed the impact of many environmental factors (including the current velocities) which affect the formation of rheophilic communities in river ecosystems. The dominant complexes of zooperiphyton include all trophic groups of invertebrates, among which the group of detritophages-collectors has the highest taxonomic diversity. The group of phytodetritophages-filterers + collectors mainly includes chironomid larvae of the genus Chironomus, which are associated with strongly polluted zones. Invertebrate filterers, sponges, and bryozoans prevail mainly in zooperiphyton in the lower reaches of the river, which are not subjected to the pollution.     

Epithelial water transport in a balanced gradient system.

The relationship between epithelial fluid transport, standing osmotic gradients, and standing hydrostatic pressure gradients has been investigated using a perturbation expansion of the governing equations. The assumptions used in the expansion are: (a) the volume of lateral intercellular space per unit volume of epithelium is small; (b) the membrane osmotic permeability is much larger than the solute permeability. We find that the rate of fluid reabsorption is set by the rate of active solute transport across lateral membranes. The fluid that crosses the lateral membranes and enters the intercellular cleft is driven longitudinally by small gradients in hydrostatic pressure. The small hydrostatic pressure in the intercellular space is capable of causing significant transmembrane fluid movement, however, the transmembrane effect is countered by the presence of a small standing osmotic gradient. Longitudinal hydrostatic and osmotic gradients balance such that their combined effect on transmembrane fluid flow is zero, whereas longitudinal flow is driven by the hydrostatic gradient. Because of this balance, standing gradients within intercellular clefts are effectively uncoupled from the rate of fluid reabsorption, which is driven by small, localized osmotic gradients within the cells. Water enters the cells across apical membranes and leaves across the lateral intercellular membranes. Fluid that enters the intercellular clefts can, in principle, exit either the basal end or be secreted from the apical end through tight junctions. Fluid flow through tight junctions is shown to depend on a dimensionless parameter, which scales the resistance to solute flow of the entire cleft relative to that of the junction. Estimates of the value of this parameter suggest that an electrically leaky epithelium may be effectively a tight epithelium in regard to fluid flow.     

Trophic network of aquatic macroinvertebrates along an altitudinal gradient in a Neotropical mountain river

Studies of trophic networks and the evaluation of processes that occur along altitudinal gradients in river systems are of great importance because they allow an understanding of energy flow dynamics and provide scientific tools for the planning and management of river ecosystems. This research describes the trophic network of aquatic macroinvertebrates along an altitudinal gradient of the Gaira River, a mountain Neotropical watercourse located in the Sierra Nevada de Santa Marta in northern Colombia. The organisms were collected in the upper, middle and lower reaches of the river during the rainy and dry seasons (between 2010 and 2013). Trophic relationships were evaluated through gut content analysis. The contents were determined and quantified using photographs and expert review, and a binary consumption matrix was used to determine the characteristics of the trophic network. We characterized the diet composition at each site for each season using discriminant analysis. Trophic networks during the dry seasons showed higher trophic species richness and linkage density, and the predominance of coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) in the high and medium sections of the river. During the dry seasons the diets had a lower number of basal species, but in the low river section there was a high percentage of fungi and microalgae. During the rainy seasons, no patterns were observed for the percentage of resources. Results indicated a direct relation between periods of hydrologic stability and an increase of CPOM during dry seasons and an increase of resource diversity consumed by macroinvertebrates at all sites during the rainy season, showing that changes in trophic networks of the Gaira River were more important during seasonal periods than along the altitudinal gradient.     

Variability in the growth rate of chub Leuciscus cephalus along a longitudinal river gradient

An increase in individual growth rate of chub Leuciscus cephalus along a longitudinal river gradient of the Rhône River basin was observed and supported by significant differences between mean growth rates of successive river size groups. The potential implications of the results for studies on species traits variability along large-scale environmental gradients are discussed.     

Response of sorghum to a water gradient and potassium variable

Summary A field study utilizing a modified line-source sprinkler system was used to investigate the hydrologic balance of a sorghum crop subjected to water deficit stress. An additional objective was to evaluate the evapotranspiration-yield relationship and to determine if the water use efficiency could be improved through the addition of increased amounts of potassium. Sixteen lysimeters packed with a Greenfield sandy loam and planted to sorghum (Sorghum bicolor) were placed under a moisture gradient irrigation system. Four levels of potassium were used in the experiment (0, 100, 200 or 300 Kg/ha). The results indicated that when the leaching fraction was set at 0.15 near the source, the predicted leaching fraction would go to zero at 10 meters from the source. Although yield was related to evapotranspiration, a better fit to the yield data was obtained when a parameter that included an availability and deficit term was used. The relationship was not linear but instead was described by an exponential fit. This parameter was also highly sensitive to leaf water potential and diffusion resistance. Finally, water use efficiency increased with yield but showed no correlation with potassium application.     

Bacterial and phytoplankton dynamics in a sub-tropical estuary

Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates and growth efficiencies were studied in July 2001 and January 2002 during both spring and neap tides, along a tidal cycle, at three sites in a subtropical estuary. Major freshwater inputs located in the Northern region led to differences in both phytoplankton and bacterioplankton biomass and activity along the estuary. While in the Northern region phytoplankton is light-limited, with mean phytoplankton production (PP) between 1.1 and 1.9 μg C l⁻¹ h⁻¹ and mean specific growth rates (PSG) between 0.14 and 0.16 d⁻¹, the Southern region registered values as high as 24.7 μg C l⁻¹ h⁻¹ for PP and 2.45 d⁻¹ (mean PP between 3.4 and 7.3 μg C l⁻¹ h⁻¹; mean PSG between 0.28 and 0.57 d⁻¹). On the other hand, maximum bacterial production (BP: 63.8 μg C l⁻¹ h⁻¹) and specific growth rate (BSG: 32.26 d⁻¹) were observed in the Northern region (mean BP between 3.4 and 12.8 μg C l⁻¹ h⁻¹; mean BSG between 1.98 and 6.67 day⁻¹). These bacterial activity rates are among the highest recorded rates in estuarine and coastal waters, indicating that this system can be highly heterotrophic, due to high loads of allochthonous carbon (mainly derived from mangrove forest). Our results also showed that, despite that BP rates usually exceeded PP, in the Southern region BP may be partially supported (∼45%) by PP, since a significant regression was observed between BP and PP (r = 0.455, P < 0.001). Handling editor: P. Viaroli     

Spatial variability overwhelms seasonal patterns in bacterioplankton communities across a river to ocean gradient

Few studies of microbial biogeography address variability across both multiple habitats and multiple seasons. Here we examine the spatial and temporal variability of bacterioplankton community composition of the Columbia River coastal margin using 16S amplicon pyrosequencing of 300 water samples collected in 2007 and 2008. Communities separated into seven groups (ANOSIM, P<0.001): river, estuary, plume, epipelagic, mesopelagic, shelf bottom (depth<350 m) and slope bottom (depth>850 m). The ordination of these samples was correlated with salinity (ρ=−0.83) and depth (ρ=−0.62). Temporal patterns were obscured by spatial variability among the coastal environments, and could only be detected within individual groups. Thus, structuring environmental factors (for example, salinity, depth) dominate over seasonal changes in determining community composition. Seasonal variability was detected across an annual cycle in the river, estuary and plume where communities separated into two groups, early year (April–July) and late year (August–Nov), demonstrating annual reassembly of communities over time. Determining both the spatial and temporal variability of bacterioplankton communities provides a framework for modeling these communities across environmental gradients from river to deep ocean.     

A hierarchical study of macroinvertebrate recolonization of disturbed patches along a longitudinal gradient in a prairie river

1. Macroinvertebrate densities and community composition were examined at three spatial scales after substratum disturbance; among reaches along a longitudinal gradient, within reaches and within plots. Reaches consisted of sandstone outcrops that were separated by approximately 2 km of highly mobile sandy silt substratum. 2. Substrata were disturbed by scraping sandstone plots (0.3 ± 0.3 m). Body-sized depressions created by Trichoptera in the sandstone were removed along with the upper 5 mm of sandstone, resulting in areas of newly exposed, smooth sandstone. 3. The spatial scale of examination determined whether patterns of macroinvertebrate distribution and densities were discernible. Initially there were no significant differences in community composition or total densities among reaches or among upstream/ downstream locations within reaches. Following substratum disturbance and 30 days recolonizarion, total macroinvertebrate densities did not differ significantly between undisturbed plots and disturbed plots. However, densities of Petrophilia (Pyralidae: Lepidoptera) differed along the longitudinal gradient and the Simuliidae had its highest density in the upstream reach. Significant differences were found in total macroinvertebrate densities between the upstream and the downstream halves of disturbed plots, with higher densities occurring in the downstream portions. 4. Recolonized plots had similar macroinvertebrate densities and community composition to undisturbed plots, suggesting that the stream community was highly resilient.     

Riverscape-level patterns of riparian plant diversity along a successional gradient, Willamette river, Oregon

Riparian forest communities dominated by Populus balsamifera ssp. trichocarpa L. (Torr. and Gray ex Hook.) Brayshaw are important contributors to biodiversity in terrestrial and aquatic ecosystems of the Western United States. Species composition along a successional gradient from stand initiation to late-succession of P. balsamifera-dominated riparian forests was investigated along 145 km of the Willamette River, Oregon. There were 151 total species encountered across 28 stands and a mean species richness of 33.3 species per stand. Young stands were dominated by P. balsamifera and Salix tree spp. and opportunistic herbaceous species. Understory trees, shrubs, and herbaceous species as well as late-successional tree species established 12–15 years after stand initiation. Fraxinus latifolia Benth. was the dominant late-successional tree species. Vertical structural diversity, P. balsamifera mean diameter at breast height, large tree biomass, and stand age were strongly correlated with understory species presence and abundance based on non-metric multidimensional scaling (NMS) ordination. There were no young stands on mid and high terraces and this was reflected in geomorphic position being strongly correlated with the stand age gradient. Abundance of Phalaris arundinacea L. an invasive grass species, was also significantly correlated with plant species composition and abundance. This study indicates that Willamette River riparian forests are diverse and therefore important to the biodiversity of the Willamette River valley and that their presence as a mosaic of communities of different successional stages may be threatened by human interventions, including influences exerted by introduced plant species.     

Relative influence of shredders and fungi on leaf litter decomposition along a river altitudinal gradient

We compared autumn decomposition rates of European alder leaves at four sites along the Lasset–Hers River system, southern France, to test whether changes in litter decomposition rates from upstream (1,300 m elevation) to downstream (690 m) could be attributed to temperature-driven differences in microbial growth, shredder activity, or composition of the shredder community. Alder leaves lost 75–87% of original mass in 57 days, of which 46–67% could be attributed to microbial metabolism and 8–29% to shredder activity, with no trend along the river. Mass loss rates in both fine-mesh (excluding shredders) and coarse-mesh (including shredders) bags were faster at warm, downstream sites (mean daily temperature 7–8°C) than upstream (mean 1–2°C), but the difference disappeared when rates were expressed in heat units to remove the temperature effect. Mycelial biomass did not correlate with mass loss rates. Faster mass loss rates upstream, after temperature correction, evidently arise from more efficient shredding by Nemourid stoneflies than by the Leuctra-dominated assemblage downstream. The influence of water temperature on decomposition rate is therefore expressed both directly, through microbial metabolism, and indirectly, through the structure of shredder communities. These influences are evident even in cold water where temperature variation is small.     

Planktonic actinobacterial diversity along a salinity gradient of a river and five lakes on the Tibetan Plateau

The diversity and community structure of planktonic Actinobacteria in a freshwater river and five fresh/saline/hypersaline lakes on the Tibetan Plateau, China were investigated with a combination of geochemical and 16S rRNA gene phylogenetic analyses. A total of 387 actinobacterial 16S rRNA gene clones were sequenced, and they could be classified into Actinobacteridae, Acidimicrobidae, and unclassified Actinobacteria. The Actinobacteridae sequences were distributed into five suborders (e.g., Corynebacterineae, Frankineae, Micrococcineae, Propionibacterineae, and Streptosporangineae) and unclassified Actinobacteridae. Some actinobacterial members (specifically Micrococcineae) were present in a wide range of salinities (from freshwater to NaCl saturation). Statistical analysis showed that salinity and salinity-related environmental variables (such as ions and total nitrogen) significantly (r > 0.5; P < 0.05) influenced the distribution of planktonic actinobacterial community in the investigated aquatic biotopes. Our data have implications for a better understanding of the distribution of Actinobacteria in high-elevation lakes.