Low-flow Sites Research Articles

Barnacles as biological flow indicators.

Hydrodynamic stress shapes the flora and fauna that exist in wave-swept environments, alters species interactions, and can become the primary community structuring agent. Yet, hydrodynamics can be difficult to quantify because instrumentation is expensive, some methods are unreliable, and accurately measuring spatial and temporal differences can be difficult. Here, we explored the utility of barnacles as potential biological flow-indicators. Barnacles, nearly ubiquitous within estuarine environments, have demonstrated notable phenotypic plasticity in the dimensions of their feeding appendages (cirri) and genitalia in response to flow. In high flow, barnacles have shorter, stockier cirri with shorter setae; in low flow, barnacles have longer, thinner cirri with longer setae. By measuring the relative differences in cirral dimensions, comparative differences in flow among locations can be quantified. We tested our hypothesis that ivory barnacles (Amphibalanus eburneus) could be useful biological flow indicators in two experiments. First, we performed reciprocal transplants of A. eburneus between wave protected and wave exposed areas to assess changes in morphology over 4 weeks as well as if changes dissipated when barnacles were relocated to a different wave habitat. Then, in a second study, we transplanted barnacles into low (<5 cm/s) and high flow (>25 cm/s) environments that were largely free of waves and shielded half of the transplanted barnacles to lessen flow speed. In both experiments, barnacles had significant differences in cirral morphologies across high and low flow sites. Transplanting barnacles revealed phenotypic changes occur within two weeks and can be reversed. Further, ameliorating flow within sites did not affect barnacle morphologies in low flow but had pronounced effects in high flow environments, suggesting that flow velocity was the primary driver of barnacle morphology in our experiment. These results highlight the utility of barnacles as cheap, accessible, and biologically relevant indicators of flow that can be useful for relative comparisons of flow differences among sites.

Going with the flow: How corals in high-flow environments can beat the heat.

Coral reefs are experiencing unprecedented declines in health on a global scale leading to severe reductions in coral cover. One major cause of this decline is increasing sea surface temperature. However, conspecific colonies separated by even small spatial distances appear to show varying responses to this global stressor. One factor contributing to differential responses to heat stress is variability in the coral's micro-environment, such as the amount of water flow a coral experiences. High flow provides corals with a variety of health benefits, including heat stress mitigation. Here, we investigate how water flow affects coral gene expression and provides resilience to increasing temperatures. We examined host and photosymbiont gene expression of Acropora cf. pulchra colonies in discrete in situ flow environments during a natural bleaching event. In addition, we conducted controlled ex situ tank experiments where we exposed A. cf. pulchra to different flow regimes and acute heat stress. Notably, we observed distinct flow-driven transcriptomic signatures related to energy expenditure, growth, heterotrophy and a healthy coral host-photosymbiont relationship. We also observed disparate transcriptomic responses during bleaching recovery between the high- and low-flow sites. Additionally, corals exposed to high flow showed "frontloading" of specific heat-stress-related genes such as heat shock proteins, antioxidant enzymes, genes involved in apoptosis regulation, innate immunity and cell adhesion. We posit that frontloading is a result of increased oxidative metabolism generated by the increased water movement. Gene frontloading may at least partially explain the observation that colonies in high-flow environments show higher survival and/or faster recovery in response to bleaching events.

Investigating benthic impacts at salmon farms using eddy covariance measurements of benthic oxygen fluxes

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 11:337-357 (2019) - DOI: https://doi.org/10.3354/aei00317 Investigating benthic impacts at salmon farms using eddy covariance measurements of benthic oxygen fluxes David R. Plew* National Institute of Water and Atmospheric Research, PO Box 8602, Christchurch 8440, New Zealand *Corresponding author: david.plew@niwa.co.nz ABSTRACT: Benthic oxygen fluxes were measured in situ using the eddy covariance method at 2 Chinook salmon farms located in the New Zealand Marlborough Sounds in 29-35 m depth and compared to sediment nutrient (C, N and P) and sulphide concentrations. Observations from 3 sites at a high-flow location (near-bed RMS velocities of 0.11 to 0.17 m s-1) showed oxygen fluxes increasing with sediment enrichment. Mean ± SE oxygen fluxes of -102 ± 4 mmol O2 m-2 d-1 were observed immediately adjacent the farm. Higher fluxes were observed at 120 m from the farm (-53 mmol O2 m-2 d-1) than at 50 m (-48 mmol O2 m-2 d-1), consistent with higher sediment nutrient concentrations at the more distant site, and reflecting the effect of currents on deposition patterns. Ratios of C, N and P in sediments indicated a reduction of N mineralisation rates as sediment enrichment increased. The low-flow site (near-bed RMS velocities of 0.033 m s-1) had similar oxygen fluxes (-108 ± 9 mmol O2 m-2 d-1) to the high-flow site closest to the farm, but sediments were more highly enriched with high sulphide concentrations. Oxygen fluxes at the low-flow site were close to the estimated maximum potential flux that could be achieved under the ambient hydrodynamic conditions. Less than 0.2% of surface PAR reached the sediments, and no evidence of benthic primary production was observed at any of the sites. By incorporating ambient hydrodynamic conditions, eddy covariance has the potential to obtain true in situ benthic oxygen fluxes, giving greater insight into aquaculture-environmental interactions. KEY WORDS: Aquaculture · Benthic impacts · Oxygen flux · Organic enrichment · Eddy covariance · Salmon Full text in pdf format PreviousNextCite this article as: Plew DR (2019) Investigating benthic impacts at salmon farms using eddy covariance measurements of benthic oxygen fluxes. Aquacult Environ Interact 11:337-357. https://doi.org/10.3354/aei00317 Export citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 11. Online publication date: July 11, 2019 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2019 Inter-Research.

Impacts of cave air ventilation and in-cave prior calcite precipitation on Golgotha Cave dripwater chemistry, southwest Australia

Speleothem trace element chemistry is an important component of multi-proxy records of environmental change but a thorough understanding of hydrochemical processes is essential for its interpretation. We present a dripwater chemistry dataset (PCO2, alkalinity, Ca, SIcc, Mg and Sr) from an eight-year monitoring study from Golgotha Cave, building on a previous study of hydrology and dripwater oxygen isotopes (Treble et al., 2013). Golgotha Cave is developed in Quaternary aeolianite and located in a forested catchment in the Mediterranean-type climate of southwest Western Australia. All dripwaters from each of the five monitored sites become supersaturated with respect to calcite during most of the year when cave ventilation lowers PCO2 in cave air. In this winter ventilation mode, prior calcite precipitation (PCP) signals of increased Mg/Ca and Sr/Ca in dripwater are attributed to stalactite deposition. A fast-dripping site displays less-evolved carbonate chemistry, implying minimal stalactite growth, phenomena which are attributed to minimal degassing because of the short drip interval (30 s).We employ hydrochemical mass-balance modelling techniques to quantitatively investigate the impact of PCP and CO2 degassing on our dripwater. Initially, we reverse-modelled dripwater solutions to demonstrate that PCP is dominating the dripwater chemistry at our low-flow site and predict that PCP becomes enhanced in underlying stalagmites. Secondly, we forward-modelled the ranges of solution Mg/Ca variation that potentially can be caused by degassing and calcite precipitation to serve as a guide to interpreting the resulting stalagmite chemistry. We predict that stalagmite trace element data from our high-flow sites will reflect trends in original dripwater solutes, preserving information on biogeochemical fluxes within our system. By contrast, stalagmites from our low-flow sites will be dominated by PCP effects driven by cave ventilation. Our poorly karstified system allows us to highlight and quantify these in-cave (PCP) processes, which are otherwise masked at sites where karstification is more developed and hydrogeology is more complex. Our modelling also shows enhanced CO2 source production in the unsaturated zone that is attributed to deeply-rooted vegetation and increasing bioproductivity which we link to forest recovery after fires impacted our site during 2006 CE.

Abstract 168: Splice Variants of Tissue Factor Determine the Coagulant State of Endothelial Cells and Modulate Vascular Stabilization/Regression

Introduction: Recruitment of pericytes (PC), critical to microvascular maturation, requires deposition of basement membrane proteins such as von Willebrand factor (vWF). Loss of PCs ultimately leads to vessel regression and rarefaction. Hypothesis: Splice variants of tissue factor (TF) stabilize the vasculature by supporting EC-PC interactions. Results: We determined that alternatively spliced TF (asTF) resides in Weibel Palade bodies (WPB) of ECs. Overexpression of KLF-2 enhanced asTF protein levels in WPBs. In confluent EC monolayers, asTF was deposited into the extracellular matrix (ECM). In assays employing vascular plexus remodelling on 3D-basement membrane and purified recombinant asTF or full-length TF (flTF), only asTF improved EC-PC interactions. At sites of low KLF-2 expression (low or turbulent flow), total TF mRNA was diminished, leading to low asTF expression. However, upon EC activation by TNFα, total TF mRNA levels rapidly increased and TF protein expression shifted from the ECM-deposited asTF to luminally expressed flTF. Interestingly, flTF-driven EC conversion to a procoagulant state was associated with the release of vWF and asTF from WPBs, depleting the intracellular depots from these proteins. Immunohistochemical staining of kidney specimens of human living donors confirmed that asTF was present at high levels in WPBs of microvascular endothelium. Moreover, after 45 minutes of reperfusion of the transplanted kidney, asTF- and vWF-staining was markedly reduced, consistent with their release from WPB following an inflammatory insult. Conclusion: We show for the first time that quiescent, KLF-2 expressing ECs predominantly generate non-coagulant asTF, which is stored in WPBs and serves to stabilize the microvasculature by supporting EC-PC interactions. In contrast, in low-flow zones or sites with disturbed flow (e.g. at bifurcations), KLF-2 is downregulated, leading to decreased production of asTF and impaired stabilization of the vasculature. During inflammation, acute perturbation of ECs (e.g. exposure to TNFα) is associated with downregulation of KLF-2 and a post-transcriptionally regulated increase in the production of flTF, resulting in a pro-coagulant phenotype and loss of EC-PC interactions.

Processes Forming Daily Lamination in a Microbe-Rich Travertine Under Low Flow Condition at the Nagano-yu Hot Spring, Southwestern Japan

A daily rhythm of microbial processes, in terms of sub-mm order lamination, was identified for a microbe-rich aragonite travertine formed at a low-flow site of the Nagano-yu Hot Spring in Southwestern Japan. Continuous observation and sampling clearly showed that the lamination consisted of diurnal microbe-rich layers (M-layers) and nocturnal crystalline layers (C-layers). The M-layers originated from biofilm formed by growth and upward migration of filamentous cyanobacteria related to Microcoleus sp., which can rapidly glide and secrete extracellular polymeric substances (EPS). During the daytime, cyanobacterial biofilm development inhibited aragonite precipitation on the travertine surface due to the calcium-binding ability of EPS. After sunset, aragonite precipitation started on the surface where aerobic heterotrophic bacteria decomposed EPS, which induced precipitation of micritic crystals. This early stage of C-layer formation was followed by abiotic precipitation of fan-shaped aragonite aggregates. Despite their major role in lamina formation, the cyanobacteria were readily degraded within 6–10 days after embedding, and the remaining open spaces in the M-layers were sparsely filled with crystal clots. These lamina-forming processes were different from those observed in a high-flow site where the travertine has a dense texture of aragonite crystals. The microbial travertine at Nagano-yu is similar to some Precambrian stromatolites in terms of in situ mineral precipitation, regular sub-mm order lamination, and arrangement of filamentous microbes; therefore, the lamination of these stromatolites possibly occur with a daily rhythm. The microbial processes demonstrated in this study may revise the interpretation of ancient stromatolite formation.

Non-Linear Interactions between Consumers and Flow Determine the Probability of Plant Community Dominance on Maine Rocky Shores

Although consumers can strongly influence community recovery from disturbance, few studies have explored the effects of consumer identity and density and how they may vary across abiotic gradients. On rocky shores in Maine, recent experiments suggest that recovery of plant- or animal- dominated community states is governed by rates of water movement and consumer pressure. To further elucidate the mechanisms of consumer control, we examined the species-specific and density-dependent effects of rocky shore consumers (crabs and snails) on community recovery under both high (mussel dominated) and low flow (plant dominated) conditions. By partitioning the direct impacts of predators (crabs) and grazers (snails) on community recovery across a flow gradient, we found that grazers, but not predators, are likely the primary agent of consumer control and that their impact is highly non-linear. Manipulating snail densities revealed that herbivorous and bull-dozing snails (Littorina littorea) alone can control recovery of high and low flow communities. After ∼1.5 years of recovery, snail density explained a significant amount of the variation in macroalgal coverage at low flow sites and also mussel recovery at high flow sites. These density-dependent grazer effects were were both non-linear and flow-dependent, with low abundance thresholds needed to suppress plant community recovery, and much higher levels needed to control mussel bed development. Our study suggests that consumer density and identity are key in regulating both plant and animal community recovery and that physical conditions can determine the functional forms of these consumer effects.

An isotopic and modelling study of flow paths and storage in Quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records

We investigated the distinctive shallow sub-surface hydrology of the southwest Western Australia (SWWA) dune calcarenite using observed rainfall and rainfall δ18O; soil moisture, cave drip rate and dripwater δ18O over a six-year period: August 2005–March 2012. A lumped parameter hydrological model is developed to describe water fluxes and drip δ18O. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation δ18O signal and discuss the implications for speleothem paleoclimate records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct δ18O values and recharge responses (‘low’ and ‘high’ flow sites). Dripwaters exhibit δ18O variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored in a secondary reservoir which is associated with high-flow dripwater that is 1‰ lower than our monitored low-flow sites (δ18O). One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). Additionally, our results yield a better understanding of the vadose zone hydrology and dripwater characteristics in Quaternary age dune limestones. We show that flow to our monitored sites is dominated by diffuse flow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock.

Novel observations of benthic enrichment in contrasting flow regimes with implications for marine farm monitoring and management

We examine macrofaunal and physico-chemical responses to organic enrichment beneath salmon farms in contrasting flow environments, and reveal pronounced flow-related differences in the magnitude and spatial extent of effects. Total macrofaunal abundances at high flow sites were nearly an order of magnitude greater than at comparable low flow sites, representing a significant benthic biomass. These very high abundances occurred in conjunction with moderate-to-high species richness, and were evident in the absence of appreciable organic matter accumulation. Biological responses to increasing sulfide were variable; however a significant biological threshold was evident at 1500μM. Macrofaunal responses at high flow sites differed substantially from the Pearson–Rosenberg model. The atypical ecological conditions were attributed to (i) limited accumulation of fine sediments, (ii) maintenance of aerobic conditions in near-surface sediments, and (iii) an abundant food supply. Thus, enhanced resilience to organic waste at well-flushed sites appears related to both biological and physical processes.

Textural transition in an aragonite travertine formed under various flow conditions at Pancuran Pitu, Central Java, Indonesia

A large travertine mound at Pancuran Pitu, Central Java, Indonesia, displays various types of lamination reflecting different hydrological conditions. To understand the geomicrobiological processes of the laminated travertines, we investigated the mound's hydrology, water chemistry, texture, microbial composition, and volume percentage of carbonate mineral (VPC). We identified four types of travertines: dense travertine (80% VPC, flow rate ~200cm/s), lithified microbial travertine (60% VPC, flow rate ~70cm/s), fragile microbial travertine (30% VPC, flow rate ~10cm/s), and poorly lithified microbial mat (<10% VPC, flow rate <5cm/s). The first two types exhibited regular lamination at sub-mm intervals that was associated with the distribution of cyanobacteria and organic substances. The dense travertine consisted of alternation of crystalline and banded layers at ~160-μm intervals, which resembles the daily lamination of an aragonite travertine from southwest Japan. The banded layer formed during the day, exhibited porosity, and contained organic substances. Lamination in the lithified microbial travertine was likely formed by the daily growth cycles of filamentous cyanobacteria. The daytime surface of this travertine appeared to be sub-mm-thick cyanobacterial biofilm with spheroidal aggregates of aragonite, which covered a crystalline layer consisting of radially expanded needle crystals. The crystalline layer was likely formed during the night when the cyanobacterial growth was interrupted. These regularly laminated travertines fabrics appeared to be similar to certain Precambrian stromatolites. Regularity of the lamination was less in the other two microbe-rich types at low-flow sites. The majority of the aragonite occurred as spherical aggregates within the microbial mat, although thin (10–50μm) crystal layers in the fragile microbial travertine may indicate daily intervals. As represented in the relationship among the VPC, flow rate, and precipitation rate of aragonite, the travertines were more consolidated with increasing hydrodynamic energy because of the activation of carbonate precipitation and the inhibition of thick cyanobacterial biofilm development.

Evaluation of carbon pathways supporting the diet of invasive Hemimysis anomala in a large river

This study is providing the first insight on the food sources supporting Hemimysis anomala in a lotic ecosystem where food webs differ from those of lentic ecosystems. Using carbon and nitrogen stable isotopes, the diet of Hemimysis was evaluated in multiple sites of the St Lawrence River where higher density swarms have been recently reported. We found that Hemimysis may feed on multiple food sources, including benthic and pelagic production, and from multiple trophic levels. Results from a mixing model applied to evaluate the importance of benthic versus pelagic food sources revealed site specific variations in diet. At the low flow sites located in the Montréal Harbour, Hemimysis fed primarily on pelagic production compared to the more open sites exposed to higher flow located outside of the harbor. This study indicates that Hemimysis may adapt its feeding behavior to environmental conditions and/or food source availability, potentially increasing its impacts on food web structure.

Biogeochemistry of Hypersaline Springs Supporting a Mid-Continent Marine Ecosystem: An Analogue for Martian Springs?

Hypersaline springs that host unique mid-continent marine ecosystems were examined in central Manitoba, Canada. The springs originate from a reflux of glacial meltwater that intrudes into underlying bedrock and dissolved buried salt beds. Two spring types were distinguished based both on flow rate and geochemistry. High flow springs (greater than 10 L/s) hosted extensive marine microbial mats, which were dominated by algae but also included diverse microbes. These varied somewhat between springs as indicated by changes in profiles of fatty acid methyl esters. Culture studies confirmed the presence of sulfate-reducing bacteria in sediments at the high flow sites. In contrast, low flow springs were affected by solar evaporation, increasing salinity, and temperature. These low flow springs behaved more like closed nutrient-limited systems and did not support microbial mats. Direct comparison of the high and low flow springs revealed interesting implications for the potential to record biosignatures in the rock record. High flow springs have abundant, well-developed microbial mats, which desiccate and are cemented along the edges of the spring pools; however, the high mass flux overwhelms any geochemical signature of microbial activity. In contrast, the nutrient-limited low flow sites develop strong geochemical signatures of sulfate reduction, even in the absence of microbial mats, due to less dilution with the lower flows. Geochemical and physical evidence for life did not correlate with the abundance of microbial life but, rather, with the extent to which the biological system formed a closed ecosystem.

NET PRIMARY PRODUCTIVITY OF A WESTERN MONTANE RIPARIAN FOREST: POTENTIAL INFLUENCE OF STREAM FLOW DIVERSION

ABSTRACT We estimated aboveground and belowground net primary productivity (NPP) for two reaches of a montane riparian ecosystem in the eastern Sierra Nevada Mountains of California with differing stream flow regimes resulting from varying degrees of stream flow diversion for hydroelectric power generation. Total understory productivity (herbaceous and shrub) was 2.5 times higher (P < 0.001) at the high-flow site than at the low-flow site (22.4 and 8.9 g C m−2 yr−1, respectively). Annual litterfall was also higher (P = 0.03) at the high-flow (235 g C m−2 yr−1) than at the low-flow site (180 g C m−2 yr−1). However, tree and total aboveground NPP, as well as annual soil respiration and belowground NPP, were all statistically similar between sites. Furthermore, total (above- plus below-ground) NPP was statistically similar between sites (903 and 643 g C m−2 yr−1 for the high- and low-flow sites, respectively). These productivity estimates are, to our knowledge, the first ever reported for a western montane r...

Biomass and Productivity of Intertidal Rockweeds (Ascophyllum nodosum LeJolis) in Cobscook Bay

Abstract Rockweeds dominate much of the New England and Canadian Maritime coasts and serve as food and habitat for numerous species. Cobscook Bay is unique in New England for its high tidal amplitude, wide intertidal expanse, diverse flora and fauna, and presumed high intertidal productivity, much of which is thought to be related to extensive intertidal fucoid populations. The goals of this study were: 1) to estimate intertidal rockweed biomass and productivity, 2) to quantify the variation in rockweed productivity over a range of temporal and spatial scales (high- vs low-flow sites at headlands and coves, respectively), and 3) to estimate the contribution of rockweeds to detrital pools. Net productivity was determined by separately weighing growth in the current year vs growth from preceding year classes. Sampling the same populations in both the fall and following spring allowed adjustment for winter mortality of thallus structures and productivity at two sites. Variation in the length of apical tips o...

Temporal and spatial components of variability in benthic recruitment, a 5-year temperate example

Deployment of artificial substrata is a common method of investigating early community development and recruitment, but rarely are such experiments of long enough duration to include even year time scales. We placed replicate, machined-slate panels (15×15 cm) in the intertidal and at depths of 6 and 12 m at two sites of differing flow rate at Lough Hyne, SW Ireland. These were serially replaced every 30–60 days for a period of 5 years (1997–2002), except in the intertidal (2000–2002). The number and identity of all recruits were recorded. Recruitment varied over several orders of magnitude both on temporal and spatial scales. The greatest source of variability was between the intertidal (with few species or recruit numbers) and the subtidal zones (many species, some with thousands of recruits per panel per 30 days). Highest levels of recruitment occurred at the low-flow site (Labhra Cliff). Here, recruitment was dominated by the serpulid polychaete, Pomatoceros sp., reaching ~4000 individuals per panel per 30 days. Highest species richness occurred, however, at the high flow site (Whirlpool Cliff). At this site more colonial forms (e.g. bryozoans) settled. Season was found to be the dominant pattern explaining subtidal recruit and species number variability. Year, however, was the dominant temporal pattern explaining change in diversity (Shannon–Wiener H′). In space, depth explained most variability of recruit numbers, whereas site explained more variation in species richness. Both these spatial factors contributed similarly to variability of diversity (H′). Recruitment has long been known to vary considerably over large spatial scales, such as with latitude and isolation, but we that show changes of a similar magnitude in recruitment can occur across small spatial scales. Individual taxa showed varied temporal patterns of recruitment including continuous, regular seasonal fluctuations and irregular pulses in particular years.

Mechanistic Linkage of Hydrologic Regime to Summer Growth of Age-0 Atlantic Salmon

Significant reductions in juvenile stream salmonid growth have been observed in association with low summer flow, but underlying mechanisms are poorly understood and predictive power is limited. We conducted a stage-specific analysis of the relationship between summer flow and the growth of age-0 Atlantic salmon Salmo salar in two rearing sites in the upper Connecticut River basin, New Hampshire. We contrasted effects of variation in foraging habitat availability and temperature on individual age-0 Atlantic salmon mass during one high-flow year and two low-flow years and from high- and low-flow sites within years. Overall age-0 Atlantic salmon mass was positively correlated with the availability of model-predicted favorable foraging locations and negatively correlated with density during the summer. Individual Atlantic salmon mass and the proportion of temperature-predicted maximum mass were lowest during the two low-flow years and were lower in upstream than in downstream sections. Between-year variation in growth was not closely associated with temperature model predictions. However, some of the difference between upstream and downstream sections appeared to be associated with lower summer temperatures in the upstream section. Our case study provides a framework for combining empirical and modeling approaches to quantify the potential impact of hydrologic change on fish growth and for linking variation in stream discharge to juvenile Atlantic salmon performance across time and space.

Climatic and stream-flow controls on tree growth in a Western montane riparian forest.

Humans have severely impacted riparian ecosystems through water diversions, impoundments, and consumptive uses. Effective management of these important areas is becoming an increasingly high priority of land managers, particularly as municipal, industrial, and recreational demands for water increase. We examined radial tree growth of four riparian tree species ( Pinus jeffreyi, Populus trichocarpa, Betula occidentalis, and Pinus monophylla) along Bishop Creek, California, and developed models relating basal area increment (BAI) and relative basal area increment (RBAI) to climatic and stream flow variables. Between years 1995-1999, univariate regression analysis with stream flow explained 29 to 61% of the variation in BAI and RBAI among all species except P. trichocarpa; growth by P. trichocarpa was not significantly related to stream flows over this period. Stepwise linear regression indicated that species responded differently to climatic variables, and models based on these variables explained between 33 to 86% of variation in BAI and RBAI during the decade of the 1990s. We examined branch growth of P. trichocarpa for sensitivity to differences in stream flow regimes and found that annual branch growth did not vary between a high- and low-flow site, but that annual branch growth was significantly higher in wet years with greater stream flows. Our results support the establishment of site-specific management goals by land managers that take into account all of the important tree species present in riparian ecosystems and their differential responses to altered hydrologic condition. Instream flow requirements for maintaining tree growth and vigor are only one of the species-specific responses that need to be evaluated, and these assessments should attempt to separate experimentally stream-flow (managed) controls from climatic (unmanaged) controls on growth.

Linking Sediment Biofilms, Hydrodynamics, and River Bed Clogging: Evidence from a Large River.

> Abstract We investigated possible effects of the hydrodynamics at the water/sediment interface on river bed biofilms within the reservoir Freudenau (Vienna, Austria) of the Danube River during the period 1996/97. Two study sites (OBB and SSF) that differed in the magnitude of surface/subsurface water exchange were selected and intersite comparisons revealed higher organic matter, bacterial cell numbers, and esterase activity in SSF with lower horizontal outflow. Concentrations of colloidal carbohydrates and uronic acids were unaffected by hydrodynamics. The relative contribution of uronic acids to bulk colloidal carbohydrates was higher in the low-flow site SSF. The distribution patterns of this relative contribution generally matched the subsurface flow pattern. Shortly after impoundment in March 1996 and along with decreased surface flow velocity, maximal biofilm carbohydrate exopolymers concurred with minimal esterase activity in OBB. We hypothesize that this inverse relationship is due to increased diffusional resistance within the exopolymer biofilm matrix that reduces mass transfer and hydrolytic activity. These results, to our knowledge, are the first evidence for microbial participation in the clogging of a large river bed. Biofilm-associated organic carbon increased significantly by a factor of approximately 3.3 to 4.4 with progressive clogging as determined by the sediment leakage coefficient, which increased approximately 3.8 times. Concomitantly, with ongoing clogging, esterase activity exhibited increasingly higher values at the interface relatively to deeper sediment layers, which translates into steeper depth gradients. Furthermore, minimal inflow from the surface water into the river bed along with steepest esterase gradients concurred with a senescent benthic algal bloom. This suggests an important role for algae in clogging. Either algae obstruct voids mechanically, or their exudates fuel heterotrophic bacteria that in turn are involved in clogging processes. However, our data do not allow unequivocal differentiation between biogenic and physical clogging mechanisms.

How recruitment, intraspecific interactions, and predation control species borders in a tidal estuary.

We examined the relative contribution of recruitment, intraspecific species interactions, and predation in controlling the upper intertidal border of the northern acorn barnacle, Semibalanusbalanoides, in a tidal estuary in Maine. We hypothesized that the contracted border at sites that experienced low tidal currents was due to flow-mediated recruitment that resulted in reduced survival due to the absence of neighbor buffering of thermal stress (i.e., positive intraspecific interactions). We tested this hypothesis by manipulating the density of recently settled barnacles and their thermal environment in a field experiment. Counter to our original hypothesis, barnacles with neighbors suffered severe mortality at low-flow sites. When density-dependent predation by the green crab (Carcinusmaenus) was experimentally eliminated, however, we did detect evidence for positive interactions at the low-flow sites but not at the high-flow sites. In spite of the close proximity of the sites, maximum daily rock temperatures at the low-flow sites were slightly, but consistently, greater than those at high-flow sites. Our findings suggest that the upper intertidal border of S. balanoides in the Damariscotta River is limited at low-flow sites by a combination of reduced recruitment, elevated mortality from thermal stress and enhanced predation by green crabs. More generally, our findings highlight how physical stress and predation interact to alter the nature of density-dependent species interactions in natural assemblages.

Macrophyte functional variables versus species assemblages as predictors of trophic status in flowing waters

A series of models was developed using functionally-derived variables (mainly based on morphological attributes of freshwater macrophytes) to predict the trophic status of river and associated channel systems. The models were compared with an existing species-assemblage based procedure for predicting British river trophic conditions (the Macrophyte Trophic Ranking scheme, MTR). We compared sites in cooler temperate conditions (in Scotland) and warmer, sub-tropical conditions (in Egypt). In total, we made measurements of 13 traits from >600 individual plant specimens of 33 species growing at 42 sites (divided into independent input and test site datasets). N status (as annual mean concentration in water of total oxidised nitrogen, TON) was only very poorly predicted by this approach. However, P (as annual mean concentration in water of soluble reactive phosphate, SRP) was better predicted: both by a model based on MTR (r = -0.585, p<0.001), and by models using functional attributes of the macrophyte vegetation. River Trophic Status Indicator (RTSI) models based on ranked plant functional group relationship to river water P concentrations (RTSI FG ), or field-measured trait sets of the plants (RTSI TR ) could also individually explain up to about 34% of the variation in P, both for the total dataset and for subsets from Egypt or Scotland alone or for high V. low-flow sites. Combining both types of RTSI measure produced the most powerful predictive model ( r = 0.72, p<0.001), explaining just over half the variability in P.