Laboratory Stream Research Articles

Monopolization in a resource queue: water striders competing for food and mates

The monopolization of resources plays an important theoretical role in the literature on competition for food and mates. We used 12 groups of male water striders (Aquariusremigis) to: (1) test the general prediction that monopolization of both food and mates decreases as the temporal clumping of resources increases, (2) compare the efficiency of two indices of resource monopolization, coefficient of variation and Q (Ruzzante et al. 1996), and (3) quantitatively assess the resource queue model of Blanckenhorn and Caraco (1992). Each group of six males competed for both food items and mates released from the upstream end of a laboratory stream. The mean inter-arrival time for resource units (food or females) was 10 min, with four levels of temporal clumping (variance in inter-arrival time: 0, 25, 50 or 320 min2). As predicted, the monopolization of both food and mates decreased as the temporal clumping of resource arrival increased, although monopolization was greater for food than for mates. Q detected the difference in monopolization of food and mates, whereas the coefficient of variation did not, because Q is independent of mean resource abundance. The resource queue model successfully predicted monopolization of both resource types, explaining 89% and 76% of variation in the proportion of food and mates acquired by the six males. The success of the model suggests that the scaling of handling time to the variance in resource inter-arrival time should play an important role in any general theory of resource monopolization.

Life History and Case-Building Behavior of Culoptila cantha (Trichoptera: Glossosomatidae) in the Brazos River, Texas

The life history of Culoptila cantha (Ross) in the Brazos River, northcentral Texas, was studied from January 1995 to December 1997. The several thousand larvae, pupae, and adults sampled from a large riffle indicated a predominately trivoltine cycle during both years. The overwintering generation spanned 6–7 mo; most larvae recruited from August and September eggs grew to 4th instar by November, resumed growth to 5th instar in February, and pupated and emerged during March. Growth of the warm-season generations was less synchronized with probable cohort spreading and overlap, and occurred steadily during respective development periods of 2–3 mo. Water temperature thresholds of 22–24 and 27–29°C appeared to synchronize emergence of overwintering and 1st warm-season generations, respectively, whereas the later warm-season generation emerged asynchronously. Emergence occurred near dusk, and adults were nocturnally active within an air temperature range of 15–30°C. Mean fecundity was highest in March and decreased throughout the summer and fall. Larvae and pupae aggregated on the downstream undersides of clean cobbles (10–20 cm diameter) in the head of the riffle. Case reconstruction progression and behavior of larvae removed from field cases were observed and videotaped; larvae constructed emergency cases of loosely attached sand particles in the first 0.5 h, and subsequently modified these into typical cases when placed overnight in a laboratory stream. Emergence success was not affected by forcing 5th instars to construct these new cases. This study provides the 1st comprehensive life history of a species of Protoptilinae.

Size-dependent foraging behaviour and use of cover in juvenile coho salmon under predation risk

Foraging and use of cover by juvenile coho salmon (Oncorhynchus kisutch) were affected by predation threat in both seminatural channels and laboratory streams. In the field, coho salmon preferred stream sections with brushy cover only when under threat from hunting common mergansers. The mergansers had their highest capture success in pools without cover. Predation threat also caused coho salmon to use cover more as foraging habitat and to aggregate more in favourable positions at the head of the pool. In the laboratory, under simulated predation threat, fish using a refuge were significantly larger than those in the risky habitat. This pattern persisted for 2 days after the predation threat was discontinued. The average growth of coho salmon under predation threat was depressed and the difference in growth between large and small individuals was less than in control groups. We argue that larger fish were more averse to predation risk than smaller fish and that the smaller fish took advantage of feeding opportunities indirectly provided as a result of the predation risk. We speculate that in natural environments, predation may depress growth rates because of risk-avoidance behaviour but may also serve to reduce growth-rate differences among size classes within a cohort.

Impact of Barge Traffic on Stream Reaeration: Laboratory Experiments

Stream reaeration has been studied extensively for several decades, yet little is known about the effect of barge traffic in rivers and canals. To address this lack of knowledge, a laboratory stream model was constructed that consists of a circular duct where model barges were towed at different traffic velocities and frequencies under various water conditions. Dissolved oxygen levels were monitored after initial deoxygenation. Excluding the effect of wind and flow, the reaeration coefficient (overall gas transfer coefficient) in the laboratory system increased as barge speed and traffic frequency (number of barge passages per hour) increased. The reaeration coefficient due to moving barges is inversely related to water depth. Finally, the reaeration coefficient due to moving barges is proportional to barge size, channel size, load, and draft.

Responses of fish exposed to a C9--11 linear alcohol ethoxylate nonionic surfactant in stream mesocosms

Bluegill sunfish (Lepomis macrochirus) and fathead minnow (Pimephales promelas) responses to a nonionic linear alcohol ethoxylate (LAE) surfactant were examined in 10-day laboratory and 30-day outdoor stream mesocosm experiments. Larval fathead minnow survival was determined in a 10-day laboratory study with nominal concentrations of 0, 1.0, 3.0, 7.0, 13.0 and 25.0 mg LAE l−1. Bluegill sunfish survival and growth and fathead minnow survival, reproduction and behaviour were evaluated in replicated stream mesocosms with mean measured concentrations of 0, 0.73, 2.04, 4.35, 5.7 and 11.24 mg LAE l−1. The 10-day laboratory No Observed Effect Concentration (NOEC) for larval fathead minnow survival was 1.0 mg l−1. The 30-day stream NOEC for bluegill sunfish survival and growth was 5.7 mg l−1. In the stream mesocosms, NOECs for adult fathead minnow survival, reproduction and behaviour were 2.04 mg l−1, 0.73 mg l−1 and 0.73 mg l−1, respectively. Surviving fathead minnows that had ceased reproduction following exposure to LAE in the streams re-established normal reproduction during a post- treatment observation period in the laboratory. Laboratory studies in conjunction with the stream mesocosm study yielded complementary results which provided a detailed assessment of threshold responses of fish to LAE

Aggressive behaviour of underyearling rainbow trout in simulated winter concealment habitat

The behaviour of wild underyearling rainbow trout Oncorhynchus mykiss in concealment habitat in a laboratory stream aquarium at 2 and 6°C was recorded daily with an infrared video camera for 90 min over dawn. Aggressive behaviours (threat nips, nips and chases) were frequent during this time as fish entered concealment habitat. Aggressiveness varied widely among fish groups, with a range of 1 to 45 aggressive acts being initiated during a 90-min filming period. Larger fish initiated most of the encounters and removed a higher proportion of fish from concealment than did smaller aggressors. Thirteen per cent of the aggressive acts resulted in the recipient being completely removed from concealment habitat. The highest combined frequency of aggressive acts was 0.64 per visible fish per 10 min period and occurred at relatively low light levels (300 lx). The data suggest that when fish density is high, such as when habitat is limited, aggressive behaviour in winter may cause some underyearling salmonids to be excluded from concealment.

Aggressive behaviour of underyearling rainbow trout in simulated winter concealment habitat

The behaviour of wild underyearling rainbow trout Oncorhynchus mykiss in concealment habitat in a laboratory stream aquarium at 2 and 6°C was recorded daily with an infrared video camera for 90 min over dawn. Aggressive behaviours (threat nips, nips and chases) were frequent during this time as fish entered concealment habitat. Aggressiveness varied widely among fish groups, with a range of 1 to 45 aggressive acts being initiated during a 90‐min filming period. Larger fish initiated most of the encounters and removed a higher proportion of fish from concealment than did smaller aggressors. Thirteen per cent of the aggressive acts resulted in the recipient being completely removed from concealment habitat. The highest combined frequency of aggressive acts was 0.64 per visible fish per 10 min period and occurred at relatively low light levels (300 lx). The data suggest that when fish density is high, such as when habitat is limited, aggressive behaviour in winter may cause some underyearling salmonids to be excluded from concealment.

Interspecific competition and habitat selection by the riverine dragonfly Onychogomphus uncatus

1. Substratum selection by the burrowing larvae of the dragonfly Onychogomphus uncatus was examined in an artificial laboratory stream at different larval densities and in the presence of one of three other dragonfly species.2. The larvae of O. uncatus, as well as those of the other species, clearly preferred gravelly sand substratum rather than gravel or stone.3. At low larval density (71.4 m–2) in the stream, 83% of the O. uncatus were found in gravelly sand. An increase of larval abundance in the stream to 202.4 specimens m–2 resulted in greater density in all substrata, but this increase was proportionally lowest in gravelly sand.4. The presence of a second species had various effects on the microdistribution of O. uncatus. In the presence of Gomphus simillimus or Cordulegaster boltoniiimmaculifrons the distribution of O. uncatus changed significantly; their density increased in the normally less preferred substrata. This effect is interpreted as asymmetric interspecific interference. The presence of a third species, Onychogomphus forcipatus unguiculatus, had no effects.

The influence of a river bird, the dipper (Cinclus cinclus), on the behaviour and drift of its invertebrate prey

1. We examined the behavioural response of stream macroinvertebrates to real and simulated predatory activity by a river bird, the Eurasian dipper, Cinclus cinclus L.2. In the field, we assessed whether invertebrate drift changed in response to live dippers in enclosures; we found no effects on the drift of any of the five families for which individual analyses were possible, either because it was infrequent, or involved distances too short to be detected (< 0.5 m).3. In a laboratory stream, we observed prey during encounters with a model dipper which simulated flight, swimming, bill contact with the prey, and stone turning. Invertebrate families varied in their response. Simuliids and hydropsychid caddis lacked effective escape behaviour, consistent with heavy losses through predation by dippers in the wild. Other families either drifted (Baetidae, Gammaridae) or moved away (Heptageniidae, Ephemerellidae, Leuctridae, Perlidae) from the model dipper, but responded only to bill contact or simulated stone turning. Such delayed responses would not protect individuals directly targeted by foraging dippers and partly explain the lack of detectable effects by dippers on drift in the field.4. We suggest why invertebrates do not show more marked escape responses to this important predator.

Interactions between fish, grazing invertebrates and algae in a New Zealand stream: a trophic cascade mediated by fish-induced changes to grazer behaviour?

Experiments in laboratory stream channels compared the behaviour of Deleatidium mayfly nymphs in the absence of fish with that in the presence of either native common river galaxias (Galaxias vulgaris Stokell) or introduced brown trout (Salmo trutta L.). Galaxias present similar predation risks to prey during day and night but are more active at night. Whereas, trout present a higher predation risk during the day. Deleatidium maintained a fixed nocturnal drift periodicity that is characteristic of streams containing visually feeding fish regardless of the nature of the predation regime presented in the laboratory. However, the number on the substratum surface, and therefore able to graze algae, was lower when fish were present than when they were absent. The number was lower during the day in the presence of trout, when they present the highest predation risk, and lower during the night compared to the day in trials with galaxias when galaxias activity disturbs Deleatidium from the substratum. Increases in the probability of Deleatidium leaving a patch, reductions in the proportion of mayflies on high quality patches and reductions in the distance travelled from refuge also reflected variations in the predation regime. Similar differences in positioning were observed under the same predation regimes in in situ channels in the Shag River and these were associated with differences in algal biomass. Algal ash-free dry mass (AFDM) and chlorophyll a (chl a) were higher on the tops of cobbles when fish were present. Fish also affected the biomass and the distribution of algae on cobbles as AFDM and chl a were higher on the sides of cobbles from channels with trout compared to those with galaxias. Changes in grazing behaviour, caused by predator avoidance, are likely to have been responsible for differences in algal biomass because no significant differences were detected between treatments in the biomass of Deleatidium or of total invertebrates.

The influence of spatial heterogeneity on the behavior and growth of two herbivorous stream insects.

Environmental heterogeneity can affect the behavior of organisms, but the consequences of patchiness for organismal energetics (e.g., growth, fitness) are not well understood. This study demonstrates that spatial heterogeneity can affect the growth of aquatic stream insects in laboratory streams, and reveals the behavioral mechanisms for these effects. In a 2×2 factorial design, I experimentally manipulated resource distribution (homogeneous vs. patchy, with the same overall resource levels) and current velocity (fast vs. slow) to investigate the direct and interactive effects of these factors on the drift behavior and growth of two mobile stream grazers, the mayflies Baetis bicaudatis and Epeorus deceptivus. B. bicaudatis nymphs grew larger in environments with homogeneously distributed resources than in patchy environments, and both species grew larger in fast than slow current environments. Patterns of drift behavior over the course of the study corresponded to observed differences in growth. Both species grew to larger body size in treatments where they drifted more successfully among substrates (fast-current treatments) and where they entered the drift less frequently (fast current for both species, and homogeneous treatments for B. bicaudatis). Overall, these results demonstrate that patchiness can significantly influence both the behavior of aquatic insects and the size to which these insects grow. In the light of previously published relationships between nymphal mayfly body mass and fecundity, these results suggest that patchiness in streams may have important consequences for mayfly populations.

Longitudinal Patterns of Nutrient Cycling and Periphyton Characteristics in Streams: A Test of Upstream-Downstream Linkage

We studied trends in nutrient cycling and periphyton characteristics (biomass, species composition, productivity, and nutrient content) along longitudinal gradients in laboratory streams to test the hypothesis that upstream-downstream linkages produce distinct longitudinal patterns in stream ecosystems. Periphyton communities were grown under uniform light and flow conditions in two 88-m-long laboratory streams created by connecting four channel segments (each 0.3 m wide and 22 m long) in series. At the end of 8 wk, large longitudinal declines in streamwater N and P concentrations were observed in each stream. Although chlorophyll a declined and the proportion of cyanobacteria in the periphyton increased with distance downstream, longitudinal trends in ash-free dry mass, gross primary productivity (GPP), and total respiration were not significant. In contrast, longitudinal trends in most of the parameters related to nutrient deficiency and cycling were significant. Chlorophyll-specific phosphatase activity and C: nutrient ratios in periphyton biomass increased from upstream to downstream, suggesting greater nutrient deficiency downstream. Ratios of net N uptake rate: GPP and net P uptake rate: GPP declined from upstream to downstream, suggesting that nutrient recycling supported a greater fraction of the algal nutrient demand downstream. Ratios of net: total P uptake rate also declined from upstream to downstream, suggesting that a larger fraction of the total P uptake from stream water downstream was met by P recycled within the segment rather than by inputs from upstream. Within-segment recycling supplied only 10-25% of P uptake from stream water in upstream segments but contributed 60-70% of the P uptake from stream water in downstream segments. Finally, total P uptake rate: GPP ratio declined from upstream to downstream, suggesting that cycling of P within the periphyton mat was greater downstream than upstream. Together, our results showed that increased nutrient cycling can compensate for longitudinal declines in nutrient concentrations in stream water, preventing large longitudinal changes in periphyton biomass and productivity. Nutrient cycling and algal species composition were the characteristics showing strongest longitudinal linkage in these periphyton-dominated streams, whereas total biomass and productivity patterns were poorly related to longitudinal position.

Influence of grazer type and abundance on plant-herbivore interactions in streams

Grazer-periphyton interactions were investigated in 11 laboratory streams holding a range of densities of three herbivore taxa during a 32-d experiment. Effects of grazers on algae were strongest with Dicosmoecus gilvipes caddisflies, intermediate with Juga silicula snails, and weakest with Baetis spp. mayflies. Algal standing crop, export, and gross primary production declined logarithmically with increasing grazer density. Algal turnover rate, however, increased with grazer abundance. At high densities of all grazers, responses in most algal parameters converged, suggesting that high grazing pressure, regardless of taxon, will similarly affect periphyton. Growth of both Dicosmoecus caddisflies and Juga snails was density-dependent, with the highest growth rates occurring at the lowest densities. Caddisflies displayed high growth rates but low efficiency in resource use. Snails had lower growth rates but were more efficient in resource use. The coexistence of Dicosmoecus and Juga, or other competing herbivores, in natural streams may be related to these fundamental differences in life history strategies.

Responses of blacknose dace (Rhinichthys atratulus) and brook char (Salvelinus fontinatis) to acidified water in a laboratory stream

As part of the Shenandoah National Park: Fish in Sensitive Habitats (SNP: FISH) project, movements of blacknose dace (Rhinichthys atratulus) and brook char (Salvelinus fontinalis) were examined during exposure to artificial acidification in paired channels of a laboratory stream. The objective of this study was to determine the ability of the fish to avoid depressions in the ambient pH and recognize and use a neutral-pH microhabitat refuge during acute reductions. Fish preference for a particular channel was statistically predictable based on the delivery of food. We tested fish avoidance behavior by manipulating food and the delivery of a pulse of acidified water. Both blacknose dace and brook char avoided the acid pulse (ambient pH reduced from 7.2 to 5.1) by sheltering in the pH-neutral refuge. Extensive field sampling in refuge microhabitats before and during episodic acidification is needed to determine changes in the distributional patterns of these species associated with acid precipitation events.

Molluscan cellulolytic activity responses to zinc exposure in laboratory and field stream comparisons

Changes in cellulolytic activity of Asiatic clams (Corbicula fluminea) and snails (Mudalia dilatata) were monitored throughout 30-d exposures to constant additions (0.0, 0.025, 0.05, 0.50, and 1.0 mg l−1) of zinc (Zn). All exposures of 0.05 mg Zn 1−1 or greater significantly reduced enzyme activity (exo- and endocellulase) in both molluscs as early as 10 d following exposures in outdoor laboratory streams incorporating New River water as diluent. More sterile laboratory stream exposures were less consistent in yielding quantifiable differences that could be attributed to metal induced stress apart from effects of nutritional stress. Tests conducted under natural field conditions during all seasons did not differ significantly with respect to changes in annual energetics of either clams or snails. However, evidence of differing uptake routes, with respect to two ecologically and physiologically distinct molluscs, was apparent in bioaccumulation, growth, and enzyme activity throughout exposure and following 60-d recovery.

EFFECTS OF FLOWING WATER ON NITROGEN‐ AND PHOSPHORUS‐LIMITED PHOTOSYNTHESIS AND OPTIMUM N:P RATIOS BY SPIROGYRA FLUVIATILIS (CHAROPHYCEAE)1,2

ABSTRACTThe effects of flowing water on net photosynthesis, dark respiration, specific growth rate, and optimum N:P ratios by Spirogyra fluviatilis Hilse were assessed. The alga was cultivated under nitrogen or phosphorus limitation in laboratory streams at three flow velocities: 3, 12, and 30 cm·s−1. The Droop equation adequately described respiration and photosynthesis (PSnet) as a function of N or P cell quota (QN or Qp). The data show that for N‐ or P‐limited Spirogyra fluviatilis, flowing water is physiologically costly. Generally, flowing water had little effect on respiration rates; however, the proportion of gross photosynthesis devoted to dark respiration did increase with flow velocity. For photosynthesis, the minimum N and P cell quotas increased with velocity, and the theoretical PSnet maxima for N and P both appeared greatest at 12 cm·s−1. The Droop models showed that for any given QN or Qp, PSnet, was reduced by the 30‐cm·s−1 treatment. Consistent with this finding, independent estimates of specific growth rates for P‐limited S. fluviatilis in the laboratory streams were inversely related to flow velocity when ambient PO4−3 was undetectable. However, growth was not diminished at the fastest velocity when PO4−3 was available for uptake. Thus, the increase in cellular phosphorus demand can be offset by flow‐enhanced P uptake when conditions permit; otherwise, growth will be impaired.The optimum N:P ratios for S. fluviatilis at 3, 12, and 30 cm·s−1 were 50, 58, and 52 by atoms, respectively, when calculated for PSnet= 0. The optimum ratios were inversely related to PSnet and decreased to approximately 20 when PSnet was near maximum. The potential for flowing water to mediate nutrient partitioning among lotic algae by altering growth rates and optimum nutrient ratios is discussed.

Effect of periphyton biomass on hydraulic characteristics and nutrient cycling in streams.

The effect of periphyton biomass on hydraulic characteristics and nutrient cycling was studied in laboratory streams with and without snail herbivores. Hydraulic characteristics, such as average water velocity, dispersion coefficients, and relative volume of transient storage zones (zones of stationary water), were quantified by performing short-term injections of a conservative tracer and fitting an advection-dispersion model to the conservative tracer concentration profile downstream from the injection site. Nutrient cycling was quantified by measuring two indices: (1) uptake rate of phosphorus from stream water normalized to gross primary production (GPP), a surrogate measure of total P demand, and (2) turnover rate of phosphorus in the periphyton matrix. These measures indicate the importance of internal cycling (within the periphyton matrix) in meeting the P demands of periphyton. Dense growths of filamentous diatoms and blue-green algae accumulated in the streams with no snails (high-biomass streams), whereas the periphyton communities in streams with snails consisted almost entirely of a thin layer of basal cells of Stigeoclonium sp. (low-biomass streams). Dispersion coefficients were significantly greater and transient storage zones were significantly larger in the high-biomass streams compared to the low-biomass streams. Rates of GPP-normalized P uptake from water and rates of P turnover in periphyton were significantly lower in high biomass than in low biomass periphyton communities, suggesting that a greater fraction of the P demand was met by recycling in the high biomass communities. Increases in streamwater P concentration significantly increased GPP-normalized P uptake in high biomass communities, suggesting diffusion limitation of nutrient transfer from stream water to algal cells in these communities. Our results demonstrate that accumulations of periphyton biomass can alter the hydraulic characteristics of streams, particularly by increasing transient storage zones, and can increase internal nutrient cycling. They suggest a close coupling of hydraulic characteristics and nutrient cycling processes in stream ecosystems.

Anti-Predator Responses of Mayfly Larvae to Conspecific and Predator Stimuli

Hydrodynamic and water-borne chemical stimuli are considered important cues that initiate anti-predator behaviours of mayfly larvae. We tested the hypotheses that chemical stimuli from conspecifics and the predacious fish Rhinichthys cataractae and hydrodynamic stimuli from a Rhinichthys model would initiate anti-predator responses by mayfly larvae of Ephemerella aurivillii, Paraleptophlebia heteronea, and Baetis tricaudatus. A second set of experiments tested the hypotheses that chemical stimuli from conspecifics and the predacious stonefly Claassenia sabulosa and hydrodynamic stimuli from a Claassenia model would initiate responses by Baetis. These hypotheses were tested in laboratory streams where mayfly larvae received either chemical stimuli, hydrodynamic stimuli, or the combination of hydrodynamic and chemical stimuli. Responses by larvae to these stimulus types, in terms of use of substratum surfaces and drift rates, were compared with those by larvae in control streams without conspecific and predator stimulus types. Responses to chemical stimuli were variable depending upon mayfly species and the chemical stimulus type. For instance, while Ephemerella and Paraleptophlebia responded to Rhinichthys odours, Baetis did not respond to either Rhinichthys or Claassenia odours. Paraleptophlebia responded to conspecific odours whereas Ephemerella and Baetis did not. In contrast, all three mayfly species responded to hydrodynamic stimuli either alone or when combined with chemical stimuli. Responses by Baetis were more complex when chemical stimuli were combined with hydrodynamic stimuli. For instance, although Baetis did not respond to conspecific odours alone, drift was significantly higher in streams receiving the three stimuli of conspecific odours, predator odours from Rhinichthys, and hydrodynamic stimuli from a Rhinichthys model when compared with streams that received only the conspecific odours combined with the fish model. Similarly, an enhanced response was observed when Baetis odours were combined with Claassenia odours and the Claassenia model. Thus, the lack of a response to a chemical stimulus did not preclude an enhanced response when combined with other stimulus types. These observations suggest that although some mayflies are capable of detecting the presence of some chemical stimuli, a response is not elicited unless the stimulus is accompanied by additional stimuli.

PHOSPHORUS UPTAKE KINETICS OF SPIROGYRA FLUVIATILIS (CHAROPHYCEAE) IN FLOWING WATER1,2

ABSTRACTThe inorganic phosphorus (Pi) uptake kinetics of Spirogyra fluviatilis Hilse were examined as a function of phosphorus cell quota (QP) and flow velocity in a laboratory stream apparatus. Short‐term uptake and the acclimation of the uptake mechanism to flow were measured by the disappearance of Pi pulses in a recirculating flow cell. Short‐term Pi uptake was biphasic. When the alga was P‐deficient, Phase 1 and 2 half‐saturation constants and maximum uptake rates were 11.0 and 47.2 μg P·L−1 and 473 and 803 μg P·g dry wt−1 h−1, respectively. Flowing water altered short‐term uptake when the alga was P‐deficient, but not when it was P‐replete. When QP was less than 0.21%, increases in flow velocity from 3 to 15 cm·s−1 enhanced uptake with maximum uptake for any Pi pulse at 12 and 15 cm·s−1. At 22 and 30 cm·s−1, uptake was reduced by 12% or more relative to the maxima. If, however, the alga was cultivated at 22 and 30 cm·s−1 and short‐term Pi uptake was measured at 12 cm·s−1, uptake was on average 33% greater than when the alga was cultivated at the latter velocity. Apparently, the alga could adjust short‐term uptake to compensate for the suboptimal conditions of the faster velocities.Long‐term Pi uptake and net phosphorus efflux were estimated by a non‐steady state application of the Droop equation. Long‐term uptake of very low Pi concentrations was not reduced by fast flowing water. Instead, uptake increased proportionately with flow velocity. Maximum phosphorus efflux from S. fluviatilis was 3% of cellular P per hour and occurred when QP was greater than 0.2%. At lower QP, the hourly efflux rate was typically less than 1%. Flowing water did not greatly enhance efflux, although when Pi was undetectable, efflux did tend to increase slightly with velocity. The data show that the effects of flowing water on Pi uptake were varied and not always beneficial. If the effects of flowing water on nutrient acquisition by other lotic algae are similarly varied and complex, flow may be an important determinant of nutrient partitioning among benthic algae in streams.

Influence of Water Temperature on Interactions between Juvenile Colorado River Cutthroat Trout and Brook Trout in a Laboratory Stream

Abstract We compared microhabitat selection by and behavioral interactions between juvenile Colorado River cutthroat trout Oncorhynchus clarki and brook trout Salvelinus fontinalis at two water temperatures in the laboratory. At 10°C, both species occupied similar depths but cutthroat trout used slightly faster current velocities. At 20°C, depth and current velocity use were similar for both species. The species were nearly equal competitors at 10°C, but brook trout showed a clear competitive dominance over cutthroat trout at 20°C. At the warmer temperature, brook trout were more aggressive, consumed more food, and occupied the lead position in a dominance hierarchy more often than cutthroat trout. Brook trout also maintained equilibrium longer during a thermal challenge test, suggesting they are more tolerant of heat stress than Colorado River cutthroat trout. Our results provide an example of condition-specific competition that may influence fish distribution patterns in streams.