Low Current Velocity Research Articles

Hydrodynamics, diffusion-boundary layers and photosynthesis of the seagrasses Thalassia testudinum and Cymodocea nodosa

Photosynthetic rates of aquatic plants frequently increase with increasing current velocities. This is presumably due to a reduction in the thickness of the diffusion boundary-layer which allows for a higher carbon availability on the plant surface. Blades of the seagrasses Thalassia testudinum and Cymodocea nodosa exposed to different current velocities under controlled laboratory conditions, showed increased photosynthetic rates with increasing flow only at low current velocities (expressed as blade friction velocities, u*). Carbon saturation of photosynthetic processes occurred at a relatively low u* level (0.25 cm s-1) for T. testudinum collected from a calm environment compared to C. nodosa (0.64 cm s-1) collected from a surf zone. No further enhancement of photosynthetic rates was observed at higher u* levels, suggesting limitations in carbon diffusion through the boundary layer below critical u* levels and possible limitations in carbon fixation by the enzymatic system at higher u* levels. These results, as well as those of previous theoretical studies, assumed the flow on the immediate seagrass-blade surface to be hydrodynamically smooth. The presence of epiphytes and attached debris causes the surface of in situ seagrass blades to be exposed to flows ranging from smooth to rough-turbulent. As a consequence, the boundary-layer thickness on moderately epiphytized blades under medium to high flow-conditions is not continuous, but fluctuates in time and space, enhancing carbon transport. In situ u* levels measured directly on blades of seagrasses indicate that T. testudinum and C. nodosa can be exposed to conditions under which the boundary layer limits photosynthesis during short periods of time (milliseconds) during low-energy events. As waves cause the thickness of the diffusion boundary-layer to fluctuate constantly, carbon-limiting conditions do not persist for prolonged periods.

Surface Flow and Particle Settling in a Coastal Reed Field

Hydraulic roughness of reed stems in shallow surface flow under low current velocity is discussed. The roughness coefficient, defined as a Manning's formula, depends upon depth and velocity in this flow. Depth and velocity, in turn, are altered by roughness through water surface slope. The interaction between these three parameters must be solved simultaneously for the prediction of the surface flow in a reed wetland. Stem roughness can be expressed as a function of Reynolds' number, similar to that for a single cylinder. Assigning this function, an iteration method easily gives us a suitable set of three parameters. Settling experiments show that stems promote vertical mixing of fine particles in shallow flow. The model of vertically uniform SS concentration is more suitable. Though the observed data fluctuated widely, the apparent settling velocity of fine particles in the flow was very close to the settling velocity in quiescent water. Using the above two models, one for flow and the other for settling in shallow wetland, we can estimate the clarification potential of a reed field under given hydraulic conditions and particle load. Numerical experiments show that fine particles tend to accumulate on the bottom of a reed field, as often observed in natural wetlands.

Spatial patterns in shallow-water crinoid communities on the central Great Barrier Reef

The crinoid communities of shallow-water areas (<12 m depth) of the central Great Barrier Reef were investigated on reefs at different locations on the continental shelf and in greater detail within one mid-shelf reef (Davies Reef). Overall, 43 comatulid crinoid species were identified, among which the family Comasteridae contributed 90% of the total number of collected specimens. High substratum complexity, in combination with high average water flows, characterized the most suitable environmental conditions for most of the crinoid species, whereas abundance and species richness were low in regions with high sedimentation rates and low current velocities. This set of environmental factors was correlated with crinoid community structures both on a local within-reef level and across the continental shelf. A few 'generalist' species (mostly comasterids) showed distribution ranges extending across the whole shelf, whereas many other species were found predominantly at the mid-shelf sites and only in low numbers, if at all, at both the inner and the outer shelf edges. Crinoid populations on reefs previously infested by Acanthaster planci were depleted in comparison with unaffected reefs. Observations suggest that the spangled emperor fish (Lethrinus nebulosus) is a major crinoid predator and that fatal predation occurs commonly among crinoids.

Food limitation in cockles ( Cerastoderma edule (L.)): Influences of location on tidal flat and of nearby presence of mussel beds

Dilution of phytoplankton concentrations by suspension-feeding organisms, and local reduction in current velocity, may lower food supply for other suspension feeders, and consequently affect their growth. Influences of food supply on stomach contents and growth of the suspension-feeding bivalve Cerastoderma edule were studied in two situations: (a) inside and outside an experimental mussel bed in a basin with a natural seawater supply, and (b) along tidal creeks in the field. Cockles showed a lower increase in shell length, body weight and condition inside the experimental mussel bed than outside. In the field, phytoplankton concentrations in the water were lower at upper than at lower tidal-creek sites. At these upper creek sites, cockles were smaller, as were their growth rates and stomach contents, compared to the lower creek sites. Largest reductions of phytoplankton concentration in the water were observed at sites near a mussel bed. It is concluded that phytoplankton uptake by suspension feeders (particularly in mussel beds), as well as relatively low current velocities, negatively affects food supply, stomach contents and growth of C. edule.

The management implications of fine fuel dynamics in bushlands surrounding Hobart, Tasmania: R. J. Fensham, Journal of Environmental Management, 36(4), 1992, pp 301–320

In this paper, we postulate a unique environmental triggering sequence for primary outbreaks of crown-of-thorns starfish (COTS, Acanthaster planci) on the central Great Barrier Reef (GBR, Australia). Notably, we extend the previous terrestrial runoff hypothesis, viz. nutrient-enriched terrestrial runoff → elevated phytoplankton ‘bloom’ concentrations → enhanced COTS larval survival, to include the additional importance of strong larvae retention around reefs or within reef groups (clusters) that share enhanced phytoplankton concentrations. For the central GBR, this scenario is shown to occur when El Niño-Southern Oscillation (ENSO) linked hydrodynamic conditions cause the ‘regional’ larval connectivity network to fragment into smaller ‘local’ reef clusters due to low ocean current velocities. As inter-annual variations in hydrodynamic circulation patterns are not amenable to direct management intervention, the ability to reduce the future frequency of COTS outbreaks on the central GBR is shown to be contingent on reducing terrestrial bioavailable nutrient loads ~ 20–40%.

Sedimentary environments and facies in an arctic basin, Itirbilung Fiord, Baffin Island, Canada

Itirbilung Fiord typifies a deep arctic basin that experiences episodic sedimentation events, cycled principally by aeolian transport, nival and glacier melt, with punctuations by mass-sediment failures. A Quaternary sedimentary fill exceeding 150 m is found in two of the fjord's four sub-basins. Basin circulation is associated with meltwater influx during the summer, wind-generated drift during the fall accompanied by deep-water exchanges, winter isohaline-generated circulation, and tidal currents which operate over the entire year. Sediment accumulation decreases down the basin; this general trend is interrupted by side-entry fan deltas that locally contribute large volumes of sediment. Basin deposits are largely ponded between the fjord walls, although two of the fjord's sills are mantled by hemipelagic deposits. Five seismo-stratigraphic units are identified and may represent sedimentary deposits associated with a single phase of ice sheet advance and retreat. The upper three seismic units were cored and contain seven sedimentary facies that show little correlation between sub-basins: pebbly-sandy-mud, from the melt of debris-laden ice; burrowed /mottled mud, reflecting the bioturbation of hemipelagic deposits; wispy laminated /mottled mud, from low-velocity currents and/or melt-out of seasonal sea ice cover, and bioturbation; laminated sand and mud, from the hypopycnal flows from rivers; graded-coarse sand/gravel, and pebbly sand and sand, reflecting very rapid deposition from high-concentration turbidity currents or from sandy debris flows; and cross-bedded sands, that may relate to the reworking of turbidity current deposited sands, possibly by reverse flow mechanisms.

The behavioural ecology of epibenthic scavenging invertebrates in the Clyde Sea area: laboratory experiments on attractions to bait in moving water, underwater TV observation in situ and general conclusions

The attractions to bait of eight species of epibenthic scavenging invertebrates, i.e. Asterias rubens L., Buccinum undatum L., Cancer pagurus L., Ophiocomina nigra (Abildgaard), Liocarcinus depurator (L.), Pandalus montagui (Leach), Hyas araneus (L.) and Pagurus bernhardus (L.), were examined in current tank experiments at high (0.3 m s −1) and low (0.1 m s −1) water current velocities. The swimming crab L. depurator showed the greatest upstream response towards bait in the low current velocity trials, while the hermit crab P. bernhardus showed the greatest attraction to bait in the high current velocity trials. There was a significant association between the rank orders of the above species caught in baited traps in the field and their relative performance in the high current velocity experiments in the laboratory. Baited TV cameras in situ confirmed the importance of P. bernhardus and B. undatum as scavengers in the Clyde Sea area.

A sediment trap intercomparison experiment in the Panama Basin, 1979

In order to compare the collection efficiency of settling particles among sediment traps in a variety of design concepts, 28 sediment traps of 11 different designs were deployed at six depths ranging from 665 to 3769 m along five rigid mooring anchored in a sill-protected marginal basin about 3865 m deep for about 4 months from August to November 1979. The traps represented three basic designs: (1) cylinders with an aspect ratio between 2 and 3; (2) funnels with a large opening covered by a baffle with a small grid and (3) open boxes whose openings were covered by a baffle. All but two of these types of participating traps had a mechanism to isolate the collected sample. Monitoring instruments indicate that all moorings provided a stable platform throughout the duration of Sediment Trap Intercomparison Experiment (STIE) with relatively low current velocity at the middle layers and very low velocity at the deep layers. Total mass flux, fluxes of three size fractions after water sieving, carbonate, combustible and noncombustible fractions, organic carbon, nitrogen and other sedimentary constituents in the individual samples were determined and evaluated with regard to the relative consistency in terms of depth and statistical tests on the similarity of the constituents.Under the conditions tested, the trapping efficiency of settling particles between a large funnel trap with baffle and an intermediate-sized cylinder trap was nearly identical considering the laboratory analytical errors. This conclusion might be extended to cylinder traps with diameters as small as 7 cm and a large aspect ratio when deployed rigidly in a low energy ocean environment. A funnel-type trap with a more effective baffle had a higher collecting efficiency than the other traps. Because of mechanical problems, comparison of the box-type traps to the other types was inconclusive.

Functional significance of variations in the central fold of shells of late Ordovician through Devonian biconvex brachiopod genera

Flume experiments with models of Mid-Paleozoic atrypids, orthids, rhynchonellids, and spiriferids indicate that the central fold functions efficiently to separate lateral-margin incurrents from the anterior-medial margin excurrent in five of nine possible life orientations of the shell relative to the current direction and substrate. Anterior-medial incurrents and lateral excurrents are effectively separated in four of the nine orientations used with models of spiriferids and atrypids, but nonpediculate brachiopods drawing in water anterior-medially could not take advantage of reversing tidal currents to feed. The risk of refiltration of medial excurrent water is reduced with increasing relief of the central fold above the commissural plane. Downcurrent turbulance increases with increasing relief of the central fold. Eddies with a large radius of curvature are generated by large chevron-shaped central folds at low current velocities (5 cm/sec) and boomerang against the downcurrent lateral margin of models. Flume experiments on models also reveal that a well-developed sinus accelerated erosion of the supporting sediment around the weight-bearing posterior of the valves. Shells with high relief in the sinus destabilized comparatively quickly from valves-erect orientations when subjected to moderate current velocities (25 cm/sec).Morphospace (ternary) diagrams of sinus shape for Late Ordovician (Caradocian) through Devonian (Famennian) genera show the realized field among the potential morphotypic extremes, namely, 1) rectimarginate (no fold), 2) isoclinal, chevron-shaped, and 3) M-shaped anterior commissural outlines. Morphospace plots through successive stages suggest centripetal selection for taxa with moderately developed folds in the atrypids and spiriferids, with occasional evolution of “outlier” genera with chevron-shaped central folds. Orthids display progressive loss of rectimarginate genera through the Devonian. Weak directional selection is suggested by the succcessive stage-level plots of the rhynchonellid genera which expanded toward the extremes of chevron- and M-shaped central folds in potential morphospace during the Devonian.

The energetics of passive suspension feeding: ecological and evolutionary consequences for crinoids

Do organismal characteristics influence evolutionary histories of taxa? This important question has been answered in the affirmative for a variety of organisms, and here it is applied to the echinoderm class Crinoidea. The approach used in answering this question involves (1) identifying a relevant trait through functional analysis, (2) demonstrating its ecological significance, and (3) testing the hypothesis that the ecological characteristics affect evolutionary rates.Crinoids, like all passive suspension feeders, depend on externally driven flows to supply them with necessary particulate nutrients. Particles are captured with feeding appendages which function as filters. Filters act as obstacles to the flow and their morphology influences how much fluid can be filtered at a given current velocity: fine filters, because of their higher resistance to flow, sample less fluid and fewer potential food items than coarse filters. This suggests that filter morphology may be important in controlling the distributions of crinoids among different environments: at low current velocities fine-filtered crinoids may not capture sufficient particulate nutrients to satisfy their energy needs.To quantify the effect of filter morphology on particulate nutrient capture, an energy budget equation was solved for two crinoid morphologies representing the fine (pinnulate) and coarse (non-pinnulate) filters. In this equation, energy inputs in the form of captured and metabolized nutrients had to exceed energy expenditures, expressed as the standard metabolic rate. The solution of the equation showed that the current velocity below which crinoids are at an energy deficit is higher for the fine-filtered (pinnulate) than the coarse-filtered (non-pinnulate) taxa.The higher minimum velocity of fine-filtered crinoids restricts them to environments with higher current velocities; coarse-filtered taxa are less constrained and may occupy a broader range of environments. This pattern is borne out by the distribution of Mississippian crinoids studied by Kammer and Ausich (1987). With regard to current velocities, the fine-filtered crinoids may be described as “specialists” and the coarse-filtered crinoids as “generalists”. The specialist/generalist strategies of the two groups suggest an evolutionary scenario in which the fine-filtered crinoids are more prone to speciation and extinction.To test the above scenario the evolutionary rates of two morphological groups, fine-filtered crinoids (camerates) and coarse-filtered crinoids (non-pinnulate Paleozoic crinoids), were compared. As predicted, the former group had higher extinction and origination rates. A “bootstrapping” technique revealed that the differences in extinction rates were statistically significant (P < 0.05).The filter morphology-evolutionary rate scenario, however, fails to predict the durations of two other fine-filtered (pinnulate) crinoid groups: the Paleozoic cladids and their descendants, the post-Paleozoic articulates. Though fine-filtered, these taxa had broad environmental distributions and the lowest extinction rates among the Crinoidea. They are also the only crinoids with muscular arm articulations. This “evolutionary innovation”, by allowing a greater degree of morphological and behavioral flexibility, allowed these crinoids to occupy a broader range of environments and may in part explain their “anomalous” evolutionary rates.

Riverine and Estuarine Migratory Behavior of Coho Salmon (Oncorhynchus kisutch) Smolts

Using radio and ultrasonic transmitters, we tracked coho salmon (Oncorhynchus kisutch) smolts in the Chehalis River and Grays Harbor estuary, Washington, to document patterns of seaward migration. In spite of dramatic differences in the two habitats, the fish behaved similarly in both systems. Migratory progress was saltatory, characterized by movement in the direction of the current and extended periods of holding in areas of low current velocity. Consequently, smolts were displaced rapidly downstream by swift, unidirectional river currents but were retained in the estuary by relatively low-velocity, reversing tidal currents. Smolts did not use tidal currents to move seaward and appeared to swim most rapidly against strong ebbing currents. Estimated mean total metabolic cost incurred by migrating smolts was 0.027 cal∙s−1∙smolt−1 (1 cal = 4.184 J). Estimated daily caloric expenditure by smolts in 1988 was twice that of smolts tracked in 1989, apparently due to faster water velocities in 1988. The observation that coho salmon migration through estuaries is slower than riverine migration suggests that a period of estuarine residence may be necessary for them to adjust their osmoregulatory capability, orient for their return migration, feed, or reduce their vulnerability to predators.

Transport and Retention of Particulate Organic Matter in Two Low-Gradient Headwater Streams

Transport and retention of particulate organic matter (POM) were examined in the channels and on the floodplains of two low-gradient headwater streams on the Coastal Plain of southeastern Virginia. During base discharge, POM was primarily retained as it settled onto the sediment surface, but during high discharge, debris dams became primary retainers. During overbank flooding much of the coarse particulate organic matter (CPOM) moved from the channels onto the floodplains. The mean distance that wood moved over a year at Colliers Creek, which had low current velocity and a broad, frequently inundated floodplain, was 23 m in the channel and 46 m on the floodplain; at Buzzards Branch, with higher current velocity and a smaller, less-frequently inundated floodplain, wood moved a mean distance of 136 m in the channel and only 2 m on the floodplain. Mean leaf transport distances in the channels ranged from 1.6 m in Colliers Creek during summer base discharge to 156 m at Buzzards Branch during a winter spate; mean leaf transport distances on the floodplains were 0.5-1.7 m. Fine particulate organic matter (FPOM) transport, studied only during base discharge, was farther than that of wood or leaves and varied from 1.8 m during summer base discharge in the Colliers Creek channel to 84.0 m during winter base discharge in the Buzzards Branch channel. Over one year, 97% and 27% of the marked wood placed in the channels was transported into the Colliers Creek and Buzzards Branch floodplains, respectively; only 4-9% of the marked wood placed in the floodplains moved into the channels. Base flow flux of POM was 24,000 kg/yr and 1700 kg/yr on the floodplains, and 7000 kg/yr and 13,000 kg/yr in the channels, at Colliers Creek and Buzzards Branch, respectively. Retention, transport distance, and magnitude of POM exchange between the channels and floodplains were dependent on the timing, frequency and extent of spates, and floodplain inundation, which thus were critical determinants of POM dynamics and hence system structure and function.

Sedimentation dynamics and redox iron-cycling: controlling factors for the apatite—glauconite association on the East Australian continental margin

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Suspended sediment transport and deposition of cyclically interlaminated sediment in a temperate glacial fjord, Alaska, U.S.A.

Abstract Cyclically interlaminated sediment is a distinctive lithofacies within the sediment package of temperate glacial fjords. The tidewater terminus of McBride Glacier is at the head of a small fjord that receives abundant suspended sediment and is a site of rapid accumulation of cyclically interlaminated sediment. Buoyant sediment-laden meltwater rises from a subglacial stream at the base of the glacier and mixes with fjord water to produce a thick, brackish overflow. Peak suspended sediment concentrations occur beneath the surface of the overflow at 3 to 10 m depth. Particle release from the overflow is controlled by semi-diurnal tidal fluctuations. The major vertical flux of suspended sediment is initiated at low tide because of low horizontal current velocities and reduced vertical eddy velocities. Sorting occurs as the particles settle; sand and coarse silt settle as single grains and finer flocculated particles settle as turbid layers at a rate between 2.5 m/h and 10.9 m/h. Each low water produces a couplet of a coarser grained lamina that is sorted as coarser single grains settle, and a finer grained lamina of flocculated particles in turbid layers. Individual couplet thicknesses and particle size decrease with distance from the discharge source. Semi-diurnal tides produce two couplets each day. Interstratified with these tidal rhythmites are coarser and poorly sorted laminae deposited by sediment gravity flows and coarse laminae contributed by peak daily discharge.

Hydrodynamic morphology and behavior of a free-living sediment-dwelling bryozoan, Alcyonidium disciforme (Smitt)

The free-living bryozoan Alcyonidium disciforme (Smitt) is a pliable slightly convex disc with a central opening and lophophores on the upper surface. Field and flow-tank experiments on live and preserved specimens from the Bering and Chukchi Seas as well as model colonies indicate that this morphology 1. (1) maintains a correct convex-side up position on the sea floor 2. (2) resists dislodgement and transport by flow 3. (3) when lifted into the water column, falls to the substratum basal-side up 4. (4) when basal-side up on substratum, is easily reorientated at low flow (≤ l0cm· s −1. Strong negative lift (> - 12 mN) was produced by flow over upright colonies, enabling them to resist suspension and maintain their orientation through a wide range of current velocities (≤ 60 cm · s −1). Experimental closure of the colonies' central opening reversed the direction of lift and resulted in colonies being swept away at much lower current velocities ( ≤ 30 cm · s −1 ). The self-righting ability of A. disciforme is the result of significantly higher drag and lift experienced by inverted vs. upright colonies at all flow speeds. Observations of live A. disciforme suggest the existence of a colony-wide escape response. When disturbed, split and coiled colonies tightened their coil and increased their height. Plastic models used to mimic this response showed “extended” colonies were transported at significantly lower flow velocities (mean = 46 cm · s −1) than colonies of normal aspect (mean = 57 cm · s −1). The shape of A. disciforme aided in directing experimental current flow between the overlapped ends and thus across the covered lophophore regions of split and coiled fragments. The high proportion of colony fragments found in the Bering Sea, many of which had grown into overlapping coils exceeding 360°, suggests that asexual reproduction is important. These findings show that the morphology and behavior of A. disciforme, unique among Bryozoans, enable it to live in high energy, wave swept, sedimentary environments, which are equally unusual for the phylum.

Development of habitat suitability criteria for trout in small streams

AbstractWe observed 2863 trout in the wild to determine habitat utilization in small streams of the Kings River basin in California's Sierra Nevada mountains. The habitat utilization data were used to develop habitat suitability functions that provide input variables to the instream flow incremental methodology (IFIM) habitat assessment model of the U.S. Fish and Wildlife Service.Observations of habitat utilization of rainbow trout (Salmo gairdneri), brown trout (Salmo trutta), and brook trout (Salvelinus fontinalis) were obtained during the summer months of 1983 and 1984. The observations were made in small streams with discharges ranging from 0.7 m3 s−1 to 0.03 m3 s−1. The streams are at elevations of 1250 to 2530 m. Equal effort was applied to observing undisturbed trout in all habitat types. Snorkeling proved to be the most effective method of observation.Individual trout of all species and life stages were most often observed in the lower half of the water column, utilizing low‐velocity currents of less than 3.0 cm s−1.From the depth and velocity utilization data, several forms of habitat suitability functions were developed and evaluated: Univariate depth and velocity functions derived from frequency histogram analysis. Univariate depth and univariate velocity exponential polynomial models. Bivariate depth and velocity exponential polynomial models. Univariate exponential polynomial models provided the best fit to the data for each species, based on visual comparisons of response surfaces and contour plots, and comparisons of computed sums of squared errors. Bivariate models fitted to the data resulted in greater sums of squared errors than multiplicative aggragation of univariate models, and frequently predicted utilization at zero depth.The habitat suitability functions derived from the univariate exponential polynomial models provided the best input to the IFIM habitat assessment models.

Local Geomorphology as a Determinant of Macrofaunal Production in a Mountain Stream.

By comparing distributions of functional group production among different habitats in an Appalachian mountain stream, the influence of site-specific geomorphology upon the overall functional group composition of the animal community was demonstrated. By replicated monthly sampling, substrate particle size distributions, current velocity, standing crops of benthic organic matter, and production of macrofauna were measured in each of three principal habitats: bedrock-outcrop, riffle, and pool. Samples were taken at randomly assigned locations and the relative number of samples taken from each habitat was assumed to be proportional to the area of the habitat within the stream. These proportions were used to weight production measured in each habitat and the resulting values were summed to obtain production per unit area of average stream bed. The bedrock-outcrop habitat was characterized by high material entertainment and export as indicated by significantly higher current velocities and lower standing crops of detritus compared to the riffle and pool habitats. Pools were sites of low entertainment and high retention of organic matter as demonstrated by significantly lower current velocities and higher accumulations of detritus than other habitats. The riffle habitat was intermediate to the bedrock-outcrop and pool habitats in all parameters measured. Annual production of collector-filterers was highest in the bedrock-outcrop (ash-free dry mass 1920 mg/m2 ), followed by riffle (278 mg/m2 ) and pool (32 mg/m2 ). Although constituting only 19% of the stream area, the bedrock-outcrop habitat contributed 68% of the habitat-weighted collector-filterer production. Annual production of shredders was highest in pools (2616 mg/m2 ), followed by riffles (1657 mg/m2 ) and bedrock-outcrop (579 mg/m2 ). The pool habitat, constituting 23% of stream area, contributed 36% of shredder production. Annual production of scrapers was highest in the riffle habitat (905 mg/m2 ), followed by bedrock-outcrop (517-mg/m2 ) and pool (238 mg/m2 ). Riffles constituted 58% of total stream area and were the source of 77% of the habitat-weighted scraper production. Annual production of engulfing predators was greatest in the pool habitat (2313 mg/m2 ), followed by riffles (1765 mg/m2 ) and bedrock-outcrop (687 mg/m2 ). The relatively lower production of engulfing predators in the bedrock-outcrop habitat reflects a functional shift in mode of resource acquisition by predators, with predaceous collector-filterers (Arcto-psychinae: Trichoptera) predominating in the bedrock-outcrop. Collector-gatherer production was more evenly distributed, with the bedrock-outcrop, riffle, and pool habitats each contributing 14, 54, and 33% to the habitat-weighted production, respectively. Unlike all other functional groups, this distribution was not significantly different from the distribution of stream area among habitats and reflected lack of dependence on specific physical attributes of the local environment for access to food by members of this functional group. Local geomorphology determined the diversity and spatial distribution of bedrock-outcrops, riffles, and pools in the study stream. In turn, the functional structure of the macrofauna, when viewed holistically, was the result of the integration of the relative contributions of each habitat type of total stream area. Total habitat-weighted annual production in the study stream was estimated at 5093 and 1921 mg/m2 for primary and secondary consumers, respectively. The distribution of habitat-weighted production among functional groups was: collector-gatherers (39%), followed by shredders (225), engulfing predators (22%), scrapers (13%), and collector-filterers (8%). This functional structure agrees favorably with current conceptual models of head water streams draining forested catchments.

Substratum preference of the caddisfly Helicopsyche borealis (Hagen) (Trichoptera: Helicopsychidae)

The position of Helicopsyche borealis (Hagen) (Trichoptera: Helicopsychidae) larvae on the substratum surface is dependent on the current regime but varies with larval size. All size classes of larvae chose significantly different positions on the substratum under high versus low current velocities. All size classes preferred exposed surfaces under low current velocities. Small larvae preferred the upper surfaces of substrata under low current velocities and were physically displaced under high current velocities. Larger larvae also occurred on upper surfaces, but were more evenly dispersed over all surfaces than smaller larvae, and tended to aggregate on down-stream faces of rocks during high flow.

The role of periphyton abundance and quality in the microdistribution of a stream grazer, Helicopsyche borealis (Trichoptera: Helicopsychidae)

SUMMARY. 1. A series of laboratory and field experiments was performed to determine if the preference of larvae of Helicopsyche borealis (Trichoptera: Helicopsychidae) for exposed rock surfaces in streams was related to patchiness of periphyton food.2. At low current velocities in the laboratory larvae preferred food‐saturated as opposed to food‐depleted portions of substrata and drifted off the substratum in response to low food levels.3. When offered a choice of three potential foods—a diatom, a green filamentous or a blue‐green filamentous alga—larvae showed no preference.4. Larval densities on artificial subtrata in field experiments were highest in areas of high algal chlorophyll a.5. In laboratory experiments more adults emerged from cultures containing green algae or diatoms than cultures with blue‐green algae or bacteria alone.6. Periphyton quality was not as important as abundance in controlling H. borealis microdistribution. By selecting exposed surfaces of rocks, H. borealis larvae place themselves in the microhabitat most likely to contain both abundant and high quality food patches.

Ecological Impacts of the Construction of Dams in an Estuary

This paper deals with the ecological impacts of alternative methods of constructing dams in an estuary. The main options available were:using concrete blocks and rubbleusing only sand. The advantages of using sand are that it is available locally and it is cheaper than concrete blocks and rubble. However, the application of sand in a tidal basin requires low current velocities. During the final closing period of the dams, these velocities can be reduced by using the storm-surge barrier (to be built in the mouth of the Eastern Scheldt estuary) in two ways: either reducing the tidal amplitude or reducing the tidal frequency. The tidal reduction influences the vertical amplitude and current velocities, thereby decreasing the intertidal area, and the biomass and production of microphytobenthos and macrozoobenthos. A reduction of the horizontal tide leads to a reduction of the exchange of water between the North Sea and the estuary, with a decrease in current velocities. These phenomena affect the concentration of suspended and dissolved material; in particular, salt concentrations decrease with possible stratification. This paper describes the effects of different closing procedures on natural ecosystems and fisheries. The study resulted in a policy analysis and was presented to decision-making politicians.