bluuebunny/crossref_metadata_2025 huggingface dataset:
The optimal stability `window': a mechanism underlying decadal fluctuations in North Pacific salmon stocks?
Authors: GARGETT ANN E.
Published: November 2003

While recent evidence suggests that North Pacific salmon stocks are influenced by decadal variability in atmospheric forcing of the ocean, the actual combination of physical and biological processes that determines this linkage has not been identified. This paper describes a possible scenario in which water column stability is the primary factor by which the physical environment influences phytoplankton production, the basis for production at higher trophic levels. Variation in the strength of the wintertime Aleutian Low pressure area affects water column stabilities, hence primary production, along the entire eastern boundary of the North Pacific. The `optimal stability window' explains the qualitative relationship between fish stocks and the strength of the winter Aleutian Low, as well as the observed out‐of‐phase variation between northern and southern salmon stocks.
Long‐term climate forcing of European herring and sardine populations
Authors: ALHEIT JÜRGEN , HAGEN EBERHARD
Published: November 2003

Records of the herring, Clupea harengus, fishery off the Swedish coast of Bohuslän, in the Skagerrak, date back to the 10th century. Nine periods, each lasting several decades, are known during which large quantities of herring were caught close to the shore. In the 1895–96 season, more than 200 000 tonnes were landed. During the `interim' periods, which stretched over 50 or more years, the herring fishery played little role in the economy of this region. Several other herring fisheries in European waters overlap with recent Bohuslän periods whereas the Norwegian spring‐spawning herring and some sardine, Sardina pilchardus, fisheries exhibit alternating periods. A study of the climatological/hydrographic scenario of all Bohuslän periods and those of herring in the English Channel and the Bay of Biscay showed that, on a decadal scale, they coincided with times when there was a strong ice cover off Iceland, severe winters in western Europe with extremely cold air and water temperatures, a reduction of westerly winds as indicated by negative anomalies in the North Atlantic Oscillation (NAO) index and a minimum of south‐westerly winds over England in response to meridional migrations of the belt of westerly winds. Periods of the Norwegian spring‐spawning herring and sardines in the English Channel coincided with inverse climatological/hydrographic situations. It is concluded that climate variation governed the alternating herring and sardine periods.
Prey selectivity and diel feeding chronology of juvenile chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon in the Columbia River plume
Authors: Schabetsberger R. , Morgan C. A. , Brodeur R. D. , Potts C. L. , Peterson W. T. , Emmett R. L.
Published: November 2003

We studied salmon feeding selectivity and diel feeding chronology in the Columbia River plume. Juvenile chinook and coho salmon were caught by trawling at 2–3 h intervals throughout a diel period on three consecutive days (21–23 June 2000) at stations located 14.8 and 37 km offshore from the mouth of the Columbia River. A total of 170 chinook salmon were caught at the inshore and 79 chinook and 98 coho salmon were caught at the offshore station. After each trawl, potential prey were sampled at different depths with 2–3 different types of nets (1‐m diameter ring net, bongo net, neuston net). Despite the variability in zooplankton abundance, feeding selectivity was surprisingly constant. Both salmon species fed selectively on larger and pigmented prey such as hyperiid amphipods, larval and juvenile fish, various crab megalopae, and euphausiids. Hyperiid amphipods were abundant in the salmon diets and we hypothesize that aggregations of gelatinous zooplankton may facilitate the capture of commensal hyperiid amphipods. Small copepods and calyptopis and furcilia stages of euphausiids dominated the prey field by numbers, but were virtually absent from salmon diet. Juvenile chinook salmon, with increasing body size, consumed a larger proportion of fish. Stomach fullness peaked during morning hours and reached a minimum at night, suggesting a predominantly diurnal feeding pattern. In general, both chinook and coho salmon appear to be selective, diurnal predators, preying mostly on large and heavily pigmented prey items, in a manner consistent with visually oriented, size‐selective predation.
Tracking environmental processes in the coastal zone for understanding and predicting Oregon coho (Oncorhynchus kisutch) marine survival
Authors: Logerwell E.A. , Mantua N. , Lawson P.W. , Francis R.C. , Agostini V.N.
Published: November 2003

To better understand and predict Oregon coho (Oncorhynchus kisutch) marine survival, we developed a conceptual model of processes occurring during four sequential periods: (1) winter climate prior to smolt migration from freshwater to ocean, (2) spring transition from winter downwelling to spring/summer upwelling, (3) the spring upwelling season and (4) winter ocean conditions near the end of the maturing coho's first year at sea. We then parameterized a General Additive Model (GAM) with Oregon Production Index (OPI) coho smolt‐to‐adult survival estimates from 1970 to 2001 and environmental data representing processes occurring during each period (presmolt winter SST, spring transition date, spring sea level, and post‐smolt winter SST). The model explained a high and significant proportion of the variation in coho survival (R2 = 0.75). The model forecast of 2002 adult survival rate ranged from 4 to 8%. Our forecast was higher than predictions based on the return of precocious males (‘jacks’), and it won't be known until fall 2002 which forecast is most accurate. An advantage to our environmentally based predictive model is the potential for linkages with predictive climate models, which might allow for forecasts more than 1 year in advance. Relationships between the environmental variables in the GAM and others (such as the North Pacific Index and water column stratification) provided insight into the processes driving production in the Pacific Northwest coastal ocean. Thus, coho may be a bellwether for the coastal environment and models such as ours may apply to populations of other species in this habitat.
The use of Percent Model Affinity to assess the effects of agriculture on benthic invertebrate communities in headwater streams of southern Ontario, Canada
Authors: BARTON DAVID
Published: November 2003

1. Benthic invertebrates were surveyed in 213 headwater streams in southern Ontario to test the utility of Percent Model Affinity (PMA) for assessing the relative impact of agriculture on invertebrate communities. Qualitative samples were collected from reference (= forested, sixty‐nine samples) and agricultural (195 samples) streams between May 1990 and August 1993. The percentage composition of samples from reference streams was averaged by season and type of substratum, and the most homogeneous groupings were used as expected communities for the calculation of PMA.

2. The lower confidence limit about the mean PMA for each expected community was calculated as for a one‐tailed t‐test comparing a single observation with the mean of a sample. Samples from agricultural streams whose PMA scores were outside these confidence limits were judged to indicate significant impact.

3. The sensitivity of PMA to differences in land use increased with taxonomic resolution: 40% of agricultural sites were significantly different from reference communities at the ordinal level, 79.5% were significantly different at the lowest practical taxonomic level. Most of the increase in discrimination between reference and agricultural sites was due to Chironomidae. Identification to species, rather than genus, yielded only a small increase in discrimination between reference and agricultural sites, largely because of taxonomic difficulties and because most members of several diverse genera were very similar in their distributions.

4. All of the samples from streams which drained orchards and vinyards (n = 7) were significantly different from the expected communities, as were 89% of those from streams draining corn (n = 94). A larger percentage of streams draining hay (78%,

n = 26) were impacted than were streams bordered by pasture (66%, n = 51), but the average magnitude of impact was greater in pasture streams. Only six of eleven samples from streams draining tobacco indicated significant impact.

5. The effect of land use adjacent to the sampling site was greatest early in the growing season. Comparison of natural channels and ditches draining agricultural land suggests that channelization and subsurface tile drainage eliminate most of the benefits of riparian buffer strips and magnify the effects of farming practices on the benthic fauna.
Seasonal and spatial variation in zooplankton community structure and their relation to possible controlling variables in Lake Okeechobee
Authors: BEAVER JOHN , HAVENS KARL
Published: November 2003

1. Lake Okeechobee is a large (1732 km2), shallow (mean depth 2.7 m), eutrophic, subtropical lake located in southern Florida. Approximately 25% of the lake surface area is occupied by an extensive littoral zone. From August 1988 to June 1992, ≈ 2000 zooplankton samples were collected throughout the lake.

2. During the study period, a severe drought lowered lake levels more than 1 m. At low and normal lake stage, the average lake‐wide abundance of rotifers (c. 2000 l–1) was greater than during high water periods (c. 1600 l–1). The average abundance of adult crustaceans (cladocerans and copepods) (c. 30 l–1) varied little regardless of lake stage.

3. Although only minor differences were apparent when lake‐wide means in rotifers and adult crustaceans for each lake stage were compared, pronounced differences were evident in the distribution of zooplankton communities within Lake Okeechobee. During high and normal lake stage, both rotifer and adult crustacean populations were more uniformly distributed throughout the lake. At low lake stage, the densest zooplankton populations were concentrated in the transition area between the central lake and the littoral fringe. The abundance of all zooplankton groups was inversely correlated with lake stage, but the relationship was much stronger for rotifers than crustaceans. Both rotifer and crustacean zooplankton population densities were positively related to increased phytoplankton biomass (as measured by chlorophyll a) but the relationship was much stronger for rotifers than crustaceans.
A hydrogeography of unregulated streams in the United States and an examination of scale‐dependence in some hydrological descriptors
Authors: POFF N.
Published: November 2003

1. Relatively undisturbed streams in continental U.S.A. were classified according to variation in ten ecologically relevant hydrological characteristics. Measures of flow variability and predictability for average conditions, as well as for low‐ and high‐flow extremes, were extracted from long‐term (15–58‐year) daily streamflow data for 806 streams.

2. Two groups of sites were analysed: all 806 sites and a subset of 420 ‘best’ sites. For each group, cluster analysis identified ten distinctive stream types, seven permanent and three intermittent. The geographical clustering exhibited by the stream types indicated regional differences in climatic and geological features. A bootstrapping technique applied to the permanent stream classes showed the majority of them were statistically robust.

3. The derived classification of U.S. streams based on ecologically relevant hydrological characteristics provides a comprehensive catalogue of small to mid‐size streams that, according to ecological theory, may differ in major aspects of their ecological organization. The classification provides a basis for hypothesis generation and affords an objective framework for matching streams for purposes of comparative ecological investigations.

4. A subset of 118 streams from the ten classification groups was selected to determine whether certain hydrological variables often used by ecologists to make cross‐system comparisons are sensitive to the temporal coarseness of the hydrological time series used to derive the variables. The three hydrological variables considered were streamflow predictability, streamflow variability and flood timing.

5. Streamflow predictability (using Colwell’s Index) was calculated at daily, weekly, monthly and seasonal time scales. Estimates of predictability showed either no change across time scales, a gradual and consistent increase across time scales, or a maximum value at the monthly time scale. Coefficient of variation of streamflow was calculated at daily, weekly, monthly, seasonal, and annual time steps. Daily values were always greatest for all streams. Some groups showed minimum variability at the monthly scale, others at the seasonal. Timing of daily peak flows could be detected with 50–90% accuracy across stream groups using coarser grain monthly and annual hydrographic data.

6. Inferences about hydrological similarity among streams across broad geographical scales can be sensitive to choice of time scale used in the hydrological characterization.
The U.K. Acid Waters Monitoring Network: an assessment of chemistry data, 1988–93
Authors: JENKINS ALAN , BOORMAN DAVID , RENSHAW MIKE
Published: November 2003

1. The current extent and distribution of acidified surface waters in the U.K. is assessed using data collected between 1988 and 1993 as part of the U.K. Acid Waters Monitoring Network (AWMN). The data have been collected against a background of constant S deposition across the U.K. throughout the monitoring period.

2. Surface waters with mean pH < 5.0 occur in south‐west Scotland, south‐east Northern Ireland, the Lake District and south‐east England. In addition, sites with mean pH < 5.5 are found in central and north‐east Scotland, the Pennines and north Wales. These areas receive, and have historically received, high levels of S deposition and xSO4 is identified as the main contribution to non‐marine acid anion concentrations.

3. Mean total oxidized nitrogen (TON) concentrations are > 10 μeq l–1 at ten of the twenty‐two sites which are found in areas broadly corresponding to areas of high N deposition. The mean contribution of TON to total non‐marine acid anion concentration is > 10% at sixteen sites and > 20% at six sites. Most sites, except for those with TON < 10 μeq l–1 and mainly located in north‐west Scotland and Northern Ireland, demonstrate strong seasonality in TON concentrations and during autumn and winter instantaneous contributions of TON to total non‐marine acid anions is up to 60%. This confirms the importance of N with respect to surface water acidification and strengthens the case for implementation of controls on future N emissions.

4. The data period is currently too short for rigorous trend analysis given the monthly/quarterly sampling frequency in streams/lakes, respectively. Nevertheless, increased SO4 and xSO4 can be observed in the time series at sites in south‐west Scotland and north Wales despite constant deposition to these areas. Similar trends are evident in longer‐term more frequent data at two sites and may reflect hydrometeorological influences, especially the duration and intensity of hot and dry weather in summer which promotes mineralization of organic S.
Palaeolimnological evidence for recent chemical and biological changes in U.K. Acid Waters Monitoring Network sites
Authors: JUGGINS S. , FLOWER R.J. , BATTARBEE R.W.
Published: November 2003

1. Palaeolimnological evidence is presented for the long‐term (post‐1850) and recent (post‐1970) trends in acidity of eleven sites in the U.K. Acid Waters Monitoring Network.

2. Sites are located throughout the U.K. in areas sensitive to acidification, and results show that all have been acidified since pre‐industrial times. Although there is considerable variation in the timing and magnitude of these changes the results are consistent with other evidence of the widespread and severe acidification of sensitive U.K. freshwaters as a result of acidic deposition.

3. The most severely acidified sites generally have the highest critical load exceedance, although there is a only a poor relationship between exceedance and post‐1850 pH change (r = 0.58, P = 0.06) or diatom floristic change (r = 0.52, P = 0.1). These results highlight the difficulty of inferring biological change or ‘damage’ in freshwater ecosystems from current national maps of critical load exceedances.

4. Evidence of chemical and biological response to the post‐1970 reduction in U.K. S emissions is variable: seven lakes show continued acidification in the 1970s and early 1980s while four appear to have been in steady state. One afforested site shows continued acidification until at least 1990, the year of coring, suggesting that at this site increased scavenging of acid anions following canopy closure and/or increased nitrate leaching have offset the benefits of reduced S deposition.

5. Five sites appear to have been in steady state since the early to mid‐1980s, and two show unambiguous evidence for a recent increase in pH and a reversal in the diatom assemblages to that of earlier levels. The results support and extend the findings of other studies and show that biological recovery is occurring in at least two chronically acidified areas of the U.K. (Galloway, SW Scotland, and north Wales), and that natural recovery can lead to the re‐establishment of biota previously present at the site.
Size‐dependent changes in phytoplankton C and N uptake in the dynamic mixed layer of Lake Biwa
Authors: FRENETTE JEAN‐JACQUES , VINCENT WARWICK , LEGENDRE LOUIS , NAGATA TOSHI
Published: November 2003

1. Short‐term (days) hydrodynamic effects of wind‐induced mixing on phytoplankton size structure, and C and N uptake characteristics, were studied in the surface mixed layer (epilimnion) of Lake Biwa (North Basin), before and during a period of high winds (typhoons).

2. The latter period was characterized by two major typhoon events associated with deepening of the seasonal thermocline, reduced water column stability, decreased underwater irradiance and increased dissolved reactive N and particulate P.

3. Nutrient concentrations, seston C/N ratios, and uptake rates indicated that phytoplankton biomass and production were limited by P and not N throughout the study. Higher C‐ and N‐based productivity during the typhoon period than before reflected the increased phytoplankton biomass and higher specific uptake rates due to increased nutrient supply.

4. Changes in the size‐structure of phytoplankton (< 2 and > 2 μm) were associated with variations in the stratification and mixing regime. When vertical stability was high (before the typhoons) concentrations of > 2 μm biomass (chlorophyll a, particulate organic C and N) were higher at the bottom of the mixed layer than at the surface whereas, when stability of the mixed layer was low (the typhoon period), the contribution of picoplankton (< 2 μm) to total Chl a increased at the surface and decreased at the bottom following the first high winds.

5. Photoadaptive adjustments of the phytoplankton provided further evidence of hydrodynamic control. The lower intracellular Chl a concentrations and C and N uptake efficiencies in the < 2 μm fraction suggest that they experienced, on average, higher irradiance than the larger cells because of their lower sinking rates. During the stability period, picoplankton exhibited higher photosynthetic efficiencies at the bottom of the mixed layer than at the surface. Such differences disappeared during the typhoon period indicating that the mixing rate was then probably higher than the photoacclimation rate in the small size fraction.

6. The present results stress the highly transient nature of biological homogeneity in the surface mixed layer of the lake.
Colonization dynamics of river benthos in response to local changes in bed characteristics
Authors: WILLIAMS D. , SMITH MARILYN
Published: November 2003

1. The effects of small‐scale variation in the nature of a river bed upon the colonization dynamics of benthic invertebrates were explored through a series of microhabitat implants placed in a small river in southern Ontario. The implants presented variations in the levels of four microhabitat features: substrate particle size, current regime, added food and light. The sixteen possible combinations of these variables were replicated five times, for a total of eighty implants.

2. After 14 days of exposure, some of the controlled variables had been modified and a number of other variables (sand, fine (FPOM) and coarse (CPOM) particulate organic matter) had been added by the river.

3. Benthic diversity differed among the implants, with sand and CPOM accounting for 15.5% of the variation. Benthic density was more uniform and was weakly related only to added food. Application of the CANOCO program to the data set suggested that, in general, the environmental variables manipulated in this study were not the key factors influencing colonization, although they may have been important in non‐linear combination.

4. A number of important positive associations were evident among colonizing species, some of which are thought to reflect those of predator and prey.

5. Control of colonization appears to be through multiple factors (abiotic, biotic, autochthonous, allochthonous) that differ in nature according to species, and which underline the complexity of selection responses of individual taxa to local variations in bed structure. Researchers must acknowledge this multifactorial organization and dynamic nature if workable models of lotic community structure and development are to be forthcoming.
Comparison of leaf processing rates under different temperature regimes in three headwater streams
Authors: ROWE JACQUELYN , MEEGAN SEAN , ENGSTROM S. , PERRY SUE , PERRY WILLIAM
Published: November 2003

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).

2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12‐week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day–1) and kd (degree day–1).

3. There were no significant differences in processing rates (k day–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.

4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.
Inland dispersal of adult aquatic insects
Authors: KOVATS ZSOLT , CIBOROWSKI JAN , CORKUM LYNDA
Published: November 2003

1. Adult caddisflies (Trichoptera) and mayflies (Ephemeroptera) were light‐trapped on summer evenings along the Detroit River and Lake St. Clair, near Windsor, Ontario, Canada. Light traps were located at the shore and at increasing distances inland up to 5 km, and were operated simultaneously for 2 h following sunset. Catches of five species of caddisflies of the family Hydropsychidae (Cheumatopsychecampyla, Cheumatopsychespeciosa, Hydropsychehageni, Hydropsychephalerata, Macrostemumzebratum) and the mayfly Hexagenia (Ephemeridae) were used to examine inland distribution.

2. Inland dispersal was limited: catches of caddisflies declined at a greater than exponential rate with increasing distance from shore. Mean dispersal distance from the shoreline ranged from 650 to 1845 m. Smaller caddisfly species dispersed shorter distances than larger caddisflies and Hexagenia.

3. Inland distribution of adult caddisflies exhibited considerable interspecific variation: distribution was inconsistent among trials for Hexagenia, possibly owing to timing of collections in relation to periods of peak emergence.

4. Sex ratios of caddisflies were female biased at most sites. No consistent bias was observed for Hexagenia. Different inland distribution patterns were observed for males and females. The differences appeared to reflect species‐specific reproductive strategies.
The effect of habitat‐specific sampling on biological assessment of water quality using a predictive model
Authors: PARSONS MELISSA , NORRIS RICHARD
Published: November 2003

1. Detection of impairment in macroinvertebrate communities using rapid biological assessment depends on the ability to compare sites, with confidence that differences obtained result from water quality. However, collections from more than one habitat type may introduce variation that can potentially mask water quality differences among sites. Data were collected from the riffle, edge, pool‐rock and macrophyte habitats at reference (minimally disturbed) and test (disturbed) stream sites throughout the Australian Capital Territory. The effect of habitat‐specific sampling on predictive models for detecting impairment in macroinvertebrate communities was determined. Four models were used: riffle only, edge only, each habitat as an individual object, and all habitats sampled at a site considered as a composite sample.

2. Macroinvertebrates from individual habitats generally clustered into separate groups because collections from the same habitat at different sites were more similar than collections from different habitats within a site. Thus, in the habitats as individual objects model, the taxa predicted to occur at a test site may be an indication of habitat type rather than water quality. The outputs of the composite habitats and riffle and edge models were similar. However, the variable number of habitats included at each site in the composite model may confound the detection of biological impairment because of unequal sampling effort. The riffle and edge models were the most robust because they were less confounded by inter‐habitat variation and were based on comparisons made between equivalent environmental units.

3. Comparison of observed/expected taxa ratios for test sites showed that each model could detect biological impairment, indicating considerable data redundancy was introduced by sampling several habitats. In particular, the pool‐rock and macrophyte habitats contributed no information with regard to macroinvertebrate taxon occurrence or detection of biological impairment that could not be obtained from either the riffle or edge habitats within the study area.
Artificial mixing prevents nuisance blooms of the cyanobacterium Microcystis in Lake Nieuwe Meer, the Netherlands
Authors: VISSER PETRA , IBELINGS BAS , VAN DER VEER BART , KOEDOOD JAN , MUR R.
Published: November 2003

1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successful in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcystis per litre and also per m2 was lower than in the two preceding control years. Hardly any nuisance scums of Microcystis occurred in the lake.

2. The phytoplankton shifted from a cyanobacteria‐dominated community in summer to a mixed community of flagellates, green algae and diatoms. Reduced sedimentation losses in the mixed lake, probably in combination with a lower pH, favoured non‐buoyant algae, while the entrainment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy.

3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences between the years. The chlorophyll content integrated through depth (m–2) increased in the artificially mixed lake.

4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 resulted in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction of 75% of the energy costs, while the mixing was still sufficient to prevent stratification.

5. Determination of the buoyancy state of the colonies on a sunny and calm day showed that the buoyancy loss was low close to the bubble plumes, and high at some distance from these plumes. This suggests that Microcystis could escape the mixing at some distance from the plumes, and could synthesize more carbohydrates during its stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy state appeared to be a good and simple method to investigate the extent of entrainment of colonies in the turbulent flow.
Endemism and ecological islands: the ostracods from Jamaican bromeliads
Authors: LITTLE TOM , HEBERT PAUL
Published: November 2003

1. The inhabitants of ecological islands, such as the phytolemata of bromeliad plants, provide an opportunity to examine the genetic patterns resulting from island radiations and draw inferences about modes of speciation.

2. Allozyme electrophoresis, as well as mitochondrial DNA and morphological analyses, were employed to delimit species boundaries and assess the population genetic structure of the ostracods from bromeliads on the island of Jamaica.

3. Nine species from the genus Elpidium were both sexually reproducing and endemic to the island. The other commonly encountered ostracod genus, Candonopsis, was represented by at least two sexually reproducing species.

4. Most Elpidium species showed very restricted distributions, low heterozygosity, and marked gene pool fragmentation, suggesting that population bottlenecks have occurred frequently. Whether founder events played a causal role in the diversification of these ostracods remains uncertain, but bromeliad habitats seem to be a source of high biodiversity, and may represent an ideal setting for extensive allopatric speciation.
Spatial and temporal variation of microbial respiration rates in a blackwater stream
Authors: FUSS CAROLYN , SMOCK LEONARD
Published: November 2003

1. The extent of spatial and temporal variation of microbial respiration was determined in a first‐order, sand‐bottomed, blackwater stream on the coastal plain of south‐eastern Virginia, U.S.A.

2. Annual mean respiration rates (as g O2 m–3 h–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.

3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.

4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.

5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O2 m–2 yr–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non‐woody detritus, stored in the stream during the year was lost to microbial respiration.
Temperature‐dependent growth and life cycle of Deleatidium (Ephemeroptera: Leptophlebiidae) in two high‐country streams in New Zealand
Authors: HURYN D.
Published: November 2003

1. Measurements of larval growth rates were used to produce an empirical model for examining factors influencing patterns of size structure and emergence period for populations of the leptophlebiid mayfly Deleatidium (lillii group) in two high‐country streams (South Island, New Zealand). Growth rates were measured in situ by enclosing groups of larvae and natural stream substrata in growth chambers.

2. Multiple regression analysis showed that temperature explained most variation among growth rates in both streams. Growth rates were also significantly related to larval size, although the effect was minor compared with temperature. A significant relationship between larval biomass per chamber and growth rate was shown at only one site.

3. Growth models based on multiple regression equations (R2 = 0.70–0.84) simulated the extended emergence period and complex size structure observed for populations of Deleatidium in the field. Larvae hatching from eggs deposited before mid‐February (austral summer) probably emerge as adults before May (minimum cohort duration ≈ 3 months). Larvae hatching after mid‐February probably do not emerge as adults until the following summer because of low growth rates during winter (maximum cohort duration ≈ 11 months).

4. On average, there are probably two generations of Deleatidium per year (bivoltine) at the study sites. The presence of numerous overlapping cohorts throughout summer, however, results in an extended emergence period and complex size structure.
Experimental studies of zooplankton–phytoplankton–nutrient interactions in a large subtropical lake (Lake Okeechobee, Florida, U.S.A.)
Authors: HAVENS KARL , EAST THERESE , BEAVER JOHN
Published: November 2003

1. Over a 1‐year period, twenty controlled experiments were performed using small mesocosms (20‐l clear plastic carboys) and plankton communities collected from four sites in shallow, subtropical Lake Okeechobee, Florida. In replicated treatments, macrozooplankton grazers were excluded by size fractionation (115 μm), and/or nutrients (N and P) were added, and impacts on phytoplankton biomass and productivity were measured after 3‐day incubations.

2. In most experiments (fifteen out of twenty), there was no significant effect of zooplankton exclusion on phytoplankton biomass or productivity, but there were significant increases in those attributes due to nutrient additions. The magnitude of the responses was a function of light availability at the collection sites.

3. In three experiments, zooplankton exclusion led to declines in phytoplankton biomass and productivity, suggesting that animals may sometimes have net positive effects on the phytoplankton, perhaps via nutrient recycling.

4. In only two experiments was there evidence of net negative impacts of grazers on the phytoplankton. In both instances, cladocerans (Daphnia ambigua and Eubosmina tubicen) were dominant in the zooplankton. However, the increases in chlorophyll a due to zooplankton exclusion were small (5–20%), probably because of the small size and relatively low grazing rates of the cladocerans.

5. The results support the hypothesis that phytoplankton biomass in Lake Okeechobee is little affected by herbivorous macrozooplankton. This may be a common feature of lowland tropical and subtropical lakes.
The effects of acid episodes on Lepomis sunfish recruitment and growth in two ponds in Massachusetts, U.S.A.
Authors: STALLSMITH BRUCE , EBERSOLE JOHN , HAGAR WILLIAM
Published: November 2003

1. The effects of transient acidity on the recruitment of Lepomis sunfish were examined in two ponds in Plymouth County, Massachusetts with on‐site monitoring systems. One pond had low buffering capacity and was sensitive to acidification and acid spikes, the other had better buffering with no acid spikes. Pond pH was continuously monitored over 3 years.

2. Young‐of‐the‐year (YOY) sunfish were collected and through otolith analysis aged in days since larval metamorphosis. Day‐classes were constructed from these age data. The YOY swim‐up day‐class distribution was more irregular with pronounced gaps in Maquan Pond than in the better‐buffered Furnace Pond.

3. The poorly buffered pond, Maquan, experienced irregular acid spikes, defined as a sudden lowering of pond pH from about 6.6 to below 5.3 for an hour or more. These spikes were associated with rainfall (P < 0.005). Acid spikes were not found at Furnace Pond.

4. The occurrence of acid spikes and diminished day‐classes from expected swim‐up values were significantly associated, based on pooled data from 3 years at Maquan Pond.

5. Transient acid spikes at Maquan Pond appear to harm Lepomis YOY at the vulnerable stage of metamorphosis, when their gills become fully functional and exposed to the environment.

6. Growth rates of Lepomis YOY were also studied. Length–age and weight–length growth relationships were examined for differences in instantaneous growth coefficients between years and between ponds. Length–age instantaneous growth coefficients ranged from 0.0114 to 0.0176 over five pond‐years. The slowest growth rates were in a warm, dry spawning season with the earliest average metamorphosis date.
Litter breakdown in streams of the Agüera catchment: influence of dissolved nutrients and land use
Authors: MOLINERO J. , POZO J. , GONZALEZ E.
Published: November 2003

1. The breakdown of oak (Quercus robur L.), chestnut (Castanea sativa Miller) and eucalypt (Eucalyptus globulus Labill.) litter enclosed in 5‐mm mesh bags was compared between first‐order headwaters (two with native riparian forest and two with eucalypt plantations) and a third‐order reach of Agüera stream. Weight loss and dynamics of phosphorus and nitrogen in litter were studied for a period of 155 days.

2. Among the different sites, processing rates ranged from 0.0045 to 0.0080 day–1 for chestnut leaf litter, from 0.0036 to 0.0051 day–1 for oak, and from 0.0027 to 0.0158 day–1 for eucalypt.

3. The availability of nutrients in water clearly influenced nitrogen and phosphorus dynamics in litter. In headwater reaches, net immobilization was not observed and losses of phosphorus and nitrogen followed mass loss. However, there was an enrichment of litter at the low reach, where influence of human settlements—located upstream—could lead to a greater availability of phosphorus in water.

4. The enhancement of litter decay by the exogenous nutrient supply depended on leaf quality, as only the processing rate of eucalypt increased at the nutrient‐rich site.

5. The processing rates differed little among headwaters, suggesting that riparian forest type, i.e. deciduous forest v eucalypt plantations, did not affect litter decay in the stream.
Top‐down effects of underyearling fish on a phytoplankton community
Authors: KURMAYER RAINER , WANZENBOCK JOSEF
Published: November 2003

1. The influence of underyearling (0+) fish on planktonic algal biomass and composition was examined during June–August in eight large enclosures (40 m3). Averaged for the entire period, Daphnia densities, transparency and phosphate concentrations were significantly higher (P < 0.05) and chlorophyll a values significantly lower in fishless controls than in enclosures with fish (bleak (Alburnus alburnus), roach (Rutilus rutilus) or perch (Perca fluviatilis)).

2. Enclosures without fish demonstrated a more heterogeneous algal community structure than enclosures with fish. Desmidiales and dinoflagellates were more abundant in controls, whereas green algae dominated the phytoplankton community in enclosures with fish. Larger grazing‐resistant algae occurred most frequently in enclosures without fish.

3. The importance of cascading trophic interactions was demonstrated even under strong eutrophic conditions. The strongest effects on Daphnia densities, phytoplankton biomass and nutrients were observed in enclosures with perch, with weaker effects in enclosures with cyprinids. Differences in initial fish size and species accounted for the top‐down effects observed. The results of this study emphasize the importance of 0+ perch as a key predator in structuring the plankton community in lakes.
Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds
Authors: ZANETELL BROOKE ANN , PECKARSKY BARBARA
Published: November 2003

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies (Megarcys signata) on fine sediment deposition and removal.

2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.

3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.

4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream‐side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.

5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.

6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.
Clonal variation in the survival and reproduction of Daphniapulicaria under low‐food stress
Authors: EPP GREGORY
Published: November 2003

1. Survivorship and reproductive parameters of eight Daphniapulicaria clones were evaluated in a life‐table experiment under four dietary regimes ranging from food limitation to complete food deprivation.

2. D. pulicaria exhibited both diet‐independent interclonal differences and genotype–environment interactions in life‐span.

3. Reproduction ceased entirely or nearly so under the three treatments with lowest food availability. Under life‐long food limitation, differences between clones were evident in clutch sizes and age at first reproduction. Some clones produced ephippia at a markedly higher frequency than others under food‐limiting conditions.

4. For most of the life‐history traits evaluated, within‐lake variation among clones exceeded differences between the two lake populations they represented.

5. Intraspecific variations in response to periods of food deprivation and to extended quantitative or qualitative food‐limitation could affect the clonal composition of Daphnia populations. They are therefore of potential importance in determining the effect of altered phytoplankton assemblages on zooplankton communities.
Food effects on life history traits and seasonal dynamics of Ceriodaphnia pulchella
Authors: BOERSMA MAARTEN , VIJVERBERG JACOBUS
Published: November 2003

1. This paper describes the effects of differences in food quantity and quality on selected life history traits of Ceriodaphnia pulchella. Animals were fed with four concentrations of two green algae, Scenedesmus obliquus and Chlamydomonas globosa, given separately as well as in a 1 : 1 mixture.

2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase, r, increased with food concentration. Life history parameters of animals raised on S. obliquus were lowest, whereas no significant differences were found between animals fed with C. globosa and with the mixture of the two algal species.

3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics of C. pulchella in Lake Tjeukemeer showed that the effect of changes in food conditions during the growing season of this species was limited, although overall food levels seemed to be poor throughout the year.

4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predator Leptodora kindtii.
The role of photosynthesis/light relationships in determining lower depth limits of Characeae in South Island, New Zealand lakes
Authors: SCHWARZ A.‐M. , I. HAWES I. , HOWARD‐WILLIAMS C.
Published: November 2003

1. The maximum depth of colonization of aquatic macrophytes (Zc) was investigated in eighteen South Island, New Zealand lakes. The downward attenuation coefficient for photosynthetically active radiation (Kd(PAR)) was calculated and the spectral characteristics of the lakes determined with a spectroradiometer.

2. Characean algae dominated the deepest communities in sixteen of the study lakes.

3. Zc was significantly related to Kd(PAR) by the relationship Zc = 4.5/Kd– 2.2.

4. From measurements of the photosynthetic properties of Chara corallina (Kl. ex Willd., em R.D.W.) and incident radiation over the course of a year we calculated the depth at which daily net photosynthesis would be equal to zero for each day of the year. An annual average of this depth was significantly related to Zc with anr2 of 0.86.

5. Correcting Kd(PAR) for spectral quality and taking into account the potential absorption spectrum of a characean meadow did not improve the relationships.

6. We suggest that relationships established between Kd(PAR) and Zc of characean algae in South Island, New Zealand lakes can be explained to a great extent by light limitation of photosynthesis.