bluuebunny/crossref_metadata_2025_split huggingface dataset:
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.
Variable egg development of Dinocras spp. (Plecoptera, Perlidae) and the stonefly seed bank theory
Authors: ZWICK PETER 
Published: November 2003

1. Temperature dependence of egg development of Dinocras cephalotes (Curtis) (three German and one Norwegian population) and Slovenian D. megacephala (Klapálek) was studied under a constant 14 : 10 light : dark photoperiod and constant temperature ranges of 4–24 °C and 4–18 °C, respectively. D. cephalotes was also incubated under seasonal field conditions; natural daylength and fluctuating temperatures had no modifying effect.2. Both species have very similar lower threshold temperatures (4 and 3.5 °C, respectively), thermal demand for development (c. 600 degree days) and high dependence of mean incubation period on temperature (exponents of regressions near 1.5). Present data on D. cephalotes agree with the literature on British and Norwegian material of the same species.3. Development occurs only at cue temperatures above the lower threshold. Cue temperatures range from 6 °C (some D. megacephala) to 14 °C (some D. cephalotes) and vary strongly within and between egg masses of D. cephalotes. Variation is not random, but seems to be genetically determined.4. The variable temperature response renders study of effects of particular experimental regimes, and comparisons between local populations, difficult.5. A latitudinal gradient in cue temperatures for development from 6 °C at c. 46 °N to 12 or even 14 °C at c. 61 °N seems to reflect reduced diversity at high latitudes.6. Average success of spontaneous hatching exceeded 90% between 12 and 20 °C, but declined towards higher and lower temperatures.7. Unhatched eggs were not dead but in parapause; development at other, higher or lower, temperatures was induced. Spontaneous plus induced hatching success approached 90%. Developing eggs rarely died; most dead eggs were apparently unfertilized.8. Dormant plecopteran eggs are proposed to form a seed bank in stream bed sediments. Highly successful development after up to 220 days of dormancy was ascertained in Dinocras, and survival for up to 3 years is reported for other Perloidea.9. Only systellognathan egg morphology provides options for long dormancy; the other plecopteran superfamilies, notably Nemouroidea, follow different strategies.
Acid‐induced acute toxicity of aluminium to three species of filter feeding caddis larvae (Trichoptera, Arctopsychidae and Hydropsychidae)
Authors: VUORI K.‐M. 
Published: November 2003

1. Arctopsyche ladogensis, Hydropsyche angustipennis and Hydropsyche siltalai larvae were exposed to nominal aluminium concentrations of 0, 625, 1250, 2500 and 5000 μg Al l–1 at pH 5.0 for 96 h. Larvae reared at pH 6.4 and without any aluminium treatment were used as controls. Morphological abnormalities in the anal papillae of the larvae were used as the response variable in estimating the median effective concentrations (EC50) of aluminium.2. No morphological abnormalities were observed in the control larvae. Only a few individuals of A. ladogensis had darkened anal papillae at pH 5.0 without additional aluminium treatment, whereas increasing aluminium concentrations significantly increased the number of individuals in all species displaying darkening and reduction of the papillae.3. A. ladogensis appeared to be the most sensitive species to increasing aluminium concentrations, as reflected by the significantly lower mean EC50 value for this species compared with those of H. siltalai and H. angustipennis. H. angustipennis larvae were the most tolerant to aluminium.4. The results indicate that interspecific differences in sensitivity to aluminium may be a key factor influencing the guild structure of filter feeding caddis flies in acidified streams. The results also imply that toxic effects of aluminium on filter feeding caddis larvae occur due to the impairment of normal osmoregulation processes via damage to the ion‐regulatory organs.
Gemmules of sponges from a warm lake
Authors: ILAN MICHA , DEMBO GAL , GASITH AVITAL 
Published: November 2003

1. Two types of gemmules were found, each in a different species of sponge, from the warm monomictic Lake Kinneret: (i) clustered gemmules, sharing the pneumatic layer of the gemmular capsule and resembling gemmules of Eunapius; (ii) gemmules that develop non‐synchronously, containing amphidisc spicules within the gemmular capsule and resembling those of Ephydatia. Algal cells were not detected within either type of gemmule although they exist in the developed sponges.2. Sponges began producing gemmules in the lake with the onset of lake drawdown and ceased when lake level was minimal. The gemmules hatched when the lake level began to rise.3. Under experimental conditions gemmules hatched between 13 and 35 °C. Germination was optimal at 20–25 °C. Chilling of young gemmules prior to incubation at 25 °C improved germination rate.4. The percentage of germinating dry gemmules diminished 4–6 months after their collection from the lake. None germinated after 10 months. Submerged gemmules maintained high viability with ageing (up to 100% germination 18 months after collection). Desiccation influenced gemmule viability over time, by both decreasing the percentage of germinating gemmules and increasing the lag time before onset of germination.5. Gemmules kept in the dark germinated significantly less than those illuminated for 12 h day
Atyid shrimps (Decapoda: Atyidae) influence the spatial heterogeneity of algal communities over different scales in tropical montane streams, Puerto Rico
Authors: PRINGLE CATHERINE 
Published: November 2003

1. Atyid (Decapoda: Atyidae) shrimps influence the distribution of algal communities over different scales in tropical montane streams of Puerto Rico. Within pools of an atyid‐dominated stream, atyid shrimps enhanced patchiness in algal communities along the depth gradient. Algal bands occurred in shallow pool margins where atyids did not forage (< 3 cm below water surface), with significantly greater standing crop, taxon richness, and structural complexity than deeper areas. In deeper water, atyids reduced small‐scale patchiness in algal community composition and maintained a low‐growing understorey turf dominated by sessile diatoms (Bacillariophyta) and, sometimes, closely cropped, filamentous blue‐greens (Cyanophyta).2. Among pools of the atyid‐dominated stream, atyids interacted with light to determine algal patchiness between stream margins and deeper areas. In sunny pools, algal standing crop was 140‐fold greater in pool margins than in deeper areas where atyids foraged. In shaded pools, however, standing crop in pool margins was only 5‐fold greater than in deeper areas. Effects of light on algal standing crop were greater outside atyid foraging areas than within, indicating that shrimp grazing overrides the positive effects of light.3. In contrast to the atyid‐dominated stream, algal communities in an atyid‐poor stream were characterized by a high biomass of loosely attached epipelic diatoms and no depth zonation. Interstream rock and shrimp transplant experiments indicated that atyids significantly reduced algal standing crop and altered community composition on rocks from atyid‐poor streams within 24 h. Results support the hypothesis that atyid shrimps play a major role in determining observed interstream differences in algal communities.
Changes in phosphorus and nitrogen cycling following food web manipulations in a shallow Dutch lake
Authors: VAN DER MOLEN DIEDERIK , BOERS PAUL 
Published: November 2003

1. The flow of phosphorus and nitrogen through the food web of the shallow, eutrophic lake Wolderwijd was analysed for 2 different years before and for 1 year after food web manipulation.2. After fish removal the water became clear and the growth of macrophytes began. Fish removal resulted in a significant reduction of the total nutrient pool in the water, but differences between the nutrient cycles before and after the experiment were mainly caused by a gradual change driven by a reduced phosphorus input.3. The zooplankton biomass before and after food web manipulation did not change significantly. Unfavourable food conditions and predation by young fish limited zooplankton biomass after the food web manipulation.4. After fish removal benthic algae, fish, zoobenthos and macrophytes form the largest pools of nutrients apart from the sediment top layer. However, they contribute only little to nutrient cycles in the water column.
Interspecific competition and habitat selection by the riverine dragonfly Onychogomphus uncatus
Authors: SUHLING FRANK 
Published: November 2003

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.
Temperature‐mediated dynamics of planktonic food chains: the effect of an invertebrate carnivore
Authors: BEISNER BEATRIX , MCCAULEY EDAWRD , WRONA FREDERICK 
Published: November 2003

1. This study involves an examination of two‐ and three‐trophic‐level food chains at two temperatures (18 and 25 °C) in order to determine how the addition of a carnivore to a predator–prey system can alter the dynamics of populations and how this effect may be temperature mediated. The system consisted of phytoplankton, Daphnia pulex and the flatworm Mesostoma ehrenbergii.2. Although the plant–herbivore system is inherently unstable at 25 °C, the addition of the carnivore led to a further destabilization of the Daphnia–algal dynamics at the higher temperature. No destabilization effect of the carnivore was noted at 18 °C. At the lower temperature, all populations persisted and the carnivore induced changes only in the age structure of the Daphnia populations rather than in overall biomass.3. The differential effects of the carnivore at two temperatures can be attributed to shifts in the life history, physiological rates and the reproductive strategy employed by Mesostoma.4. Previous theoretical work has predicted that the addition of a third trophic level to an unstable predator–prey system should stabilize dynamics. Our results indicate that the effect of a carnivore on plant–herbivore dynamics can be significantly affected by ambient temperature.