The formation and degradation of cyanobacterium Aphanizomenon flos-aquae blooms: the importance of pH, water temperature, and day length

Investigation of annual changes in phytoplankton community structure in a small artificial eutrophic pond was carried out from May 2002 to April 2003. A heavy bloom of Aphanizomenon flos-aquae var. klebahnii Elenk. (Cyanobacteria) persisted in most of the water column from June to the end of October. In November, the A. flos-aquae bloom suddenly crashed and green algae were predominant until the end of spring. Weekly monitoring suggested strong involvement of the changes in abiotic factors in the cyanobacterial bloom degradation. To clarify the effects of pH, water temperature, and day length on the growth of A. flos-aquae, laboratory batch experiments were conducted. The results showed that A. flos-aquae could not grow below pH 7.1 and 11°C, and the growth tended to be suppressed under a 10L:14D photoperiod. pH, water temperature, and day length are vital factors in the growth of A. flos-aquae and, additionally, grazing by cyclopoid copepods also seemed important in bloom collapse.     

Differences in feeding selectivity and efficiency between young-of-the-year European perch (Perca fluviatilis) and roach (Rutilus rutilus) — field observations and laboratory experiments on the importance of prey movement apparency vs. evasiveness

Feeding selectivity and efficiency of young-of-the-year European perch and roach were compared under field and laboratory conditions. In laboratory experiments, the importance of prey evasiveness versus prey movement conspicuousness for fish selectivity was evaluated with respect to changing Cladocera/Copepoda prey ratio. Feeding efficiency was additionally investigated in relation to feeding time (5, 10, 20 min) and prey density (approx. 50, 200, 700 ind. L⁻¹). In Říov Reservoir, the diet of both fish species was nearly exclusively composed of crustacean zooplankton. In roach, diet shifted from rotifers and bosminids in May, towards Daphnia sp. and Leptodora kindtii in June and July. Daphnia contributed almost exclusively to the roach diet since June, composing on average more than 94% of total prey. Cyclopoid copepods, occurred in the roach’s diet only on the first sampling date; later on both cyclopoid and calanoid copepods were completely absent. On the other hand, copepods played an important role in the diet of perch. In early and mid-June when their share in the zooplankton was particularly high, copepods contributed by more than 50% to the diet of perch. Although their contribution dropped with their decline in zooplankton in June/July, by the end of July they again comprised about one third of perch’s diet. In both fish species, the increase in numbers of cladocerans in their diet was related to increase in SL. In roach, the numbers of consumed prey were doubled every twenty days during the investigated period. In perch the increase was not so consistent, but significantly higher efficiency of perch was reported on three out of six sampling dates. In laboratory experiments, roach showed a distinct avoidance for copepods and a preference for cladocerans. Both prey categories were only fed non-selectively when they dominated the prey mixture. Perch selectivity was more diversified. Contrary to roach, perch were fed copepods non-selectively on a balanced prey ratio. Further, with an increasing share of Cladocera, a situation resembling that of roach and Copepoda was avoided. However, when the share of copepods in the prey mixture dropped below ten percent, they were consumed non-selectively and with their ongoing decline in the prey mixture their preference even increased. Feeding efficiency differed significantly between perch and roach when foraging on copepods exclusively or on a prey mixture where copepods predominated. In the short time feeding experiment (5 min) with copepods, perch consumed on average 5.9 times more prey than roach. Although roach increased their success with increasing time it was still 1.7 times greater than for perch in the long time feeding experiment (20 min). Total numbers of prey consumed were positively affected by prey density and feeding time. With increasing feeding time, the consumption rate generally declined. With a fourfold increase in feeding time, the numbers of consumed prey increased on average only two times. Only in roach feeding on copepods did the numbers of prey consumed per minute of feeding increase with increasing feeding time. However, the overall numbers were low. Differences in feeding selectivity and efficiency between perch and roach juveniles were found to be significant both in the field and laboratory experiments. In roach, selectivity was determined solely by prey evasiveness. By contrast, perch’s selectivity was influenced by prey movement conspicuousness; prey escape abilities did not play an important role. Perch were more efficient foragers on evasive prey, but its feeding efficiency for non-evasive prey was not lower than that of roach. According to our observations, we suggest feeding behaviour to be responsible for the roach’s inefficiency in capturing evasive copepods.     

Ciliates as a food source for marine planktonic copepods

Copepods of the genusEurytemora, isolated from the Patuxent River, a tributary of Chesapeake Bay, were fed suspensions of the ciliateUronema isolated from the Rhode River, a subestuary of Chesapeake Bay. Grazing by copepods was determined by the decrease in numbers of ciliates, which were monitored by both direct counting and particle size analysis. Results from both methods of analysis showed significant reduction in the numbers ofUronema in the suspension whenEurytemora was present. Survival of copepods with ciliates added as food source was significantly longer than without ciliates. Analysis of field samples collected in the fall showed that ciliates comprised approximately 20% of the total plankton biomass at selected sampling sites. The results of the laboratory and field studies indicate that copepods can feed on ciliates and suggest that, in nature, ciliates may comprise an important source of food for copepods.     

Coexistence of two similar copepod species, <Emphasis Type="Italic">Eudiaptomus gracilis</Emphasis> and <Emphasis Type="Italic">E. graciloides</Emphasis>: the role of differential predator avoidance

Habitat choice in relation to environmental factors of two coexisting calanoid copepod species, Eudiaptomus gracilis and E. graciloides, was studied in a mesotrophic lake and in large indoor mesocosms. Both species and sexes showed pronounced diel vertical migration (DVM) in the field. In 12 m deep mesocosms with free ranging fish DVM was observed and species increased day depth over time. No changes were observed in copepod day depth over time in experiments with fish kairomone. It is hypothesized that fish kairomone acts as an early warning system to copepods which respond by moving deeper, but only as far as the thermocline. For full DVM, a nearby mechanical stimulus is necessary. Thus, as fish go deeper to feed, copepods retreat. The response of copepods to fish predation, in the presence of low and high numbers of Daphnia,shows that copepods effectively use Daphniaas living shields to avoid predation. The two species adopt different vertical migration strategies depending on whether there are high or low numbers of Daphniapresent. A dominant feature of mesocosm experiments was the night time aggregating (lekking) of E. gracilis males at the surface. When the spring and autumn percentages of risk takers in the epilimnion were compared, E. gracilis, particularly males, suffered the greatest cost.     

Using space‐for‐time substitution and time sequence approaches in invasion ecology

1. Invasion biologists use two main approaches to evaluate the effects of non‐native species (NNS) on diversity of native species (DNS), namely space‐for‐time and time approaches. These approaches have pitfalls related to lack of controls: the former lacks pre‐invasion data, while the latter often lacks data from non‐invaded sites. 2. We propose a framework that combines space‐for‐time and time approaches and which should result in more focused mechanistic hypotheses and experiments to test the causes of invasibility and the effects of NNS on DNS. We illustrate the usefulness of our framework using two case studies: one with the submersed macrophyte, Hydrilla verticillata, in reservoir and the other with the fish, Geophagus proximus, in a large river–floodplain system. 3. Hydrilla verticillata invaded sites with DNS similar to that found in non‐invaded sites, indicating that biotic and/or abiotic factors did not influence invasion success; however, DNS increased over time in invaded sites compared with non‐invaded sites, suggesting that H. verticillata facilitated natives. In contrast, G. proximus invaded sites with higher DNS than non‐invaded sites, suggesting that biotic and/or abiotic factors favouring natives were important for invasion success, but DNS increased in invaded and non‐invaded sites over time, indicating that an independent factor contributed to DNS increases. 4. Conclusions from both studies would have been inaccurate or incomplete if the space‐for‐time and time approaches had not been used in combination as proposed in our framework.     

Effects of food limitation and temperature on cladocerans from a tropical Brazilian lake

Food limitation was tested in the laboratory by individual growth and reproduction of two cladoceran species, Ceriodaphnia richardi and Daphnia gessneri, from the shallow tropical Brazilian Lake Monte Alegre. The cladocerans were fed cultivated green alga Scenedesmus spinosus in concentrations of 0.20, 0.10, 0.05, and 0.025 mg C l⁻¹. Higher biomass and growth rates occurred in the two highest-food concentrations; the two lowest ones negatively affected clutch size and first reproduction. The threshold food concentration is lower than 0.025 mg C l⁻¹ and the incipient limiting level is a value between 0.10 and 0.20 mg C l⁻¹. The largest species, D. gessneri, was more sensitive to low food concentrations. The effects of low and high temperatures (19 and 27°C) were evaluated by life table experiments with three cladocerans from the lake—Daphnia ambigua, D. gessneri, and Moina micrura—with no food limitation (1 mg C l⁻¹ of S. spinosus). Higher population growth rates for the three species were found at 27°C; better performance in most life table parameters was observed for the former two species at the highest temperature, D. gessneri being the most sensitive to the lowest temperature. There are indications that temperature is an important abiotic factor that constrains populations of cladocerans for a short period in winter in the lake, when temperature decreases to 18–19°C. However, its influence cannot be separated from a biotic factor such as food, whose effect is stronger in the cool season, when concentrations are lower and contribution of inedible algae is relatively higher.     

Ecological correlates of invasion by <Emphasis Type="Italic">Arundo donax</Emphasis> in three southern California riparian habitats

Arundo donax L. (Poaceae) is an aggressive invader in California’s riparian habitats. Field experiments were conducted to examine invader and site attributes important in early invasion. One hundred A. donax rhizomes were planted along five transects into each of three southern California riparian habitats. Pre-planting rhizome weight was recorded, along with site variables including percent bare ground, litter depth, PAR, soil moisture, soil temperature, incidence of herbivory, native canopy cover, and plant community richness and diversity. A. donax shoot emergence, survival time, and shoot height were recorded for approximately 10 months. The experiment was repeated over three years in different locations within each site. When years and sites were pooled to reveal large-scale patterns, A. donax performance was explained by rhizome weight, soil moisture, bare ground, soil temperature, and herbivory. When each site was considered singly, A. donax was positively correlated with different variables in each location. Species richness was correlated with A. donax performance in only one site. Our results indicate that A. donax establishment in riparian habitats is promoted by both vegetative reproduction and favorable abiotic environmental factors and relatively unaffected by the composition of the native community. The positive response of A. donax to disturbance (bare ground) and high resource availability (soil moisture), combined with a competitive perennial habit suggest that this species takes advantage of a competitive-ruderal life history. The ability of A. donax to respond to different conditions in each site combined with low genetic and phenotypic variation seen in other studies also suggests that a high degree of environmental tolerance contributes to invasion success.     

Effects of the blue-green alga Microcystis aeruginosa on zooplankton competitive relations

Summary Field distribution patterns and laboratory feeding experiments have suggested that blooms of colonial blue-green algae strongly inhibit relatively large-bodied daphnid cladocerans. We conducted laboratory experiments to test the hypothesis that blooms of the colonial blue-green alga Microcystis aeruginosa would shift competitive dominance away from large-bodied daphnid cladocerans toward smaller-bodied cladocerans, copepods, and rotifers. In laboratory competition experiments, increasing the proportion of M. aeruginosa in the algal food supply resulted in a shift from dominance by the relatively largebodied cladoceran Daphnia ambigua to dominace by the copepod Diaptomus reighardi. The small-bodied cladoceran Bosmina longirostris was always numerically heavily dominant over D. ambigua, but its estimated population biomasses were only slightly higher than those of D. ambigua. Daphnia ambigua consistently outcompeted the rotifer Brachionus calyciflorus. Our results demonstrate that blooms of M. aeruginosa can alter zooplankton competitive relations in laboratory experiments, favoring small-bodied cladocerans and copepods at the expense of large-bodied cladocerans. However, contrary to predictions, blooms of M. aeruginosa did not improve the competitive ability of rotifers.     

Temperature-dependent Allee effects in a stage-structured model for <Emphasis Type="Italic">Bythotrephes</Emphasis> establishment

Whether the invasive freshwater cladoceran Bythotrephes longimanus can establish after introduction into a water body depends on several biotic and abiotic factors. Among these, water temperature is important because both development rates and mode of reproduction (parthenogenetic or sexual) in Bythotrephes are influenced by temperature. We built a stage-structured model for the population dynamics of Bythotrephes based on the temperature-dependency of events in its life cycle and used the density of resting eggs at the end of each year to track changes in population density. The model was parameterized using data from published laboratory experiments and data on the Bythotrephes population in Harp Lake, Canada, from 1994 to 2005. The parameterized model was then used to simulate the outcome of invasions with different initial resting egg densities under different temperature regimes. A strong Allee effect emerged from the model, i.e. there is a critical threshold density above which the population can establish and below which it goes extinct. We showed analytically that the existence of an Allee effect arises from the model structure and is therefore robust to the parameter values. An increase in temperature reduces the establishment threshold for introductions in the same year as well as for introductions in the previous years. We therefore hypothesize that climate warming might facilitate Bythotrephes invasions. Finally, we study how the establishment threshold is influenced by the timing of the introduction event and thus identify time periods during the year when lakes may be particularly susceptible to Bythotrephes invasions.     

Interspecific variations in responses of <Emphasis Type="Italic">Festuca rubra</Emphasis> and <Emphasis Type="Italic">Trifolium pratense</Emphasis> to a severe clipping under environmental changes

Understanding and predicting possible responses of grassland species to global change is of important meaning for adapting grassland management to a changed and changing environment. A laboratory clipping experiment was conducted to examine the interspecific responses in an ecological context of competition and environmental changes. Festuca rubra and Trifolium pratense, either in monoculture or two- and three-species mixtures, were grown in three environmental combinations (ambient and increased temperature, repetitive N supply, and simulated acid rain), respectively. After a growth time of three months, plants were clipped at the height of 1.0 cm above soil surface. Plant height and aboveground biomass prior clipping, and survival rate and regrowth (height and biomass) after clipping were analyzed. F. rubra and T. pratense responded differently in compensatory growth and competition intensity to environmental change and co-existing species. The differences in their physiological and ecological traits may account for species-dependent responses. The present study emphasizes that predicting the plant assemblage response in the face of global change requires in understanding the integrating effects of abiotic and biotic factors.     

Calanoid copepod grazing on phytoplankton: seasonal experiments on natural communities

Calanoid copepods are major components of most lacustrine ecosystems and their grazing activities may influence both phytoplankton biomass and species composition. To assess this we conducted four seasonal, in situ, grazing experiments in eutrophic Lake Rotomanuka, New Zealand. Ambient concentrations of late stage copepodites and adults of calanoid copepods (predominantly Calamoecia lucasi, but with small numbers of Boeckella delicata) were allowed to feed for nine days on natural phytoplankton assemblages suspended in the lake within 1160 litre polyethylene enclosures. The copepods reduced the total phytoplankton biomass of the dominant species in all experiments but were most effective in summer (the time of highest grazer biomass) followed by spring and autumn. In response to grazing pressure the density of individual algal species showed either no change or a decline. There were no taxa which increased in density in the presence of the copepods. The calanoid copepods suppressed the smallest phytoplankton species (especially those with GALD (Greatest Axial Linear Dimension) < µm) and there appeared to be no selection of algae on the basis of biovolume. Algal taxa which showed strong declines in abundance in the presence of the copepods include Cyclotella stelligera, Coelastrum spp., Trachelomonas spp., Cryptomonas spp., and Mallomonas akrokomos. Calanoid copepods are considered important grazers of phytoplankton biomass in this lake. The study supports the view that high phytoplankton:zooplankton biomass ratios and large average algal sizes characteristic of New Zealand lake plankton may, at least partly, be caused by year round grazing pressure on small algae shifting the competitive balance in favour of larger algal species.     

Species‐specific mediation of temperature and community interactions by multiple foundation species

Foundation species can provide habitat that modify abiotic and biotic processes that contribute to ecosystem function. While many studies have focused on the processes and consequences of a focal foundation species, understanding the ecological equivalence of co‐occurring foundation species is important to identify key species responsible for ecosystem function. Here, we investigated the relative contributions of co‐occurring foundation species on abiotic (temperature) and biotic responses of invertebrate species (recruitment, persistence, growth and survival). In a series of experimental field studies, we manipulated foundation species to measure invertebrate recruitment, persistence, and predation. A laboratory experiment measured foundation species effects on herbivore growth. Results demonstrated that macroalgal (Fucus vesiculosus ecad and Ascophyllum nodosum ecad scorpioides) intermediate foundation species provide habitat, food, and alleviate abiotic stress for dominant littorinid herbivores that surpass that provided by the primary species (Spartina alterniflora). These foundation effects were species‐specific with F. vesiculosus ecad important for early life‐history stages (enhanced recruitment and early growth of littorinid snails) and A. nodosum ecad important later on as a refuge from predators (Carcinus meanas) and stressful temperature. Understanding of the different effects of co‐occurring foundation species on population and community processes is necessary for predicting community response to natural disturbance, species invasion, and ecosystem‐based management actions.     

Short-term variation of nutritive and metabolic parameters in <Emphasis Type="Italic">Temora longicornis</Emphasis> females (Crustacea,Copepoda) as a response to diet shift and starvation

Changes in fatty acid patterns, digestive and metabolic enzyme activities and egg production rates (EPR) were studied in the small calanoid copepod Temora longicornis. Female copepods were collected in spring 2005 off Helgoland (North Sea). In the laboratory one group of copepods was fed with the cryptophycean Rhodomonas baltica for a period of 3 days. Another group of copepods was maintained without food. According to the fatty acid patterns, animals from the field were feeding on a more detrital, animal-based and to a minor extent to a diatom-based diet. Under laboratory conditions, females rapidly accumulated fatty acids such as 18:4 (n-3), 18:3 (n-3) and 18:2 (n-6) which are specific of R. baltica. Diatom-specific fatty acids such as 16:1 (n-7) were strongly reduced. In fed animals the activities of digestive and metabolic enzymes remained constant and egg production rates were highest on day 2. Starving animals, in contrast, showed significantly reduced faecal pellet production and EPR. Proteolytic enzyme activity decreased rapidly within 24 h and remained at a low level until the end of the experiment. Citrate synthase decreased continuously as well. T. longicornis rapidly reacts to dietary changes and food depletion. It has limited energy stores and, thus, strongly depends on continuous food supply.     

Population dynamics of the spruce bark beetle: a long‐term study

Bark beetle population dynamics is thought to be primarily driven by bottom‐up forces affecting insect performance and host tree resistance. Although there are theoretical predictions and empirical evidences that predation and parasitism may play an important role in driving bark beetle population fluctuations, long‐term studies testing the role of both biotic and abiotic controls on population dynamics are still rare. The aim of the study was to quantify the relative importance of predation, negative density feedback and abiotic factors in driving Ips typographus population dynamics. We analyzed a unique time series of population density of I. typographus and its main predator Thanasimus formicarius over almost two decades in four regions across Sweden. We used a discrete population model and a multi‐model inference approach to evaluate the importance of both bottom up and top down factors. We found that availability of breeding substrates in the form of storm‐felled trees was the main outbreak trigger, while strong intra‐specific competition for host trees was the main endogenous regulating factor. Although temperature‐related metrics are known to have strong individual effect on I. typographus development and number of generations, they did not emerge as important drivers of population dynamics. A positive effect of low summer rainfall was evident only in the region located in the southernmost and warmest part of the spruce distribution range in Sweden. Predator density did not emerge as an important prey regulating factor. As the reported damage from storms seems to have increased across whole Europe, spruce forests are expected to be increasingly susceptible to large outbreaks of I. typographus with important economic and ecological consequences for boreal ecosystems. However, the observed negative density feedback seems to be a natural regulating mechanism that impedes a strong long‐term propagation of the outbreaks.     

Spatial and temporal variation of <Emphasis Type="Italic">Hypnea musciformis</Emphasis> carrageenan (Rhodophyta - Gigartinales) from natural beds in Rio de Janeiro State,Brazil

Hypnea musciformis (Wulfen in Jacqu.) J. V. Lamour. is a native source of kappa-carrageenan, a commercially important galactan, exploited for decades on the northeastern Brazilian coast. Further studies to obtain information about the effects of abiotic factors on the carrageenan yield and quality of specimens from the southeastern Brazilian coast (Rio de Janeiro State) are needed. In this study, the carrageenan yield and quality of H. musciformis was analyzed over a period of 1 year in populations from Rio de Janeiro state. One epiphytic population was collected at a site situated at the entrance of the Sepetiba Bay (Praia Vermelha), another in the middle of the bay (Praia Grande) and a third (Praia Rasa) in a place exposed to the open sea. An epilithic population was also collected at Praia Rasa. In the sampling period, different changes were observed in the carrageenan yield and quality (viscosity and sulphate content) of the H. musciformis (August 1994 to June 1995) at the different sites and habitats (epilithic and epiphytic). Kappa carrageenan was the only one of its type detected. Only one population showed a correlation between viscosity and degree of sulphate content. Our analyses suggest that abiotic factors, such as water movement, desiccation stress, low salinity and extreme temperatures, were the main abiotic factors that influenced the changes in carrageenan of the populations studied. Thus, for a rational use of natural beds of H. musciformis, it is desirable to estimate the changes in carrageenan and algal biomass in the place to be exploited.     

Spatio-temporal distribution of mesozooplankton in a sub-Antarctic coastal basin of the Kerguelen Archipelago (southern Indian Ocean)

Seventeen surveys were performed fortnightly from January to December 1990 and covered a grid of 16 stations in the large Morbihan Bay at Kerguelen Island. Temperature and salinity were measured at four or five standard depths. Chlorophyll, carbon and nitrogen contents of surface water particulate matter were determined along with mesozooplanktonic biomass and abundance of copepods. The dominant species (90%) of mesozooplankton wasDrepanopus pectinatus, and two other taxa,Oithona spp. andCalanus simillimus, accounted for 8.6 and 0.4% respectively. A Multiple Correspondence Analysis was used to determine the influence of abiotic (temperature and salinity) and biotic factors (potential food supply) on the spatio-temporal distribution of copepods and, more specifically, on that of the growth stages ofD. pectinatus. No specific hydrological features were found. The distribution of copepods was fairly homogeneous in the whole bay while the quantitative changes were influenced by the seasonal rhythm. Only a slight trend of increasing values for the biotic or abiotic parameters and the neriticD. pectinatus was observed from the central to the inner back area of the bay.     

Should we expect a relationship between primary production and fisheries? The role of copepod dynamics as a filter of trophic variability

It is frequently put forward that variability in fisheries productivity is related to interannual variation in physical processes affecting phytoplankton productivity. Here, alternative views of the role of copepods as an intermediary link in North Atlantic marine food chains are discussed. Following Bainbridge & McKay (1968) and Cushing (1982), a strong link between phytoplankton and fisheries variability is proposed for some fish stocks, like cod and redfish, that spawn in spring in regions where Calanus finmarchicus dominates the plankton. Otherwise, in regions where small copepods and other microzooplankton dominate the prey field productivity for larval fish, a weak link is proposed. Experimental studies, including laboratory observations of copepod reproductive response to food concentration and incubation techniques for measuring in situ reproductive rates, are important for understanding how copepod dynamics may filter year-to-year differences in phytoplankton production cycles.     

Pisonia grandis monocultures limit the spread of an invasive ant—a case of carbohydrate quality?

The mechanisms by which invasive species are able to spread into and dominate natural communities are poorly understood and remain a focus of invasion research. In this quest, studying invasions that are limited by a controlling factor will be more informative than will studies documenting unabated spread and impacts. Some ant species are very successful invaders, and research demonstrating abiotic and biotic factors limiting their success has aided the understanding of invasion ecology. We report here a study showing the highly invasive African big headed ant Pheidole megacephala having a novel distribution on coral cays within Australia’s Great Barrier Reef. These patterns displayed a clear limitation of its distribution with monocultures of the tree Pisonia grandis. This distribution was contrary to the known environmental limitations of the ant, and the limitation could not be associated with an underlying abiotic determinant of the vegetation type. We present these distributional patterns, and following consideration of all known biotic and abiotic limitations of ant invasions we discuss the potential that the peculiar ecophysiology of P. grandis is the causal factor. Specifically, we suggest that the quality of carbohydrate supply to ants is a limitation to invasive spread in much the same way that carbohydrate quantity is known to affect ant population densities in other ecosystems.