The effect of Daphnia on filament length of blue-green algae

Water from a hypertrophic lake rich in filamentous blue-green algae was passed through a continuous-flow system of aquaria containing Daphnia magna, and a control system without Daphnia. Daphnia caused a significant decrease in the blue-green algal density, and a two-fold reduction in filament length. It is suggested that feeding activity of Daphnia may result in an increase in the availability of blue-green filaments to filter-feeding cladocerans.     

Phytoplankton–zooplankton seasonal timing and the 'clear-water phase' in some English lakes

SUMMARY 1. An examination is made of the relative seasonal timing of the postwinter increase of phytoplankton and zooplankton populations in four English lake basins. It centres upon weekly sampling over 20 years and rough counts of larger Crustacea, as copepods and cladocerans, from filtered samples that were used for chlorophyll a (Chl) estimation. 2. Typically, a spring maximum of phytoplankton, dominated by diatoms and earlier in the shallower lakes, is accompanied or followed by a maximum of copepods and then one of cladocerans dominated by the Daphnia hyalina–galeata complex. Regarding timing, the maximum of copepods has no apparent relation with phytoplankton abundance (Chl). The maximum of cladocerans appears to be largely independent of variation in the phytoplankton maximum, but is generally associated with a minimum in Chl. Evidence for some direct causality in this inverse correlation after the spring phytoplankton maximum is best displayed by the shallow Esthwaite Water in which the peaks of Chl and cladocerans are separated further than in the deep Windermere basins where phytoplankton growth is delayed. In Esthwaite Water, and possibly often in Windermere, a principal minimum in Chl is ascribable to grazing by Daphnia. 3. The typical inverse relationship of Chl and cladocerans is lost in some years when relatively inedible large phytoplankters (e.g. colonial chrysomonads, filamentous cyanophytes) are abundant and Chl minima are less pronounced, although maxima of cladocerans still occur. Conversely, available edible phytoplankters include various small forms grouped as μ‐algae and Cryptomonas spp.; their probable depletions by Daphnia appear to be sequential and may limit the latter's maxima, whose inception is temperature‐dependent. 4. The spring–summer maxima of cladocerans and minima of Chl are generally coincident with a main seasonal maximum of Secchi disc transparency and light penetration – to which removal of non‐phytoplankton particles by filtering cladocerans may contribute.     

Interactions between metazoans,autotrophs, mixotrophs and bacterioplankton in nutrient‐depleted high DOC environments: a long‐term experiment

相似文献   

Top-down control of natural phyto- and bacterioplankton prey communities by Daphnia magna and by the natural zooplankton community of the hypertrophic Lake Blankaart

Biomanipulation, through the reduction of fish abundance resulting in an increase of large filter feeders and a stronger top-down control on algae, is commonly used as a lake restoration tool in eutrophic lakes. However, cyanobacteria, often found in eutrophic ponds, can influence the grazing capacity of filter feeding zooplankton. We performed grazing experiments in hypertrophic Lake Blankaart during two consecutive summers (1998, with and 1999, without cyanobacteria) to elucidate the influence of cyanobacteria on the grazing pressure of zooplankton communities. We compared the grazing pressure of the natural macrozooplankton community (mainly small to medium-sized cladocerans and copepods) with that of large Daphnia magna on the natural bacterioplankton and phytoplankton prey communities. Our results showed that in the absence of cyanobacteria, Daphnia magna grazing pressure on bacteria was higher compared to the grazing pressure of the natural zooplankton community. However, Daphnia grazing rates on phytoplankton were not significantly different compared to the grazing rates of the natural zooplankton community. When cyanobacteria were abundant, grazing pressure of Daphnia magnaseemed to be inhibited, and the grazing pressure on bacteria and phytoplankton was similar to that of the natural macrozooplankton community. Our results suggest that biomanipulation may not always result in a more effective top-down control of the algal biomass.     

Effectiveness of phytoplankton control by large-bodied and small-bodied zooplankton

Employingin situ enclosures containing inocula of the lake zooplankton (mainlyDaphnia galeata, Daphnia cucullata andBosmina spp.) from a moderately eutrophic Lake Ros (Northern Poland) or large-bodiedDaphina magna, the following observations on succession of phytoplankton were made: 1) whereasD. magna could control the density of all the photoplankton size classes, the lake zooplankton could not suppress the large-sized phytoplankters or net phytoplankton; 2) the lake zooplankton was able to control the density of small algae (< 50μm), but its effect on large algae may be opposite: a promotion of net phytoplankton growth by removing small-sized algae which can out-compete net phytoplankton for limited PO₄-P resources (<5μg P l⁻¹). Since efficiency of phytoplankton density control byD. magna decreased with an increase in net phytoplankton abundance, biomanipulation could not be successful without introducing or maintaining a high population of large-bodied cladoceran species before high densities of large algae would make the control of phytoplankton inefficient.     

The impact of year-to-year changes in the weather on the dynamics of Daphnia in a thermally stratified lake

The factors influencing the seasonal dynamics of Daphnia in a thermally stratified lake (Esthwaite Water) are described and related to long-term changes in the weather. The Daphnia produced three cohorts in the year and the strength of the cohorts was determined by year-to-year variations in the physical characteristics of the lake and the abundance of edible algae. Food was most abundant in early summer when small, fast-growing flagellates were particularly common. In late summer, the phytoplankton community was dominated by large, inedible species but edible forms re-appeared when nutrients were entrained by wind mixing. Examples are presented to demonstrate the effect that year-to-year variations in the weather have on the growth of the phytoplankton and the dynamics of the Daphnia. In ‘good’ years, when the lake stratifies early and there are periods of episodic mixing in summer, there are two ‘pulses’ of edible algae and two strong cohorts of Daphnia. In ‘bad’ years when stratification is delayed and there is little episodic mixing, the growth of the edible algae is suppressed and the Daphnia produce two weak cohorts. The results are discussed in relation to the impact of intermediate disturbances on growth of phytoplankton and current theories of population regulation in Daphnia. The evidence suggests that the dynamics of the Daphnia in the lake are strongly influenced by seasonal variations in the mixing regime, the recycling of nutrients and the episodic growth of edible algae.     

Lake restoration by biomanipulation using piscivore and Daphnia stocking; results of the biomanipulation in Japan

Biomanipulation has been employed in numerous locations throughout the world as a means for reducing phytoplankton biomass; however, it has not been employed very often in Japan. A common approach involves the introduction of piscivorous fish to reduce the abundance of planktivorous fish. In our study, to first apply biomanipulation, we stocked Lake Shirakaba (a high-altitude, protected area in a park) in central Japan with rainbow trout fingerlings and cladoceran Daphnia (Daphnia galeata) in 2000. A “pre-biomanipulation” data set (1997–1999) and “a post-biomanipulation” data set (2000–2006) allowed us to evaluate the lake's response to biomanipulation. After the biomanipulation, zoo-planktivorous pond smelt disappeared and a large population of Daphnia had been established, which substantially reduced the number of the previously dominant small cladocerans and rotifers. Water transparency increased from about 2 m (before biomanipulation) to more than 4 m (after biomanipulation). Reductions in algal biomass and increased transparency led to expansion of the submerged macrophyte Elodea nuttallii. Total phosphorus concentrations declined as well over this time period. Based on these results, we concluded that biomanipulation using piscivore and Daphnia stocking succeeded in improving lake water quality by reducing algal abundance and providing favorable conditions for the establishment of rooted plants.     

Relationships between zooplankton community structure and phytoplankton in two lime-treated eutrophic hardwater lakes

1. North Halfmoon Lake and Lofty Lake (Alberta, Canada) were chosen for whole-lake liming experiments as a new restoration technology to enhance calcite precipitation and reduce eutrophication. During a 3-year study (1991–93) the relationships between zooplankton and phytoplankton were assessed, together with the effects of lime additions. 2. Zooplankton communities were numerically dominated by rotifers, while the major contribution to biomass was due to large filter-feeding Daphnia during the first half of the summer season. In Lofty Lake, cladocerans made up to 93% of biomass, whereas in North Halfmoon Lake both cladocerans and calanoids were strongly represented. 3. Total zooplankton and cladoceran biomasses were inversely correlated with chlorophyll a (chl a). The same relationship was found between large Daphnia (≥ 1 mm) and chl a. These relationships suggest that the decline in Daphnia may have been caused by an increase in cyanobacteria biomass during bloom events. 4. There were minor changes in rotifer populations after liming; however, these changes have been caused by natural year-to-year variation rather than liming. In general, cladocerans showed an increase in body size and population biomass when pre and post-treatment data were compared by means of ANCOVA. Statistical analysis showed that there were more cladocerans per unit of chl a after liming; however, further research is needed to relate these patterns unambiguously to the application of lime as a restoration technology.     

Trophic Cascades Uncoupled in a Coastal Marsh Ecosystem

The Mediterranean population of the exotic eastern mosquitofish Gambusia holbrooki (Agassiz 1859) (Osteichthyes, Poeciliidae) has been held responsible for causing eutrophication due to zooplankton removal and phytoplankton enhancement, however no experimental evidence exists of this. To test this allegation, an enclosure experiment was conducted in spring in an oligohaline coastal marsh. The manipulation of fish density had profound effects on zooplankton, whose density greatly decreased when the occurrence of mosquitofish increased. Cladocerans and ostracods were more affected by mosquitofish than cyclopoid copepods, whilst rotifer density was not modified. Changes in zooplankton density did not cascade to lower trophic levels as no differences were observed between the chlorophyll concentration in fish and fish-less enclosures. This is because zooplankton was dominated by species with low filter-feeding rates, such as small cladocerans. In consequence, the total macrophyte standing crop was not affected. The only benthic macroinvertebrate species whose density increased in the absence of eastern mosquitofish was the mud snail P. acuta. Higher numbers of snails explain why the standing crop of the filamentous green algae Oedogonium sp. decreased in fish-less enclosures. The density of chironomid midge larvae did not increase in fish-less enclosures, because eastern mosquitofish forage on them mainly during summer, when zooplankton has already been depleted; nor were damselflies, probably because they are too large. Nitrogen concentration decreased after fish exclusion, but phosphorus concentration remain unchanged. In conclusion, it was found that the eastern mosquitofish affect zooplankton of the Mediterranean oligohaline lagoons considerably, but they do not enhance phytoplankton growth, because the system is bottom-controlled by submerged macrophytes.     

The role of solid waste materials as habitats for macroinvertebrates in a lowland dam reservoir

Eight hypereutrophic phytoplankton dominated ponds from the Brussels Capital Region (Belgium) were biomanipulated (emptied with fish removal) to restore their ecological quality and reduce the risk of cyanobacterial bloom formation. Continuous monitoring of the ponds before and after the biomanipulation allowed the effects of the management intervention on different compartments of pond ecosystems (phytoplankton, zooplankton, submerged vegetation and nutrients) to be assessed. Fish removal resulted in a drastic reduction in phytoplankton biomass and a shift to the clear-water state in seven out of eight biomanipulated ponds. The reduction in phytoplankton biomass was associated with a marked increase in density and size of large cladocerans in six ponds and a restoration of submerged macrophytes in five ponds. The phytoplankton biomass in the ponds with extensive stands of submerged macrophytes was less affected by planktivorous fish recolonisation of some of the ponds later in the summer. The two non-vegetated ponds as well as one pond with sparse submerged vegetation showed a marked increase in phytoplankton biomass associated with the appearance of fish. Phytoplankton biomass increase coincided with the decrease in large Cladocera density and size. One pond lacking submerged macrophytes could maintain very low phytoplankton biomass owing to large Cladocera grazing alone. The results of this study confirmed the importance of large zooplankton grazing and revegetation with submerged macrophytes for the maintenance of the clear-water state and restoration success in hypereutrophic ponds. They also showed that large Cladocera size is more important than their number for efficient phytoplankton control and when cladocerans are large enough, they can considerably restrain phytoplankton growth, including bloom-forming cyanobacteria, even when submerged vegetation is not restored. The positive result of fish removal in seven out of eight biomanipulated ponds clearly indicated that such management intervention can be used, at least, for the short-term restoration of ecological water quality and prevention of noxious cyanobacterial bloom formation. The negative result of biomanipulation in one pond seems to be related to the pollution by sewage water. Guest editors: B. Oertli, R. Cereghino, A. Hull & R. Miracle Pond Conservation: From Science to Practice. 3rd Conference of the European Pond Conservation Network, Valencia, Spain, 14–16 May 2008