Z. Maciej Gliwicz

Food thresholds in Daphnia species in the absence and presence of blue-green filaments

Three Daphnia species of different body sizes were grown on a single food resource in steady-state conditions. They differed in food threshold concentrations nec- essary for body growth and reproduction. The largest species, D. pulicaria, had the lowest thresholds and the smallest, D. cucullata, had the highest thresholds, while D. hyalina was intermediate. This implies competitive superiority of the largest species and inferiority of the smallest one. The sequence, however, was reversed in the presence of blue-green algal filaments, when the thresholds become lowest for D. cucullata and highest for D. pulicaria. Thus the inhibitory effect of filaments is body-size dependent. High densities of filaments that mechanically interfere in food collection may cause shifts in competitive ability among congeneric cladocerans. This may be the reason why, in Europe, D. pulicaria is most successful in ultraoligotrophic lakes while D. cucullata is most successful in eutrophic lakes.

The Role of Predation in Zooplankton Succession

For the last two decades predation has been considered to be a major driving force in shaping zooplankton communities and in determining their density and structure. In early studies, species succession was usually considered to be the result of differences in ecological tolerance to various abiotic environmental factors, such as light intensity and water density or viscosity (Hutchinson, 1967). By the early 1960s, the time of worldwide “productivity” research, the competition for resources was usually considered to be the primary driving force. Later, after the pioneering papers by Hrbacek (Hrbacek, 1962; Hrbacek et al., 1961) were cited by Brooks and Dodson (1965) and Hall et al. (1976), predation was considered to be the major factor responsible for successional events.

Family planning in Daphnia: resistance to starvation in offspring born to mothers grown at different food levels

SummaryWe observed a shift in maternal investment per offspring in clonal cultures of twoDaphnia species. Mothers grown at high food levels produced large clutches of smaller eggs but their offspring could not survive long under starvation conditions. Genetically identical mothers grown at low food levels produced small cultches of larger eggs, and their offspring, albeit low in numbers, were able to survive long periods of starvation. Our data show thatDaphnia mothers are capable of assessing food level and use this information in adjusting their fractional peroffspring allocation of reproductive resources.

Limnological control of brine shrimp population dynamics and cyst production in the Great Salt Lake, Utah

In the Great Salt Lake of Utah, the brine shrimp Artemia franciscanaKellogg is an important food resource for birds and they produce dormant cysts that are harvested and used extensively in the aquaculture industry. We analyzed the limnological factors controlling Artemia growth and cyst production over 12 months in 1994 and 1995. Laboratory experiments showed that inter-brood intervals were highly dependent on temperature and slightly on food level. At optimal temperatures and nutritious food, juveniles reached reproductive size within 7 d in the laboratory. In winter when temperatures were less than 3 °C, Artemia were absent from the lake, phytoplankton abundance was high (≥13 Chl a μg l−1), and the dominant grazers were ciliated protozoans. In the spring, cysts hatched when phytoplankton was abundant (15–30 μg Chl a l−1), and the Artemia grew and produced large clutches of ovoviviparous eggs. Estimated naupliar production from these eggs was 80 l−1 from April to May. Despite the high production of nauplii, Artemia densities declined to 8 l−1by June and the growing shrimp population grazed down the phytoplankton resource to <1 μg Chl a l−1. With the depleted phytoplankton food resource during the summer, Artemia growth slowed, lipid indices decreased, clutch sizes declined, and females switched primarily to oviparous cyst production. During the summer, there was limited production of ovoviviparous eggs, and limited recruitment of juveniles, probably due to low food. Although oviparous reproduction began in June, more than 90% of the cysts were produced after July when female densities had declined to 1.5 l−1, but nearly all of them were producing cysts. Estimated cyst production was 650 000 m−2, or 4.54 × 106 kg dry weight for the entire lake. The reported commercial harvest took 21% of the 1994 cyst production. That harvest had little impact on the subsequent years population, as Artemia densities were ultimately controlled by algal production in the lake.

Relative significance of direct and indirect effects of predation by planktivorous fish on zooplankton

One of the most obvious features of tropical lakes and reservoirs is the small body size of their zooplankton taxa. It is believed that this is the result of high and persistent predation by abundant planktivorous fish, which select large-bodied zooplankton prey thus making them more vulnerable to extinction in tropical as compared to temperate habitats. Do these extinctions result directly from fish predation? Could the high predation-induced mortality alone be responsible for an extermination of the population from a habitat? Or could indirect effects of predation be responsible? Some important indirect effects can be seen at the demographic level; these include reduced reproduction in the population resulting from higher vulnerability of ovigerous females to predation by visually oriented planktivores. Other important indirect effects can be observed at the individual level; these include shifts in behavior (from foraging to predator avoidance) and adjustments in physiology (from high to low feeding rate) in those planktonic animals which detect danger from their predators by sensing either the ‘predator odor’ or an ‘alarm substance’ originating from injured conspecific prey. Although a zooplankton species density may mostly result from the brutal force of direct predator impact on the population (mortality), it is more likely that its distribution in time and space could be attributed to a combination of indirect effects of predation on individual behavior and physiology. An example of periodicity in density and depth distribution patterns of Cahora Bassa zooplankton species and their periodic exterminations seems to confirm the role of indirect effects of predation by planktivorous fish.

Food and predation as major factors limiting two natural populations of Daphnia cucullata Sars

A detailed analysis of two euplanktonic populations carried out in the same period, 18 May–11 August, 1978, in two neighboring Mazurian lakes revealed different patterns of changes in population densities and significant differences in fecundity, age structure and mean sizes of individuals despite similar epilimnion temperatures in the lakes. Similar densities were maintained in both populations although food limitation was less important and predation more intense in one than in the other of the two lakes. The relative importance of food limitation and predation is discussed in lakes of low and high fertility. It is also suggested that both low food concentration and high selective predation pressure favors small clutch sizes in cladocerans.

Food of brook charr in extreme oligotrophic conditions of an alpine lake

SynopsisA stunted form of Salvelinus fontinalis has persisted since its introduction in 1948 in ultra oligotrophic Zielony Lake in the Tatra Mountains, southern Poland, despite extremely poor available food. The charr shifts from feeding on airborne insects to benthic chironomid larvae when the lake is frozen in October, and to the sole planktonic crustacean survivor (Cyclops abyssorum tatricus) when access to the sediments becomes difficult due to oxygen depletion in April–May. The plankton food resource enables the charr to survive till June, when the ice-cover breaks up. Due to charr predation the old Cyclops generation becomes extinct in May, but Cyclops eggs hatch after passing through the charr intestine unharmed. Each new Cyclops generation explodes due to the absence of cannibalistic adults, which have been removed by the charr, and persists in high densities until the next April–May when the egg-carrying females (highly selected by the charr) appear again to focus the charr attention on planktonic food resources.

Predation‐Mediated Coexistence of Large‐ and Small‐Bodied Daphnia at Different Food Levels

Using an individual‐based age‐structured population model (a combination of O’Brien’s apparent‐prey‐size approach, Eggers’s reactive‐field‐volume model, and Holling’s disk equation), we could predict that (1) a Daphnia population could be kept at low density by fish predation irrespective of food level, with greater recruitment at higher food being instantly compensated for by raised mortality reflecting increased predation, and (2) Daphnia density levels are species specific and inversely related to both body size at first reproduction and the reaction distance at which a foraging fish sees its Daphnia prey. These two hypotheses were experimentally tested in outdoor mesocosms with two Daphnia species of different body sizes grown in the absence or presence of fish that were allowed to feed for 2–3 h each evening. While each Daphnia quickly reached high density with reproduction halted by food limitation in the absence of fish, the populations stayed at much lower species‐specific density levels, similar in low and high food concentrations, in the presence of fish. This suggests that our model offers a reasonable mechanistic explanation for the coexistence of large‐ and small‐bodied zooplankton in proportions reflecting their body sizes throughout habitats comprising a wide productivity spectrum, with each species at a density level at which it becomes included in a predator’s diet.

Cladoceran Densities, Day-to-Day Variability in Food Selection by Smelt, and the Birth-Rate-Compensation Hypothesis

Late-evening gut inspection of a dominant planktivore (smelt) and evaluation of densities, fecundities, and body-size distributions in dominant zooplankton prey (cladocerans) were made in day-to-day sequences in June–July (24 days in 1999 and 24 days in 2000). This was conducted as a field test of the hypothesis that species-specific population densities in cladocerans result from size-selective predation by a dominant fish assumed to be a general predator, switching from one prey to another as relative abundance changes. Little of the expected coincidence has been revealed between population density declines and increased numbers of a given prey in smelt diet. However, the data were consistent with the notion that fish would switch from one prey to another depending on the prey relative abundance (the number of prey a fish would see in its reaction field volume). Each cladoceran population fluctuated around its species-specific density level, lower or higher, depending on individual susceptibility to smelt predation, from 0.2 ind. l−1 in large-bodied Daphnia hyalina and Leptodora kindtii, to 30.0 ind. l−1 in small-bodied Daphnia cucullata andBosmina thersites. In spite of high fish-to-fish and day-to-day variability in both smelt diet and smelt selectivity for different prey, all cladocerans (also copepods and midge larvae) were equally persistent in smelt diet, and smelt selectivity was similar for small- and large-bodied prey categories, but lower for elongated-(Daphnia, Diaphanosoma) than for compact-body (Bosmina, Chydorus) species, when integrated for the entire sampling time. Closer examination of D. cucullata and B. thersites revealed strong smelt selection for later instars and females with greater clutches, showing that size distribution in a cladoceran population might be structured by fish predation in a similar way to that a cladoceran community (species relative abundance) is structured in a lake habitat. The “birth-rate-compensation hypothesis” is offered to explain why the value of food selectivity index in a planktivorous fish would remain the same for alternate prey categories with similar life-history traits, unless they differ in susceptibility to predation before the time of first reproduction.

Can ecological theory be used to improve water quality

The role of ecology in environmental management has been more to warn of the dangers of pollution than to propose technical solutions for environmental problems, this being mostly left to chemists and engineers. Only limited aspects of the broad and diverse theory of ecology have been applied in the present bottom-up-control approach to water management (nutrient reduction). Another aspect of ecological theory which has recently become applicable is the top-down-control approach known as ‘biomanipulation’ (promoting efficient herbivores). Both approaches are likely to be reinforced in the future by further application of ecological theory to water management. Of the multiple possibilities of applying ecological theory to environmental problems, one is the use of our knowledge on chemical communication in aquatic habitats, the phenomena by which prey detect the presence of their predators. I postulate that the phytoplankton standing crop might be reduced by chemical regulation of the physiology and life cycles of algae and cyanobacteria, as well as by chemical regulation of the behaviour and life histories of planktonic herbivores and their predators. At present planktivorous fish seem to be the most logical target for such chemical manipulation. A desirable effect, i.e. a reduction in algal standing crop, could be achieved by scaring fish from the lake pelagial thus releasing predation pressure on the offshore herbivorous Zooplankton.