Piotr Dawidowicz

Biomanipulation in shallow lakes: state of the art

The current state of biomanipulation was the subject of muchdiscussion at Shallow Lakes ‘95. This led to a workshop focusing onthe factors influencing the establishment of macrophytes and themechanisms responsible for their stability followingbiomanipulation. The purpose of the current paper is to distilcurrent knowledge on biomanipulation in shallow lakes gleaned fromdiscussions at the conference and recent literature.Biomanipulation should be used in the theoretical context of twoaltemative stable equilibria, as the extreme perturbation requiredto move from a phytoplankton dominated state to one dominated bymacrophytes. Understanding the nature of the factors and mechanismsresponsible for turbid water, is critical if biomanipulation is tobe appropriate. We suggest that with sufficient information,particular components of the fish community may be targeted andprecise figures for removal, designed to exceed critical thresholdvalues, may be set. Without this knowledge, a ‘play-safe’ strategyshould be adopted and at least 75% of the fish removed. Stockingwith piscivores may be a useful additional measure to fish removal.The principal objective of biomanipulation in shallow lakes is togenerate a period of clear water of sufficient length to allowmacrophytes to establish. To this aim, as well as for technicalreasons, biomanipulation is best undertaken in winter and earlyspring to generate clear water as early as possible in the season.In the cases where grazing is important, this coincides with thespring peak of Daphnia spp. Biomanipulation may have to berepeated if macrophytes do not colonise effectively within thefirst season. The factors responsible for the lag in response ofmacrophytes in some cases and the potential mechanisms contributingto the maintenance of clear water in macrophyte beds are discussed.From empirical data sets from many lakes, both a relative increasein the piscivorous fish stock and a reduction in nutrient levelsare thought to be important in stabilising the system in thelong-term. Whether biomanipulation may lead to alternativeequilibria (i.e. high diversity macrophyte communities withpiscivorous fish at high P) is unknown. Further study ofexceptional cases, theoretical modelling and development andanalysis of more long-term (>10 years) case histories isrecommended.

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 lack of vertical migration in Daphnia: the effect of homogenously distributed food

Neither Daphnia hyalina, Daphnia cristata, nor Daphnia cucullata vertically migrated in Lake Mikołajskie and Lake Majcz. We suggest that even under strong fish predation pressure there is no reason to migrate when seston is relatively homogenously distributed throughout the water column.

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.

Behavioural uniformity as a response to cues of predation risk

When confronted with a predator, individuals often alter their behaviour to minimize predation risk. To investigate behavioural changes induced by predator presence and light in a zooplankton species, we video-recorded the swimming behaviour of waterfleas, Daphnia pulicaria. Chemical cues from two predators with different hunting techniques (a visual predator: bluegill sunfish, Lepomis macrochirus; and a tactile predator: phantom midge larvae, Chaoborus sp.) were used in two sets of experiments. We filmed Daphnia individually in either control or predator-treated water, in a light or dark environment (four treatments in total) and recorded swimming speed, hop rate, sink rate and net angle of swimming. All swimming parameters in the fish experiment and two in the Chaoborus sp. experiment changed significantly between treatments. The coefficient of variation (CV) of all parameters except sink rate decreased significantly in the presence of light and/or a predator. In the presence of light, CV values declined by at least two-fold in both fish and Chaoborus experiments, while the changes in CV values between treatments varied depending on the type of predator used in the experiment. Waterfleas increased their swimming uniformity while isolated from other individuals. We therefore conclude that this swimming behaviour pattern is based on individual perception of both kairomones and light in the environment rather than on social interactions. Our results suggest that uniformity in swimming behaviour may be an important defence mechanism in zooplankton, aiming to minimize oddity among group members in order to decrease predation risk.

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

Water from a hypertrophie 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.

Shortened lifespan: another cost of fish-predator avoidance in cladocerans?

Life-history theory predicts adaptive shifts in response to size-selective predation, namely earlier reproduction, smaller age/size at maturity, and higher relative investment into reproduction. Such shifts should bring about reduced lifespan of potential prey. We tested this prediction in life-table experiments with clones of Daphnia hyalina and Diaphanosoma brachyurum, two species of contrasting anti-predatory strategies. The clones were derived from seven lakes of different trophy and held in water with and without fish kairomone, under standard laboratory conditions. Exposure to the kairomone caused a decrease in age of first reproduction and an increase in early-life reproductive effort but also an about 20% decrease of longevity in both species. Although shortened lifespan did not result in significant decrease in fitness of the tested species (in terms of lifetime reproductive output) it should be taken into account in considerations of costs and benefits of inducible defenses in cladocerans.

How perceived predation risk shapes patterns of aging in water fleas

Predation is an important selection pressure which shapes aging patterns in natural populations, and it is also a significant factor in the life history decisions of individuals. Exposure to the perceived threat of size-dependent fish predation has been shown to trigger adaptive responses in animal life history including an increase in early reproductive output. In water fleas, this response to perceived predation risk appears to have a cost, as a lifespan in an environment free of predation cues is 20% longer. The aim of this study is to establish the biodemographic basis of phenotypic differences in the water flea lifespan which are induced by the cues of fish predation. We examined mortality by fitting the Gompertz-Makeham model of mortality to large cohorts of two cladoceran species, Daphnia longispina and Diaphanosoma brachyurum. Our findings indicate that perceived exposure to the threat of fish predation (induced through chemical cues) only accelerated the rate of aging in Diaphanosoma, and not in Daphnia where the treatment led to an earlier onset of aging. The second of these two phenotypic responses is consistent with the genetically based differences between Daphnia from habitats that differ with respect to predation risk. In contrast, the response of Diaphanosoma demonstrates that the cue of extrinsic mortality-in this case, fish predation-is a key factor in shaping these cladoceran life histories in the wild, and is one of the few interventions which has been shown to induce a plastic change in the rate of aging.

The effect of temperature and kairomone on Daphnia escape ability: a simple bioassay

We propose a simple and sensitive bioassay for analyzing zooplankton escape behavior and present results of its testing on four Daphnia species under varying temperature, and in either the presence or absence of chemical cues of fish predation. In the assay, the animals are blindly transferred with a pipette through subsequent containers and their final distributions are compared. For testing the effects of a single factor, comparing distributions of the animals is a sensitive tool to detect differences in escape ability. We propose single-value measures to be used in larger experiments with multigroup comparisons in order to enable studying global effects and reduce the loss of statistical significance due to correction for multiple testing. Our results show that if escape ability is considered an important fitness component, among the tested species, which were D. longispina, D. lumholtzi, D. magna, and D. pulicaria, the last may be the most vulnerable to increased fish predation associated with increased lake water temperatures.

Depth-selection behavior and longevity in Daphnia: an evolutionary test for the predation-avoidance hypothesis

The classic evolutionary theory of senescence predicts that long lifespan evolves under low risk of extrinsic mortality. As lakes present planktonic animals with vertical gradients of mortality risk associated with fish predation, we expected the individual lifespan of Daphnia of the “hypolymnetic” clones to be longer than that of “epilimnetic” Daphnia. In order to test this prediction, we performed a laboratory study on 14 clones from the D. longispina species complex, taken during the daytime either from epilimnion or hypolimnion of three mesotrophic lakes. “Epilimnetic” Daphnia started reproduction earlier, aged faster, and lived shorter than their “hypolimnetic” conspecifics. Our results indirectly support the predation-avoidance hypothesis as the ultimate explanation for depth-selection behavior in Daphnia.