Richard S. Stemberger

Body Size, Food Concentration, and Population Growth in Planktonic Rotifers

Population growth rates of eight species of planktonic rotifers were assessed for five to seven food concentrations using daily renewed batch cultures. The food concentration for which population growth rate was zero (the threshold food level) varied by a factor of 17 among species. The log of threshold food concentration was positively and significantly related to the log of body mass. Similarly, a strong positive log-log relationship was found between rotifer body mass and the food concentration supporting one-half the maximum population growth rate (rax/2); this food con- centration varied by a factor of 35 among species. There was a positive relationship between rotifer body mass and maximum population growth rate. Because the smallest species have the lowest threshold food levels and the food levels necessary for them to attain rm,,a/2 are lowest, they appear well adapted to living in food-poor environments. Large species appear to be restricted to food-rich environments but may thrive there because of their high reproductive potential. These conclusions are consistent with observations on species distribution and community structure of rotifers in nature.

Multiple-species induction of morphological defenses in the rotifer Keratella testudo

Filtrates of cultures of 10 of 12 common freshwater zooplankton species induced posterior spines in the rotifer Keratella testudo. Filtrates of cultures of Asplanchna spp. and those of crustacean zooplankton, including cladocerans and cyclopoid and calanoid copepods, generally produced the strongest induction responses. The filtrates of Daphnia pulex cultures, at densities as low as 0.4 individuals/L, promoted significant posterior spine development. This is the first known case of a competitor-controlled developmental poly- morphism in zooplankton. Filtrates of cultures of the herbivorous rotifer Synchaeta pec- tinata and the fourth instar larvae of the dipteran Chaoborus punctipennis were ineffective as spine-promoting agents. In some, but not all, interactions with inducing species, the posterior-spined Keratella phenotype had higher survivorships than the unspined one. Spined individuals were sig- nificantly more protected than unspined ones against injury by mechanical interference by Daphnia. Direct observations showed that the spined phenotype was more rapidly rejected after entering the bronchial chambers of Daphnia. This decreased the retention time in the bronchial chambers and, consequently, reduced the likelihood of injury by the filtering limbs and mouthparts of Daphnia. The ability of this rotifer to respond to many zooplank- ton species seems advantageous, because the spined phenotype is less vulnerable to pre- dation and interference competition from a variety of larger, co-occurring zooplankton than is the unspined phenotype.

Rotifer Threshold Food Concentrations and the Size‐Efficiency Hypothesis

Using published data, we develop a physiological explanation for the positive relationship between threshold food concentration (the food concentration at which the population growth rate is zero) and body mass in planktonic rotifers. The exponent de? scribing maximum clearance rate as a function of body mass is similar to the exponent expressing energy intake rates at low food concentrations. The former exponent (0.42) is considerably lower than the exponent for respiration (0.66). Consequently, the difference between energy intake and metabolism should decrease with increasing body size at low food concentrations. Rotifer swimming speeds are rather uniform for a wide range of body sizes and species. Consequently, length-specific swimming speeds (body lengths moved per unit time) and, particularly, mass-specific swimming speeds (distance moved per unit time per unit mass) decrease rapidly with increasing body size. We conclude that swimming speed, which is under the energy constraints of ciliary locomotion and which directly determines the rate of food encounters, provides the mechanism for the positive relationship between body size and threshold food concentration.

A zooplankton-N:P-ratio indicator for lakes

We develop the conceptual and empirical basis for a multi-level ecosystem indicator for lakes. The ratio of total N to total P in lake water is influenced or regulated by a variety of ecosystem processes operating at several organizational levels and spatial scales: atmospheric, terrestrial watershed, lake water, and aquatic community. The character of the pelagic zooplankton assemblage is shown to be well correlated with lake water N:P ratio, with species assemblages arrayed along the N:P gradient in accordance with resource supply theory. Features of specific zooplankton assemblages or deviations from expected assemblages can provide information useful for lake managers, such as the efficiency of pollutant transfer and biomagnification of toxins, loss of cool-water refuge areas, degree of zooplanktivory and food web simplification related to changes in fisheries, and assemblage changes due to anthropogenic acidification. Evaluation of the influence of watershed land use, forest cover and vegetation type, atmospheric deposition, and basin hydrology on the supply of N and P to lake ecosystems provides a means to couple changes in the terrestrial environment to potential changes in aquatic ecosystems. Deviations of lake water N:P values from expected values based on analysis of watershed and lake basin characteristics, including values inferred from appropriate diatom microfossil deposits, can provide an independent validation and baseline reference for assessing the extent and type of disturbance. Therefore, the N:P ratio of lake water can serve as a potentially useful and inexpensively obtained proxy measure for assessing changes or shifts in the biological and nutrient status of lakes.

Body size, ration level, and population growth in Asplanchna

Abstract1.)The daily ration required to maintain a population growth rate, rm, of zero (threshold ration) increased with increasing Asplanchna body mass. This relationship is described by the equation T=0.342 W0.797 where T=threshold ration (μg day-1 dry mass) and W=Asplanchna body mass (μg adult-1 dry mass).2.)The threshold ration of large campanulate morphs of A. silvestrii was 3.7 times greater than that of conspecific saccate morphs suggesting that campanulates may be restricted to food-rich habitats.3.)The daily ration required to maintain rm that is half the maximal population growth rate increased with increasing Asplanchna body mass and is described by the equation H=1.107 W1.103 where H=ration level and W=Asplanchna body mass. This population growth characteristic may reflect adaptations of rotifers to resource level.4.)The relationships between ration level, food concentration, and Asplanchna body mass do not support the predictions of the size-efficiency hypothesis but are consistent with observed patterns of species distribution in nature.

The potential for population growth of Ascomorpha ecaudis

The intrinsic rate of natural increase (rm) of the rotifer Ascomorpha ecaudis was found to be a hyperbolic function of food concentration. The threshold food concentration and rmax/2-food concentration determined for this species were significantly lower than values predicted from allometric models. These growth characteristics may be related to the mucus house in which Ascomorpha lives and/or the symbiotic algae living in its body tissues. The maximum rate of population growth recorded (0.71 d−1) was similar to that of other soft-bodied rotifers of similar body mass. These population growth characteristics and the resistance of this species to invertebrate predation should allow it to become a dominant member of freshwater zooplankton communities. However, field observations suggest that it is not. Reasons for this are suggested.

The influence of mixing on rotifer assemblages of Michigan lakes

Seasonal changes in the rotifer assemblages of 42 lakes in northern lower Michigan was closely related to lake mixing characteristics, basin morphometry, and the presence of an oxygenated coldwater refuge. Three major classes of lakes (dimictic, discontinuous polymitic, and continuous polymictic) were evident by their capacity to maintain coldwater species as seasons progressed from winter through fall. The disappearance of coldwater assemblages from dimictic lakes coincided with oxygen depletion in the hypolimnion or with erosion of the hypolimnion through mixing. Coldwater species disappeared from large discontinuous polymictic lakes when deep epilimnetic mixing occured in late summer and fall. Species assemblages of nearly all stratified lakes converged with those of continuous polymictic lakes when the hypolimnetic refuge deteriorated in summer and fall. Local weather conditions, however, between years had a pronounced effect on the persistence of cold water species through the seasons by affecting the temperature and oxygen conditions of the hypolimnion. Large lakes of the region contain many of the coldwater species of the Laurentian Great Lakes but some taxa are conspicuosly absent. Cold stenothermal rotifers persist in the lakes of the region despite adverse environmental conditions. Their life histories and ability to form resting stages permit them to escape periods of oxygen depletion and thermal stress. In contrast, the crustacean glacial marine fauna (i.e. Mysis relicta, Limnocalanus macrurus, and Scenecella calanoides) was absent from all of the study lakes even though many of the present-day basins were once connected to the Laurentian Great Lakes. These species long life cycles, lack of diapausing stages, and limited dispersal may make them vulnerable to local extinction with the deterioration and loss of the coldwater refuge.