Inedible Algae Research Articles

Mass mortality of the Japanese pearl oyster Pinctada fucata martensii in relation to water temperature, chlorophyll a and phytoplankton composition.

Mass mortalities of the Japanese pearl oyster Pinctada fucata martensii have widely occurred in western Japan since 1994. The causes of these mass mortalities are at present not thoroughly understood. In this study, we investigated oyster survival in relation to some environmental factors such as water temperature, concentration of chlorophyll a and density or composition of phytoplankton. The examined mass mortality occurred from September to December 1998, and the color on the adductor muscle of the oysters was red-brown, suggesting an infectious disease. Oysters that became moribund during the experiment lost weight, while the weight of unaffected oysters increased. The cell density of Nitzschia spp., an inedible algae for the oyster, in Uchiumi Bay increased before and during the mass mortality event. From the results of our study, we hypothesize that P. fucata martensii was weakened by starvation because of the dominance of inedible food and then contracted an infectious disease that resulted in mortality.

Minimal models of top‐down control of phytoplankton

1. A set of models describing the dynamics of top‐down control of phytoplankton by Daphnia in lakes is reviewed. The basis of these models is a simple and well‐known model that has been used, among other things, to demonstrate the paradox of enrichment. 2. We discuss minimal extensions that allow this model to mimic the effects of spatial heterogeneity, planktivory, seasonality and inedible algae. 3. These models generate hypotheses about mechanisms that may cause patterns observed in the field such as: Spatial heterogeneity stabilizes plankton dynamics, and causes enrichment to result in a gradual increase of both zooplankton and algal biomass. Daphnia populations can collapse at a threshold level of predation pressure from fish, rather than gradually become less abundant. Absence of Daphnia in summer may typically be caused by the combination of food shortage followed by suppression by fish. The spring clear‐water phase and various other aspects of the typical seasonal dynamics of Daphnia and algal biomass can simply be explained from the seasonal rise and fall of rates of biological processes and the reproduction cycle of fish. Depending on the conditions, cyanobacterial dominance can be stimulated but also suppressed by Daphnia. 4. We discuss the way in which such very simple models may contribute to the building of theories about plankton dynamics in the field, and the caveats of interpreting wrongly the message from models.

Large-amplitude cycles of Daphnia and its algal prey in enriched environments

Ecological theory predicts that stable populations should yield to large-amplitude cycles in richer environments1,2,3. This does not occur in nature. The zooplankton Daphnia and its algal prey in lakes throughout the world illustrate the problem4,5,6. Experiments show that this system fits the theory's assumptions7,8,9, yet it is not destabilized by enrichment6. We have tested and rejected four of five proposed explanations10. Here, we investigate the fifth mechanism: inedible algae in nutrient-rich lakes suppress cycles by reducing nutrients available to edible algae. We found three novel results in nutrient-rich microcosms from which inedible algae were excluded. First, as predicted by theory, some Daphnia-edible algal systems now display large-amplitude predator-prey cycles. Second, in the same environment, other populations are stable, showing only small-amplitude demographic cycles. Stability is induced when Daphnia diverts energy from the immediate production of young. Third, the system exhibits coexisting attractors—a stable equilibrium and large-amplitude cycle. We describe a mechanism that flips the system between these two states.

Plankton Abundance and Dynamics across Nutrient Levels: Tests of Hypotheses

In lakes and reservoirs in which Daphnia is able to suppress the biomass of edible algae far below the level set by nutrients, the interaction is stable across the range of nutrient-poor to nutrient-rich environments. This phenomenon contradicts standard consumer–resource models, which predict that dynamics should become increasingly unstable with enrichment. We test four hypotheses that might account for stability at high-nutrient levels: (1) greater abundance of inedible algae with enrichment interferes with Daphnia’s feeding; (2) Daphnia’s death rate increases with enrichment; (3) Daphnia’s death rate increases with Daphnia density; (4) Daphnia’s functional response depends on Daphnia’s density. All hypotheses are rejected because they predict much higher biomass of edible algae at high-nutrient levels than is observed. Additional evidence on Daphnia death rates strengthens the case against hypotheses (2) and (3). We consider other hypotheses and conclude that three in particular would repay further investigation. (a) Inedible algae act as a nutrient “sponge,” reducing the effective carrying capacity for edible algae; (b) limited spatial movement can enhance stability through a metapopulation-like effect, and (c) stochastic variation among individuals can be stabilizing. The central problem investigated here is a general one, with implications for many consumer–resource systems.

PLANKTON ABUNDANCE AND DYNAMICS ACROSS NUTRIENT LEVELS: TESTS OF HYPOTHESES

In lakes and reservoirs in which Daphnia is able to suppress the biomass of edible algae far below the level set by nutrients, the interaction is stable across the range of nutrient-poor to nutrient-rich environments. This phenomenon contradicts standard con sumer-resource models, which predict that dynamics should become increasingly unstable with enrichment. We test four hypotheses that might account for stability at high-nutrient levels: (1) greater abundance of inedible algae with enrichment interferes with Daphnia's feeding; (2) Daphnia's death rate increases with enrichment; (3) Daphnia's death rate increases with Daphnia density; (4) Daphnia's functional response depends on Daphnia's density. All hypotheses are rejected because they predict much higher biomass of edible algae at high-nutrient levels than is observed. Additional evidence on Daphnia death rates strengthens the case against hypotheses (2) and (3). We consider other hypotheses and conclude that three in particular would repay further investigation. (a) Inedible algae act as a nutrient sponge, reducing the effective carrying capacity for edible algae; (b) limited spatial movement can enhance stability through a metapopulation-like effect, and (c) stochastic variation among individuals can be stabilizing. The central problem investigated here is a general one, with implications for many consumer-resource systems.

How Daphnia cope with algae selected for inedibility in long- running microcosms

Macro video records of restrained feeding Daphnia enabled us to measure simultaneously carapace gape, claw rejection rate, filter limb beat rate, and mandible movement rate. We compared the effects of high and low concentrations of highly edible algae and of inedible algae, the latter selected by long-term Daphnia grazing in oligotrophic microcosms. Inedible algae slowed the filtering process and influenced the carapace gape (widening at low concentration and narrowing at high), but did not affect the rejection rate.

Predicted versus actual determinations of algal production, algal biomass, and zooplankton biomass in a hypereutrophic, hyposaline prairie lake

We compared the accuracy of various regression models in predicting algal production, algal biomass and composition, and zooplankton biomass in a hypereutrophic, hyposaline prairie lake. The total phosphorus (TP) models investigated underestimated mean summer algal biomass and inedible biomass: the models overestimated mean summer edible algae biomass and annual primary production in the euphotic zone. Differences between predicted and actual biomass values are attributed to intense zooplankton grazing on the edible algal community and to the gradual accumulation of slow-growing, inedible algae. The TP model investigated provided an accurate prediction of zooplankton biomass. The algal biomass model overestimated zooplankton biomass, possibly because edible algae accounted for a very small fraction of algal biomass in Humboldt Lake during the ice-free season. The chlorophyll model investigated underestimated zooplankton biomass, apparently because Humboldt Lake algae have a relatively low chlorophyll content. The use of a 0.01 conversion factor to estimate algal biomass on the basis of chlorophyll appears to be inadequate and requires further study. There was no evidence that hyposaline Humboldt Lake has a relatively high zooplankton to phytoplankton biomass ratio when compared with freshwater lakes.

A Predator-Prey Model for Zooplankton Grazing on Competing Algal Populations

The typical freshwater algal community can be roughly categorised into two groups according to the distribution of biomass with cell size. Algae in the small size group are known to be edible for zooplankton grazers; large algae are much less edible. To model population dynamics of a size selective zoaplankton predator like Daphnia we thus consider a 3-species predator-prey model with an "edible" and an "inedible" prey. We show that a system which in the absence of inedible algae exhibits cyclic dynamics may exhibit a stable equilibrium if inedible algae are introduced. This stabilisation can be caused by one or both of the following mechanisms: (a) "indirect interaction" involving interspecific competition between the prey which reduces the effective carrying capacity of the edible species, and (b) "direct interaction" in which there is a reduction of the attack rate on the edible prey when the predator spends time handling inedibles. We study the behaviour of the system under enrichment. With only the indirect interaction, the equilibrium density of the edible prey is unaffected by enrichment, but the density of both inedible prey and predator increases. With sufficient enrichment the system can always be destabilised. If there is direct interaction, the equilibrium density of all three species increases with enrichment: the edible fraction decreases, but has a lower bound. If the direct interaction is sufficiently strong there are regions of parameter space for which the system can never be destabilised by enrichment.

Nutrient Enrichment and Grazer Effects on Phytoplankton in Lakes

I derived predictions about how the magnitude of Daphnia effects on total phytoplankton biomass should vary across a gradient of enrichment (expressed as algal carrying capacity) using two, simple predator—prey models. These predictions were then compared with data from a survey of field experiments in temperate lakes. Algal responses to Daphnia manipulation were quantified as an Algal Response Factor (ARF), defined as total algal biomass in the low—Daphnia treatment divided by total algal biomass in the high—Daphnia treatment. Total phosphorus concentration (TP) was used as an index of algal carrying capacity, ranging from 10 to 460 mg/L over the 22 experiments surveyed. The Algal Response Factor ranged from 1 to 40 and was a positive, linear function of TP: ARF = ‐0.14 + 0.08(TP), r2 = 0.81; log ARF = ‐0.81 + 0.83(log TP), r2 = 0.75. Thus, algal carrying capacity, as quantified by TP, explains much of the variation in Daphnia effects on total algal biomass across lakes. The survey results supported the prediction of the simpler model, a two—species, Lotka—Volterra model of pure exploitation. Incorporating the additional complexity of inedible algae into this model did little to improve its predictive power. The conclusion that inedible algae are not of major importance to the prediction of Daphnia effects across an enrichment gradient was supported by a survey of Daphnia effects on the proportionate biomass of inedible algae. There was no evidence that Daphnia grazing typically favors dominance by inedible algae at equilibrium in highly enriched lakes. My results suggest that the well—established positive relationship between TP and equilibrium algal biomass in lakes is more likely to be a consequence of increases in Daphnia's death rate with enrichment, rather than decreases in Daphnia's feeding and assimilation rates. When zooplanktivorous fish are rare, Daphnia should be able to prevent phytoplankton biomass from responding to nutrient addition at equilibrium. As a consequence, the success of biomanipulation in eutrophic lakes should critically depend upon the effectiveness of strategies aimed at reducing zooplanktivory.

Short term experiments on calanoid-cyclopoid-phytoplankton interactions

To investigate their potential effects on each other in nature, calanoid (Diaptomus clavipes and D. siciloides) and cyclopoid (Acanthocyclops vernalis and Mesocyclops edax) copepod populations were manipulated in 5 liter aquaria in laboratory experiments of 20–60 days duration. Diaptomus generally had a strongly negative effect on both cyclopoid species. The cyclopoids established populations more successfully when introduced to aquaria before calanoids than they did when calanoids were already present. On the other hand, whether introduced earlier or later than the cyclopoids, Diaptomus populations were unaffected by Acanthocyclops and were strongly depressed by Mesocyclops. Diaptomus effects on the phytoplankton were often strong but varied markedly among experiments. They included reduction of populations of edible algae, such as Chlamydomonas, which are essential for both calanoid and cyclopoid nauplii, and large increases in inedible algae, such as Kirchneriella. Feeding experiments revealed that under conditions of food scarcity Acanthocyclops nauplii survived less well than did Diaptomus nauplii. Competition for edible phytoplankton seemed to be a key factor in the calanoid-cyclopoid interactions, since the survival of herbivorous cyclopoid larvae determined the abundance of the predaceous adults. This indicates that the competitive effects of calanoids on cyclopoids often may exceed the predative effects of cyclopoids on calanoids.

The relative effects of enrichment and climate change on the long‐term dynamics of Daphnia in Esthwaite Water, Cumbria

SUMMARY. 1. The factors influencing the seasonal and inter‐annual variations in the numbers of Daphnia hyalina in Esthwaite Water between 1956 and 1972 are analysed. Esthwaite Water has always been eutrophic. but the phosphorus and nitrogen loadings to the lake increased significantly in the mid 1960s.2. Qualitatively, the phytoplankton and zooplankton populations in the lake changed relatively little during the period of study. Quantitatively, however, eutrophic species of algae became more abundant and the numbers of Eudiaptomus declined as the numbers of Daphnia increased.3. The seasonal dynamics of the Daphnia was governed partly by the seasonal temperature cycle, and partly by the periodicity of edible algae. The birth rate of the Daphnia was constrained by temperature from January to April and from October to December. At other times their rate of increase was governed by the relative abundance of edible and inedible algae.4. Edible and inedible species of algae tended 10 appear in a recurring annual sequence. Diatoms such as Asterionella were abundant in the spring, the early summer phytoplankton was dominated by edible flagellates, but inedible algae such as Aphanizomenon and Microcystis become dominant later in the year.5. Daphnia could only reproduce in late summer when there were periodic regrowths of edible algae. Such regrowths were most likely to occur when there had been some entrainment of deep nutrients by episodic wind mixing. Calm weather encouraged the growth of blue‐green algae that effectively‘blocked’the development of the Daphnia for the remainder of the summer.6. The factors that controlled the seasonal dynamics of the Daphnia also influenced the average number recorded in a particular year. The average number of Daphnia increased in the early sixties when Cryptomonas was abundant and decreased in the late sixties when blooms of Aphanizomenon appeared in mid summer. Detailed analyses showed that a similar increase in the numbers of Aphanizometion had occurred in the late fifties. The critical factor throughout was the prolonged period of calm resulting in stable stratification.7. This ‘weather’ effect was highlighted by comparing de‐trended timeseries of Daphnia and Aphanizometion numbers with a simple measure of thermocline stability. De‐trending removed the superimposed effects of progressive enrichment and revealed a 10‐year cycle of thermocline stability that matched the temperature cycle recently reported in Windermere, These cycles are related to the movement of weather systems in the Atlantic so could change if the pattern of atmospheric circulation is altered by global warming.8. The possible effects of climate change on Daphnia dynamics are discussed in relation to the findings in Esthwaite Water.

Simple Models and Variation in Plankton Densities Among Lakes

To explore the idea that the dynamics of the zooplankter Daphnia and its algal food supply can be accounted for by simple predator-prey models, we tested the ability of such models to predict how the average densities change along a gradient of nutrient enrichment. Total algal carrying capacity is known to increase from nutrient-poor to nutrient-rich lakes. Here we show that this is true also for edible algae alone. We also provide evidence that the realized per capita growth rate of edible algae increases with trophy (trend 1). A synthesis of laboratory and field data suggests two other trends associated with increasing nutrient status: (2) Daphnia's attack rate decreases in response to increasing concentrations of inedible algae; and (3) its death rate increases in response to greater predation pressure. In models of the Lotka-Volterra type, either trend 2 or a combination of trends 1 and 3 leads to the prediction that the abundance of both Daphnia and their algal prey should increase along a nutrient gradient in natural habitats where inedible algae and predators of Daphnia both occur. In contrast, where these features are missing, the original "paradox of enrichment" prediction is expected: Daphnia density should increase but algal density should not. Our analysis of more than 30 study-years of data from natural lakes confirmed the first of these predictions; results from outdoor artificial habitats, lacking inedible algae and predators of Daphnia, confirmed the second. We cannot evaluate the relative importance of the two mechanisms competing to explain the result from natural habitats, but our analyses suggest new observations and experiments that could test these mechanisms. These results are consistent with our general hypothesis that the dynamics of the Daphnia-algae interaction can be explained by simple predator-prey models. In particular, they support two assumptions: (1) other species in the system can be modeled simply as part of the environment of the predator-prey interaction; and (2) variation in the dynamics from one habitat to another results from merely quantitative differences in parameter values and does not require us to postulate qualitative, or "structural," changes among habitats.

Effects of a carbamate insecticide, carbaryl, on the summer phyto- and zooplankton communities in ponds

A carbamate insecticide, carbaryl (1-naphthyl- N-methylcarbamate), was applied in concrete ponds and the effects on plankton communities were studied. In a control pond, Cladocera declined following the increase in the density of inedible algae after a cladoceran peak. Once the density of Cladocera became low, Chaoborus larvae suppressed the increase of Cladocera and consequently supported the rotifer dominance in the zooplankton community by their selective predation on cladocerans. In a treated pond, the plankton community and its succession were similar to those in the control pond until the chemical application. 1 ppm of carbaryl killed all zooplankton and Chaoborus larvae. Cladocera reappeared soon and increased rapidly due to the absence of Chaoborus larvae. Consequently, rotifer populations were suppressed. Thus, the chemical application altered the dominance of rotifers to that of cladocerans. The same phenomenon was observed again after the second chemical application 12 days after. Although apparent direct effects of the chemical application on phytoplankton were not found, the phytoplankton community structure changed following the changes in the zooplankton density.

The effect of food quality on feeding and respiration by Daphnia and Diaptomus1

Filtering, ingestion, and respiration rates for Daphnia pulex and Diaptomus siciloides were measured in southern Green Bay water mixed with algal cultures to produce varying proportions of blue‐green algae. These measurements along with observations of Daphnia mouthpart responses to these foods were used to assess the relative competitive potential of Daphnia and calanoid copepods in lakes of varying trophy. Ratios of filtering and ingestion rate to respiration rate were used as a measure of energy input per unit of energy expended. Passive filtering and subsequent rejection of inedible algae by Daphnia result in higher ratios than those shown by Diaptomus in high quality foods, suggesting a competitive advantage in oligotrophic or mesotrophic lakes. Diaptomus selective feeding, on the other hand, results in higher ratios in the low quality food characteristic of eutrophic conditions. These results and data from the literature are used to propose a two‐dimensional niche model to demonstrate the interaction between food quality and abundance on the competitive interaction between calanoid copepods and Daphnia‐like cladocerans.

Zooplankton grazing and phytoplankton species richness: Field tests of the predation hypothesis1

The hypothesis that herbivory plays a major role in the maintenance of high algal diversity was tested over a 2‐year period in polyethylene enclosures at Heney Lake, Quebec. Reduction of grazing pressure resulted in a significant decrease in the number of inedible species, whereas the diversity of edible algae remained unaffected. Lowering the herbivore levels may cause an intensification of exploitative competition among phytoplankton, which favors edible species and a few inedible algae like Synedra at the expense of many inedible species that are eliminated.