Keystone predator effects and grazer control of planktonic primary production

If prey species exhibit trade-offs in their ability to utilize resources versus their ability to avoid predation, predators can facilitate prey turnover along gradients of productivity, shifting dominance from edible to inedible prey (the keystone predator effect). I tested this model under controlled, laboratory conditions, using a model aquatic system composed of zooplankton as the top consumer, a diverse community of algae as prey, and nutrients as basal resources. Nutrient manipulations (low and high) were crossed with presence–absence of zooplankton. Results supported theoretical predictions. Algal biomass increased in response to enrichment regardless of predator presence/absence. However, predators and nutrients had an interactive effect on algal biomass and size structure. At the low nutrient level, algal-prey were dominated by edible forms and attained similar biomass regardless of zooplankton presence/absence. At the high level of enrichment, presence of zooplankton favored higher levels of algal biomass and shifted dominance to large, inedible taxa. At the termination of the experiment, I performed a series of lab-based assays on the resultant algal community in order to quantify trade-offs among algal size classes in maximal population growth rates (as a measure of competitive ability for nutrients) and susceptibility to zooplankton grazing. Assays provided support for a size-based keystone trade-off. Small size classes of algae displayed higher maximal growth rates but were more susceptible to grazing effects. Large size classes were protected from grazing but showed low rates of population growth in response to enrichment.     

萼花臂尾轮虫培养滤液对铜绿微囊藻、斜生栅藻和小球藻群体形成及生长的影响

为探索浮游动物和藻类之间可能存在的信息传递,研究了萼花臂尾轮虫培养滤液对铜绿微囊藻、斜生栅藻和小球藻的生长及群体形成的影响.把萼花臂尾轮虫按1000个·L^-1的初始密度置于小球藻中培养24h后,用孔径0.15μm的微孔滤膜抽滤,得到轮虫培养滤液,此滤液中含有轮虫在生活过程中释放的一些信息化学物质.将轮虫培养滤液以20%的比例分别加入纯培养的铜绿微囊藻、斜生栅藻和小球藻中,进行为期7d的试验.结果表明,萼花臂尾轮虫培养滤液能显著地促进斜生栅藻的群体形成,而对铜绿微囊藻和小球藻在群体形成方面没有显著作用.另外,该滤液能显著提高小球藻种群的增长,对铜绿微囊藻和斜生栅藻的生长无明显影响.3种藻类对萼花臂尾轮虫的潜在牧食采取了不同的生态策略:斜生栅藻形成群体,增大摄食阻力,从而降低被摄食的风险;小球藻通过提高增长率来抵消被取食的损失;铜绿微囊藻是通过其它方式来降低被牧食(例如毒素).这些方式分别是这些藻类维持种群规模的反牧食防御策略之一.     

Linking herbivore-induced defences to population dynamics

1. Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi‐ and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator–prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2. We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3. A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4. We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities.     

Are Trade-Offs Among Species’ Ecological Interactions Scale Dependent? A Test Using Pitcher-Plant Inquiline Species

Trade-offs among species' ecological interactions is a pervasive explanation for species coexistence. The traits associated with trade-offs are typically measured to mechanistically explain species coexistence at a single spatial scale. However, species potentially interact at multiple scales and this may be reflected in the traits among coexisting species. I quantified species' ecological traits associated with the trade-offs expected at both local (competitive ability and predator tolerance) and regional (competitive ability and colonization rate) community scales. The most common species (four protozoa and a rotifer) from the middle trophic level of a pitcher plant (Sarracenia purpurea) inquiline community were used to link species traits to previously observed patterns of species diversity and abundance. Traits associated with trade-offs (competitive ability, predator tolerance, and colonization rate) and other ecological traits (size, growth rate, and carrying capacity) were measured for each of the focal species. Traits were correlated with one another with a negative relationship indicative of a trade-off. Protozoan and rotifer species exhibited a negative relationship between competitive ability and predator tolerance, indicative of coexistence at the local community scale. There was no relationship between competitive ability and colonization rate. Size, growth rate, and carrying capacity were correlated with each other and the trade-off traits: Size was related to both competitive ability and predator tolerance, but growth rate and carrying capacity were correlated with predator tolerance. When partial correlations were conducted controlling for size, growth rate and carrying capacity, the trade-offs largely disappeared. These results imply that body size is the trait that provides the basis for ecological interactions and trade-offs. Altogether, this study showed that the examination of species' traits in the context of coexistence at different scales can contribute to our understanding of the mechanisms underlying community structure.     

Different Nutritional Histories Affect the Susceptibility of Algae to Grazing

We hypothesize that algae with different cell compositions are differently perceived by their predators and consequently subjected to selective grazing. Five populations of the diatom Phaeodactylum tricornutum that differed in organic and elemental composition, but were otherwise identical, were generated by acclimation to distinct growth regimes. The different populations were then mixed in pairs and subjected to predation by either the rotifer Brachionus plicatilis or the copepod Acartia tonsa. The presence of rotifers had no impact on the ratio between any two algal populations. The presence of copepods, however, affected the ratio between algae previously acclimated to a medium containing 1 mM NH₄⁺ and algae acclimated to 0.5 mM NO₃^?, and to either a lower irradiance or a higher CO₂ concentration. We discuss the possible reason for the influence of different nutritional histories on the vulnerability of algae to predators. The differential impact of grazers on the growth of algae with different nutritional histories may result from direct selective grazing (i.e., grazers can detect algae with the most palatable cell composition), alone or combined to an asymmetric utilization of the nutrients regenerated after predation by co‐existing algal populations. Our results strongly suggest that the nutritional history of algae can influence the relationships between phytoplankton and grazers and hint at the possibility that algal cell composition is potentially subject to natural selection, because it influences the probability that algae survive predation.     

Population growth rates of two closely related rotifer species: effects of food quantity, particle size, and nutritional quality

SUMMARY. 1. The relationship between population growth rates and the concentrations of several algal species was determined in laboratory experiments with the rotifers Brachionus rubens and B. calyciflorus .
2. The effects of food quantity were well described by a modified Monod model with a threshold for zero population growth. The model parameters depended on particle size and nutritional quality of the food algae. Differences between the rotifer species were significant and reflected their varying food-size preferences.
3. For each rotifer species, thresholds were lowest for algae in the most readily ingested size range. The lowest thresholds were 0.07–0.09 mgC 1⁻¹ with algae of about 5 μm equivalent spherical diameter (ESD) for B. rubens , and 0.19 mgC 1⁻¹ with algae of about 10 μm ESD for B. calyciflorus .
4. Maximal growth rates ( r _max) were slightly below 0.8 day⁻¹ for both rotifers with most algal species. The highest r _max values for both rotifers were observed when Cyclotella meneghiniana was provided as food. With this alga, B. calyciflorus had a significantly higher r_max (1.02 day⁻¹) than B. rubens (0.838 day⁻¹).
5. From a comparison of the relationship between growth rates and ingestion rates, Chlamydomonas reinhardii appeared to be of low nutritional quality for B. rubens .
6. Egg ratios were related to growth rate and were not influenced by the algal food used. Egg development times and average mortality rates were estimated from the relationship between egg ratio and growth rate. B. calyciflorus appeared to have a high average mortality rate (0.383 day⁻¹) compared to B. rubens (0.083 day⁻¹).     

Promotion of harmful algal blooms by zooplankton predatory activity

The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients. Eutrophication raises nutrient levels, but does not simply increase normal predator-prey activity; rather, harmful algal bloom (HAB) events develop often with serious ecological and aesthetic implications. Generally, HAB species are outwardly poor competitors for nutrients, while their development of grazing deterrents during nutrient stress ostensibly occurs too late, after the nutrients have largely been consumed already by fast-growing non-HAB species. A new mechanism is presented to explain HAB dynamics under these circumstances. Using a multi-nutrient predator-prey model, it is demonstrated that these blooms can develop through the self-propagating failure of normal predator-prey activity, resulting in the transfer of nutrients into HAB growth at the expense of competing algal species. Rate limitation of this transfer provides a continual level of nutrient stress that results in HAB species exhibiting grazing deterrents protecting them from top-down control. This process is self-stabilizing as long as nutrient demand exceeds supply, maintaining the unpalatable status of HABs; such events are most likely under eutrophic conditions with skewed nutrient ratios.     

Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom‐up (e.g. nutrients and light, which determine growth rates) or from the top‐down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient‐enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient‐enriched stream habitats of the Owennagearagh River, south‐west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high‐voltage localised electric pulses. Replicates of treatment (grazer‐excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2‐week Cladophora cover, periphytic chlorophyll a and biofilm ash‐free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer‐excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient‐enriched site. 4. Our results demonstrate that in eutrophic, high‐light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.     

Effect of Detachment on the Palatability of two Kelp Species

Many species of macroalgae survive after becoming dislodged from their primary substratum, but little is known about their capacity to express anti-herbivore defences after detachment. We examined the effect of detachment on the relative palatability of the two kelp species Lessonia nigrescens and Macrocystis integrifolia to mesograzers. Laboratory and field experiments were conducted on the northern-central coast of Chile to investigate whether (i) time after detachment and (ii) grazing on detached and attached algae could trigger internal defence mechanisms in the algae, which may have acted as deterrents to grazing. In order to examine palatability, feeding assays were run after each experiment using fresh algal pieces and artificial food. Time after detachment had a significant influence on palatability of L. nigrescens but not of M. integrifolia. During the first 12 days of detachment, detached L. nigrescens held in grazer-free laboratory tanks were not significantly more palatable than attached conspecifics from the field but thereafter detached individuals became more palatable. Floating individuals of M. integrifolia showed no effect of detachment, indicating that this alga maintains its defence after detachment. An experiment conducted in the field confirmed these results for M. integrifolia. An additional laboratory experiment confirmed that attachment status plays an important role on algal defence reaction for L. nigrescens when exposed to grazers. Detached and previously grazed individuals of this species were less palatable than grazer-free control algae, but grazing had no effect on palatability of attached algae. Our results indicate that kelps have varying capacities for development of anti-grazing responses once they become detached, possibly depending on their capacity to float and survive after detachment.     

Sea-urchin feeding fronts

Sea-urchin feeding fronts are a striking example of spatial pattern formation in an ecological system. If it is assumed that urchins are asocial, and that they move randomly, then the formation of these dense fronts is an apparent paradox. The key lies in observations that urchins move further in areas where their algal food is less plentiful. This naturally leads to the accumulation of urchins in areas with abundant algae. If urchin movement is represented as a random walk, with a step size that depends on algal concentration, then their movement may be described by a Fokker–Planck diffusion equation. For certain combinations of algal growth and urchin grazing, traveling wave solutions are obtained. Two-dimensional simulations of urchin algal dynamics show that an initially uniformly distributed urchin population, grazing on an alga with a smoothly varying density, may form a propagating front separating two sharply delineated regions. On one side of the front algal density is uniformly low, and on the other side of the front algal density is uniformly high. Bounds on when stable fronts will form are obtained in terms of urchin density and grazing, and algal growth.     

食物种类对萼花臂尾轮虫种群增长率的影响

在短期慢性观测过程中,食物类型可能是造成萼花臂尾轮虫(Brachionuscalyciflorus)种群繁殖率变化的一种原因。共观测了分别单独投喂10种不同绿藻对轮虫种群增长率的影响。为验证藻青菌是藻类饵料(如绿藻Scenedesmus)有价值的佐剂这一假说,还用蓝细菌单独投喂或与斜生栅藻(Scenedesmusobliquus)混合投喂轮虫进行实验观测。结果发现食物种类对萼花臂尾轮虫种群增长率影响显著。斜生栅藻组获得最大种群增长(1.6/d),而Desmodesmus组增长率最低(0.3/d)。以占斜生栅藻组最大增长的百分率来表示,其它几种绿藻组种群增长由高到低依次为:Desmodesmussubspicatus88%,小球藻(Chlorellavulgaris)83%,单壳缝藻(Monoraphidiumminutum)77%,D.quadricauda74%,S.falcatus71%,S.acuminatus69%,S.pectinatus64%,莱茵衣藻(Chlamydomonasreinhardtii)57%,D.abundans19%。轮虫增长率的差异不能用藻类饵料的大小差异来解释。蓝细菌(Microcystisaeruginosa)和(Synechococcuselongates)不论是单独投喂还是与优良藻类饵料(斜生栅藻)混合投喂都对萼花臂尾轮虫种群增长有抑制作用。这种副作用似乎与微囊藻素无关。该结果不支持无毒蓝细菌可作为与其他绿藻饵料配合使用的优良佐剂这一假说。本研究所观察到的生长变化显示了饵料种类对萼花臂尾轮虫种群增长的影响,也预示了对毒性实验结果的影响     

Steady-state rotifer growth in a two-stage, computer-controlled turbidostat

Steady-state populations of rotifers (Brachionus calyciflorus)were maintained in twostage, continuous-flow turbidostatic cultureon the green alga Chlorella pyrenoidosa. In this system, themaximum specific growth rate,µ^max of the rotifers wasmaintained by using a computer to control the concentrationof algae, as rotifer food, in the rotifer culture. As rotifersconsumed algae, the turbidity decreased until a set-point wasreached. Then fresh algal suspension (supplied from a steady-statealgal chemostat) was metered into the rotifer culture, whichwas held in the dark. Rotifer and algal populations, as wellas rotifer µ^max entered steady states. These steady-stateresults were consistent with previous data from chemostat studies,but growth transients indicated that the of the µ^maxrotifersmay be subject to selection. The system is unique in providinga means to explore population dynamics of a metazoan maintainednear its µ^max.     

The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

Defenses in phytoplankton against grazing induced by nutrient limitation, UV-B stress and infochemicals

It is becoming increasingly evident that the efficiency of zooplankton grazing on algae is not only a matter of quantity of the grazer relative to its food. Planktonic primary producers are not defenseless food-particles that are easily harvested by the consumers. Several algal species are able to adjust their phenotype (colony formation, spines, size) in such a way that it results in a reduced grazing pressure. It was recently demonstrated that morphological changes in the cell wall of green algae, induced by nutrient limitation and UV-B stress, may reduce their digestibility. A high fraction of induced cells pass intact and viable through the gut of the zooplankters, such that the grazing impact on the population is strongly reduced. It was also found that the presence of exudates (infochemicals) released by daphnids may change the morphology of algae. Unicellular green algae of the genus Scenedesmus were induced to form eight-cell coenobial types, heavily armed with spines, within three to five days after adding filtered water from an algal culture with Daphnia present. Both defence mechanisms may play an important role in zooplankton production and competition, and may serve as an example of highly efficient strategies to resist heavy grazing pressure.     

Seasonal shifts in the importance of bottom–up and top–down factors on stream periphyton community structure

We examined the importance of temporal variability in top–down and bottom–up effects on the accumulation of stream periphyton, which are complex associations of autotrophic and heterotrophic microorganisms. Periphyton contributes to primary production and nutrient cycling and serves as a food resource for herbivores (grazers). Periphyton growth is often limited by the availability of nitrogen and phosphorus, and biomass can be controlled by grazers. In this study we experimentally manipulated nutrients and grazers simultaneously to determine the relative contribution of bottom–up and top–down controls on periphyton over time. We used nutrient diffusing substrates to regulate nutrient concentrations and an underwater electric field to exclude grazing insects in three sequential 16–17 day experiments from August to October in montane Colorado, USA. We measured algal biomass, periphyton organic mass, and algal community composition in each experiment and determined densities of streambed insect species, including grazers. Phosphorus was the primary limiting nutrient for algal biomass, but it did not influence periphyton organic mass across all experiments. Effects of nutrient additions on algal biomass and community composition decreased between August and October. Grazed substrates supported reduced periphyton biomass only in the first experiment, corresponding to high benthic abundances of a dominant mayfly grazer (Rhithrogena spp.). Grazed substrates in the first experiment also showed altered algal community composition with reduced diatom relative abundances, presumably in response to selective grazing. We showed that top–down grazing effects were strongest in late summer when grazers were abundant. The effects of phosphorus additions on algal biomass likely decreased over time because temperature became more limiting to growth than nutrients, and because reduced current velocity decreased nutrient uptake rates. These results suggest that investigators should proceed with caution when extending findings based on short‐term experiments. Furthermore, these results support the need for additional seasonal‐scale field research in stream ecology.     

Evolution of nutrient uptake reveals a trade-off in the ecological stoichiometry of plant-herbivore interactions

Nutrient limitation determines the primary production and species composition of many ecosystems. Here we apply an adaptive dynamics approach to investigate evolution of the ecological stoichiometry of primary producers and its implications for plant-herbivore interactions. The model predicts a trade-off between the competitive ability and grazing susceptibility of primary producers, driven by changes in their nutrient uptake rates. High nutrient uptake rates enhance the competitiveness of primary producers but also increase their nutritional quality for herbivores. This trade-off enables coexistence of nutrient exploiters and grazing avoiders. If herbivores are not selective, evolution favors runaway selection toward high nutrient uptake rates of the primary producers. However, if herbivores select nutritious food, the model predicts an evolutionarily stable strategy with lower nutrient uptake rates. When the model is parameterized for phytoplankton and zooplankton, the evolutionary dynamics result in plant-herbivore oscillations at ecological timescales, especially in environments with high nutrient availability and low selectivity of the herbivores. High herbivore selectivity stabilizes the community dynamics. These model predictions show that evolution permits nonequilibrium dynamics in plant-herbivore communities and shed new light on the evolutionary forces that shape the ecological stoichiometry of primary producers.     

Interactions between red tide microalgae and herbivorous zooplankton: effects of two bloom-forming species on the rotifer <Emphasis Type="Italic">Brachionus plicatilis</Emphasis> (O.F. Muller)

Experiments were carried out to investigate interspecific interactions between the rotifer Brachionus plicatilis and two harmful algal bloom (HAB) species using single and mixed culture methods. B. plicatilis populations and the growth of two algae were compared at different algal cell densities. The results demonstrate that B. plicatilis obtained sufficient nutrition from Alexandrium tamarense to support net population increase. When exposed to a density of 8 × 10⁴ cells ml⁻¹ A. tamarense, the number of B. plicatilis increased faster than it did when exposed to other four algal densities (16 × 10⁴, 24 × 10⁴, 32 × 10⁴, and 40 × 10⁴ cells ml⁻¹). Cell densities of A. tamarense decreased due to the grazing of B. plicatilis. In contrast, Heterosigma akashiwo had an adverse effect on the B. plicatilis population and its growth was largely unaffected by rotifer grazing. In this case, the B. plicatilis population decreased and H. akashiwo grew at a rate similar to that of a control without addition of rotifers. Mixed culture experiments showed that A. tamarense could partly counteract the effect of H. akashiwo in limiting the rate of population increase of rotifer. In addition, the effect of different initial cell densities on interspecific competition between A. tamarense and H. akashiwo in mixed culture(s) was also investigated. The results show that A. tamarense competed very successfully when the inoculation proportions of A. tamarense and H. akashiwo were 40:5 and 40:30. Handling editor: D. Hamilton     

Effects of consumers and enrichment on abundance and diversity of benthic algae in a rocky intertidal community

Human alteration of nutrient cycling and the densities of important consumers have intensified the importance of understanding how nutrients and consumers influence the structure of ecological systems. We examined the effects of both grazing and nutrient enrichment on algal abundance and diversity in a high-intertidal limpet-macroalgal community on the South Island of New Zealand, a relatively nutrient-poor environment. We used a fully factorial design with three levels each of grazing (manipulations of limpet and snail densities) and nutrients (nutrient-diffusers attached to the rock). Top-down control by grazers appears to be the driving organizing mechanism for algal communities in this system, with strong negative effects of grazing on algal diversity and abundance across all levels of nutrient enrichment. However, in contrast to the conclusions drawn from the analysis of the whole algal community, there was an interactive effect of grazing and enrichment on foliose algae, an important component of the algal system. When herbivory was reduced to very low levels, enrichment generated increases in the abundance and biomass of foliose algae. As expected, top-down control was the primary determinant of algal community structure in this system, controlling abundance and diversity of macrophytes on the upper shore. Contrary to expectations, however, increased nutrients had no community-wide effects, although foliose algal abundance increases were greatest with high nutrients and reduced grazing. It seems likely that most of the corticated algal species have limited capacity to respond to nutrient pulses in this nutrient-poor environment.     

The effects of barley straw (Hordeum vulgare) on the growth of freshwater algae

Bioassays were conducted to determine the efficacy of barley straw liquor in controlling algal growth of 12 freshwater species of algae representing three divisions. Barley straw liquor inhibited the growth of three nuisance algae common in freshwater: Synura petersenii, Dinobyron sp., and Microcystis aeruginosa. However, Selenastrum capricornutum, Spirogyra sp., Oscillatoria lutea var. contorta, and Navicula sp. had significantly increased growth in the presence of straw liquor. The growth of the remainder, Ulothrix fimbriata, Scenedesmus quadricauda, Chlorella vulgaris, Anabaena flos-aquae, and Synedra sp. showed no significant difference from controls. In a related field study, we treated four of six ponds with barley straw and monitored their chlorophyll a levels for one growing season. While phytoplankton populations in all ponds decreased in midsummer, the phytoplankton biomass in treated ponds did not differ significantly from that of control ponds, suggesting that the application of barley straw had no effect on algal growth in these systems.