Grazer Growth Research Articles

Stoichiometric microplastics models in natural and laboratory environments

Microplastics pose a severe threat to marine ecosystems; however, relevant mathematical modeling and analysis are lacking. This paper formulates two stoichiometric producer-grazer models to investigate the interactive effects of microplastics, nutrients, and light on population dynamics under different settings. One model incorporates optimal microplastic uptake and foraging behavior based on nutrient availability for natural settings, while the other model does not include foraging in laboratory settings. We establish the well-posedness of the models and examine their long-term behaviors. Our results reveal that in natural environments, producers and grazers exhibit higher sensitivity to microplastics, and the system may demonstrate bistability or tristability. Moreover, the influences of microplastics, nutrients, and light intensity are highly intertwined. The presence of microplastics amplifies the constraints on grazer growth related to food quality and quantity imposed by extreme light intensities, while elevated phosphorus input enhances the system’s resistance to intense light conditions. Furthermore, higher environmental microplastic levels do not always imply elevated microplastic body burdens in organisms, as organisms are also influenced by nutrients and light. We also find that grazers are more vulnerable to microplastics, compared to producers. If producers can utilize microplastics for growth, the system displays significantly greater resilience to microplastics.

Increased salinity stress changes plant productivity and biomass by altering the top‐down controls in eelgrass beds

AbstractUnderstanding biological community responses to disturbances such as extreme rainfalls under climate change is crucial when predicting ecosystem changes and functions. However, it is still unclear how species interaction changes with increasing disturbances and how disturbance susceptibility varies among organisms from habitats with different stress levels along stress gradients such as salinity gradients. We examined whether (1) higher frequency (i.e., pulse disturbance) and longer duration (i.e., press disturbance) of salinity change affected the primary producers, animals, and their interactions, and whether (2) organisms from naturally stressful habitats with unstable salinity were more tolerant to salinity changes. We used eelgrass Zostera marina, epiphytic microalgae, and grazer gastropod Lacuna decorata from two sites in each of the two lagoons; Akkeshi with a strong salinity gradient, and Notoro with a weak gradient to do indoor mesocosm experiments for testing the effects of salinity change and grazer presence on eelgrass growth and biomass of epiphytic microalgae, as well as grazer survival, growth, and consumption rates. We established treatments with low, intermediate, and high levels of salinity stress to test the effects of press and pulse disturbances in terms of salinity changes. The findings suggest the presence of a top‐down control of epiphytic microalgae by grazers. Eelgrass growth rate was higher with the presence of grazers but declined with higher salinity stress. The biomass of epiphytic microalgae was higher with the absence of grazers and with higher salinity stress across all sites. Grazer survival varied among individuals raised in habitats with naturally stable or unstable salinity regime and decreased with more frequent and longer salinity changes. Grazer growth rates and grazing rates also decreased with higher salinity stress, but grazing on epiphytic microalgae was highest at the intermediate stress levels across all sites. This study demonstrates that salinity regime shifts that are expected to occur with extreme rainfalls due to ongoing climate change could affect coastal ecosystems such as eelgrass beds, and we call for further investigations on the different stress responses of organisms raised in habitats with stable and unstable salinity to understand the potential effects of the disturbances on ecosystem function.

Feedback between bottom-up and top-down control of stream biofilm mediated through eutrophication effects on grazer growth

Algal biofilms in streams are simultaneously controlled by light and nutrient availability (bottom-up control) and by grazing activity (top-down control). In addition to promoting algal growth, light and nutrients also determine the nutritional quality of algae for grazers. While short-term experiments have shown that grazers increase consumption rates of nutrient-poor algae due to compensatory feeding, nutrient limitation in the long run can constrain grazer growth and hence limit the strength of grazing activity. In this study, we tested the effects of light and phosphorus availability on grazer growth and thus on the long-term control of algal biomass. At the end of the experiment, algal biomass was significantly affected by light, phosphorus and grazing, but the interactive effects of the three factors significantly changed over time. At both high light and phosphorus supply, grazing did not initially reduce algal biomass, but the effect of grazing became stronger in the final three weeks of the experiment. Snail growth was enhanced by light, rather than phosphorus, suggesting that algal quantity rather than quality was the main limiting factor for grazer growth. Our results highlight the role of feedback effects and the importance of long-term experiments in the study of foodweb interactions.

Investigating the influences of quorum quenching and nutrient conditions on activated sludge flocs at a short-time scale

Quorum sensing signals regulate various functions within activated sludge processes such as formation of microbial aggregates. Disturbance of this signaling system, known as quorum quenching (QQ), provides opportunities for eliminating some problems related to biological wastewater treatment (e.g., biofouling and excess sludge production). However, it is poorly understood how and to what extent QQ systems can affect the microbial aggregation processes and the following floc formation. In particular, an in-depth structural characterization at the scale of microbial aggregate while considering nutrient conditions in the reactor is still largely disregarded. Here, we evaluated the QQ effects at the short-term time scale (i.e., after 4 h for the exogenous period and 19 h for exogenous/endogenous period), by combining advanced techniques for microbial characterization (flow cytometry, CARD-FISH, and confocal laser scanning microscopy) and conventional physical-chemical assessments. The results indicated that by implementing QQ agents (immobilized Acylase I enzyme in porous alginate beads) the abundance of single cells and suspended microbial aggregates in the supernatant did not show significant changes during the exogenous period. Conversely, at the end of the exogenous/endogenous period a significant increase of single prokaryotic cells, small and large microbial aggregates favored the growth of grazers, including free-living nanoflagellates and ciliates. Flocs became looser and thinner than those in the control reactor, thus affecting the sludge settling behavior. Inability of microbial community in degradation of soluble protein during the endogenous period confirmed that the QQ agents are likely to inhibit the secretion of protease enzyme within microbial communities of activated sludge.

Macrophyte‐specific effects on epiphyton quality and quantity and resulting effects on grazing macroinvertebrates

Abstract Aquatic macrophytes can have a significant impact on their associated community of epiphytic algae and bacteria through the provisioning of structural habitat complexity through different growth forms, the exudation of nutrients and the release of allelochemicals. In turn, this effect on epiphytic biofilm biomass and nutrient content has a potential effect on the macroinvertebrates that depend on epiphyton as a food source. We studied the effect of living macrophytes and their growth form on biofilm development in a semi‐controlled replicated microcosm experiment. Conditions of a nutrient‐poor water layer and nutrient‐rich sediment were created to study the effects of nutrient exudation by living macrophytes. We compared biofilm quantity and quality on structurally simple (Vallisneria spiralis) versus complex (Egeria densa) living plants and artificial analogues. Subsequently, the biofilm that had developed on the plants was fed, in a laboratory growth experiment, to two species of macroinvertebrate grazers (the snail Haitia acuta and the mayfly nymph Cloeon dipterum). This enabled us to assess if and how the macrophyte‐induced effects on the epiphyton can influence macroinvertebrate grazers. Living macrophytes were found to have a significant effect on epiphytic algal cover, which was mostly expressed by a lower cover on living macrophytes compared to their artificial analogues. Additionally, epiphyton cover on artificial macrophytes was found to be higher on complex structures compared to simple ones, yet this was not observed on living macrophytes. Plant specific traits, such as the release of allelopathic substances, competition for nutrients and DIC, and the amount of CaCO3 deposition on plant surfaces might explain these results. The density of epiphytic bacteria was found to be negatively correlated with biofilm Ca content from macrophytes in every treatment except living E. densa, which differed in leaf anatomy from the other plants by possessing polar leaves. Furthermore, biofilm on living macrophytes had lower C:N:P molar ratios compared to that on artificial plants, which is likely to be explained by nutrient exudation by the living plants. Although it was expected that a more nutritious biofilm would lead to increased grazer growth, this was observed only for H. acuta on E. densa. Because biofilm quantity was not a limiting factor, this lack of effect may be caused by compensatory feeding. It can be concluded that, depending on their traits, living macrophytes can have a positive effect on macroinvertebrate grazers by providing a large surface area for colonisation by epiphytic algae and bacteria, by improving biofilm stoichiometry and by stimulating bacterial growth.

Dynamics of a Producer-Grazer Model Incorporating the Effects of Phosphorus Loading on Grazer's Growth.

Phosphorus is an essential element for all life forms, and it is also a limiting nutrient in many aquatic ecosystems. To keep track of the mismatch between the grazer's phosphorus requirement and producer phosphorus content, stoichiometric models have been developed to explicitly incorporate food quality and food quantity. Most stoichiometric models have suggested that the grazer dynamics heavily depends on the producer phosphorus content when the producer has insufficient nutrient content [low phosphorus (P):carbon (C) ratio]. However, recent laboratory experiments have shown that the grazer dynamics are also affected by excess producer nutrient content (extremely high P:C ratio). This phenomenon is known as the "stoichiometric knife edge." While the Peace et al. (Bull Math Biol 76(9):2175-2197, 2014) model has captured this phenomenon, it does not explicitly track P loading of the aquatic environment. Here, we extend the Peace etal. (2014) model by mechanistically deriving and tracking P loading in order to investigate the growth response of the grazer to the producer of varying P:C ratios. We analyze the dynamics of the system such as boundedness and positivity of the solutions, existence and stability conditions of boundary equilibria. Bifurcation diagram and simulations show that our model behaves qualitatively similar to the Peace etal. (2014) model. The model shows that the fate of the grazer population can be very sensitive to P loading. Furthermore, the structure of our model can easily be extended to incorporate seasonal P loading.

Compensatory grazing by Daphnia generates a trade‐off between top‐down and bottom‐up effects across phytoplankton taxa

AbstractTrade‐offs among traits that shape different components of fitness determine the potential for species to coexist in communities and the regulation of ecosystem productivity by consumers vs. resources. We compared functional and structural traits of 14 species of freshwater phytoplankton to test the influence of life history (exponential growth rate, r, and asymptotic density, K), cell volume, cellular fatty acid (FA) biochemistry, and stoichiometry (cellular C:N and C:P ratios) on grazing and growth rate of Daphnia consumers. Grazers exerted the strongest top‐down effects on algae with the highest C:N and C:P, and lower levels of FA unsaturation. In contrast, the grazer's growth rate was most associated with phytoplankton species’ FA unsaturation and chain length. Different traits were related to grazing rate and trophic transfer efficiency, and as a result, the grazing rate on a particular phytoplankton taxon was unrelated to the grazers’ growth rate. The best nutrient competitors (those with high C:N, C:P, and K, which previous experiments showed had lower minimum requirements for nutrients) were most heavily affected by grazing, but did not contribute the most to grazer growth. Our results indicate that Daphnia increase their food consumption rate in order to compensate for low stoichiometric food quality. Strong competitors with low nutrient demands may therefore be subject to the most intense grazing by individual consumers but not support greater production of higher trophic levels.

The effect of light and nutrients on algal food quality and their consequent effect on grazer growth in subtropical streams

Recent investigators have emphasized that food quality has greater importance than food quantity for herbivore somatic growth. Herbivore growth is thought to be regulated by light and nutrients through changes in food elemental ratios, but our knowledge of the mechanisms driving grazer growth in stream ecosystems is scarce. We manipulated light (through shading) and nutrients (fertilizer addition) in 3 headwater streams in southeastern Queensland, Australia, and measured the responses of natural grazer communities. The growth responses of Austrophlebioides and Helicopsyche to light and nutrients differed with their body size and periphyton food quality. Large larvae were more sensitive to increased light and nutrient availability than were small larvae. Light induced a significant negative effect on Austrophlebioides growth, and this impact increased with increasing nutrient addition. Nutrients had a pronounced positive effect on Helicopsyche growth regardless of light intensity. These responses reflected changes in food quality. Periphyton food quality in terms of C∶N played a more decisive role in grazer growth than algal food quantity (as chlorophyll a). The growth of large larvae was significantly greater under low light intensity and nutrient-enriched conditions where high-quality food was more abundant than under high-light and low-nutrient conditions where periphyton food quality was low. Our results suggest that changes in riparian vegetation or nutrient inputs can significantly influence grazer growth through changes in periphyton C∶N. We recommend that future investigators of foodweb responses to riparian canopy change or nutrient enrichment on stream ecosystems pay more attention to the effects of algal food quality rather than quantity.

Microzooplankton grazing experiments in the subtropical Indian River Lagoon, Florida challenge assumptions of the dilution technique

The most widely accepted method for measuring microzooplankton grazing rates, the dilution technique, was developed in oligotrophic, open-ocean environments. Applications of the technique in productive waters have resulted in varied responses that often complicate the interpretation of results in light of potential violations of assumptions. Grazing experiments using the dilution technique were carried out for 12months at two sites in the Indian River Lagoon (IRL), Florida, an underrepresented ecosystem type with regard to global knowledge of microzooplankton grazing. Positive slopes were observed in the majority of experiments implying that grazing activity actually enhanced phytoplankton growth. In two such experiments, plankton community analysis supported the hypothesis that grazer abundances were not constant over the incubation period and net grazer growth rates were related to dilution level, i.e., an important assumption was violated. Trophic cascades (i.e., grazers eating grazers) are the most likely explanation for these observations. The frequency of positive slopes found in this study highlights the need for more detailed studies of trophic interactions within plankton communities of the IRL, and estuarine systems in general. The IRL has experienced persistent harmful algal blooms, the causes of which are currently under investigation; however, the impact of microbial grazers on harmful algal species cannot be quantified until appropriate methods for resolving complex trophic interactions are identified.

Compensatory feeding and low nutrient assimilation efficiencies lead to high nutrient turnover in nitrogen-limited snails

According to stoichiometric principles, the ratios at which consumers recycle nutrients depend on the elemental compositions of the consumer and its food. However, nutrient assimilation efficiencies and ingestion rates can vary among consumer species and, thus, can affect the rates of consumer-mediated nutrient recycling (CNR). The grazer Theodoxus fluviatilis has high nutrient excretion rates of either P or N, depending on grazer growth limitation, and has a high body N. I examined how a grazer with a high proportion of N in its body tissues can assimilate enough N to maintain that N content despite high N excretion rates by estimating the mass balance for nutrient recycling including nutrient excretion through fecal pellets. I used the snail species Theodoxus fluviatilis and Lymnea peregra fed nutrient-enriched periphyton in a 2-d grazing experiment done in 48 experimental units. I estimated periphyton and grazer nutrient stoichiometry and nutrient excretion rates and ratios in dissolved and fecal-pellet form, and calculated nutrient assimilation efficiencies of the limiting nutrient (N). Theodoxus fluviatilis had higher N excretion rates, lower N assimilation efficiency, and higher ingestion rates than L. peregra. Thus, T. fluviatilis recycled more N by ingesting and processing a larger amount of food per unit time than L. peregra. My study shows that grazers with low nutrient assimilation efficiencies and high nutrient demands can assimilate sufficient nutrients via high ingestion rates. The consequence of this strategy (high ingestion and excretion rates) could be a more rapid nutrient turnover in ecosystems dominated by these grazers.

The influence of moss on grazers in high‐altitude streams: food, refuge or both?

Summary Mosses in streams attach primarily to stable substrata subject to low hydrological perturbation, serving as shelter and as substratum for epiphytic algae, which are a food source for many grazing macro‐invertebrates. Given the changes in the hydrodynamic conditions occurring at substrata surfaces and the increasing habitat complexity, predator–prey interactions may be altered in streams dominated by moss, compared with those dominated by diatom‐covered rocks. However, little is known about the relative effects of the food and refuge functions of moss and how they influence grazers in streams. Results from a field survey indicated that streams with greater moss cover supported higher densities of less mobile, armoured, predator‐resistant but disturbance‐vulnerable caddisfly grazers than streams where moss was rare. However, the abundance of more mobile, disturbance‐resistant but predator‐vulnerable mayfly grazers was unrelated to the extent of moss cover. To distinguish potential mechanisms explaining those observed relationships, we performed a microcosm experiment comparing the responses of two grazer species, one caddisfly (Allomyia gnathos) and one mayfly (Baetis bicaudatus), to variation in substrata and risk of predation. We estimated grazer growth rates, algal accrual rates and the per capita effect of grazers on algal biomass in circular flow‐through chambers that differed in substratum (artificial moss‐covered tiles versus diatom‐covered tiles) and predation risk (non‐feeding predatory stoneflies present versus absent). Results showed that, although artificial moss‐covered tiles provided more food (epiphytic algae biomass – chl‐a), only the mayflies grew faster with higher food resources available on the artificial moss compared with those feeding on diatom‐covered tiles. Furthermore, algal resource depression was greater on diatom‐covered tiles, which suggests that diatoms on bare tiles were more vulnerable to grazing than epiphytes on artificial moss. Although non‐feeding predators did not affect the biomass of either grazer species, algal accrual rates were higher in chambers where mayflies were exposed to predation risk, which is consistent with a behavioural trophic cascade. Mayflies also had a higher per capita impact on algal biomass in the absence of predatory stoneflies. We conclude that armoured, less mobile and disturbance‐vulnerable caddisfly grazers primarily colonise mossy substrata for protection, because we found no evidence that they benefit from the higher epiphytic food supply on moss. However, although disturbance‐resistant mayflies can exploit increased food resources provided by moss, in nature they do not aggregate on mossy substrata in preference to diatom‐covered mineral substrata. Therefore, classical trade‐offs between food acquisition and safety do not explain the observed distributions of caddisflies, which prefer habitats that both are safe and have the best food resources, nor of mayflies, which prefer neither the safest nor the most resource‐rich habitats. These results demonstrate that mayflies and caddisflies have evolved different solutions to the problems of foraging under risks of predation and disturbance.

The snail Potamopyrgus antipodarum grows faster and is more active in the shade, independent of food quality

Ecological stoichiometry has advanced food web ecology by emphasising the importance of food quality over food quantity for herbivores. Here, we focus on the effects of abiotic factors such as nutrients and light (known to influence food quality) on grazer growth rates. As model organism we used the mudsnail Potamopyrgus antipodarum that is native to New Zealand but invasive elsewhere. In a stream channel experiment in New Zealand, we manipulated light (two levels) and nutrients (four levels) to achieve a range of primary producer carbon: nutrient ratios and added mudsnails (3 densities + ungrazed control) to 128 periphyton covered stream channels in a 2 × 4 × 4 full factorial design. We measured snail growth rate and activity, food quality and nutritional imbalance, to test the predictions that (1) less light and more nutrients increase periphyton food quality and thus snail growth rates, and (2) less crowding leads to higher food availability and thus higher snail growth rates. We found that snail growth rates were higher under low light than under high light intensities and this difference increased with increasing nutrient addition. These changes in growth rate were not mediated by food quality in terms of periphyton nutrient ratios. Furthermore, experimental treatments strongly affected snail behaviour. Snails grazed more actively in the low light treatments, and thus it is more likely that snail growth rates were directly affected by light levels, maybe as a result of innate predator avoidance behaviour or as a reaction to high UV intensities. We conclude that in our stream channels snail growth rate was limited by factors other than food quality and quantity such as UV exposure, algal defences or the relatively low ambient water temperature.

Stoichiometric Constraints Do Not Limit Successful Invaders: Zebra Mussels in Swedish Lakes

BackgroundElemental imbalances of carbon (C): nitrogen (N): phosphorus (P) ratios in food resources can constrain the growth of grazers owning to tight coupling between growth rate, RNA allocation and biomass P content in animals. Testing for stoichiometric constraints among invasive species is a novel challenge in invasion ecology to unravel how a successful invader tackles ecological barriers in novel ecosystems.Methodology/Principal FindingsWe examined the C∶P and N∶P ratios and the condition factor of a successful invader in lakes, the zebra mussel (Dreissena polymorpha), collected from two Swedish lakes. Concurrently, we analyzed the elemental composition of the food (seston) and tissue of the mussels in which nutrient composition of food and mussels varied over time. Zebra mussel condition factor was weakly related to the their own tissue N∶P and C∶P ratios, although the relation with the later ratio was not significant. Smaller mussels had relatively lower tissue N∶P ratio and higher condition factor. There was no difference in C∶P and N∶P ratios between seston and mussels' tissues. Our results indicated that the variation in nutrient stoichiometry of zebra mussels can be explained by food quality and quantity.Conclusions/SignificanceOur study suggests that fitness of invasive zebra mussels is not constrained by nutrient stoichiometry which is likely to be important for their proliferation in novel ecosystems. The lack of imbalance in C∶P and N∶P ratios between seston and mussels along with high tissue C∶P ratio of the mussel allow them to tolerate potential P limitation and maintain high growth rate. Moreover, zebra mussels are able to change their tissue C∶P and N∶P ratios in response to the variation in elemental composition of their food. This can also help them to bypass potential nutrient stoichiometric constraints. Our finding is an important step towards understanding the mechanisms contributing to the success of exotic species from stoichiometric principles.

TRANSIENT DYNAMICS OF PELAGIC PRODUCER–GRAZER SYSTEMS IN A GRADIENT OF NUTRIENTS AND MIXING DEPTHS

Phytoplankton-grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton-grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon:nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light-nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the "paradox of energy enrichment" (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment.

Role of nutrient supply in grazer–periphyton interactions: reciprocal influences of periphyton and grazer nutrient stoichiometry

Grazer–periphyton interactions are shaped, in part, by indirect effects of nutrient regeneration. They are an important model system with which to test predictions of ecological stoichiometry and the Growth Rate Hypothesis. We conducted a laboratory experiment to test how nutrient enrichment and grazer identity interact to regulate the nutrient content and stoichiometry of both periphyton and consumers. We considered a situation in which P concentration in the water column was high, in contrast to previous experiments in which P was the limiting nutrient. We added N and P, alone and in combination, to the water in experimental aquaria that contained periphyton communities on clay tiles and grazers (1 of 3 snail species) or no grazers (ungrazed control). Benthic algae incorporated nutrients in close proportion to their availability in each nutrient treatment. Algal biomass increased significantly with +N+P enrichment, but not with +N or +P enrichment alone. Grazers had no effect on periphyton C:N ratios and positive effects on periphyton C:P and N:P ratios. P content of grazers (% dry mass) increased and C:P and N:P molar ratios of grazers decreased in response to N enrichment of the water. Grazer P content increased in response to N enrichment, probably because of increased grazer growth rates. We hypothesize that the addition of N under N-limiting conditions led to increased P uptake or retention by grazers because of high growth rates and RNA production, consistent with the Growth Rate Hypothesis.

Complex interactions of light, nutrients and consumer density in a stream periphyton–grazer (tailed frog tadpoles) system

Summary Consumer–resource interactions may be modified by complex environmental interactions. We experimentally manipulated light, nutrients, consumer density (tailed frog tadpoles − Ascaphus truei), and enclosure sizes in two small and steep streams of coastal British Columbia, Canada. We used flow‐through, screened enclosures in situ as experimental units and measured the responses of algae (as chlorophyll a), periphyton (ash‐free dry mass) and growth rates of tailed frog tadpoles using a factorial model design. Light exerted a strong, positive effect on algae and periphyton production with 30–40% higher biomass over shaded treatments, and a 14% higher relative growth rate of tadpoles. Nutrient additions had little to no effect on any measure, alone or in interaction with other factors. Tadpole growth, algae and periphyton standing crop were significantly reduced along a gradient of increasing consumer density. Enclosure size had a large effect on all three measures. Algae and periphyton production were under simultaneous top‐down (tadpole grazing) and bottom‐up control (light); however, the relative importance of these effects was stream‐dependent. At Klondike Creek algae and periphyton production was influenced primarily by grazing, and by light levels at Dipper Creek. Mean growth rate of tadpoles was on average 45% higher at Klondike Creek than at Dipper Creek. Most of the effects of the abiotic treatments and consumer density interacted statistically with stream identity, despite these streams being very similar and < 6 km apart. The strong effect of stream on the outcomes may be a consequence of large temperature differences. This study demonstrated that light, consumer density, stream, enclosure size and nutrients alone and in combination affected algae and periphyton production, and grazer growth rate. While the interactions between consumer and resources were tightly coupled, the relative importance of the driving factors varied significantly between nearby streams.

Heterotrophic nanoflagellate enhancement of bacterial growth through nutrient remineralization in chemostat culture

Heterotrophic nanoflagellates are the principal consumers of picoplankton in the ocean. Their role as nutrient remineralizers is also well established. However, the coupled interactions be- tween grazer consumption and prey growth are less well understood. In this work, we demonstrate a tight coupling among flagellate grazing, nitrogen remineralization, and prey growth, resulting in bac- terial growth rates averaging 2- to 14-fold higher in the presence of flagellate grazers. These results were obtained using 2-stage, nitrogen-limited chemostats containing a mixed culture of heterotrophic bacteria enriched from seawater and Paraphysomonas bandaiensis, a chrysomonad flagellate. Abun- dance and biovolume of the flagellates were monitored on a daily basis, as was bacterial abundance. Grazing rates were measured using short-term tracer uptake experiments (fluorescently-labeled bac- teria and beads), and these data were used to calculate gross bacterial growth rates in the presence of grazers. A mass balance approach was used to estimate reduced nitrogen regenerated by the protist and nitrogen demand of the heterotrophic bacteria. These independent methods of assessing grazer growth and feeding, coupled with estimates of flagellate gross growth efficiency, provided strong, in- ternally consistent constraints on the estimates of bacterial growth rates in the presence of grazers. Un- der these culture conditions, P. bandaiensis had a carbon-based gross growth efficiency averaging 28%. This work shows that independently measured grazing rates are essential in protist culture work if system dynamics are to be understood. It also underscores the necessity of including protist remineralization pathways in models if realistic simulations are to be obtained.

CRYPTIC TROPHIC CASCADE ALONG A GRADIENT OF LAKE SIZE

Lake size and depth mediate the strength of interaction between fish and zooplankton. We test whether this variation in zooplanktivory indirectly affects the phy- toplankton by comparing 19 lakes that represent two food webs resulting from the absence of piscivores in small, partial-winterkill lakes. Lakes with piscivorous fish are further distinguished by thermal stratification, which provides a refuge for zooplankton to avoid fish predation. We contrasted phytoplankton abundance in these three categories of lakes over six years, using both direct measures of concentration and a growth bioassay that measures phytoplankton from the perspective of a standard grazer (Daphnia). Contrary to expectations, phytoplankton abundance was largely unaffected by trophic structure or the presence of a deep-water refuge. However, grazer growth differed dramatically among the three categories of lakes. Consistent with trophic cascade, increased fish planktivory resulted in more phytoplankton food as measured from the grazer's perspective. This effect was independent of lake productivity or total abundance and size of phytoplankton. Instead, variation in food quality for grazers was associated with compositional differences in phytoplankton. These results indicate that persistent trophic cascades are more dramatic in the plankton than previously realized but primarily influence composition, rather than bio- mass. Although cryptic, such top-down effects create functional variation in grazer-resource coupling.

Dilution effects on microzooplankton in dilution grazing experiments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 200:127-139 (2000) - doi:10.3354/meps200127 Dilution effects on microzooplankton in dilution grazing experiments J. R. Dolan1,2,*, C. L. Gallegos1, A. Moigis1,** 1Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA 2Marine Microbial Ecology Group, CNRS ESA 7076, Université Paris VI, Station Zoologique, BP 28, 06230 Villefranche-sur-Mer, France *E-mail: dolan@obs-vlfr.fr **Present address: Ulmenstrasse 73, 24306 Ploen, Germany ABSTRACT: In dilution experiments, filtered seawater is used to create a gradient of grazing pressure on phytoplankton. Microzooplankton grazing is estimated by examining phytoplankton growth within the gradient. However, the dilution series also represents a resource gradient for microzooplankton. Here we report the effects of dilution on grazers. In 2 standard dilution experiments, using communities from the eutrophic (chlorophyll a = 12 to 15 µg l-1) Rhode River Estuary, we examined the effects of dilution on different groups of microzooplankters: rotifers, tintinnid ciliates, oligotrich ciliates, predacious ciliates, and Mesodinium rubrum. Apparent growth rates of tintinnids and oligotrichs varied with prey concentration, decreasing with the dilution factor from about +0.5 d-1 in undiluted whole water to about -1 d-1 in the 5% whole water, closely resembling numerical response curves. Among tintinnids, there was an increase in the relative abundance of larger tintinnids in the time 24 h samples of dilute treatments compared to the less dilute treatments. No consistent dilution effect was shown by rotifers or predacious ciliates. The growth rates of the photosynthetic ciliate M. rubrum increased with dilution, resembling the typical pattern of chlorophyll a and autotrophic nanoplankton. Grazer growth in undiluted waters and grazer mortality in dilute water may be common and result in uncertainty in measured grazing rates. We urge that grazers be examined in grazing experiments not only to assess possible artifacts in grazing rate estimates, but also to provide information, beyond a simple grazing rate, on the grazer populations. KEY WORDS: Ciliate · Oligotrich · Tintinnid · Phytoplankton · Nanoplankton · Rotifer · Flagellate Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 200. Publication date: July 14, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Light Limitation in a Stream Ecosystem: Responses by Primary Producers and Consumers

Heavy shade presents serious challenges for primary producers and food—limited herbivores in forest streams. In this study, we examined the response of periphyton and grazing snails (Elimia clavaeformis) to summer shade in White Oak Creek (WOC), a second—order stream in a Tennessee deciduous forest. Three experiments were performed: (1) in situ manipulation of light and snail density to test the effects of light limitation and grazing; (2) construction of photosynthesis—irradiance (P—I) curves to test for shade adaptation by periphyton; and (3) measurements of snail growth vs. irradiance, to qualify the indirect relationship between grazers and an abiotic constraint on photosynthesis. In the first experiment, light and snail densities were manipulated in a 2 X 2 factorial design: two light treatments were created by removing streamside vegetation from four sites in WOC and by pairing each of these sites with an adjacent, shade site; two snail density treatments at each site were created by adding snails at normal (970 individuals/m2) and low (50 individuals/m2) density to the two sides of Plexiglas channels. Snails at normal densities cropped periphyton biomass to low levels regardless of light regime, but periphyton productivity was higher at the open sites where snails grew faster and accumulated more lipid. Snail growth and lipid accumulation were strongly affected by intraspecific competition in both light regimes. In the second experiment, photosynthesis—irradiance curves for periphyton from shaded and open sites illustrated considerable shade adaptation: shaded periphyton was 2 times more efficient at low irradiance than was periphyton from open sites. Despite the greater efficiency of shade periphyton at low irradiance, integrated primary production estimated with photosynthetic models was 4 times greater in the open because shade adaptation provided only partial compensation for the very low irradiances in the shade. In the third experiment, in situ snail growth again increased with decreasing shade. The growth vs. irradiance response resembled a P—I curve: snail growth increased almost linearly with increased light and then leveled off at a photon flux density of °7 mol°m—2·d—1. If this curve primarily reflects rates of food supply, then periphyton production and grazer growth in WOC and similar streams is light—limited at a photon flux density <7 mol·—2·d—1. Bottom—up effects of light limitation were propagated very strongly in WOC, where the invertebrate fauna is dominated by a grazer that appears to escape top—down control.