Planktonic Systems Research Articles

Beyond biomass: Resource effects on primary production and consumer nutrient assimilation in streams

Abstract Primary production in aquatic ecosystems is strongly controlled by resource availability (bottom‐up). At the same time, grazers exert top‐down pressure on algae. The effects of both resources and grazers on algal communities have been investigated extensively for both benthic and planktonic systems. However, most studies focus exclusively on net effects on algal standing stock, while the underlying ecological functions and processes often remain unknown. Here, we tested the effects of the two resources, light (L) and the limiting nutrient phosphorus (P), on both primary production (net oxygen production) by benthic algae (periphyton) and matter assimilation by the next trophic guild (particularly nitrogen assimilation by grazers). For this, we grew natural periphyton communities in stream mesocosm flumes together with two native grazer taxa and 15N as a tracer. Both resources resulted in an initial increase in net primary production, but this effect equalised after 36 days in the more mature communities. While both resources still showed strong positive effects on algal biovolume, the biovolume‐specific net primary production decreased with resource level and was lowest in the double elevated (+L, +P) treatment. The response of nitrogen assimilation by grazers was more closely related to the response of periphyton quality (stoichiometric and taxonomic composition) than to its biomass. In addition, light affected the nitrogen assimilation of both grazers negatively. These findings demonstrate that the responses of ecological functions, here primary production and assimilation by grazers, are not necessarily proportional to algal biomass and are thus not easily predictable from the common use of biomass as a proxy for functions.

Community-specific diffusion characteristics determine resistance of biofilms to oxidative stress

Biofilms are multicellular communities with a spatial structure. Different from single-cell scale diffusion in planktonic systems, the diffusion distance becomes the dimension of multicellular clusters in a biofilm. Such differences in diffusion behavior affect the tolerance and response to exogenous stress. Here, we found that at the same doses of exogenous hydrogen peroxide (H2O2), planktonic Escherichia coli were completely killed within twohours, whereas the biofilm resumed growth in sixhours by building a catalase barrier to block H2O2 penetration, despite the growth burden. Unexpectedly, when we changed the carbon source from glucose to glycerol, H2O2 instantly counterintuitively boosted biofilm growth due to supplemental oxygen, which was the growth-limiting factor. We further demonstrated that the energy metabolism modes determined the growth-limiting factor, which then determined the two patterns of biofilms resistances to H2O2.

Marine primary producers in a darker future: a meta‐analysis of light effects on pelagic and benthic autotrophs

The availability of underwater light, as primary energy source for all aquatic photoautotrophs, is (and will further be) altered by changing precipitation, water turbidity, mixing depth, and terrestrial input of chromophoric dissolved organic matter (CDOM). While experimental manipulations of CDOM input and turbidity are frequent, they often involve multiple interdependent changes (light, nutrients, C‐supply). To create a baseline for the expected effects of light reduction alone, we performed a weighted meta‐analysis on 240 published experiments (from 108 studies yielding 2500 effect sizes) that directly reduced light availability and measured marine autotroph responses. Across all organisms, habitats, and response variables, reduced light led to an average 23% reduction in biomass‐related performance, whereas the effect sizes on physiological performance did not significantly differ from zero. Especially, pigment content increased with reduced light, which indicated a strong physiological plasticity in response to diminished light. This acclimation potential was also indicated by light reduction effects minimized if the experiments lasted longer. Nevertheless, the performance (especially biomass accrual) was reduced the more the less light intensity remained available. Light reduction effects were also more negative at higher temperatures if ambient light conditions were poor. Macrophytes or benthic systems were more negatively affected by light reduction than microalgae or plankton systems, especially in physiological responses were microalgae and plankton showed slightly positive responses. Otherwise, the effect magnitudes remained surprisingly consistent across habitats and aspects of experimental design. Therefore, the strong observed log–linear relationship between remaining light and autotrophic performance can be used as a baseline to predict marine primary production in future light climate.

Chaos control of chaotic plankton dynamics in the presence of additional food, seasonality, and time delay

In marine ecology, plankton systems have rich and complex dynamics. The control of chaos in plankton systems is one of the main motives of recent research. In this article, the chaos control mechanisms is explored using internal parameters and external forces. Also, the availability of additional food for both zooplankton and fish is incorporated. The present work enhances the existing literature on the chaos control dynamics of available additional food for predator population by focusing on the consequences of predation delay and seasonal perturbations. It presents a comprehensive view of the different ranges of alternative food, periodic fluctuations, and time delay on the dynamics of the proposed system. The stability analysis of the systems with and without predation delay is studied. The Hopf-bifurcation analysis is performed by taking time delay as a bifurcation parameter. Control of chaos in plankton-fish dynamics is explored with respect to additional food for plankton and fish population, predation delay, seasonal perturbations and delayed feedback control. The numerical simulation presents the validation of theoretical results.

Chemical contamination-mediated regime shifts in planktonic systems

Abrupt transitions leading to algal blooms are quite well known in aquatic ecosystems and have important implications for the environment. These ecosystem shifts have been largely attributed to nutrient dynamics and food web interactions. Contamination with heavy metals such as copper can modulate such ecological interactions which in turn may impact ecosystem functioning. Motivated by this, we explored the effect of copper enrichment on such regime shifts in planktonic systems. We integrated copper contamination to a minimal phytoplankton–zooplankton model which is known to demonstrate abrupt transitions between ecosystem states. Our results suggest that both the toxic and deficient concentration of copper in water bodies can lead to regime shift to an algal-dominated alternative stable state. Further, interaction with fish density can also lead to collapse of population cycles thus leading to algal domination in the intermediate copper ranges. Environmental stochasticity may result in state transition much prior to the tipping point and there is a significant loss in the bimodality on increasing intensity and redness of noise. Finally, the impending state shifts due to contamination cannot be predicted by the generic early warning indicators unless the transition is close enough. Overall the study provides fresh impetus to explore regime shifts in ecosystems under the influence of anthropogenic changes like chemical contamination.

Substrate properties as controlling parameters in attached algal cultivation.

There is growing interest in attached algae cultivation systems because they could provide a more cost- and energy-efficient alternative to planktonic (suspended algae) cultivation systems for many applications. However, attached growth systems have been far less studied than planktonic systems and have largely emphasized algae strains of most interest for biofuels. New algal biorefinery pathways have assessed the commercial potentials of algal biomass beyond biofuel production and placed more emphasis on value-added products from that biomass. Therefore, algal strain selection criteria and biomass cultivation methods need to be updated to include additional strains for improved efficiency. One possible way of improving attached cultivation systems is through engineering substrate surface characteristics to boost algal adhesion and enable strain selective algal colonization and growth. This review explores the effect of substrate chemical and topographical characteristics on the cultivation of attached algae. It also highlights the importance of considering algal community structure and attachment mechanisms in investigating attached algae systems using the example of filamentous algae found in algal turf scrubber (ATS™) systems. KEY POINTS : • Attached algal cultivation is a promising alternative to planktonic cultivation. • Performance increase results from tuning surface qualities of attachment substrates. • Attachment adaptation of periphytic algae has innate potential for cultivation.

Microrheology reveals microscale viscosity gradients in planktonic systems

Microbial activity in planktonic systems creates a dynamic and heterogeneous microscale seascape that harbors a diverse community of microorganisms and ecological interactions of global significance. In recent decades great effort has been put into understanding this complex system, particularly focusing on the role of chemical patchiness, while overlooking a physical parameter that governs microbial life and is affected by biological activity: viscosity. Here we reveal spatial heterogeneity of viscosity in planktonic systems by using microrheological techniques that allow measurement of viscosity at length scales relevant to microorganisms. We show the viscous nature and the spatial extent of the phycosphere, the region surrounding phytoplankton. In ∼45% of the phytoplankton cells analyzed we detected increases in viscosity that extended up to 30 µm away from the cell with up to 40 times the viscosity of seawater. We also show how these gradients of viscosity can be amplified around a lysing phytoplankton cell as its viscous contents leak away. Finally, we report conservative estimates of viscosity inside marine aggregates, hotspots of microbial activity, more than an order of magnitude higher than in seawater. Since the diffusivities of dissolved molecules, particles, and microorganisms are inversely related to viscosity, microheterogeneity in viscosity alters the microscale distribution of microorganisms and their resources, with pervasive implications for the functioning of the planktonic ecosystem. Increasing viscosities impacts ecological interactions and processes, such as nutrient uptake, chemotaxis, and particle encounter, that occur at the microscale but influence carbon and nutrient cycles at a global scale.

Top-down versus bottom-up control in planktonic systems: some case studies from the English Lake District

In this review, I use examples drawn from long-term studies in the English Lakes to highlight the limitations of top-down control in planktonic systems. These limitations are most obvious at the base of the food chain where there are large differences in the potential growth rates of the phytoplankton and zooplankton. Three of the examples are taken from a 40-year study of the impact of perch on Daphnia in the North Basin of Windermere, one on the rates of change measured in an experimental enclosure and one from an 18-year study of Daphnia dynamics in Esthwaite Water. The Windermere examples show that the commercial perch fishery established in 1941 had little effect on the abundance of Daphnia but the enrichment of the lake by war-time ploughing led to a short-lived increase in their reproductive rate. The other examples demonstrate that time plays an important part in all trophic interactions and shows that established patterns of phytoplankton succession are frequently disrupted by changes in the intensity of wind-induced mixing. The results are discussed in relation to recent reviews of the efficacy of biomanipulation and the challenges of analysing complex interactions in dynamic systems.

Phytoplankton and dimethylsulfide dynamics at two contrasting Arctic ice edges

Abstract. Arctic sea ice is retreating and thinning and its rate of decline has steepened in the last decades. While phytoplankton blooms are known to seasonally propagate along the ice edge as it recedes from spring to summer, the substitution of thick multiyear ice (MYI) with thinner, ponded first-year ice (FYI) represents an unequal exchange when considering the roles sea ice plays in the ecology and climate of the Arctic. Consequences of this shifting sea ice on the phenology of phytoplankton and the associated cycling of the climate-relevant gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) remain ill constrained. In July–August 2014, two contrasting ice edges in the Canadian High Arctic were explored: a FYI-dominated ice edge in Barrow Strait and a MYI-dominated ice edge in Nares Strait. Our results reveal two distinct planktonic systems and associated DMS dynamics in connection to these diverging ice types. The surface waters exiting the ponded FYI in Barrow Strait were characterized by moderate chlorophyll a (Chl a, <2.1 µg L−1) as well as high DMSP (115 nmol L−1) and DMS (12 nmol L−1), suggesting that a bloom had already started to develop under the markedly melt-pond-covered (ca. 40 %) FYI. Heightened DMS concentrations at the FYI edge were strongly related to ice-associated seeding of DMS in surface waters and haline-driven stratification linked to ice melt (Spearman's rank correlation between DMS and salinity, rs=-0.91, p<0.001, n=20). However, surface waters exiting the MYI edge at the head of Nares Strait were characterized by low concentrations of Chl a (<0.5 µg L−1), DMSP (<16 nmol L−1), and DMS (<0.4 nmol L−1), despite the nutrient-replete conditions characterizing the surface waters. The increase in autotrophic biomass and methylated sulfur compounds took place several kilometers (ca. 100 km) away from the MYI edge, suggesting the requisite for ice-free, light-sufficient conditions for a phytoplankton bloom to fully develop and for sulfur compound dynamics to follow and expand. In light of the ongoing and projected climate-driven changes to Arctic sea ice, results from this study suggest that the early onset of autotrophic blooms under thinner, melt-pond-covered ice may have vast implications for the timing and magnitude of DMS pulses in the Arctic.

Nearshore-offshore trends in Lake Superior phytoplankton

Changes in phytoplankton community composition and structure can have broad-scale ecosystem effects; however, drivers of species diversity in planktonic systems are not well understood. In lakes, a common but not thoroughly tested assumption is that shallow, nearshore waters are much more diverse and productive, and contribute considerably more material and energy to pelagic food webs than deeper waters farther offshore. Lake Superior is a large, cold, oligotrophic freshwater system which can provide insight into community organization under oligotrophic conditions. We used epilimnion and deep chlorophyll layer phytoplankton data from a lake-wide sampling program conducted in 2011 and 2016 to test whether assemblage composition, total algal biovolume, cell concentrations, diversity, and richness vary with depth. Although lake depth was an important factor in structuring assemblage composition, there were no clear nearshore-offshore gradients in cell density or biovolume despite the exposure of nearshore areas to higher concentrations of watershed-derived nutrients. Shannon diversity increased slightly with increasing depth, whereas richness was uncorrelated. Understanding of the nearshore-offshore patterns in phytoplankton community characteristics in the Great Lakes has implications for designing monitoring strategies and for considering how further changes in climate and nutrient deposition would affect the base of the food web.

The Influence of Trophic Interactions in the Plankton Community on Its Spatiotemporal Dynamics

The present work considers a model of spatiotemporal dynamics for plankton community in a vertical column of water. Usually mesozooplankton (e.g. copepods) is considered as the main grazer in models of plankton systems. However, numerous studies have appeared recently that microzooplankton, not copepods, are the primary grazers in oceanic ecosystems. This work presents the model with microzooplankton and copepods as phytoplankton predators. Functional response for copepod feeding includes its preference for phytoplankton and microzooplankton. The effects of half saturation constants of zooplankton and diet variants on the stability of spatial homogeneous equilibriums was studied. In the case of effective grazing by mesozooplankton the system demonstrates stable coexistence of both zooplankton groups if phytoplankton carrying capacity is sufficient for it. In the case if microzooplankton protists are ineffective grazers – they are displaced by copepods or survive in a small number if their portion in the copepods diet is insignificant. The phenomenon of multistability has been identified in the system for copepod diet with microzooplankton preference. It means that several equilibria are stable at the same time, and the dynamics of the system are determined by the initial conditions. Allowing for spatial dependence in the modeled plankton community causes spatial structures appearance induced by Turing instability. It means that the system with diffusion only is able to induce stationary vertical profiles of plankton biomass. Such system state is possible if both predators are effective grazers and the microzooplankton prevails in the copepods diet.

Prey taxonomy rather than size determines salp diets

AbstractSalps are gelatinous planktonic suspension feeders that filter large volumes of water in the food‐dilute open ocean. Their life cycle allows periodic exponential growth and population blooms. Dense swarms of salps have a high grazing impact that can deplete the photic zone of phytoplankton and export huge quantities of organic matter to the deep sea. Previous studies described their feeding manner as mostly nonselective, with larger particles retained at higher efficiencies than small particles. To examine salp diets, we used direct in situ sampling (InEx method) of undisturbed solitarySalpa maxima. Aggregates (“chains”) ofSalpa fusiformisandThalia democraticawere studied using in situ incubations. Our findings suggest that in situ feeding rates are higher than previously reported and that cell removal is size independent with ∼ 1μm picoeukaryotes preferentially removed over both larger eukaryotes and smaller bacteria. The prey : predator size ratios we measured (1 : 104–1 : 105) are an order of magnitude smaller than previously reported values and to the best of our knowledge, are the smallest values reported so far for any planktonic suspension feeders. Despite differences among the three species studied, they had similar prey preferences with no correlation between salp body length and prey size. Our findings shed new light on prey : predator relationships in planktonic systems—in particular, that factors other than size influence filtration efficiency—and suggest that in situ techniques should be devised and applied for the study of suspension feeding in the ocean.

Mixotrophs: Dynamic disrupters of plankton systems?

We consider a simple plankton model with one phytoplankton autotroph population, one mixotrophic phytoplankton population that both photosynthesises and consumes the other phytoplankton, and one zooplankton predator population. The zooplankton predator may be either a specialist or a generalist grazer on the phytoplankton. We examine the influence on the system's equilibrium points and dynamical properties of varying degrees of mixotrophy by the phytoplankton and grazing strategy by the zooplankton, both with and without the presence of seasonal forcing.We find that the strength of the mixotrophy interaction does not substantially change the equilibrium properties of the model across all grazing strategies, but it can have a substantial effect on the dynamical properties. Further, external forcings on typical phytoplankton or zooplankton time scales can in some circumstances substantially modify these dynamics. Our analyses suggest that plankton models developed to represent bio-geochemical processes in the ocean for applications in climate change modelling should be subjected to thorough equilibrium and dynamical analyses before being used for climate prediction.

Spatio-temporal dynamics near the steady state of a planktonic system

The study of spatio-temporal behaviour of ecological systems is fundamentally important as it can provide deep understanding of species interaction and predict the effects of environmental changes. In this paper, we first propose a spatial model with prey taxis for planktonic systems, in which we also consider the herb behaviour in prey and effect of the hyperbolic mortality rate. Applying the homogeneous Neumann boundary condition to the model and using prey-tactic sensitivity coefficient as bifurcation parameter, we then detailedly analyse the stability and bifurcation of the steady state of the system: firstly, we carry out a study of the equilibrium bifurcation, showing the occurrence of fold bifurcation, Hopf bifurcation and the BT bifurcation; then by using an abstract bifurcation theory and taking prey-tactic sensitivity coefficient as the bifurcation parameter, we investigate the Turing–Hopf bifurcation, obtaining a branch of stable non-constant solutions bifurcating from the positive equilibrium, and our results show that prey-taxis can yield the occurrence of spatio-temporal patterns; finally, numerical simulations are carried out to illustrate our theoretical results, showing the existence of a periodic solution when the prey-tactic sensitivity coefficient is away from the critical value.

Grazers, pathogens and shelf-shading enhance phytoplankton species richness more and reduce productivity less when environments are less dynamic: A theoretical study

Human activities, such as reservoir construction, can result in hydraulically less dynamic systems and cause downstream systems to shift from sudden resource supply transitions to gradual. In this research, how such human activities might influence phytoplankton is further explored theoretically. Building on previous modeling results, new findings suggest that preferential grazing, pathogens, self-shading, and all these combined interact differentially with the mode of resource loading (sudden and gradual transitions) to shape phytoplankton assemblage characteristics. Most notably, phytoplankton species richness was much greater in scenarios that considered pathogen effects, self-shading effects, and combined preferential grazing, pathogen and shelf-shading effects when resource supply transitions were gradual compared to when they were sudden, i.e., 2.7-fold increase, 2.4-fold increase and 1.7-fold increase, respectively. Furthermore, reduced productivity with the additions of preferential grazing, pathogens, self-shading, and all these combined was lessened when resource supply transitions were gradual. Smaller differences to phytoplankton species evenness, overyielding, species interactions, niche breadth, and resource drawdown were also observed when comparing scenarios with sudden and gradual resource supply transitions. The nuanced details of preferential grazing, pathogens and shelf-shading uncovered here underscore the fundamental principle framed in the philosophy of Okham's Razor. How much complexity must we account for to understand how plankton systems will respond to altered land use in watersheds, and associated changed hydrology and nutrient loading? In this cursory effort, with simplistic additions of preferential grazing, pathogens and self-shading, findings highlight the need to explore empirically the effects of impoundment construction on downstream estuarine ecosystems.

Exploitative competition of invaders differentially influences the diversity of neutral, lumpy and intransitive phytoplankton assemblages in spatially heterogeneous environments

Recent theoretical research suggested that species-supersaturated phytoplankton assemblages sustained through neutrality, lumpy coexistence or intransitivity were sensitive to immigration, quickly losing diversity following invasions. The presence of source patches, however, may facilitate persistence of species-supersaturation in the face of invaders. We explore that notion here through simulation analyses of more detailed spatially explicit models depicting one- and two-dimensional systems (“pipe” and “two-eddy” models, respectively) where water circulation is governed over a range of advection and turbulent diffusion. For the pipe model, when advection and turbulent diffusion are low, and when advection is high and turbulent diffusion is low, resident assemblages are more resistant to invaders. But at intermediate rates of advection and with increases to turbulent diffusion, assemblages are more quickly impacted by invaders. For the two-eddy model, when advection and turbulent diffusion are low, resident assemblages are again resistant to invaders, but only for lumpy assemblages. In contrast to the “pipe” model, resistance to invaders of neutral and lumpy assemblages is high when advection and turbulent diffusion are high. For both model configurations, intransitive assemblages were much more sensitive to invaders, suggesting that intransitivity might not be as prevalent a biodiversity-sustaining mechanism in spatially heterogeneous plankton systems.

Norfloxacin disrupts Daphnia magna-induced colony formation in Scenedesmus quadricauda and facilitates grazing

Environmental pollutants can negatively affect grazer-induced defenses in phytoplankton species and lead to the disruption of phytoplankton-zooplankton relationships. However, it is still not known whether norfloxacin (NOR), a widely used antibiotic, has such effects on plankton systems when released into the environment. Here, we investigated the colony-forming ability of the green alga Scenedesmus quadricauda as well as the behavior (hopping frequency and duration of quiescence) and grazing rate of the cladoceran Daphnia magna, when exposed to different NOR concentrations (0, 50, 500, 5000 and 50,00ugL−1), to determine the effects of NOR on the S. quadricauda-D. magna relationship. Colony size was generally unaffected by NOR but significantly increased when subjected to the stress of adding grazers; however, the combined effects of adding grazers and increasing NOR concentrations led to consistently decreased colony size compared to only adding grazers. The hopping frequency and the time spent on swimming of D. magna individuals decreased while their grazing rate increased with increasing NOR concentrations. Thus, NOR seemed to inhibit grazer-induced colony formation in S. quadricauda, resulting in more algal cells being consumed by grazers, despite inhibiting grazers’ feeding behavior. Considering that phytoplankton-zooplankton interaction plays an important role in determining ecological process of aquatic ecosystems, ecosystem function might be more affected by NOR contamination than previously recognized.

Lethal/sublethal responses of Daphnia magna to acute norfloxacin contamination and changes in phytoplankton-zooplankton interactions induced by this antibiotic

Although the well-known antibiotic norfloxacin (NOR) is recognized as an important environmental pollutant, little is known about its impacts on ecological processes, particularly on species interactions. In this paper, we quantified Daphnia magna (Crustacea, Cladocera) responses in mortality rate at lethal NOR concentrations (0, 25, 50, 100, 200, 300 and 400 mg L−1), and in heartbeat rate, swimming behavior and feeding rate (on the green alga Chlorella pyrenoidosa) at sublethal NOR concentrations (0, 25, 50 and 100 mg L−1) to determine the effects of this antibiotic in plankton systems. In 96-h-long lethal experiment, mortality rates of D. magna increased significantly with increasing NOR concentration and exposure time. In sublethal experiments, heartbeat rate decreased, while time ratio of vertical to horizontal swimming (TVH) and the duration of quiescence increased in D. magna individuals exposed to increasing NOR concentrations after 4 and 12 h of exposure. These collectively led to decreases in both average swimming ability and feeding rate, consistent with the positive relationship between average swimming ability and feeding rate. Overall, results indicate that, by affecting zooplankton heartbeat rate and behavior, NOR decreased feeding efficiency of D. magna even at low doses, therefore, it might seriously compromise ecosystem health and function.

The effect of photon flux density on algal biofilm growth and internal fatty acid concentrations

Algal biofilm systems offer potential advantages over planktonic systems for growing, harvesting and processing algal biomass. To determine how feasible these systems can be, however, researchers must determine optimal biomass and lipid productivities. In this paper the effect of photon flux density (PFD) on algal biofilm biomass productivities, internal fatty acid concentrations, and overall fatty acid productivity was studied. It was determined that as PFD increased, algal biofilm biomass productivities significantly increased until they peaked i.e. at about 300μmol/m2/s, presumably due to light saturation. Increasing PFDs from 50 to 150μmol/m2/s produced statistically significant increases in fatty acid concentrations from 5% to 8%; however, increasing PFDs beyond that range did not enhance lipid accumulation. Because of significant increases in biomass productivity with increased PFD, there was also a significant increase in fatty acid productivity with increased PFD. Lastly, increasing PFD resulted in diminishing returns of biomass productivity, suggesting that photons are not being effectively utilized at higher light intensities. This finding has implications for photobioreactor design and operation with artificial lighting.

Minimal Model of Plankton Systems Revisited with Spatial Diffusion and Maturation Delay.

This study revisits the minimal model for a plankton ecosystem proposed by Scheffer with spatial diffusion of plankton and the delay of the maturation period of herbivorous zooplankton. It deepens our understanding of effects of the nutrients and the predation of fish upon zooplankton on the dynamical patterns of the plankton system and also presents new phenomena induced by the delay with spatial diffusion. When the nutrient level is sufficient low, the zooplankton population collapses and the phytoplankton population reaches its carrying capacity. Mathematically, the global stability of the boundary equilibrium is proved. As the nutrient level increases, the system switches to coexistent equilibria or oscillations depending on the maturation period of zooplankton and the predation rate of fish on herbivorous zooplankton. Under an eutrophic condition, there is a unique coexistent homogeneous equilibrium, and the equilibrium density of phytoplankton increases, while the equilibrium density of herbivorous zooplankton decreases as the fish predation rate on herbivorous zooplankton is increasing. The study shows that the system will never collapses under the eutrophic condition unless the fish predation rate approaches infinite. The study also finds a functional bifurcation relation between the delay parameter of the maturation period of herbivorous zooplankton and the fish predation rate on herbivorous zooplankton that, above a critical value of the fish predation rate, the system stays at the coexistent equilibrium, and below that value, the system switches its dynamical patterns among stable and unstable equilibria and oscillations. The oscillations emerge from Hopf bifurcations, and a detailed mathematical analysis about the Hopf bifurcations is carried out to give relevant ecological predications.