Planktonic Food Web Structure Research Articles

Killing the predator: impacts of highest-predator mortality on the global-ocean ecosystem structure

Abstract. Recent meta-analyses suggest that microzooplankton biomass density scales linearly with phytoplankton biomass density, suggesting a simple, general rule may underpin trophic structure in the global ocean. Here, we use a set of highly simplified food web models, solved within a global general circulation model, to examine the core drivers of linear predator–prey scaling. We examine a parallel food chain model which assumes microzooplankton grazers feed on distinct size classes of phytoplankton and contrast this with a diamond food web model allowing shared microzooplankton predation on a range of phytoplankton size classes. Within these two contrasting model structures, we also evaluate the impact of fixed vs. density-dependent microzooplankton mortality. We find that the observed relationship between microzooplankton predators and prey can be reproduced with density-dependent mortality on the highest predator, regardless of choices made about plankton food web structure. Our findings point to the importance of parameterizing mortality of the highest predator for simple food web models to recapitulate trophic structure in the global ocean.

Flow event size influences carbon, nutrient and zooplankton dynamics in a highly regulated lowland river

AbstractRiver regulation and water extraction has significantly altered flow regimes and reduced flood events in many inland river systems. Environmental flows have been adopted in many systems to mitigate the ecological impacts of river regulation, however a lack of knowledge regarding the interrelationship between flow regimes, carbon transport and instream productivity make prioritising water management difficult. To address this knowledge gap, we conducted a study on the Namoi River in the Murray-Darling Basin, Australia, monitoring changes in dissolved organic carbon (DOC), nutrient dynamics and planktonic food web structure during a period of variable flows. Nutrient and DOC concentrations were positively correlated with river discharge and zooplankton concentrations were highest post flow events. Planktonic chlorophyll-a, increased DOC concentration and higher discharge were the most influential drivers of change in zooplankton communities. Further, our results indicated that flow events increased production through both heterotrophic and autotrophic pathways, significantly boosting zooplankton concentration compared to base flow conditions across all measured flow events. We suggest even small in-channel flow events can be important for increasing basal and zooplankton production in rivers, and therefore should be protected or promoted by environmental flow management, particularly during drought conditions.

Seasonality of modelled planktonic food web structure in the Strait of Georgia, Canada

When attempting to model an ecosystem, compromises must be made to retain the essence of the system and its functioning. Here, we explore the implications of these aggregations on a modelled ecosystem by using Ecopath to model the plankton food web in the Strait of Georgia (SoG), a temperate coastal system on the west coast of Canada, in three seasons. The food web is defined by fifteen functional groups representing autotrophic phytoplankton, the microbial loop, and mesozooplankton in three seasons, differentiated by node biomass, productivity, and diet. Variations in the community composition lead to shifts in trophic behaviour and food-web structure seasonally. In spring, autotrophic phytoplankton (i.e. diatoms) are the main source of carbon for the system. In summer and winter, the microbial loop becomes more important due to limited primary production. These structural changes within the plankton food web throughout a year have implications for higher trophic levels, including the seasonal availability and quality of food for planktivorous fish. Finally, we compared this detailed seasonal approach to plankton modelling with a more common simplified approach and examined its impacts on the entire ecosystem. The microbial loop is often excluded from coastal ecosystem models but is an important component, influencing trophic positions and transfer efficiencies. However, aggregating plankton groups appears to be an adequate approach to plankton modelling in the SoG, but modelling decisions should be driven by the research question.

Trophic ecology of midwater zooplankton along a productivity gradient in the Southeast Pacific

The environmental regulation of planktonic food web structure is a key determinant for the efficiency of energy transfer through trophic levels, with direct implications for the amount of energy that is available to top-consumers and deep-sea communities. Yet, the complex trophic interplay between the different components of plankton communities under contrasting environmental conditions remains unresolved. Here, we present a comprehensive field study based on compound-specific isotope analysis of amino acids that aims to explore depth-resolved changes in the trophic strategies of various size-classes of mesozooplankton from distinct habitats across the Southeast Pacific. According to the δ15N values of the so-called source amino acids such as phenylalanine, we first identified a clear shift in nitrogen sources for biological production, from nitrate in the productive upwelling waters of the Humboldt Current System to increased inputs of diazotroph-N within the ultra-oligotrophic South Pacific gyre. These shifts in the productivity and in the source of N at the base of the food web were not accompanied by significant changes in the trophic position (TP) of mesozooplankton, which were mainly omnivores (TP = 2.8 ± 0.3, n = 65) in all ecoregions. However, although the planktonic food web length remained relatively unaltered along the productivity gradient, the microbial contribution to mid-trophic level consumers appeared to be more important at relatively high productivities, likely due to more intense remineralization processes in a late successional stage of the upwelling throughout the coastal-transition zone. Altogether, this research work will contribute to a better understanding of the food web functioning in one of the least explored marine regions of the world’s oceans.

Effects of experimental sediment resuspension on protozooplankton grazing activity: implication for the planktonic food web structure

Effects of experimental sediment resuspension on protozooplankton grazing activity: implication for the planktonic food web structure

Interactive effects of nutrients and temperature on herbivorous predation in a coastal plankton community

AbstractMarine microbial communities in coastal environments are subject to both seasonal fluctuations and anthropogenic alterations of environmental conditions. The separate influences of temperature and resource‐dependency on phytoplankton growth, community, and ecosystem metabolism are relatively well understood. However, winners and losers in the ocean are determined based on the interplay among often rapidly changing biological, chemical and physical drivers. The direct, indirect, and interactive effects of these conditions on planktonic food web structure and function are poorly constrained. Here, we investigated how simultaneous manipulation of temperature and nutrient availability affects trophic transfer from phytoplankton to herbivorous protists, and their resulting implications at the ecosystem level. Temperature directly affected herbivorous protist composition; ciliates dominated (66%) in colder treatment and dinoflagellates (60%) at warmer temperatures. Throughout the experiments, grazing rates were < 0.1 d−1, with higher rates at subzero temperatures. Overall, the nutrient–temperature interplay affected trophic transfer rates antagonistically when nutrients were amended, and synergistically, when nutrients were not added. This interaction resulted in higher percentages of primary production consumed under nutrient unamended compared to nutrient amended conditions. At the ecosystem level, these changes may determine the fate of primary production, with most of the production likely exported out of the pelagic zone in high‐temperature and nutrient conditions, while high‐temperature and low‐nutrient availability strengthened food web coupling and enhanced trophic transfer. These results imply that in warming oceans, management of coastal nutrient loading will be a critical determinant of the degree of primary production removal by microzooplankton and dependent ecosystem production.

Plankton energy flows using a global size-structured and trait-based model

Plankton community models are critical tools for understanding the processes that shape marine plankton communities, how plankton communities impact biogeochemical cycles, and the feedbacks between community structure and function. Here, using the flexible Marine Biogeochemistry Library (MARBL), we present the Size-based Plankton ECological TRAits (MARBL-SPECTRA) model, which is designed to represent a diverse plankton community while remaining computationally tractable. MARBL-SPECTRA is composed of nine phytoplankton and six zooplankton size classes represented using allometric scaling relationships for physiological traits and interactions within multiple functional types. MARBL-SPECTRA is embedded within the global ocean component of the Community Earth System Model (CESM) and simulates large-scale, emergent patterns in phytoplankton growth limitation, plankton phenology, plankton generation time, and trophic transfer efficiency. The model qualitatively reproduces observed global patterns of surface nutrients, chlorophyll biomass, net primary production, and the biogeographies of a range of plankton size classes. In addition, the model simulates how predator:prey dynamics and trophic efficiency vary across gradients in total ecosystem productivity. Shorter food chains that export proportionally more carbon from the surface to the ocean interior occur in productive, eutrophic regions, whereas in oligotrophic regions, the food chains are relatively long and export less organic matter from the surface. The union of functional type modeling with size-resolved, trait-based modeling approaches allows MARBL-SPECTRA to capture both large-scale elemental cycles and the structure of planktonic food webs affecting trophic transfer efficiency.

The influence of macrophyte ecological groups on food web components of temperate freshwater lakes

Aquatic macrophyte taxonomic composition, species abundance and cover determine the physical structure, complexity and heterogeneity of aquatic habitats – the structuring role of macrophytes. These traits influence richness, distribution, feeding and strength of the relationships between food web communities in lakes. The aim of this study was to determine how lakes with different dominating macrophyte ecological groups affect planktonic food web components, emphasising the influence on young of year (YOY) fish and large (≥1 +) fish community. We hypothesised that different dominating macrophyte ecological groups have different structural effects on food web components and YOY fish growth, abundance and feeding. Studied lakes categorised into three different macrophyte ecological groups – lakes dominated by emergent, floating+floating-leaved or submerged vegetation. We found that all dominating ecological groups had a strong influence on plankton communities (except heterotrophic bacterioplankton and nanoflagellates), YOY fish and large fish. Floating-leaved plant dominance was positively related to planktonic food web structure and YOY fish weight, length, abundance and the consumption of zooplankton as a prey of all major species of YOY fishes. Larger fish tended to favour the presence of emergent vegetation. This conclusion has important implications for local managers and conservationists in respect to the maintenance and protection of littoral habitats and fish resources.

Controls and characteristics of biomass quantization in size-structured planktonic ecosystem models

Strong relationships between size and other traits have long motivated studies of the size structure and dynamics of planktonic food webs. Size structured ecosystem models (SSEMs) are often used to represent the behavior of these ecosystems, with organism size as a first order approximation of the axis of biological diversity. Previous studies using SSEMs have reported the emergence of localized “peaks” in the size spectrum, a phenomenon that will be referred to in this study as “quantization”. However, SSEMs that are used routinely in Earth System Models (ESMs), they tend to be too coarsely discretized to resolve quantization. Observational studies of plankton biomass have also shown qualitatively similar patterns, with localized peaks along the size spectrum. The conditions under which quantization occurs and the ecosystem parameters that control the locations of the biomass “peaks” along the size spectrum have not been systematically explored. This study serves to simultaneously advance our understanding of the constraints on quantization in size-structured ecosystems, and to suggest an approach to discretizing SSEMs that leverages quantization to select a greatly reduced number of size classes. A size-structured model of the pelagic food web, similar to those implemented in global models, is used to investigate the sensitivity of biomass peaks to predator–prey interactions, and nutrient forcing. This study shows that the location of biomass peaks along the size spectrum is strongly controlled by the size selectivity of predation, and the location of biomass peaks along the size spectrum is less sensitive to variations in nutrient supply, external ecosystem forcing, and vertical heterogeneity. Taking advantage of a robust localization of biomass peaks, the dynamics of a continuous planktonic size spectrum to be represented using a few selected size classes, corresponding to locations of the peaks along the size spectrum. These findings offer an insight on how to approach discretization of size structured ecosystem model in Earth system models.

An integrated multiple driver mesocosm experiment reveals the effect of global change on planktonic food web structure

Global change puts coastal marine systems under pressure, affecting community structure and functioning. Here, we conducted a mesocosm experiment with an integrated multiple driver design to assess the impact of future global change scenarios on plankton, a key component of marine food webs. The experimental treatments were based on the RCP 6.0 and 8.5 scenarios developed by the IPCC, which were Extended (ERCP) to integrate the future predicted changing nutrient inputs into coastal waters. We show that simultaneous influence of warming, acidification, and increased N:P ratios alter plankton dynamics, favours smaller phytoplankton species, benefits microzooplankton, and impairs mesozooplankton. We observed that future environmental conditions may lead to the rise of Emiliania huxleyi and demise of Noctiluca scintillans, key species for coastal planktonic food webs. In this study, we identified a tipping point between ERCP 6.0 and ERCP 8.5 scenarios, beyond which alterations of food web structure and dynamics are substantial.

Seasonal variability in planktonic food web structure and function of the Northeast U.S. Shelf

AbstractHerbivorous consumption of primary production is a key transformation in global biogeochemical cycles, directing matter and energy either to higher trophic levels, export production, or remineralization. Grazing by microzooplankton is often poorly constrained, particularly in dynamic coastal systems. Temperate coastal areas are seasonally and spatially variable, which presents both challenges and opportunities to identify patterns and drivers of grazing pressure. Here we report on two winter and one summer week‐long cruises (2018–2019), as part of the new Northeast U.S. Shelf Long‐Term Ecological Research program. During both seasons, coastal waters were colder and fresher, and had higher phytoplankton biomass than waters at the shelf break. The phytoplankton community was dominated by large cells in winter and by small cells in summer. Phytoplankton growth rates ranged from < 0.5 d−1 in winter and up to 1.4 d−1 in summer and were strongly correlated to temperature, to light availability, and to phytoplankton community size‐structure. Grazing rates were not correlated with total chlorophyll a, which points to other biological drivers, including species composition in predator‐prey interactions at the first trophic level. The percentage of primary production consumed (%PP) indicated higher trophic transfer in winter (%PP > 50%) than during summer (%PP < 20%), highlighting seasonal shifts in planktonic food web structure and function. These results imply that predictable shifts in environmental conditions can be linked to ecosystem shifts in net primary production. Hierarchies of variability, from localized to interannual and long‐term climate driven, can be understood within the context of sustained measurements of ecosystem properties and function.

Temporal changes in food web structure in semiarid waterholes

ABSTRACT High seasonality of semiarid aquatic ecosystems may be due to large-amplitude water level fluctuations. The resulting habitat changes impact aquatic communities and food web structures as a result of species replacement or loss. To study the impact of water level fluctuations on planktonic food web structure, we compared food web descriptors in contrasting seasons. We examined the planktonic food webs associated with semiarid natural and artificial waterholes sampled in Hwange National Pak, Zimbabwe, during the hot dry (HDS), wet (WS), and cold dry seasons (CDS). Natural waterholes are filled by rainwater, while underground water is pumped into artificial waterholes to provide drinking water to wildlife during the dry season. We computed the descriptors of 66 food webs. WS food webs tended to be more complex than those of HDS and CDS; the percentage of basal species was lower, whereas the food web connectance, percentage of intermediate species, omnivory index, mean trophic level, and maximum trophic height of species were higher. Pumping water to maintain pans in the dry season had no effect on food web properties. The drying up of waterholes in semiarid environments simplifies planktonic food webs. The dominance of small rotifers may explain the observed low omnivory index and food web connectance in HDS compared to those observed in other studies.

Decrease in diatom dominance at lower Si:N ratios alters plankton food webs

Abstract Many coastal oceans experience not only increased loads of nutrients but also changes in the stoichiometry of nutrient supply. Excess supply of nitrogen and stable or decreased supply of silicon lower silicon to nitrogen (Si:N) ratios, which may decrease diatom proportion in phytoplankton. To examine how Si:N ratios affect plankton community composition and food web structure, we performed a mesocosm experiment where we manipulated Si:N ratios and copepod abundance in a Baltic Sea plankton community. In high Si:N treatments, diatoms dominated. Some of them were likely spared from grazing unexpectedly resulting in higher diatom biomass under high copepod grazing. With declining Si:N ratios, dinoflagellates became more abundant under low and picoplankton under high copepod grazing. This altered plankton food web structure: under high Si:N ratios, edible diatoms were directly accessible food for copepods, while under low Si:N ratios, microzooplankton and phago-mixotrophs (mixoplankton) were a more important food source for mesograzers. The response of copepods to changes in the phytoplankton community was complex and copepod density-dependent. We suggest that declining Si:N ratios favor microzoo- and mixoplankton leading to increased complexity of planktonic food webs. Consequences on higher trophic levels will, however, likely be moderated by edibility, nutritional value or toxicity of dominant phytoplankton species.

Planktonic food web structure and trophic transfer efficiency along a productivity gradient in the tropical and subtropical Atlantic Ocean

Oligotrophic and productive areas of the ocean differ in plankton community composition and biomass transfer efficiency. Here, we describe the plankton community along a latitudinal transect in the tropical and subtropical Atlantic Ocean. Prochlorococcus dominated the autotrophic community at the surface and mixed layer of oligotrophic stations, replaced by phototrophic picoeukaryotes and Synechococcus in productive waters. Depth-integrated biomass of microzooplankton was higher than mesozooplankton at oligotrophic stations, showing similar biomasses in productive waters. Dinoflagellates dominated in oligotrophic waters but ciliates dominated upwelling regions. In oligotrophic areas, microzooplankton consumed ca. 80% of the production, but ca. 66% in upwelling zones. Differences in microzooplankton and phytoplankton communities explain microzooplankton diel feeding rhythms: higher grazing rates during daylight in oligotrophic areas and diffuse grazing patterns in productive waters. Oligotrophic areas were more efficient at recycling and using nutrients through phytoplankton, while the energy transfer efficiency from nutrients to mesozooplankton appeared more efficient in productive waters. Our results support the classic paradigm of a shorter food web, and more efficient energy transfer towards upper food web levels in productive regions, but a microbially dominated, and very efficient, food web in oligotrophic regions. Remarkably, both models of food web exist under very high microzooplankton herbivory.

Stratification, nitrogen fixation, and cyanobacterial bloom stage regulate the planktonic food web structure

Changes in the complexity of planktonic food webs may be expected in future aquatic systems due to increases in sea surface temperature and an enhanced stratification of the water column. Under these conditions, the growth of unpalatable, filamentous, N2‐fixing cyanobacterial blooms, and their effect on planktonic food webs will become increasingly important. The planktonic food web structure in aquatic ecosystems at times of filamentous cyanobacterial blooms is currently unresolved, with discordant lines of evidence suggesting that herbivores dominate the mesozooplankton or that mesozooplankton organisms are mainly carnivorous. Here, we use a set of proxies derived from amino acid nitrogen stable isotopes from two mesozooplankton size fractions to identify changes in the nitrogen source and the planktonic food web structure across different microplankton communities. A transition from herbivory to carnivory in mesozooplankton between more eutrophic, near‐coastal sites and more oligotrophic, offshore sites was accompanied by an increasing diversity of microplankton communities with aging filamentous cyanobacterial blooms. Our analyses of 124 biotic and abiotic variables using multivariate statistics confirmed salinity as a major driver for the biomass distribution of non‐N2‐fixing microplankton species such as dinoflagellates. However, we provide strong evidence that stratification, N2 fixation, and the stage of the cyanobacterial blooms regulated much of the microplankton diversity and the mean trophic position and size of the metabolic nitrogen pool in mesozooplankton. Our empirical, macroscale data set consistently unifies contrasting results of the dominant feeding mode in mesozooplankton during blooms of unpalatable, filamentous, N2‐fixing cyanobacteria by identifying the at times important role of heterotrophic microbial food webs. Thus, carnivory, rather than herbivory, dominates in mesozooplankton during aging and decaying cyanobacterial blooms with hitherto uncharacterized consequences for the biogeochemical functions of mesozooplankton.

Frequency of virus-resistant hosts determines experimental community dynamics.

Parasites, such as bacterial viruses (phages), can have large effects on host populations both at the ecological and evolutionary levels. In the case of cyanobacteria, phages can reduce primary production and infected hosts release intracellular nutrients influencing planktonic food web structure, community dynamics, and biogeochemical cycles. Cyanophages may be of great importance in aquatic food webs during large cyanobacterial blooms unless the host population becomes resistant to phage infection. The consequences on plankton community dynamics of the evolution of phage resistance in bloom forming cyanobacterial populations are still poorly studied. Here, we examined the effect of different frequencies of a phage-resistant genotype within a filamentous nitrogen-fixing Nodularia spumigena population on an experimental plankton community. Three Nodularia populations with different initial frequencies (0%, 5%, and 50%) of phage-resistant genotypes were inoculated in separate treatments with the phage 2AV2, the green alga Chlorella vulgaris, and the rotifer Brachionus plicatilis, which formed the experimental plankton community subjected to either nitrogen-limited or nitrogen-rich conditions. We found that the frequency of the phage-resistant Nodularia genotype determined experimental community dynamics. Cyanobacterial populations with a high frequency (50%) of the phage-resistant genotype dominated the cultures despite the presence of phages, retaining most of theintracellular nitrogen in the plankton community. In contrast, populations with low frequencies (0% and 5%) of the phage-resistant genotype were lysed and reduced to extinction by the phage, transferring the intracellular nitrogen held by Nodularia to Chlorella and rotifers, and allowing Chlorella to dominate the communities and rotifers to survive. This study shows that even though phages represent minuscule biomass, they can have key effects on community composition and eco-evolutionary feedbacks in plankton communities.

Single-Cell View of Carbon and Nitrogen Acquisition in the Mixotrophic Alga Prymnesium parvum (Haptophyta) Inferred From Stable Isotope Tracers and NanoSIMS

Nutritional modes of unicellular eukaryotes range from pure photoautotrophy of some phytoplankton to pure heterotrophy of species typically called protozoa. Between these two extremes lies a functional continuum of nutrient and energy acquisition modes termed mixotrophy. Prymnesium parvum is an ecologically important mixotrophic haptophyte alga that can produce toxins and form ecosystem disruptive blooms that result in fish kills and changes in planktonic food web structure. We investigated carbon and nitrogen acquisition strategies of single cells of P. parvum using a combined experimental-imaging approach employing labeling of live cells with stable isotope tracers (13C and 15N) followed by measurement of cellular isotopic ratios using nanometer-scale secondary ion mass spectrometry (NanoSIMS). With this method, we were able to quantify the relative contributions of photosynthesis and heterotrophy to the nutrition of the alga. Our results suggest that P. parvum relies on predation primarily for nitrogen, while most carbon for cellular building blocks is obtained from inorganic sources. Our analysis further revealed that nitrogen assimilation can vary up to an order of magnitude among individual cells, a finding that would be difficult to determine using other methods. These results help to improve our understanding of mixotrophy across the enormous diversity of eukaryotes, one cell and one species at a time.

Planktonic food web structure at SSTF and PF in the Indian sector of the Southern Ocean during austral summer 2011

ABSTRACTThis study aims to describe the planktonic food web structure with respect to phytoplankton biomass (chlorophyll a) and prevailing environmental conditions at the South Subtropical Front (S...

Photosynthetic oxygen production in a warmer ocean: the Sargasso Sea as a case study.

Photosynthetic O2 production can be an important source of oxygen in sub-surface ocean waters especially in permanently stratified oligotrophic regions of the ocean where O2 produced in deep chlorophyll maxima (DCM) is not likely to be outgassed. Today, permanently stratified regions extend across approximately 40% of the global ocean and their extent is expected to increase in a warmer ocean. Thus, predicting future ocean oxygen conditions requires a better understanding of the potential response of photosynthetic oxygen production to a warmer ocean. Based on our own and published observations of water column processes in oligotrophic regions, we develop a one-dimensional water column model describing photosynthetic oxygen production in the Sargasso Sea to quantify the importance of photosynthesis for the downward flux of O2 and examine how it may be influenced in a warmer ocean. Photosynthesis is driven in the model by vertical mixing of nutrients (including eddy-induced mixing) and diazotrophy and is found to substantially increase the downward O2 flux relative to physical-chemical processes alone. Warming (2°C) surface waters does not significantly change oxygen production at the DCM. Nor does a 15% increase in re-mineralization rate (assuming Q10 = 2; 2°C warming) have significant effect on net sub-surface oxygen accumulation. However, changes in the relative production of particulate (POM) and dissolved organic material (DOM) generate relatively large changes in net sub-surface oxygen production. As POM/DOM production is a function of plankton community composition, this implies plankton biodiversity and food web structure may be important factors influencing O2 production in a warmer ocean.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Anthropogenic shift of planktonic food web structure in a coastal lagoon by freshwater flow regulation

Anthropogenic modification of aquatic systems has diverse impacts on food web interactions and ecosystem states. To reverse the adverse effects of modified freshwater flow, adequate management of discharge is required, especially due to higher water requirements and abstractions for human use. Here, we look at the effects of anthropogenically controlled freshwater flow regimes on the planktonic food web of a Ramsar listed coastal lagoon that is under recovery from degradation. Our results show shifts in water quality and plankton community interactions associated to changes in water flow. These shifts in food web interactions represent modifications in habitat complexity and water quality. At high flow, phytoplankton-zooplankton interactions dominate the food web. Conversely, at low flow, bacteria, viruses and nano/picoplankton interactions are more dominant, with a substantial switch of the food web towards heterotrophy. This switch can be associated with excess organic matter loading, decomposition of dead organisms, and synergistic and antagonistic interactions. We suggest that a lower variability in flow amplitude could be beneficial for the long-term sustaining of water quality and food web interactions, while improving the ecosystem health of systems facing similar stresses as the Coorong.