Food chain efficiency ( We conducted a large‐scale, 6‐week mesocosm experiment in which we manipulated light and nutrient (nitrogen and phosphorus) supply and the identity of the carnivore in a 2 × 2 × 2 factorial design. We quantified the response of We predicted that bluegill would be more likely to experience P‐limitation due to higher P requirements, and hence that Carnivore identity mediated the effects of light and nutrients on Comparison of our results with those from previous experiments showed that
The relative importance of top‐down vs. bottom‐up control of phytoplankton biomass in aquatic ecosystems has been long debated and studied. However, few studies have considered the relative importance of top‐down vs. bottom‐up control on phytoplankton vertical distributions and characteristics of deep chlorophyll maxima (DCMs), and fewer still have investigated the importance of these drivers for multiple phytoplankton groups. We examined depth profiles of four phytoplankton spectral groups and a suite of top‐down (zooplankton) and bottom‐up (nutrients, temperature, and light) drivers from 51 north temperate lakes varying on gradients of size, trophic state, light availability, and thermal stratification. We used regression trees to identify the most important drivers of different vertical distribution metrics for each phytoplankton spectral group. The relative importance of top‐down vs. bottom‐up control varied across spectral groups and was related to the characteristics of the dominant taxa within each spectral group, as assessed by microscope counts. Zooplankton biomass was the most important driver of brown algae vertical distributions, likely because this group contained highly edible taxa (primarily chrysophytes), while thermal stratification predicted vertical distributions of buoyancy‐regulating cyanobacteria. Our work highlights the importance of examining phytoplankton community composition to improve understanding of DCM characteristics and top‐down vs. bottom‐up control of phytoplankton in aquatic systems.
more » « less- NSF-PAR ID:
- 10455092
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 65
- Issue:
- 10
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 2485-2501
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Summary FCE ), the proportion of primary production converted to production of the top trophic level, can influence several ecosystem services as well as the biodiversity and productivity of each trophic level. AquaticFCE is affected by light and nutrient supply, largely via effects on primary producer stoichiometry that propagate to herbivores and then carnivores. Here, we test the hypothesis that the identity of the top carnivore mediatesFCE responses to changes in light and nutrient supply.FCE and the biomass and productivity of each trophic level (phytoplankton, zooplankton, and carnivore). We used an invertebrate,Chaoborus americanus , and a vertebrate, bluegill sunfish (Lepomis macrochirus ), as the two carnivores in this study because of the large difference in phosphorus requirements between these taxa.FCE would be lower in the bluegill treatments than in theChaoborus treatments. We also expected the interactive effect of light and nutrients to be stronger in the bluegill treatments. Within a carnivore treatment, we predicted highestFCE under low light and high nutrient supply, as these conditions would produce high‐quality (low C:nutrient) algal resources. In contrast, if food quantity had a stronger effect on carnivore production than food quality, carnivore production would increase proportionally with primary production, thusFCE would be similar across light and nutrient treatments.FCE , and as predictedFCE was higher in food chains withChaoborus than with bluegill. Also as predicted,FCE inChaoborus treatments was higher under low light. However,FCE in bluegill treatments was higher at high light supply, opposite to our predictions. In addition, bluegill production increased proportionally with primary production, whileChaoborus production was not correlated with primary production, suggesting that bluegill responded more strongly to food quantity than to food quality. These carnivore taxa differ in traits other than body stoichiometry, for example, feeding selectivity, which may have contributed to the observed differences inFCE between carnivores.FCE responds similarly to light and nutrients in food chains withChaoborus and larval fish (gizzard shad: Clupeidae), but very differently in food chains with bluegill. These findings warrant further investigation into the mechanisms related to carnivore identity (e.g., developmental stage, feeding selectivity) underlying these responses, and highlight the importance of considering both top‐down and bottom‐up effects when evaluating food chain responses to changing light and nutrient conditions. -
null (Ed.)Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.more » « less
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Abstract Tropical floodplains secure the protein supply of millions of people, but only sound management can ensure the long‐term continuity of such ecosystem services. Overfishing is a widespread threat to multitrophic systems, but how it affects ecosystem functioning is poorly understood, particularly in tropical freshwater food webs. Models based on temperate lakes frequently assume that primary producers are mostly bottom‐up controlled by nutrient and light limitations, with negligible effects of top‐down forces. Yet this assumption remains untested in complex tropical freshwater systems experiencing marked spatiotemporal variation.
We use consolidated community‐based fisheries management practices and spatial zoning to test the relative importance of bottom‐up versus top‐down drivers of phytoplankton biomass, controlling for the influence of local to landscape heterogeneity. Our study focuses on 58 large Amazonian floodplain lakes under different management regimes that resulted in a gradient of apex‐predator abundance. These lakes, distributed along ~600 km of a major tributary of the Amazon River, varied widely in size, structure, landscape context, and hydrological seasonality.
Using generalised linear models, we show that community‐based fisheries management, which controls the density of apex predators, is the strongest predictor of phytoplankton biomass during the dry season, when lakes become discrete landscape units. Water transparency also emerges as an important bottom‐up factor, but phosphorus, nitrogen and several lake and landscape metrics had minor or no effects on phytoplankton biomass. During the wet‐season food pulse, when lakes become connected to adjacent water bodies and homogenise the landscape, only lake depth explained phytoplankton biomass.
Synthesis and applications . Tropical freshwaters fisheries typically assume that fish biomass is controlled by bottom‐up mechanisms, so that overexploitation of large predators would not affect overall ecosystem productivity. Our results, however, show that top‐down forces are important drivers of primary productivity in tropical lakes, above and beyond the effects of bottom‐up factors. This helps us to understand the enormous success of community‐based ‘fishing agreements’ in the Amazon. Multiple stakeholders should embrace socio‐ecological management practices that shape both bottom‐up and top‐down forces to ensure biodiversity protection, sustainable fisheries yields and food security for local communities and regional economies. -
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Abstract We investigate where bottom‐up and top‐down control regulates ecological communities as a mechanism linking ecological gradients to the geography of consumer abundance and biomass. We use standardized surveys of 54 North American grasslands to test alternate hypotheses predicting 100‐fold shifts in the biomass of four common grassland arthropod taxa—Auchenorrhyncha, sucking herbivores, Acrididae, chewing herbivores, Tettigoniidae, omnivores, and Araneae, predators.
Bottom‐up models predict that consumer biomass tracks plant quantity (e.g. productivity and standing biomass) and quality (nutrient content) and that ectotherm access to food increases with temperature. Each of the focal trophic groups responded differently to these drivers: the biomass of sucking herbivores and omnivores increased with plant biomass; that of chewing herbivores tracked plant quality; and predator biomass did not depend on plant quality, plant quantity or temperature.
The Exploitation Ecosystem Hypothesis is a top‐down hypothesis that predicts a shift from resource limitation of herbivores when plant production is low, to predator limitation when plant production is high. In grasslands where spider biomass was low, herbivore biomass increased with plant biomass, whereas bottom‐up structuring was not evident when spiders were abundant. Furthermore, neither predator biomass nor trophic position (via stable isotope analysis) increased with plant biomass, suggesting predators themselves are top‐down limited.
Stable isotope analysis revealed that trophic position of the chewing herbivore and omnivore increased significantly with plant biomass, suggesting these groups increased scavenging and meat consumption in grasslands with higher carbohydrate availability.
Taken together, our snapshot sampling documents gradients of food web structure across 54 grasslands, consistent with multiple hypotheses of bottom‐up and top‐down regulation.
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Abstract. Oligotrophic regions play a central role in global biogeochemical cycles, with microbial communities in these areas representing an important term in global carbon budgets. While the general structure of microbial communities has been well documented in the global ocean, some remote regions such as the western tropical South Pacific (WTSP) remain fundamentally unexplored. Moreover, the biotic and abiotic factors constraining microbial abundances and distribution remain not well resolved. In this study, we quantified the spatial (vertical and horizontal) distribution of major microbial plankton groups along a transect through the WTSP during the austral summer of 2015, capturing important autotrophic and heterotrophic assemblages including cytometrically determined abundances of non-pigmented protists (also called flagellates). Using environmental parameters (e.g., nutrients and light availability) as well as statistical analyses, we estimated the role of bottom–up and top–down controls in constraining the structure of the WTSP microbial communities in biogeochemically distinct regions. At the most general level, we found a
typical tropical structure
, characterized by a shallow mixed layer, a clear deep chlorophyll maximum at all sampling sites, and a deep nitracline. Prochlorococcus was especially abundant along the transect, accounting for 68±10.6% of depth-integrated phytoplankton biomass. Despite their relatively low abundances, picophytoeukaryotes (PPE) accounted for up to 26±11.6% of depth-integrated phytoplankton biomass, while Synechococcus accounted for only 6±6.9%. Our results show that the microbial community structure of the WTSP is typical of highly stratified regions, and underline the significant contribution to total biomass by PPE populations. Strong relationships between N2 fixation rates and plankton abundances demonstrate the central role of N2 fixation in regulating ecosystem processes in the WTSP, while comparative analyses of abundance data suggest microbial community structure to be increasingly regulated by bottom–up processes under nutrient limitation, possibly in response to shifts in abundances of high nucleic acid bacteria (HNA).
