Lecture 4 Aquatic food web structure and the flow of carbon lecture slides, lecture

Page 1: Overview

• Title: Aquatic food web structure and the flow of carbon

• Authors: Vladimir Matveev and Barbara J. Robson

• Affiliation: CSIRO Land and Water, Canberra, Australia

• Corresponding Author: barbara.robson@csiro.au

• Publication Dates: Received 23 December 2013; Accepted 29 August 2014; Published 3 November 2014

Page 2: Carbon Cycling in Freshwater Ecosystems

• Importance of Carbon Cycling:

  • Crucial for climate change, ecosystem health, and human activities.

  • Carbon can limit primary production even in CO2-supersaturated waters.

• Net Heterotrophy:

  • Most lakes and rivers are net heterotrophic due to terrestrial carbon (allochthonous) subsidies.

• Limitations of Terrestrial Carbon Utilization:

  • Low nutritional quality and inability of bacteria to synthesize PUFAs (polyunsaturated fatty acids) essential for metazoans.

• Role of Bacterivorous Nanoflagellates:

  • Can synthesize PUFAs, potentially linking terrestrial carbon to metazoan production.

Page 3: Influence of Food Web Structure

• Food Web Structure and Carbon Dynamics:

  • Influences carbon fluxes, accumulation, distribution, burial, and sequestration.

  • Affects carbon emission/sequestration ratio, indicating whether an ecosystem acts as a carbon source or sink.

• Significance of Small Lakes:

  • Dominant type of freshwater bodies with high carbon burial rates.

• Impact of Human Activities:

  • Human activities modify productivity and food web structure, which could control carbon emissions.

Page 4: Introduction to Carbon Cycling

• Global Importance of Carbon Cycling:

  • Essential for understanding global climate change and ecosystem transformations.

  • Inland waters significantly influence global carbon cycle, comparable to oceanic CO2 uptake.

Page 5: Food Web Structure's Role

• Understanding Food Webs:

  • Essential for regulating the carbon cycle.

  • Aims of the paper:

    • Analyze carbon entry and processing in aquatic ecosystems.

    • Review influence of internal/external carbon sources.

    • Evaluate current models of carbon pathways in freshwater food webs.

Page 6: Biogeochemical Cycles - The Carbon Cycle

• Carbon as Energy Currency:

  • Holds a large proportion of an organism’s mass (45-55% in plants and freshwater invertebrates).

  • Carbon bonds are key for energy transfer in aquatic ecosystems.

Page 7: Ecosystem Metabolism and Carbon Input

• Lakes as Input-Output Systems:

  • Viewed as energy systems, not isolated entities.

• Ecosystem Characteristics:

  • Contains autotrophic and heterotrophic components capturing energy, carbon, and nutrients.

• Key Processes:

  • Primary production (carbon acquisition), secondary production (carbon transfer), decomposition (carbon re-mineralization), and respiration (carbon loss).

Page 8: Pathways of Carbon Input

• Metabolic Gates for Carbon Entry:

  • Autochthonous primary production.

  • Microbial decomposition.

  • Consumption of live terrestrial organisms.

  • Consumption of dissolved organic carbon (DOC).

  • Consumption of imported detritus and colloidal microparticles.

• Control of Carbon Mobilization:

  • Pathways dictate how carbon is mobilized and incorporated into ecosystem metabolism.

Page 9: Sources, Gates, and Sinks of Carbon

• Table 1 Overview:

  • Sources: Atmospheric CO₂, terrestrial DOC, etc.

  • Gates: Autochthonous primary production, microbial respiration, consumption pathways.

  • Sinks: Respiration by plants and animals, burial in sediments.

Page 10: Bypassing Metabolic Gates

• Charcoal's Role:

  • Can enter ecosystems bypassing metabolic gates.

  • Provides little nutritional value but alters sediment properties affecting benthic communities.

Page 11: Influence of Carbon Composition

• Benthic Community Dynamics:

  • Affected by substratum particle size.

  • Recalcitrant carbon modifies ecosystem functioning indirectly.

Page 12: Carbon Balance and P/R Ratios

• Defining Ecosystem Metabolism:

  • Primary production (P) vs. respiration (R) indicates ecosystem's carbon balance.

  • Positive NEP: Ecosystem accumulates carbon, acts as a CO2 sink.

  • Negative NEP: Ecosystem acts as a CO2 source.

Page 13: NEP Indicators

• Heterotrophic Systems:

  • Most lake/river ecosystems are heterotrophic; median ratios for streams (approx. 0.5) and lakes (approx. 0.6).

• Variability in P/R Ratios:

  • Conditions may lead to variant P/R ratios within the same system.

Page 14: P/R Ratios in Freshwaters

• Variability by Location:

  • Shallow eutrophic lakes/rivers may show P/R ≥ 1 while oligotrophic lakes show P/R <1.

• Factors Affecting Ratios:

  • Regions of elevated microbial decomposition may reveal different P/R ratios.

Page 15: P/R Ratios Dependency

• Factors Contributing to Heterotrophy:

  • P/R ratios can reflect the ecosystem's reliance on external organic carbon.

• Examples of Variability:

  • Example from the Murray River indicates P/R ratios can shift with flooding events.

Page 16: River Continuum Concept

• P/R Variability Along Rivers:

  • P/R ratios differ in forested headwaters (<1) vs downstream regions (>1).

• Ecosystem Changes:

  • Stressed ecosystems may demonstrate unbalanced P/R ratios.

Page 17: Reliability of P/R Ratios

• Natural Variation of P/R Ratios:

  • Most freshwater ecosystems exhibit P/R < 1, questioning reliability as an ecosystem health indicator.

• Effects of Fertilization and Acidification:

  • Human interventions can modify P/R ratios significantly.

Page 18: Food Web Concepts

• Key Concepts:

  • River Continuum, Serial Discontinuity, Flood Pulse, Functional Process Zones.

  • Each concept describes different influences on food web functions and organism distribution based on environmental conditions.

Page 19: Types of Terrestrial Subsidies

• Terrestrial Subsidies' Impact:

  • Reduced P/R ratios linked to terrestrial organic matter import.

  • Types of subsidies:

    • Terrestrial dissolved organic carbon (t-DOC)

    • Terrestrial particulate organic carbon (t-POC)

    • Live terrestrial prey (t-prey) consumed by aquatic animals.

Page 20: Terrestrial Organic Matter Dynamics

• Bacterial Respiration Boosted:

  • t-DOC and t-POC provide substrates for microbial decomposition.

  • Marine carbon subsidies can also influence freshwater systems vastly, with t-DOC often exceeding POC significantly.

Page 21: Bioavailability of Terrestrial Organic Matter

• Factors Affecting Bioavailability:

  • Origin and chemical form of organic matter.

• Labile vs Recalcitrant DOC:

  • Labile DOC is readily utilized by bacteria, while recalcitrant DOC may be flocculated and stored.

Page 22: Bacterial Production and Metabolic Gates

• DOC Consumption Role:

  • Major carbon flux in ecosystems, leading to bacterial production.

  • Mechanisms like photo-oxidation and photolysis help transfer recalcitrant carbon through metabolic gates.

Page 23: Consistency in Ecosystem Dynamics

• Stabilization by Recalcitrant Carbon:

  • Maintains ecosystem dynamics by providing low-quality food sources.

• Chromatic Dissolved Organic Material (CDOM):

  • Can inhibit photosynthesis, thus affecting nutrient cycling and primary production.

Page 24: Effects of Colored DOC

• Impact on Benthic Algae:

  • Increased colored DOC can result in reduced photosynthesis and primary production.

• Role of Humic Substances:

  • Important regulators of primary production in lakes.

Page 25: Carbon Limitation of Primary Production

• Nutrient Limitation:

  • Phosphorus and nitrogen are critical limiting factors; carbon's role is less understood.

• Super saturation of CO2 in Lakes:

  • Strong correlation between organic carbon and partial pressure of CO2 (pCO2).

Page 26: Revising Carbon Limitation Perspectives

• New Insights on Carbon Limitation:

  • Even in CO2-supersaturated lakes, additional CO2 can boost production, suggesting it's a limiting factor.

• Impact of Terrestrial Carbon Subsidies:

  • Changes in carbon input can noticeably affect primary productivity in freshwater lakes.

Page 27: Cultural Eutrophication

• Definition:

  • Increased algal productivity driven by human activity, primarily through nutrient inputs.

  • Carbon co-limitation adds complexity to eutrophication understanding.

Page 28: Factors Influencing Phytoplankton Growth

• Terrestrial DOC's Role:

  • Increase in phytoplankton productivity driven by carbon input, microbial activity is temperature-dependent.

• Global Warming Effects:

  • Might accelerate CO2 release and primary production in freshwater ecosystems.

Page 29: Carbon Flux and Food Chains

• Trophic Level Simplification:

  • Food chains represent carbon paths from primary producers to consumers.

  • Non-whole numbers reflect animals feeding at multiple levels.

Page 30: The Microbial Loop

• Function of the Microbial Loop:

  • DOC feeds bacteria, facilitating a return flow of carbon to herbivore food chains.

• Efficiency of Carbon Utilization:

  • Bacteria show greater growth efficiency on internal carbon compared to external carbon.

Page 31: Food Chain Length (FCL)

• Definition:

  • FCL measures carbon transformation and energy use in an ecosystem.

• Impacts of FCL:

  • Affects community structure and ecosystem functions, alongside contaminant accumulation.

Page 32: Human Activity and FCL Impact

• FCL Variability:

  • Changes from human activities can affect various ecosystems based on their FCL, altering ecological dynamics.

Page 33: Primary Production's Effect on FCL

• Influence of Ecosystem Size:

  • Lake size (volume) serves as a primary predictor of FCL rather than productivity levels.

Page 34: Omnivory in Food Webs

• Understanding Omnivory:

  • Widespread in freshwater plankton, with organisms feeding across multiple trophic levels.

• Dietary Changes in Consumers:

  • Ontogeny can influence diets, e.g., bony bram (Nematalosa erebi) changes from carnivorous to herbivorous.

Page 35: Key Terms in Carbon Cycling

• Definitions:

  • Terms related to carbon cycling include food chain, herbivorous food chain, microbial food chain, etc.

• Function of Each Term:

  • Describes roles in carbon movement through aquatic systems.

Page 36: Food Web Structure Effects

• Impact on Ecosystem Functions:

  • FWS influences prey community size, carbon fluxes, nutrient recycling, productivity, and P/R balance in ecosystems.

Page 37: Control Mechanisms in Food Webs

• Top-Down and Bottom-Up Controls:

  • Predators affect community structure and productivity; nutrient availability dictates primary productivity.

  • Manipulating fish community structures can significantly change water quality and ecosystem characteristics.

Page 38: Role of Top Predators

• Impact on Carbon Fixation:

  • Top predators can reduce atmospheric carbon influx into lakes, influencing net autotrophy.

• Fish Influence:

  • They play critical roles in phosphorus cycling and overall lake dynamics.

Page 39: Nutrient Recycling Dynamics

• Role of Nutrient Enrichment:

  • Promotes primary production and makes lakes carbon sinks.

  • Interplay of food web structure and nutrient levels determines carbon sources or sinks.

Page 40: Ecological Efficiency in Food Chains

• Trophic Transfer Efficiency (TTE):

  • Influences biomass production and water quality, quantifying efficiency at each trophic level.

Page 41: Evaluating TTE Across Ecosystems

• Meta-Analysis Findings:

  • Typical TTE across aquatic ecosystems averages around 10%.

Page 42: Bacterial Carbon Controversy

• Link or Sink Controversy:

  • Debate on whether bacteria serve as a nutrient source or a carbon sink, defined by their feeding interactions and contributions to carbon flow.

Page 43: Efficiency of Bacterial Grazing

• Bacterial Biomass Control:

  • Ciliates can regulate bacterial populations, ensuring efficient carbon transfer between microbial groups.

Page 44: Carbon Flow Rates and Community Structure

• Connections between Flow Rate and Community:

  • Community composition impacts carbon movement rates, influenced by environmental factors like temperature and organism size.

Page 45: Herbivory and Specific Production

• Link to Primary Production:

  • Plant communities' properties significantly impact herbivory patterns due to palatability and production rates.

Page 46: General Conclusions

• Key Takeaways from the Study:

  • Carbon cycling is crucial for ecosystem health and climate understanding.

  • Inland waters play a major role in the global carbon cycle.

• Food Web Structures:

  • Regulate carbon flow; most ecosystems are net heterotrophic, relying on external carbon sources.

Page 47: Implications for Management

• Small Lakes' Importance:

  • Crucial for carbon burial, often overlooked.

• Management Strategies:

  • Human actions can significantly alter carbon cycling, and proper management could mitigate carbon emissions.