Aquatic Food Webs, Isotopes, and Trophic Cascades

Administrative Details

• Paper Deadlines:
      * The Karski paper is due today.
      * The Morton's paper is due this Friday.
• Grading Strategy: Students are required to submit four of the six assigned papers. If a student submits five papers, only four will be counted towards the grade.
• Points Update: Students who participated in the field trip or completed the paper summary have been awarded $20$ points toward their total of $50$ points.
• Upcoming Lectures:
      * Friday: Antonio will discuss bioassessment.
      * Monday: Discussion of a follow-up paper by Mike Bogan.

Presenter Background: Taylor Cain

• Current Status: Master’s student in Water Resources working with Becky; currently in the final semester and graduating this summer.
• Professional Project: Installing Beaver Dam Analogs (BDAs) on a creek in Northern New Mexico at Bandelier National Historical Park (noted phonetically as Bendegos in transcript, located approximately $40$ minutes east of Santa Fe).
• Project Goals: Monitor surface water and groundwater levels resulting from these man-made structures that mimic natural beaver dams.
• Academic and Career Path:
      * Initially an art major before dropping out and working in customer service.
      * Earned a Bachelor's degree in Environmental Science from Humboldt State University (now Cal Poly Humboldt).
      * Worked various field jobs in botany, fisheries, and forestry.
      * Moved to New Mexico in 2020 for a job with the BLM (Bureau of Land Management) conducting native plant seed collection throughout the state.
      * Worked for the Institute for Applied Ecology, a nonprofit in Santa Fe focused on revegetation, restoration, and creating native seed mixes.

Fundamentals of Food Webs

• Definition: Food webs are the conceptual tools ecologists use to understand "who is eating what" in aquatic systems and the subsequent transfer of energy between organisms.
• Food Chains vs. Food Webs:
      * Food Chains: A simplified, linear representation of energy transfer. Often considered an oversimplification because relationships are rarely perfectly linear.
      * Food Webs: A holistic description of complex feeding relationships. They account for the fact that organisms rarely occupy a single, fixed trophic level.
• Trophic Levels:
      * Primary Producers: The base of all food chains; includes photosynthetic organisms like algae, aquatic plants, and detritus.
      * Primary Consumers: Organisms that consume organic matter/vegetation (e.g., herbivores or grazers).
      * Secondary Consumers: Organisms that eat primary consumers.
      * Decomposers: A specialized group consuming dead organic material, vital for nutrient cycling.
      * Fractional Trophic Levels: Because some organisms do not fit neatly into levels (e.g., level $2$ vs. level $3$), ecologists may assign fractional levels like $2.5$ to provide nuance.
• Case Study: Gizzard Shad:
      * This fish, common in warm US reservoirs, changes its trophic level based on life stage.
      * Young Fish: Feed on zooplankton (Secondary Consumers).
      * Juveniles: Develop a specialized long gut with a grinding chamber and gill adaptations to filter algae, detritus, and sediment (Primary Consumers).
      * Adults: Revert to filtering zooplankton (Secondary Consumers).

Measuring Feeding Relationships

• Gut Content Analysis:
      * Method: Dissecting the animal to examine the stomach contents.
      * Pitfalls: Provides only a short time window (the last one or two meals); depends heavily on the state of digestion; individuals may eat something unusual that does not reflect their consistent diet.
• Stable Isotope Analysis:
      * Concept: Based on the phrase "you are what you eat."
      * Mechanisms: Focuses on atoms of the same element with different numbers of neutrons (heavier vs. lighter).
      * Key Isotopes: Nitrogen-15 ($^{15}N$) and Carbon-13 ($^{13}C$). These are rarer and heavier than the common $^{14}N$ and $^{12}C$.
      * Fractionation: The unequal processing of isotopes. Because heavier isotopes ($^{15}N$ and $^{13}C$) break down more slowly in chemical reactions, organisms preferentially excrete the lighter isotopes and retain the heavier ones.

Stable Isotope Ratios and Formulas

• Delta Notation ($\delta$): Used to express the ratio of a sample to a known reference standard, measured in parts per thousand (per mil).
• Carbon-13 Formula:
      * $\delta^{13}C = \left( \frac{(\frac{^{13}C}{^{12}C})<em>{\text{sample}}}{(\frac{^{13}C}{^{12}C})</em>{\text{standard}}} - 1 \right) \times 1000$
• Reference Standards:
      * Carbon: Derived from PDB (Pee Dee Belemnite), a fossil marine organism. Its ratio is approximately $0.0112$.
      * Nitrogen: Use atmospheric nitrogen ($N_2$) as the reference.
• Interpretations:
      * Most biological materials have negative $\delta^{13}C$ values, meaning they are depleted relative to the PDB standard.
      * Nitrogen values increase predictably as one moves up the trophic levels (bioaccumulation of heavy nitrogen).

Carbon Signatures and Photosynthetic Pathways

• C3 Plants: (e.g., wheat, rice, barley) typically have $\delta^{13}C$ values between $-25$ and $-30$ per mil.
• C4 Plants: Have a different photosynthetic signature than C3 plants.
• CAM Plants: Have a wider range of values.
• Aquatic Plants: Exhibit the largest range of carbon isotope ratios, making them difficult to discern specifically via carbon signatures alone.
• Application: Researchers match the $\delta^{13}C$ values from an organism (e.g., fish tissue) to these ranges to determine what type of plants or primary producers supported the food web.

Ecosystem Case Studies: Lotic vs. Lentic

• Lookout Creek, Oregon (Lotic/Stream):
      * Predators, primary consumers, and periphyton align roughly vertically on a plot of $\delta^{15}N$ vs. $\delta^{13}C$.
      * Vertical alignment indicates a direct feeding relationship within that carbon signature.
• Toolik Lake, Alaska (Lentic/Lake):
      * Fish at the top are vertically aligned with copepods.
      * Copepods (primary consumers) align with phytoplankton rather than periphyton.
• Advantages of Isotopes over Gut Contents:
      * Non-lethal: Can use fin clips, feathers, or hair samples instead of killing the animal.
      * Lower Cost: Modern mass spectrometers have reduced analysis prices.
      * Temporal Data: Different tissues provide different time scales (e.g., blood = weeks, muscle = months, bone/collagen = years).

Trophic Cascades

• Top-Down Cascade: Predators at the top influence every lower level.
      * Example: Large Piscivores (fish-eating fish) suppress populations of smaller Zooplanktivores (e.g., perch, shiners). With fewer small fish, Zooplankton populations increase in size and abundance. This leads to increased grazing on Phytoplankton, which subsequently decreases.
      * If large piscivores are removed, the reverse happens: small fish increase, zooplankton decrease, and phytoplankton bloom.
• Bottom-Up Cascade: Driven by nutrient enrichment at the base of the food web.
      * Increased nutrients leads to more phytoplankton/algae, which supports more herbivores and eventually more fish.

Biomanipulation as a Management Tool

• Objective: Using trophic cascades to improve water quality (e.g., reducing algal blooms to achieve clear water).
• Strategies:
      1. Introduce Large Piscivores: Artificially triggers a top-down cascade to reduce phytoplankton.
      2. Reduce External Nutrients: Limits fertilizer runoff or wastewater effluent (Bottom-up control).
      3. Transplant Aquatic Plants: Submerged plants stabilize sediment to prevent nutrient re-suspension and compete with phytoplankton for light and nutrients.
• Lake Success Considerations:
      * Oligotrophic Lakes (Nutrient-poor): Primary production is already low; grazing might not significantly change abundance.
      * Eutrophic Lakes: May have large Cyanobacteria (blue-green algae) populations that are toxic or too large for zooplankton to eat, rendering top-down biomanipulation less effective. Bottom-up approaches are preferred here.

Questions & Discussion

• Question: What merits do food chains have despite being an oversimplification?
      * Response: They provide a general idea of trophic level categories and help predict the impact of removing a specific species from the system.
• Question: Why do most food webs only have 3 to 5 levels?
      * Response: Energy transfer is the limiting factor. Only about $10\%$ of energy is transferred between levels, so by the time you reach the 4th or 5th level, there is insufficient energy to support a higher level.
• Question: Do stream and lake food webs look different?
      * Response: Yes. Streams involve more allochthonous inputs (nutrients from outside the system/land), while lakes rely more on internal (autochthonous) production.
• Question: Can we identify keystone species via food webs?
      * Response: Yes. For example, large piscivores qualify as keystone species because their presence or absence fundamentally reshapes the entire food web structure and the abundance of primary producers.