Biomagnification and Trophic Transfer Comprehensive Study Guide

Fundamentals of Biomagnification and Trophic Transfer

Definition and Progression of Biomagnification

Biomagnification is defined as the process by which a contaminant increases in concentration within the tissues of organisms as it travels up the food chain. This process results in an order of magnitude increase in toxin levels at higher trophic levels.

The DDT Case Study (Concentration Progression)

Using DDT as a primary example, the concentration of the contaminant increases significantly at each step of the aquatic food web:

• Water: $0.000003\,ppm$

• Zooplankton: $0.04\,ppm$

• Small Fish: $0.5\,ppm$

• Large Fish: $2\,ppm$

• Fish-eating Birds of Prey: $25\,ppm$

Quantifying Trophic Status and Transfer

Understanding how contaminants move requires defining the trophic status of organisms. Traditionally, this was achieved through:

• Field Surveys: Defining status using information found in natural history literature.

• Visual Observations: Observing species interactions directly.

• Gut Content Analysis: Analyzing what an organism has eaten. However, this method often renders complex interactions into highly simplified representations of the real world.

During the last 25 years, a more accurate method of quantifying trophic status has become available through the study of stable isotopes.

Isotopic Discrimination and Trophic Webs

Principles of Isotopic Analysis

Trophic transfer is often studied through the isotopic discrimination of light isotopes of Nitrogen ($N$) or Carbon ($C$). The rate or extent to which an isotope participates in a biological or chemical process depends on the isotope's mass.

Average Soft Tissue Composition

• Carbon ($C$): Approximately $40\%$

• Nitrogen ($N$): Approximately $10\%$

Trophic Discrimination in Wildlife: Case Studies

Khutzeymateen Bear Tissue Analysis

Data regarding Western Canadian bears demonstrates how diet influences isotopic signatures (Data source: G. Cabana):

• Grizzly Bears: Exhibit higher $\delta^{15}N$ (ranging approximately from $10\%$ to $14\%$). Their diet includes marine salmon, which occupies a higher trophic level.

• Black Bears: Exhibit lower $\delta^{15}N$ (ranging approximately from $2\%$ to $6\%$). Their diet typically excludes salmon, consisting primarily of nuts and berries.

• Carbon Signatures ($\delta^{13}C$): Grizzly bears show a wider range of $\delta^{13}C$ (approaching $-18\%$ to $-22\%$) compared to Black Bears (clumped around $-24\%$ to $-26\%$).

Bone Collagen in Modern and Prehistoric Organisms

Isotopic signatures allow for comparisons between modern animals and prehistoric remains (400 yr BP and 850-5500 yr BP). Observed organisms include:

• Lower Trophic Levels: Rabbit, woodchuck, squirrel, deer, and lamb.

• Mid-to-High Trophic Levels: Dog, bobcat, human, and mountain lion (which shows high $\delta^{15}N$ indicating a predator status).

Estimating Human Paleodiet

The percentage of $C_4$ plants in a diet can be calculated using the $\delta^{13}C$ values found in bone collagen (Data source: Schwarz et al., 1985).

Paleodiet Formula: $\%\text{ C. Plants in Diet} = \frac{\delta^{13}C_{\text{sample}} - \delta^{13}C_{\text{C3-org}}}{\delta^{13}C_{\text{C4-org}} - \delta^{13}C_{\text{C3-org}}} \times 100$

Constants for calculation:

• $\delta^{13}C_{\text{C3-org}} = -26\%$

• $\delta^{13}C_{\text{C4-org}} = -9.7\%$

Historical data shows a dramatic shift in human diets (increase in $C_4$ plants like maize) occurring between 1000 BC and 1000 AD.

Contaminants in Aquatic Ecosystems

Mercury in the Great Lakes Food Web

Studies of Lake Trout trophic structures show a direct correlation between trophic level (represented by $\delta^{15}N$) and total mercury/methyl mercury concentrations (Data source: Lepak et al., 2015).

Food Web Hierarchy (Increasing Mercury):

1. Zooplankton / Mysis

2. Prey fish: Sculpin, Alewife, Smelt, Cisco

3. Apex Predator: Lake Trout

Sources of Mercury

Stable isotope signatures are used to determine mercury sources in the Great Lakes Watershed, distinguishing between industrial sources and atmospheric precipitation across Lakes Superior, Huron, Michigan, Erie, and Ontario.

Principles of Bioaccumulation and Bioconcentration

Key Distinctions

• Bioaccumulation: Affects a single organism throughout its lifetime. It is the accumulation of toxins inside living organisms faster than they can be excreted.

• Biomagnification: Affects many organisms across multiple levels and depends on the dynamics of the hierarchical food chain.

Criteria for Trophic Accumulation

For a compound to bioaccumulate or biomagnify, it must be:

1. Fat-soluble (Lipophilic): It must be able to build up in the fatty tissues of an organism.

2. Stable: The toxin does not readily decompose or degrade through metabolic processes.

3. Mobile: The toxin is easily found and transported throughout the environment.

Measuring Contaminant Levels

Biomagnification Factor (BMF)

$BMF$ is calculated by comparing lipid-normalized tissue concentrations in an organism to those in its prey. $BMF = \frac{\text{Compound in organism}}{\text{Compound in prey}}$

• BMF > 1: Indicates biomagnification has occurred (higher numbers indicate more contaminant).

Bioconcentration Factor (BCF)

Bioconcentration refers to a chemical being absorbed by an aquatic species such that its internal concentration is higher than the surrounding water. Note that concentration can decrease through "Growth Dilution." $BCF = \frac{\text{Compound in organism}}{\text{Compound in water}}$

• BCF < 1: Compound favors the water phase.

• BCF > 1: Compound favors the organism.

Octanol Partitioning Coefficient ($K_{ow}$)

$K_{ow}$ measures hydrophilicity (water-loving) versus lipophilicity (fat-loving).

• High $K_{ow}$: Chemical tends to accumulate in fatty tissue or sediment.

• Low $K_{ow}$: Chemical tends to accumulate in the water column.

Environmental Fate and Chemical Degradation

Persistent Organic Pollutant (POP) Movement

POPs move between environmental compartments (soil, water, air, biota) via:

• Soil Sorption: Binding to soil particles.

• Bioaccumulation/Bioconcentration: Entering the food web.

• Degradation mechanisms:     * Photolysis: Breakdown by light.     * Hydrolysis: Breakdown by water.     * Biotransformation: Breakdown by biological organisms.

Chemical Half-life

The half-life is the time required to reduce a contaminant's concentration by $50\%$. The rate is summarized by the rate constant ($k$), where the transformation rate is independent of initial concentration.

Half-life Equation: $t_{1/2} = \frac{\ln(2)}{k}$

Health and Environmental Implications

Polychlorinated Biphenyls (PCBs)

Research from "Our Stolen Future" and various health risk assessments (Carpenter 2006; Eghbaljoo et al. 2023) highlights the dangers of PCBs:

• Physiological Effects: Alteration of thyroid and reproductive functions in both males and females.

• Disease Risk: Increased risk of cardiovascular disease, liver disease, and diabetes.

• Maternal Health: High risk of infants born with low birth weight, leading to lifetime disease risks.

• Carcinogenic Risk: High consumption rates and tissue accumulation contribute to significant cancer risks, even if producers do not deliberately cause harm.

Global PCB Usage and Trophic Correlation

Global usage of PCBs reached massive levels until 1993, with cumulative usage exceeding $500\,tons$ in specific high-use geographic regions.

Congener 105 and Trophic Level

In the Great Lakes, lipid-corrected congener 105 (a specific PCB) shows a positive linear correlation with trophic level. Regression Analysis: $y = 0.5458x - 0.6696$

• Organisms sampled: Zooplankton, Mysis, Deepwater Sculpin, Slimy Sculpin, Bloater Chub, Yellow Perch, Alewife, Lake Whitefish, and Lake Trout.