18.1 Environmental Toxicology: Fate, Transport, Bioaccumulation (log Kow, BCF) & Ecotoxicological Testing

Applied toxicology on the Diplomate of the American Board of Toxicology (DABT) examination is not a second copy of mammalian study design. Domain IV of the American Board of Toxicology (ABT) 2026 Candidate Handbook asks you to characterize ecotoxicological effects at several levels of biological organization, and to connect ecological disruption to human public health through ecosystem services and animal-human disease contact. This independent OpenExamPrep chapter covers those IV.1 and IV.3 tasks together with the environmental-fate tools every candidate is expected to use when interpreting residues, transport, and food-web transfer. It is OpenExamPrep teaching material for DABT candidates; it is not an ABT publication and does not speak for the Board.

On exam items, the trap is almost always a scale error: treating a 96-hour fish LC50 as if it were a population forecast, treating log Kow as if it were a measured bioconcentration factor (BCF), or treating a single-species laboratory result as if it already described a wetland, a fishery, or a drinking-water supply.

Why ecological scale changes the answer

A chemical can kill, impair, or spare organisms, and those organism-level facts still leave the public-health and ecological question open. The same water concentration can produce different stories at four nested scales.

Individual responses are what standard laboratory tests actually measure: immobilization, death, reduced growth, or reduced brood size in a defined species under controlled water or soil conditions. They answer "is this substance acutely toxic to this organism?" They do not, by themselves, answer "will the fishery collapse?"

Population responses add demography. A modest reduction in juvenile survival can shrink a population if recruitment was already limiting; the same mortality can be invisible if density dependence was strong and survivors have more food. Conversely, a chemical that does not kill adults can still drive a decline if it cuts reproduction (for example, eggshell thinning in raptors, discussed with DDT in the next section of this chapter).

Community responses include indirect effects: toxicity that hits one group and then harms another group that was not the most sensitive species in a single-bottle test. Classic patterns include:

• An insecticide that is only moderately toxic to fish but eliminates aquatic insects, so insectivorous fish starve or emigrate even though the fish 96-hour LC50 looked reassuring.
• A herbicide that removes submerged plants, collapsing habitat and dissolved-oxygen buffering for invertebrates and larval fish.
• Predator loss that releases a grazer population, which then strips algae or plants.
• Secondary poisoning, such as anticoagulant rodenticide residues moving from target rodents into raptors and scavengers.

ABT IV.1 specifically flags indirect effects from exposure in an ecological community on susceptible populations. When a stem asks why a fish kill occurred despite a fish LC50 well above the measured water concentration, look for a food-web or habitat mechanism, not only a mis-measured LC50.

Ecosystem responses are changes in processes and services: primary production, decomposition, nutrient retention, and the production of clean water and breathable air. Those services are the bridge from ecology into IV.3 (human health), covered later in this section.

Fate, transport, persistence, and bioavailability

Before a residue can poison a daphnid, a fish, an earthworm, or a person who eats fish, the chemical has to get there in a form that can be taken up. Environmental toxicology therefore starts with fate and transport, not with the LC50 table.

Chemicals move among air, water, soil, sediment, and biota. The properties that govern that movement are the same ones you use in screening-level risk work:

Persistence is resistance to breakdown. Persistent organochlorines (DDT and its DDE metabolite, many polychlorinated biphenyl (PCB) congeners, polychlorinated dibenzo-p-dioxins) occupy environmental reservoirs for years. That reservoir is why a banned insecticide can still appear in sediment cores and in the lipids of long-lived predators.

Biotransformation is the organism's (or the microbial community's) chemical alteration of the parent. In mammals you already think of Phase I and Phase II metabolism as a detoxification or bioactivation story. In the environment, the same idea has two exam-relevant twists:

1. Microbial transformation can create a more toxic or more bioaccumulative product (elemental or inorganic mercury → methylmercury; some parent pesticides → persistent metabolites such as DDE).
2. Rapid biotransformation in fish lowers BCF relative to what log Kow would predict. A high log Kow chemical that is readily metabolized may never reach the tissue residue the partition model expects.

Bioavailability is the fraction of the environmental load that can actually cross a biological membrane. Total sediment concentration is not dose. Freely dissolved water concentration, pore-water concentration, and (for metals) the free ion or a well-chosen speciation metric are closer to the effective exposure. Organic carbon, black carbon, sulfides (for metals), pH, and dissolved organic matter all change bioavailability without changing a bulk chemistry result. Exam items that give a high total soil concentration and a quiet earthworm study are often asking you to notice sequestration, not to declare the chemical non-toxic in every matrix.

Bioaccumulation, BCF, BAF, and biomagnification

These four terms are not synonyms. Mixing them is a high-yield DABT error.

log Kow as a screening predictor of BCF. For many neutral, poorly metabolized organic chemicals, partitioning into octanol is a usable analog of partitioning into organism lipid. That is why log Kow is used as a screen for bioconcentration potential, not as a substitute for a measured BCF. Several chemical-management schemes treat a log Kow of about 4.5 as a flag that a laboratory BCF study may be needed. Independent OpenExamPrep teaching for this exam is: quote log Kow as a predictor and a screen, then say what would falsify it.

A historical fish QSAR of the Veith type (approximately $\log \mathrm{BCF} \approx 0.85,\log K_{ow} - 0.70$ for nonpolar organics) is the conceptual relationship, not a number you should treat as a regulatory criterion. Two physical limits matter more than the slope:

• In the mid-hydrophobic range (roughly log Kow 3 to 6), BCF often rises with log Kow.
• At very high log Kow (often >7–8), measured BCF can flatten or fall because the chemical is so insoluble and so particle-bound that the freely dissolved concentration—and therefore uptake—drops. Large molecular size and steric hindrance add to that ceiling. This bilinear behavior is why "higher log Kow always means higher BCF" is false.

log Kow is the wrong screen for metals and metalloids (speciation and specific transporters dominate), for many ionizable acids and bases (the neutral fraction, not the total, partitions), for surfactants, and for chemicals that are rapidly biotransformed. For those classes, a measured BCF, a dietary bioaccumulation study, or a metal-specific bioavailability model is the honest next step. This chapter does not walk the full REACH PBT/vPvB machinery; the DABT-relevant point is the logic of the screen, not a checklist of every European information requirement.

Food-web transfer of persistent organochlorines and methylmercury. Biomagnification is the ecological process that turns a water concentration too low to kill a daphnid into a residue that poisons a piscivorous bird or a person who eats fish.

• Organochlorines (DDT/DDE, many PCBs, dioxin-like compounds) are persistent, lipid-soluble, and slowly metabolized. They partition into plankton lipid, concentrate in forage fish, and reach high lipid-normalized concentrations in raptors, marine mammals, and human adipose tissue. DDE's later role in avian eggshell thinning is a community- and population-level story built on this transfer.
• Methylmercury (MeHg) is the exam's important exception to "biomagnification = high log Kow." Inorganic mercury is methylated by anaerobic microbes (often in sediments). MeHg binds thiol groups on proteins, not only storage lipid, and is efficiently assimilated from diet. It therefore biomagnifies through aquatic food webs even though a Kow-only organic screen is the wrong mental model. Human Minamata disease, discussed in the next section, is the public-health endpoint of that food web.

When a question gives a low water concentration, a high fish residue, and a neurologic syndrome in people who eat those fish, the mechanism is dietary biomagnification of MeHg, not an acute water LC50 exceedance.

Ecotoxicological testing: OECD 202, 203, and 207

ABT IV.1 expects you to interpret effects of ecotoxicological concern. The acute tests that still appear as the handbook-level reference list for aquatic and soil invertebrates/fish are OECD Test Guidelines 202, 203, and 207. They are screening and classification tools. They are not a full aquatic-terrestrial dossier (this independent OpenExamPrep section does not inventory algal growth, fish early-life, sediment chironomid, or the rest of a complete chemical-registration eco package).

Read the endpoint names. OECD 202 is immobilization, not 96-hour fish death. OECD 203 is fish lethality, not a Daphnia test. OECD 207 is earthworm acute lethality, not a reproduction study (chronic earthworm reproduction is a different guideline). Mixing those three is a common multiple-choice construction.

Single-species tests also miss the indirect community effects already discussed. That is why mesocosms exist.

Mesocosms: between the beaker and the lake

A mesocosm is an experimental ecosystem—tanks, ponds, or enclosed water columns—that holds multiple trophic levels under semi-controlled outdoor or greenhouse conditions. Compared with OECD 202/203:

• You can measure community composition, primary production, nutrient cycling, and recovery after a pulse, not only a 48- or 96-hour EC50/LC50.
• Exposure can include sorption, photolysis, and volunteer species that a laboratory never stocked.
• Control is weaker than a laboratory beaker: weather, volunteer colonization, and tank-to-tank variability complicate statistics.
• Cost and time are higher, so mesocosms are a higher-tier tool when laboratory tests and simple food-web reasoning are not enough.

A DABT stem that asks which study best captures an insecticide's effect on a pond community is pointing at a mesocosm or a well-designed field study, not at repeating OECD 202 at one more concentration.

Ecosystem services and zoonoses (IV.3)

ABT IV.3 asks you to assess the public-health impact of environmental toxicants that follow ecological disruption. Two conceptual channels matter.

Ecosystem services. Wetlands, forests, soils, and intact aquatic food webs supply clean and safe water, clean air, flood buffering, pollination, and food. A toxicant that kills filter-feeding bivalves, strips riparian vegetation, or collapses decomposer communities is not only an ecological injury. It can raise human exposure to pathogens, particulates, and chemical residues by removing the biological machinery that used to intercept them. Harmful algal blooms after nutrient-plus-pesticide disturbance are a compact example: the ecological shift produces toxins (microcystins, saxitoxin in some systems) that close drinking-water intakes and fisheries.

Zoonoses and animal-human interaction. Toxicants can change disease prevalence at the animal-human interface without the chemical itself being an infectious agent. Habitat damage and wildlife die-offs can concentrate reservoir hosts (rodents, some bats and birds) around remaining food and water, increasing spillover contact. Immunotoxic chemicals can raise pathogen loads in wildlife. Pesticide impacts on scavengers can leave carcasses in the landscape longer. The exam-level claim is conceptual: ecological disruption is a public-health exposure pathway, not a separate "wildlife-only" topic.

Independent OpenExamPrep practice for this material lives with the rest of the DABT bank at /practice/dabt. Use those items to test whether you can move a fact from an LC50 table to a food web without inventing a REACH-sized testing list.