When a cold Arctic stream turns orange and acidic with dissolved metals after permafrost thaw, which fish hazard does that chemistry point to first?

Answer: Metals hit gill tissue

Metals hit gill tissue ✓ — Correct. Fish gills are thin exchange surfaces, so metal-rich acidic water can injure them before temperature or eyesight becomes the issue. Studies of rusting Arctic streams note that iron and aluminum particles can interfere with gill respiration, while broader fish-toxicology work shows waterborne metals bind to gills. The threat is chemical stress in cold water.

Algae drain oxygen — Algae can drain oxygen in warm, nutrient-rich blooms, so it is a real fish-kill pattern elsewhere. But orange rusting streams are not mainly an algae-bloom story. The clue is dissolved metals plus acidity, which points more directly to contact with gill tissue.

Acid removes oxygen — Low oxygen is a real fish killer in warm or stagnant waters, so this is a plausible water-quality guess. But the rusting-stream mechanism is not acid removing oxygen from the water. The more direct threat here is metal-rich acidic water interacting with gill tissue.