Coral reef Study Guide

📖 Core Concepts Coral reef – underwater ecosystem built by reef‑building corals (class Anthozoa, phylum Cnidaria) that secrete a calcium‑carbonate (aragonite) skeleton. Zooxanthellae (Symbiodinium) – photosynthetic dinoflagellates living inside coral tissues; supply 90 % of coral nutrition (glucose, glycerol, amino acids) in exchange for CO₂ and shelter. Reef zones – Fore reef (seaward slope, high wave energy). Reef crest / flat (shallow platform that dissipates waves). Lagoon (back reef) (calm water, may be deep). Off‑reef floor & drop‑off (surrounding seabed, steep slope). Reef types – Fringing, Barrier, Atoll, Platform (patch); distinguished by relation to land/lagoon and geometry. Darwin’s atoll formation – three‑stage subsidence model: fringing → barrier → atoll. Habitat requirements – warm (26–27 °C), shallow (< 150 m), clear, low‑nutrient (oligotrophic), agitated water. Reef growth – horizontal $1$–$3\ \text{cm yr}^{-1}$, vertical $1$–$25\ \text{cm yr}^{-1}$ (only while within light zone). Reef biodiversity – houses ≥ 25 % of marine species; > 4 000 fish species, myriad invertebrates, microbes. Key stressors – thermal bleaching, ocean acidification, excess nutrients, overfishing, physical damage, harmful chemicals (e.g., oxybenzone). --- 📌 Must Remember Optimal temperature: 26–27 °C; reefs rarely exist below 18 °C. Light limit: ≈ 150 m depth (photic zone for zooxanthellae). Reef‑building material = aragonite (CaCO₃). Growth rates – horizontal $1$–$3\ \text{cm yr}^{-1}$; vertical $1$–$25\ \text{cm yr}^{-1}$. Zooxanthellae clades – A (shallow, UV‑protective), B/C (deeper, heat‑sensitive), D (high heat tolerance). Bleaching trigger – sustained water‑temperature rise of ≈ 1–2 °C above summer maximum for weeks. Economic value – reefs provide $29.8$–$375 billion yr⁻¹ globally; protect shorelines by up to 97 % wave energy. Global reef distribution – ≈ 92 % in Indo‑Pacific; latitudinal range ≈ 30° N–30° S. --- 🔄 Key Processes | Process | Step‑by‑step sequence | |---------|----------------------| | Darwin’s atoll formation | 1. Volcanic island with fringing reef forms.<br>2. Island/substrate subsides → reef becomes barrier with lagoon.<br>3. Island sinks completely → atoll (ring reef surrounding lagoon). | | Coral spawning (external fertilization) | 1. Seasonal cues: temperature rise + lunar phase (few days after full moon).<br>2. Synchronous release of eggs & sperm into water column.<br>3. Fertilization → planula larvae. | | Planula settlement | 1. Larva swims (days‑weeks) using acoustic (long‑range) & chemical (short‑range) cues.<br>2. Finds suitable hard substrate.<br>3. Metamorphoses into a polyp → starts skeleton secretion. | | Bleaching | 1. Thermal stress → breakdown of coral‑zooxanthellae symbiosis.<br>2. Expulsion of zooxanthellae → coral appears white.<br>3. If stress persists → mortality; if stress lifts → possible re‑symbiosis. | | Microfragmentation growth | 1. Cut coral into tiny fragments (increasing surface‑area‑to‑volume).<br>2. Place on stable substrate (often rough).<br>3. Fragments grow rapidly and fuse if from same parent, forming a larger colony within 2 years. | | Nutrient recycling on reefs | 1. Phytoplankton/seaweed fix carbon at surface.<br>2. Sponges filter phytoplankton → excrete usable nutrients.<br>3. Coral tentacles capture zooplankton at night → supply N & P.<br>4. Rough coral surfaces create turbulence, enhancing nutrient flux. | --- 🔍 Key Comparisons Fringing vs Barrier vs Atoll Fringing: directly attached to shore, narrow lagoon (< 100 m). Barrier: separated from shore by deep, wide lagoon (km‑wide, 30–70 m deep). Atoll: circular reef with lagoon, no central island. Zooxanthellae Clade A vs D Clade A: shallow, UV‑protective compounds, moderate heat tolerance. Clade D: high temperature tolerance, better survival during bleaching. Sexual (spawning) vs Asexual (budding/fragmentation) Sexual: creates genetic diversity, requires synchronized cues. Asexual: rapid colony expansion, clones parent genotype, useful for restoration. Nutrient‑poor vs Nutrient‑rich water Oligotrophic: promotes coral dominance, high light penetration. Eutrophic: fuels macroalgal overgrowth, smothers corals. Marine Protected Area (MPA) vs Active Restoration MPA: protects existing ecosystems, reduces fishing/impact. Restoration: adds new coral structures (gardening, substrate) where reefs are degraded. --- ⚠️ Common Misunderstandings “Reefs need lots of nutrients.” – True productivity occurs despite low nutrients; excess nutrients cause algal overgrowth. All corals bleach → die. – Some corals can recover if stress is brief; others develop heat‑tolerant symbionts. Barrier reefs are always larger than fringing reefs. – Size varies; classification is based on lagoon presence, not absolute area. Only tropical reefs exist. – Deep‑water corals occur at greater depths and higher latitudes (outside the 30° N–S range). All reef‑building organisms are scleractinian corals. – Coralline algae, bivalve reefs, and even cyanobacteria contribute to carbonate frameworks. --- 🧠 Mental Models / Intuition Reef as a skyscraper: Coral polyps lay down “floors” (aragonite) upward while the sea‑level rise is the “elevator” – if the elevator moves faster than the building can grow, the “top floor” drowns. Spawning calendar: Think of a full‑moon alarm clock; the coral community “wakes up” a few days later to release gametes. Nutrient “pump”: Internal waves act like a submarine pump, pushing cool, nutrient‑rich water up the reef slope, similar to a surfboard creating lift. --- 🚩 Exceptions & Edge Cases Deep‑water coral reefs can exist > 150 m (e.g., Blake Plateau) – they lack zooxanthellae photosynthesis. Clade D zooxanthellae are rare but dominant in heat‑stressed regions. Reef drowning may occur even with moderate sea‑level rise if vertical growth is limited by light or substrate. Microfragment fusion only works when fragments share the same parent genotype; otherwise aggressive competition occurs. --- 📍 When to Use Which | Situation | Recommended Approach | |-----------|----------------------| | Identifying reef type in the field | Look at proximity to land and lagoon characteristics → fringing (shore‑adjacent, narrow), barrier (deep wide lagoon), atoll (circular, no island), platform/patch (isolated, oval). | | Mitigating algal overgrowth | First increase herbivore density (parrotfish, sea urchins). If insufficient, apply manual removal + urchin deployment. | | Restoring a heavily damaged reef | Combine coral gardening (microfragmentation) with artificial substrate (rough concrete/Biorock) and heat‑tolerant symbiont inoculation for climate resilience. | | Choosing monitoring method | Use satellite imagery for large‑scale health trends; deploy in‑situ water‑quality sensors for nutrient/temperature spikes. | | Deciding between MPA vs active restoration | If reef still has living coral cover but faces fishing pressure → MPA. If substrate is bare or coral cover < 10 % → active restoration. | --- 👀 Patterns to Recognize Spawning cue pattern: temperature rise + full moon → mass gamete release within 2–5 days. Spur‑and‑groove on fore reef → indicates strong wave‑driven sediment transport. Macroalgal bloom following rainy season → likely nutrient runoff event. Internal wave signatures (periodic temperature dips) → potential nutrient pulses to reef surface. Bleaching events clustered during El Niño years (4–7 yr cycle). --- 🗂️ Exam Traps Distractor: “Coral reefs thrive in nutrient‑rich waters.” – Wrong: they prefer oligotrophic conditions; excess nutrients cause algal overgrowth. Distractor: “All atolls form from sinking volcanic islands.” – Partial: atolls can also arise from sea‑level rise without subsidence. Distractor: “Bleaching always kills the coral.” – Wrong: corals can recover if stress is short and symbionts are re‑acquired. Distractor: “All reef‑building organisms are animals.” – Wrong: coralline algae and bivalve reefs also contribute carbonate structures. Distractor: “A single reef type corresponds to a single depth range.” – Wrong: platform/patch reefs can occur at various depths, including deep‑water sites. ---