# Module D Summary: Nearshore Ecosystems — Stress, Disturbance, and Who Controls Whom ## Key takeaway The key dynamic of this module is the tension between **abiotic stress** and **biotic pressure**, and how **disturbance** resets the competition. Nearshore ecosystems sit where light and nutrients are abundant, so life is dense — but space, oxygen, or substrate stability are always limited. Each ecosystem has a **foundation species** that builds the habitat (mussels/seaweeds, kelp, seagrass, corals, mangrove trees) and usually a **keystone species** whose small biomass controls the whole system (seastars, sea otters, polychaete worms, herbivorous crabs). Remove the keystone and the ecosystem flips to a different state: seastar loss → mussel monoculture; otter loss → urchin barrens; urchin loss on reefs → algal overgrowth. Disturbance is not just destructive: the **Intermediate Disturbance Hypothesis** says diversity peaks at moderate disturbance, because no single competitor can take over. Finally, these ecosystems are connected — kelp detritus feeds mudflats, and mangroves, seagrasses, and coral reefs filter, shelter, and protect each other. ## Key concepts ### General concepts - **Intertidal vs. subtidal.** The **intertidal (littoral)** lies between the highest high tide and the lowest low tide; organisms there alternate between immersion and air exposure. The **subtidal** is always underwater and much more stable. Immersion time changes fast over short vertical distances, so neighbors a few meters apart face very different conditions. **Spring tides** (new and full moons) give the largest ranges; in BC, the lowest summer low tides occur at hot midday and the lowest winter ones at freezing night. - **Substrate controls the community.** **Rocky (hard) substrate** allows attachment. **Soft bottoms** (sand, mud) shift constantly, so sessile organisms are absent and most life is **infaunal** (lives in the sediment). Terms: **epiflora** (plants/algae on the surface), **epifauna** (animals on the surface), **infauna** (animals in the sediment). - **Diversity vs. biomass.** High biomass is not high diversity: a rock covered in one barnacle species has high biomass and low diversity. - **Succession and disturbance.** After a disturbance clears space, organisms replace each other in a regular sequence: bare rock → bacterial film → algal film → seaweeds and barnacles → mussels/seastars (the **climax community**). True climax states are rare because communities keep changing. - **Intermediate Disturbance Hypothesis (IDH).** Low disturbance → one competitive dominant wins → low diversity. High disturbance → only a few stress-tolerant species survive → low diversity. **Moderate disturbance → maximum diversity.** - **Foundation vs. keystone species.** A **foundation species** is the dominant primary producer that defines the ecosystem (kelp, seagrass, mangroves). A **keystone species** (never a primary producer; usually a predator) controls the ecosystem far beyond its biomass. Removing it causes radical change. ### Rocky intertidal - **Vertical zonation.** Distinct bands form because each organism has upper and lower limits. General rule: the **upper limit is set by abiotic factors** (desiccation, temperature, UV), and the **lower limit is set by biotic factors** (competition, predation). Zones: spray zone (lichens, cyanobacterial mats) → high tide zone (periwinkles, limpets) → middle tide zone (acorn barnacles above, then mussels, seaweeds, gooseneck barnacles) → low tide zone (high diversity: urchins, anemones, macroalgae, small fish) → subtidal. - **Abiotic defenses.** Against **desiccation**: "run and hide" (tide pools, crevices, under seaweed) or "clam up" (seal the shell; limpets scrape a **home scar**). Some tolerate huge water loss (chitons ~70%, seaweeds ~90%). Against **waves**: glue, threads, holdfasts, suction, thicker shells, low profiles, flexible bodies, and safety in numbers (mussel/barnacle clumps). Against **salinity swings** (hypersaline hot tide pools, hyposaline rain): intertidal species are **euryhaline** (tolerate wide salinity range); **stenohaline** species cannot live there. - **Space competition strategies.** Get there first (planktonic larvae), out-grow, undercut/kick out, or grow over competitors. Most intertidal organisms are **meroplankton** — the planktonic larval stage gives dispersal and avoids competition with the parents. - **The classic predator-prey control.** The blue mussel (*Mytilus*) is a **competitive dominant** — it would monopolize the middle intertidal. The purple seastar (*Pisaster ochraceus*) is the **keystone predator**: it eats mussels and keeps diversity high. Remove seastars and species richness drops by ~50%. The seastar's own upper limit is abiotic (desiccation), which creates the mussel band above it. - **Post-disturbance outcomes.** A cleared patch can reach three stable states: (A) limpet/grazer-dominated, (B) mixed community maintained by seastar predation (highest diversity), or (C) mussel monoculture if seastars are absent. ### Kelp forests - **Setting.** Temperate, cold, nutrient-rich water, often with upwelling. Kelp are the **foundation species** and among the fastest-growing organisms on Earth. Kelp forests have some of the highest primary production rates of any ecosystem. - **Three vertical sections.** **Canopy** (surface fronds; most light, most wave action, most production) → **understory** (shaded; highest diversity; fish and filter feeders on stipes) → **algal turf** (the bottom: holdfasts, red algae, invertebrates). - **The trophic cascade.** Sea urchins graze kelp (from the holdfast up, dislodging it). **Sea otters are the keystone species** — they eat urchins and keep them in check. Remove otters (fur trade) → urchin population explodes → **urchin barrens** (no kelp). Modern wrinkle: overfishing reduced the prey of orcas, so some orcas switched to eating otters, releasing the urchins again (Estes et al. 1998). Remove urchins instead and kelp overgrows, reducing diversity. El Niño warmth cuts nutrients and increases kelp mortality. ### Sandy shores and mudflats (soft bottoms) - **Sediment size is set by energy.** High wave/current energy → coarse sediment (gravel, sand). Low energy → fine sediment (silt, clay). Grain sizes: gravel >2 mm, sand 2–0.062 mm, silt 0.062–0.004 mm, clay <0.004 mm. - **The oxygen-food trade-off.** Coarse sand drains and flushes at each tide: oxygen is replenished, but little food (POM) stays. Fine mud retains water, so flushing is slow: **POM/detritus accumulates (lots of food) but oxygen runs out**. Most mudflat food arrives as detritus from other ecosystems (kelp, rivers), so these are **detritivore communities** that recycle nutrients back to the water column. - **Adaptations to low oxygen.** Stay small (high SA/V); use anaerobic metabolism (sulfate-reducing bacteria make the rotten-egg H₂S smell); pump oxygen-rich water through siphons (clams) or U-shaped burrows (lugworms); evolve more efficient hemoglobin; or simply be sluggish. - **Polychaete worms as keystone species.** Their burrows pump oxygen deep into sediment, letting other aerobic organisms use the rich organic carbon there. Lugworms (*Arenicola*) are common BC deposit feeders. - **Feeding types.** Filter feeders (clam siphons, sand crab antennae), **deposit feeders** (eat sediment and digest the POM: sea cucumbers, lugworms), surface deposit feeders, scavengers (crabs), and predators (moon snails drill bivalves with a **radula**). - **Seagrasses.** The **foundation species** of many soft bottoms. Roots stabilize sediment, beds slow currents and trap particles (clearer water, more light), and they store large amounts of **blue carbon** because the anoxic sediment below them slows decomposition. ### Coral reefs - **The paradox.** Reefs are the most productive marine ecosystem and hold ~25% of marine species, yet they grow in warm, clear, **oligotrophic** (nutrient-poor) tropical water (30°N–30°S, 18–30°C, 32–40‰ salinity) with almost no phytoplankton. - **The solution: mutualism with zooxanthellae.** **Zooxanthellae** are photosynthetic dinoflagellates (*Symbiodinium*) living inside coral tissue (up to 1 million cells per cm²). The coral supplies CO₂, waste nutrients, shelter, and a sunny position. The zooxanthellae supply up to ~90% of the coral's energy and raise local pH, which speeds CaCO₃ skeleton deposition. This tight nutrient recycling is why reefs thrive in a "marine desert." Corals are also **mixotrophs**: they catch zooplankton with **nematocysts** (single-use stinging harpoons inside **cnidocytes**) and absorb DOM directly. - **Coral biology.** Corals are cnidarians with radial symmetry and no medusa stage — only **polyps**. Each polyp secretes a CaCO₃ cup (**theca**); polyps connect by living tissue (**coenosarc**). **Hermatypic** (stony) corals build reefs, are colonial, and host zooxanthellae. **Ahermatypic** corals (soft/deep-sea) lack symbionts and feed only heterotrophically. Know the four symbiosis types: **mutualism, commensalism, parasitism, amensalism**. - **Reproduction.** **Budding** (asexual) grows the colony — fast but genetically identical. **Broadcast spawning** (sexual) gives diversity and dispersal: whole reefs release gametes on one night, cued by temperature, day length, and the lunar cycle. The **planula** larva drifts, then settles where it senses **coralline algae** by **chemotaxis** — coralline algae also cement the reef together. - **Threats.** **Bleaching**: stress (usually warming) expels the zooxanthellae; the coral turns white and can starve (the **adaptive bleaching hypothesis** proposes corals can take up better-adapted strains). **Ocean acidification**: slower, weaker CaCO₃ growth — and coralline algae are even more pH-sensitive, so larval settlement fails too. **Sedimentation** blocks light and smothers polyps. **Sea level change**: too fast a rise outpaces coral growth toward the light. **Predation**: crown-of-thorns seastars eat living coral and regenerate if cut up. **Disease** (black band, white pox, aspergillosis) increases with temperature. **Herbivore loss**: the 1983 *Diadema* urchin die-off let algae overgrow Caribbean reefs — grazers are essential to keep algae from beating corals for space and light. ### Mangroves - **What they are.** Tropical flowering trees (the **foundation species**) that live in the salty intertidal of protected, muddy coasts; the forest is called a **mangal**. They cannot tolerate frost. - **Abiotic problems and adaptations.** Salty, waterlogged, low-oxygen mud. Adaptations: **salt glands** excrete excess salt; waxy leaves limit water loss; **prop roots** anchor the tree in shifting sediment; **pneumatophores** are aerial roots that grow up out of the anoxic mud to absorb oxygen; seeds germinate while still attached to the parent, so the seedling drops ready-made into the mud. - **Food web.** Few animals eat live mangrove. Instead, leaf **detritus** feeds the system: herbivorous crabs eat the litter and their burrows aerate the sediment — crabs are the **keystone species**. Top predators are large fish (seabass, gar) plus terrestrial hunters (birds, crocodiles). Mangroves store up to 5× more carbon than an equal area of tropical forest, protect coasts from storms and erosion, filter runoff, and serve as fish nurseries. - **Ecosystem connectivity.** Mangroves, seagrass beds, and coral reefs form a chain: mangroves filter land runoff, seagrasses dissipate currents and raise juvenile fish, reefs block wave energy and house the adults. Damage to one threatens all three. ### Production ranking (know the order) - Primary production (g C/m²/yr): **coral reefs (1500–3700) > kelp (640–1800) > seagrass/sandflats (550–1100) > estuaries (250–2000) > tropical rainforest (460–1600) > mangroves (370–450) > coastal upwelling (110–370) > coastal temperate seas (110–220) > Southern Ocean (40–260) > temperate pelagic (70–180) > Arctic (<1–100) > central ocean gyres (4–40)**.