# Module C Summary: Marine Ecology — Light, Nutrients, and Energy Flow ## Key takeaway The key dynamic of this module is the balance between light and nutrients, and how it controls all production in the ocean. Phytoplankton need both. Light exists only near the surface, but nutrients accumulate at depth. Wind mixing and density stratification decide which resource is available at which time. This balance explains the seasonal bloom cycle: winter mixing brings nutrients up but mixes cells too deep for light, spring calms and shallows the mixed layer so blooms start, summer stratification traps nutrients below and growth stops, fall storms restart mixing for a small second bloom. Energy then moves up the food web, but only ~10% survives each step, so food chain length controls how much top-predator biomass an ecosystem can support. Meanwhile, the **microbial loop** recycles dissolved waste back into the food web, so almost nothing is lost. Human CO₂ emissions are now pushing this whole system: warming the surface, raising sea level, and acidifying the water. ## Key concepts - **Realms and zones (review from Module A).** **Benthic** = bottom; **pelagic** = water column. Pelagic splits into **neritic** (over the continental shelf) and **oceanic** (open ocean). By depth: epipelagic (0–200 m), mesopelagic (200–1000 m), bathypelagic (1000–4000 m), abyssalpelagic (4000–6000 m), hadalpelagic (6000–11000 m). About 90% of marine life lives in the epipelagic. Deep benthic communities eat **marine snow** (sinking organic particles). - **Light zones.** **Euphotic (photic) zone**: enough light for photosynthesis; the base is where light is 1% of surface light. **Dysphotic zone** ("twilight zone"): light is present but <1%, too weak for photosynthesis. **Aphotic zone**: permanent darkness. Clear tropical water lets light reach ~200 m; productive coastal water is shallower because plankton and sediment cause **shading**. - **Compensation depth vs. critical depth.** The **compensation depth** is the depth where photosynthesis equals respiration (P = R) for cells held at that depth. The **critical depth** is the depth where total photosynthesis of the whole mixed population equals total respiration, integrated from the surface down. Scientists measure the first and calculate the second. - **The bloom rule.** If the **mixed layer** is deeper than the critical depth, cells spend too much time in darkness: total P < R, and no bloom can occur. If the mixed layer is shallower than or equal to the critical depth, total P > R, and a bloom can occur (if nutrients are available). This is the most important rule of the module. - **Seasonal cycle in temperate regions.** Winter: storms deepen the mixed layer, nutrients are resupplied, but light limits growth. Spring: storms weaken, the mixed layer shallows above the critical depth, days lengthen → **spring bloom** using winter nutrients. Summer: strong **stratification** (thermocline/pycnocline) blocks nutrient resupply → growth becomes nutrient-limited (usually nitrate). Fall: cooling and storms erode the thermocline → small **fall bloom**. Polar regions follow the same pattern but are controlled mostly by light. Tropical regions are stratified all year, so production stays low except in **upwelling** zones. - **Ecological vocabulary.** Know these terms: trophic (feeding), food chain (linear), food web (realistic network), primary producer (autotroph), primary/secondary consumer, biomass (standing stock), grazer, predator, omnivore, planktivore, piscivore, filter feeder. - **Biodiversity.** Three levels: **genetic diversity** (within a species), **species diversity** (between species), **ecosystem diversity** (interactions plus environment). A **species** is a group that can reproduce together and make fertile offspring. ~10 million species may exist; fewer than 2 million are described. Main threats: coastal development, overfishing, pollution and plastics, climate change, nutrient loading (anoxic zones), aquaculture, tourism. - **Redfield ratio.** Phytoplankton take up carbon, nitrogen, and phosphorus in the ratio **106 C : 16 N : 1 P**. The same ratio is found in their cells, in other organisms, and in dissolved seawater nutrients. If one element runs short (usually nitrogen, as nitrate NO₃⁻), it becomes the **limiting nutrient** and growth stops until mixing, upwelling, or runoff resupplies it. Diatoms also need silica (silicic acid) for their frustules. The **nutricline** is the depth below the mixed layer where nutrient concentration rises fast; it often matches the pycnocline. - **Gases in seawater.** Oxygen dissolves poorly in water (the atmosphere holds ~100× more). CO₂ is very soluble (the ocean holds ~60× more than the atmosphere), so marine producers rarely run out of carbon, but oxygen can run low where bacteria decompose a lot of organic matter. - **Trophic pyramids and transfer efficiency.** Each **trophic level** passes only about **10%** of its energy to the next level (**trophic transfer efficiency, TTE**); the rest is lost to respiration, heat, and waste. Carbon transfer follows the same rule. Real TTE varies from ~2.5% (long open-ocean chains) to >20% (short upwelling chains). Short food chains (diatom → krill → whale) support much more top-predator biomass than long chains. Some pristine ecosystems with large generalist predators are **"top-heavy"** because apex predators feed across several trophic levels. - **DOM/DOC and the microbial loop.** Dissolved organic matter (DOM ≈ DOC in this course) comes from **viral lysis**, **sloppy feeding**, cell leakage, and excretion. It is too small to sink and too small for most animals to eat. **Heterotrophic bacteria** absorb DOM and turn it into bacterial biomass (POM). Nanoflagellates and microzooplankton eat the bacteria, and the energy re-enters the classical food web. This recycling pathway is the **microbial loop**. It exists in every ecosystem; without it, waste would accumulate and nutrients would run out. - **DOM reactivity and carbon storage.** **Labile** DOC is eaten in hours–days. **Semi-labile** DOC lasts weeks–months. **Recalcitrant** DOC (RDOC) resists microbes and can persist for years to millennia, acting as a long-term ocean carbon reservoir. DOM that becomes particles can sink and feed the **biological pump**. - **Food webs beat food chains.** Real feeding is not linear: one species eats from several trophic levels, and diet changes with life stage (e.g., salmon eat zooplankton as fry and fish as adults). Food webs show these links and are the better model of ecosystem interactions. - **Climate change and the ocean.** Greenhouse gases trap heat. CO₂ is ~75% of emissions and lasts centuries; methane traps ~85× more heat over 20 years but lasts ~10 years; nitrous oxide is ~300× CO₂; fluorinated gases are the most potent. Atmospheric CO₂ rose from a pre-industrial maximum of ~300 ppm to >420 ppm today. The ocean absorbs ~90% of the excess heat. Effects: sea surface warming, **sea level rise** (glacier melt plus **thermal expansion**), and **ocean acidification** (CO₂ + water releases H⁺ ions, lowering pH, which dissolves CaCO₃ shells of corals, clams, coccolithophores, and others). Indirect effects include poleward plankton shifts, loss of ice albedo, changing currents and upwelling, stronger storms, and coastal damage (e.g., the 2021 BC heat dome). The **IPCC** reports are the standard source for the science.