# Module E Summary: Energy Flow, Limitation, and Blooms ## The big picture Energy flow through pelagic ecosystems is gated at the bottom: phytoplankton fix carbon only as fast as their **single most limiting resource** allows (Liebig's Law of the Minimum), and that resource is almost always light *or* one nutrient (usually nitrate, silicate for diatoms, or iron in HNLC regions). Light availability is set by season, latitude, weather, and how deep the mixed layer carries cells below the photic zone; nutrient availability is set by a tug-of-war between uptake at the surface, recycling by heterotrophic bacteria in the mixed layer, and physical resupply of deep nutrient-rich water — which the pycnocline normally blocks. A bloom is what happens when a limiting factor is suddenly relieved (spring sun + winter-accumulated nitrate, wind-driven upwelling, an iron pulse from dust or whale feces): fast-growing cells (diatoms first) explode until they re-exhaust the next limiting nutrient, then crash. That pulse propagates upward through food webs whose shape is dictated by the nutrient regime — nutrient-rich systems grow large cells and short, efficient chains; nutrient-poor systems grow small cells and long, leaky chains — while ~10–20% trophic transfer efficiency compounds at every step. Whatever sinks out of the mixed layer (cells, fecal pellets, marine snow) is exported to depth by the biological pump, which is why surface production also controls deep-ocean food supply, nutrient profiles, and atmospheric CO₂. ## Causal rules / linkages - **One limiter at a time.** Raising anything except the most limiting factor does nothing (e.g., adding nitrate to winter temperate water won't help — it's light-limited). - **Temperate seasonal flip:** winter = light-limited (short days + deep mixing), nutrients accumulate; spring = shallowing mixed layer + light + leftover nutrients → **bloom**; summer = stratification locks out deep nutrients → nitrate-limited; fall storms + residual light → small **fall bloom**, then reset. - **Stratification starves, mixing feeds.** A strong pycnocline = nutrient trap. Only an external force (winter storms, upwelling via Ekman transport, hurricanes) can break it and resupply the surface. - **Depth rule:** nutrients are depleted at the surface (uptake) and increase with depth (bacterial remineralization), peaking around 1,000 m where they coincide with the oxygen minimum zone. - **Nitrate builds blooms; ammonium only sustains.** Regenerated (recycled) N keeps tropical/open-ocean systems ticking at low levels but is never enough to trigger a bloom — that takes "new" nitrate from mixing or upwelling. - **Iron is the hidden gatekeeper.** In HNLC regions (subarctic Pacific, equatorial Pacific, Southern Ocean — ~20% of the ocean), nitrate and phosphate are abundant but useless without iron. Add iron → community flips from small cells to diatoms → bloom. - **Nutrient regime shapes the whole food web.** Rich supply → large diatoms → short chain (diatom → anchovy → tuna, ~3 levels, ~20% TTE). Poor supply → small cyanobacteria → long chain (~6 levels, ~10% TTE) → vanishingly little top-predator biomass for the same primary production. - **Shocks cascade and persist.** El Niño shuts down upwelling → no diatom bloom → zooplankton fail → anchovies/tuna leave → fisheries collapse, with effects lasting years after the event ends. Bottom-up control all the way. - **Bloom strength = pump strength.** Large, dense cells and big fecal pellets sink fast → strong carbon export (biological pump); small cells stay suspended and get remineralized in place → weak pump. Whales partially run this in reverse, hauling deep nutrients back to the surface (whale pump).