Plankton Study Guide

📖 Core Concepts Plankton – organisms that drift in water/air; cannot overcome currents by active swimming. Nekton, Benthos, Neuston – nekton swim against flow; benthos live on/within the seafloor; neuston float at the surface (a plankton subgroup). Life‑cycle categories – Holoplankton (entire life as plankton), Meroplankton (planktonic larval stage only), Ichthyoplankton (fish eggs & larvae). Size classes – picoplankton (0.2–20 µm), nanoplankton (≈20–200 µm), micro‑/macroplankton (< 1 mm / > 1 mm). Trophic modes – Phytoplankton (autotrophs), Zooplankton (heterotrophs), Mixoplankton (both), Decomposers (fungi & bacteria). Key ecological loops – Microbial loop, Viral shunt, Mycoloop, Biological pump. Nutrient limitation – growth controlled by nitrate ($N$), phosphate ($P$), silicate ($Si$); iron limits HNLC (high‑nutrient, low‑chlorophyll) regions. Carbon sequestration – sinking of dead cells, fecal pellets, and marine snow transports carbon to the deep ocean (biological pump). --- 📌 Must Remember Plankton vs. nekton: plankton drift; nekton swim. Holoplankton stay planktonic whole life (e.g., copepods, salps). Meroplankton are only planktonic as larvae (e.g., sea urchin larvae). Silicate is essential for diatom frustules; its shortage limits diatom growth. Iron fertilization can trigger blooms in HNLC zones but has limited, short‑lived carbon drawdown. Mixotrophy = simultaneous photosynthesis & heterotrophy; confers resilience in low‑light conditions. Biological pump efficiency depends on fecal pellet density: high absorption efficiency → dense pellets → deep carbon export. Viral shunt redirects carbon from the classic food web into dissolved organic matter (DOM). Mycoloop: chytrid fungi make otherwise inedible phytoplankton consumable for zooplankton. --- 🔄 Key Processes Primary Production → Carbon Fixation Light + nutrients → photosynthesis by phytoplankton → $CO2 + H2O \rightarrow CH2O + O2$. Microbial Loop Phytoplankton release DOM → heterotrophic bacteria consume DOM → small zooplankton graze bacteria → recycled nutrients back to phytoplankton. Viral Shunt Virus infects phytoplankton → cell lysis → DOM release → bacterial uptake → bypasses higher trophic levels. Mycoloop Fungal parasite infects large phytoplankton → produces zoospores or fragments → zooplankton ingest → energy transferred up the food web. Biological Pump (Carbon Export) Sinking of: a. Dead phytoplankton & calcite plates (coccolithophores). b. Zooplankton fecal pellets (density set by absorption efficiency). c. Marine snow aggregates. Iron Fertilization (HNLC response) Add Fe → stimulates phytoplankton growth → temporary bloom → limited carbon sequestration; potential side effects (harmful algal blooms, O₂ depletion). Temperature Influence on Microbial Respiration Higher temperature → ↑ bacterial respiration & nanoflagellate grazing → faster carbon turnover, lower growth efficiency, reduced deep‑sea carbon retention. --- 🔍 Key Comparisons Phytoplankton vs. Zooplankton Phytoplankton: autotrophic, photosynthesize, base of food web. Zooplankton: heterotrophic, consume other plankton, link to higher trophic levels. Holoplankton vs. Meroplankton Holoplankton: planktonic entire life (e.g., copepods). Meroplankton: only larval stage planktonic (e.g., fish larvae). Silicate‑limited vs. Iron‑limited (HNLC) waters Silicate‑limited: diatom growth curtailed, may favor non‑siliceous phytoplankton. Iron‑limited: all macronutrients abundant, but phytoplankton bloom only after Fe addition. Mixotrophs: Constitutive vs. Non‑constitutive Constitutive: own photosynthetic apparatus. Non‑constitutive: acquire plastids from prey (kleptoplasty). Dense vs. Fluffy fecal pellets Dense (high absorption efficiency, low feeding rate) → sink fast, deep carbon export. Fluffy (low absorption efficiency, high feeding rate) → slower sinking, more remineralization. --- ⚠️ Common Misunderstandings “All plankton are tiny.” Macroplankton (> 1 mm) includes jellyfish and large copepods. “Plankton are only animals.” Includes bacteria, archaea, fungi, viruses, and protists. “More nutrients always mean more carbon sequestration.” In HNLC zones, iron—not nitrate/phosphate—limits bloom; excess nutrients without Fe give little carbon drawdown. “Viruses only harm plankton.” Viral lysis fuels the microbial loop, supporting bacterial productivity. “Mixotrophy is rare.” Now recognized as widespread; many dinoflagellates and some diatoms exhibit it. --- 🧠 Mental Models / Intuition “Drift vs. Swim” – Imagine a leaf (plankton) vs. a fish (nekton) in a river: the leaf goes wherever the current carries; the fish can swim upstream. “Carbon’s Elevator” – Primary production is the ground floor; the biological pump is the elevator moving carbon down to the deep‑sea storage basement. “Nutrient Limitation Triangle” – Visualize a triangle with corners N, P, Si (or Fe). If any corner is low, growth stalls. “Loop vs. Shunt” – Picture the microbial loop as a circular conveyor belt; the viral shunt is a side door that drops material onto the floor (DOM) instead of moving it up the belt. --- 🚩 Exceptions & Edge Cases HNLC Regions – High macronutrients but Fe‑limited; iron addition may cause blooms but often short‑lived. Ocean Acidification – Can shrink phytoplankton size, paradoxically increasing carbon export efficiency (smaller cells sink slower but produce denser pellets). Temperature‑DOC Feedback – Warmer water raises sloppy feeding rates, producing more DOC, which fuels bacteria and can offset carbon export. Coccolithophore Calcification – Releases CO₂ during CaCO₃ formation, partially offsetting photosynthetic O₂ production. --- 📍 When to Use Which Identify primary producer → use phytoplankton classification (diatoms, cyanobacteria, etc.). Assess nutrient limitation → check silicate for diatoms, iron for HNLC, nitrate/phosphate for general growth. Predict carbon export potential → evaluate fecal pellet density (high absorption → deep export) and presence of large silica/calcite shells. Explain sudden DOM spikes → consider viral shunt or sloppy feeding rather than direct grazing. Model ecosystem response to warming → prioritize temperature effects on bacterial respiration and nanoflagellate grazing. --- 👀 Patterns to Recognize Surface bloom → deep‑sea carbon pulse (biological pump). High nitrate + low silicate → non‑diatom dominance (e.g., dinoflagellates). Elevated Fe + abundant macronutrients → rapid phytoplankton bloom (HNLC response). Seasonal temperature rise → shift toward smaller phytoplankton (size‑structured community change). Presence of chytrid fungi + large, defended phytoplankton → mycoloop activation. --- 🗂️ Exam Traps “All plankton are photosynthetic.” – Wrong; zooplankton and many protists are heterotrophic. “Silicate limitation only matters for diatoms.” – True, but iron limitation can dominate even when silicate is abundant. “The biological pump only involves phytoplankton sinking.” – Misses zooplankton fecal pellets and marine snow contributions. “Higher nutrient concentrations always increase carbon sequestration.” – Ignoring Fe limitation or stratification leads to overestimation. “Mixotrophs are always better competitors.” – While flexible, they may be outcompeted under stable, high‑light conditions by strict autotrophs. ---