Biological oceanography: plankton and primary productivity

Marine Biology / Oceanography (B.Sc) · Ocean science · free study notes for exams, placements and interviews

In short

Biological oceanography studies the organisms of the open water column, how much organic carbon they create, and how that carbon moves through the food web and into the deep sea. Its core quantity is primary productivity — the rate at which phytoplankton convert dissolved CO₂ into organic matter by photosynthesis — because every fishery, every oxygen-minimum zone and the ocean's uptake of atmospheric CO₂ depends on it. In Indian waters the subject explains the monsoon-driven blooms of the Arabian Sea, the stratified low-productivity Bay of Bengal, the Kerala–Karnataka upwelling that supports the oil-sardine fishery, and recurring *Noctiluca* and *Trichodesmium* blooms. It is examined in B.Sc/M.Sc marine biology, oceanography and fisheries science papers, in CSIR-UGC NET (Earth, Atmospheric, Ocean and Planetary Sciences), ICAR-ARS and IIT JAM, and is standard interview ground for CSIR-NIO Goa, CMLRE and INCOIS (MoES), ICAR-CMFRI and CIFE, FSI, state fisheries departments and environmental consultancies doing marine EIA.

Explained

What plankton are, and how they are classified

Plankton: organisms drifting in the water column whose horizontal movement is governed by currents (from Greek *planktos*, wandering). Contrast nekton (actively swimming — fish, squid) and benthos (bottom-dwelling).
Trophic classification: phytoplankton (photoautotrophs), zooplankton (heterotrophs), bacterioplankton (heterotrophic and chemoautotrophic bacteria/archaea), virioplankton (viruses), mycoplankton (marine fungi), mixoplankton (organisms both photosynthesising and feeding, e.g. many dinoflagellates).
Size classes: femtoplankton 0.02–0.2 µm (viruses); picoplankton 0.2–2 µm (*Prochlorococcus*, *Synechococcus*); nanoplankton 2–20 µm (coccolithophores, small flagellates); microplankton 20–200 µm (most diatoms, dinoflagellates, ciliates); mesoplankton 0.2–20 mm (copepods, chaetognaths); macroplankton 2–20 cm (krill, salps); megaplankton >20 cm (large jellyfish, *Pyrosoma*).
Life-history classification: holoplankton — planktonic for the whole life (copepods, chaetognaths); meroplankton — only larval stages are planktonic (nauplius of barnacles, zoea of crabs, veliger of molluscs, trochophore of polychaetes, bipinnaria of starfish, fish eggs and larvae = ichthyoplankton); tychoplankton — accidental, resuspended benthic forms.
Habitat classification: neritic (over the shelf) vs oceanic; by depth — epipelagic, mesopelagic, bathypelagic; neuston/pleuston at the air–sea interface (*Physalia*, *Velella*, *Halobates*).
Flotation adaptations: small size and high surface-to-volume ratio, spines and setae (*Chaetoceros* chains), gas vacuoles (*Trichodesmium*), oil and lipid droplets, light siliceous or absent skeletons, ionic regulation (ammonium in *Noctiluca*), gelatinous bodies.

The main groups

Diatoms (Bacillariophyceae): silica frustule in two valves, no flagella, chain-forming; dominate nutrient-rich, turbulent, upwelled and coastal waters; fast growers and the base of productive fisheries. Indian genera: *Chaetoceros*, *Skeletonema*, *Thalassiosira*, *Coscinodiscus*, *Rhizosolenia*, *Pseudo-nitzschia* (produces domoic acid).
Dinoflagellates: two dissimilar flagella, many thecate (armoured); prefer warm stratified water; include bioluminescent *Noctiluca scintillans* (green *Noctiluca* blooms in the winter Arabian Sea with its endosymbiont *Pedinomonas*), *Ceratium*, *Gymnodinium*, *Karenia*, *Prorocentrum*; cause red tides and shellfish poisoning (PSP, DSP, NSP, ciguatera via *Gambierdiscus*).
Coccolithophores: calcite plates (coccoliths), e.g. *Emiliania huxleyi*; important for the carbonate counter-pump and DMS production.
Cyanobacteria: *Prochlorococcus* and *Synechococcus* dominate oligotrophic picoplankton; diazotrophs *Trichodesmium erythraeum* and the symbiont *UCYN-A/Richelia* fix N₂ and bloom in the pre-monsoon Arabian Sea and Lakshadweep seas.
Zooplankton: copepods (60–80% of mesozooplankton biomass — *Calanus*, *Acartia*, *Temora*, *Oithona*, *Paracalanus*); euphausiids (krill); cladocerans; chaetognaths (arrow worms, carnivores, indicators of water masses); appendicularians and salps (mucous-net filter feeders, fast carbon export); pteropods (shelled, vulnerable to acidification); protozoan foraminifera, radiolaria, tintinnids and heterotrophic dinoflagellates; gelatinous cnidarians and ctenophores.
Grazing and behaviour: suspension feeding by copepod second maxillae; diel vertical migration (DVM) — nocturnal ascent to feed, daytime descent to escape visual predators, giving the deep scattering layer on echosounders and an active flux of carbon to depth.

Primary productivity: definitions and controls

Primary production: synthesis of organic carbon from inorganic carbon by autotrophs. Expressed as a rate, mg C m⁻³ h⁻¹ (volumetric) or mg C m⁻² d⁻¹ / g C m⁻² yr⁻¹ (areal, depth-integrated). Standing crop (chlorophyll, cells per litre, biomass) is *not* production.
GPP = total carbon fixed; NPP = GPP minus autotrophic respiration; NCP/net community production = GPP minus respiration by the whole community. Secondary production = growth of herbivores and higher consumers.
New vs regenerated production (Dugdale & Goering): production supported by nitrate from below the pycnocline or by N₂ fixation is new; production on ammonium/urea recycled in the euphotic zone is regenerated. The f-ratio = new/total production — high (0.5–0.8) in upwelling, low (0.1–0.2) in oligotrophic gyres. At steady state new production ≈ exportable production.
Light: photosynthetically active radiation (PAR, 400–700 nm) attenuates exponentially (Beer–Lambert). Euphotic zone = surface to the 1% surface-light depth; compensation depth = where photosynthesis of a cell equals its respiration (net production zero); critical depth (Sverdrup) = depth at which integrated photosynthesis over the mixed layer equals integrated respiration — a bloom begins when the mixed-layer depth becomes shallower than the critical depth.
P–I (photosynthesis–irradiance) curve: light-limited initial slope α, saturation at P\_max, decline at high light by photoinhibition; the saturation parameter Ik = P\_max/α. Shade-adapted cells raise chlorophyll per cell (photoacclimation).
Nutrients: nitrogen is the usual limiting nutrient in the sea, phosphorus in some coastal and estuarine systems, silicate for diatoms, and iron in HNLC (high-nutrient low-chlorophyll) regions — Southern Ocean, equatorial and subarctic Pacific. Liebig's law of the minimum fixes which one limits; Michaelis–Menten kinetics (µ = µ\_max S/(Ks+S)) describe uptake, with low-Ks species winning in oligotrophic water.
Physical supply mechanisms: coastal upwelling (Ekman transport offshore under alongshore wind), equatorial and curl-driven upwelling, winter convective mixing, cyclonic eddies and cyclone wakes, river and groundwater inputs, tidal and island-wake mixing. Stratification (fresh or warm surface layer) shuts the supply off.
Fate of production: microbial loop — DOC released by phytoplankton and sloppy feeding is taken up by bacteria, grazed by flagellates and ciliates, and respired, so carbon is recycled rather than exported; biological (soft-tissue) pump — sinking faecal pellets, aggregates and marine snow plus DVM transport carbon to depth, where remineralisation consumes oxygen and creates oxygen minimum zones — the Arabian Sea OMZ (roughly 150–1,000 m, with denitrification and anammox) being among the world's most intense.

Measuring productivity and biomass

Light-and-dark bottle (oxygen) method (Gaarder & Gran, 1927): paired clear and blackened BOD bottles incubated in situ at several depths; oxygen measured by Winkler titration or optode. Light bottle change = NPP, initial minus dark = respiration, light minus dark = GPP. Simple and calibration-free, but insensitive in oligotrophic water.
¹⁴C method (Steemann Nielsen, 1952): a known activity of NaH¹⁴CO₃ is added, incubated a few hours, filtered, and the particulate activity counted by liquid scintillation; dark-bottle counts are subtracted. Highly sensitive — the standard for open-ocean work, including on FORV *Sagar Sampada* — but needs a radiological licence; handling of radioisotopes in India is regulated by the atomic-energy regulator (the SHANTI Act, 2025 replaces the former Atomic Energy Act, 1962).
¹⁸O and ¹⁷O methods give true GPP; ¹⁵N uptake separates nitrate (new) from ammonium (regenerated) production and yields the f-ratio.
Chlorophyll a: filter a known volume on GF/F, extract in 90% acetone, read on a spectrophotometer (trichromatic Jeffrey–Humphrey equations) or fluorometer (acidification method for phaeopigments); HPLC resolves marker pigments (fucoxanthin = diatoms, peridinin = dinoflagellates, zeaxanthin = cyanobacteria, 19'-hexanoyloxyfucoxanthin = prymnesiophytes). CHEMTAX apportions groups from pigments.
In-situ and remote sensing: fluorometers on CTD rosettes and Argo/BGC-Argo floats, FRRF (variable fluorescence, Fv/Fm) for instantaneous photophysiology, flow cytometry for pico/nanoplankton, FlowCam and imaging systems for cell counts, eDNA metabarcoding for community composition. Ocean-colour satellites retrieve surface chlorophyll from water-leaving radiance ratios — India's OCM-2 (Oceansat-2) and OCM-3 (Oceansat-3/EOS-06, launched November 2022), processed by NRSC and used by INCOIS for the Potential Fishing Zone advisories; VGPM-type models convert satellite chlorophyll to depth-integrated production.
Limitations: bottle incubations enclose and exclude large grazers ("bottle effect"), trace-metal contamination depresses rates, satellite chlorophyll sees only about the first optical depth and fails in turbid Case-2 coastal water, and chlorophyll-to-carbon ratios vary several-fold with light and nutrients.

Indian Ocean patterns, and the Indian institutional and legal frame

Arabian Sea — two blooms a year: (i) south-west monsoon (June–September) — strong Findlater-jet-driven upwelling off Somalia, Oman and the south-west Indian coast, diatom-dominated, 1–3 g C m⁻² d⁻¹; (ii) winter/north-east monsoon (January–February) — dry cold continental air causes convective deepening of the mixed layer and nutrient entrainment, with large green *Noctiluca scintillans* blooms now recurring annually. Spring inter-monsoon brings *Trichodesmium*.
Bay of Bengal — low productivity despite high nutrient input: Ganga–Brahmaputra–Meghna and peninsular runoff creates a strong low-salinity barrier layer and halocline that suppresses mixing; typical production is a few hundred mg C m⁻² d⁻¹, rising sharply in cyclone wakes and cold-core eddies. Sediment-laden water also limits light.
Coastal India: south-west coast upwelling (with the Kerala mud banks or *chakara*) underpins the oil-sardine and mackerel fishery; east-coast upwelling is weaker; Gulf of Kachchh, Gulf of Mannar, Chilika, Pulicat and the Sundarbans are high-productivity estuarine–lagoon systems. Eutrophication-driven HABs and seasonal hypoxia occur off Mumbai, Kochi, Chennai and Visakhapatnam.
Programmes and bodies: CSIR-NIO (Goa) and its regional centres; CMLRE, Kochi (MoES Marine Living Resources Programme, FORV *Sagar Sampada*); INCOIS, Hyderabad (ocean-colour products, PFZ, ocean state forecasts); ICAR-CMFRI, CIFE, CIFT; FSI; NCPOR for the Southern Ocean; MoES Deep Ocean Mission; international IIOE-2 and SIBER programmes. The Indo-German LOHAFEX (2009) iron-fertilisation experiment in the Southern Ocean found copepod grazing largely prevented carbon export.
Regulation (name the instrument, verify the current text before citing in practice): the Biological Diversity Act, 2002 (amended 2023) and its rules govern access to marine biological resources and benefit sharing; the CRZ Notification, 2019 under the Environment (Protection) Act, 1986 controls coastal activity and requires marine EIA including plankton and productivity baselines; the Wild Life (Protection) Act, 1972 schedules corals and some marine invertebrates; MPEDA and FSSAI regulate shellfish-toxin monitoring for export; ballast-water transfer of plankton (an invasive-species pathway) follows the IMO Ballast Water Management Convention, administered in India by the Directorate General of Maritime Administration under the Merchant Shipping Act, 2025 (which replaced the 1958 Act from 15 March 2026). Water-quality sampling methods follow the relevant Indian Standards for water and wastewater sampling and examination; quote a specific IS number only after checking its current edition.

Key points

Plankton — drifters controlled by currents; nekton swim, benthos live on the bottom.
Phytoplankton — microscopic autotrophs producing roughly half of global photosynthesis on under 1% of global photosynthetic biomass, because turnover is days not years.
Size spectrum — femto (virus) → pico → nano → micro → meso → macro → megaplankton; picoplankton dominate oligotrophic seas, microplankton diatoms dominate upwelling.
Holoplankton vs meroplankton — lifelong vs larval-only planktonic existence.
Diatoms vs dinoflagellates — silica frustule, non-motile, turbulent nutrient-rich water vs flagellate, motile, warm stratified water and red tides.
Euphotic zone — down to 1% of surface PAR; a few metres in turbid estuaries, over 100 m in clear oligotrophic water.
Compensation depth — single-cell photosynthesis = respiration; critical depth — integrated over the mixed layer; bloom when MLD < critical depth (Sverdrup, 1953).
GPP, NPP, NCP — total fixation, minus autotrophic respiration, minus community respiration.
New vs regenerated production; f-ratio = new/total; new production sets the exportable carbon.
Redfield ratio — C:N:P = 106:16:1 by atoms in average plankton; N:P ≈ 16 diagnoses N or P limitation.
Limiting nutrients — N generally, Si for diatoms, P in some coastal waters, Fe in HNLC regions.
Upwelling — Ekman offshore transport brings nitrate up; the single biggest control on fishery productivity off south-west India.
Microbial loop — DOC → bacteria → flagellates → ciliates; recycles carbon and reduces export.
Biological pump — sinking particles plus DVM move carbon to depth; remineralisation drives the Arabian Sea OMZ.
Measurement — light/dark O₂ (GPP and R), ¹⁴C (sensitive NPP), ¹⁵N (new production), ¹⁸O (true GPP), chlorophyll by spectrophotometry/fluorometry/HPLC.
Satellite chlorophyll — OCM-3 on Oceansat-3 (EOS-06); INCOIS Potential Fishing Zone advisories; fails in turbid Case-2 water.
Two Arabian Sea blooms — SW-monsoon upwelling and winter convective mixing; the Bay of Bengal stays stratified and less productive.
HABs — *Noctiluca*, *Trichodesmium*, *Karenia*, *Pseudo-nitzschia*; cause fish kills, hypoxia and shellfish toxins.

Formulas and examples

Light attenuation: I\_z = I₀ e^−K\_d·z — I\_z = PAR at depth z (µmol photons m⁻² s⁻¹), K\_d = diffuse attenuation coefficient (m⁻¹), z in m.
Euphotic depth: Z\_eu = ln(100)/K\_d = 4.605/K\_d ; from a Secchi disc, K\_d ≈ 1.7/Z\_sd (Z\_sd = Secchi depth, m).
Oxygen method: NPP = (LB − IB)/t ; R = (IB − DB)/t ; GPP = (LB − DB)/t — LB, DB, IB = light, dark, initial O₂ (mg L⁻¹), t in hours.
O₂ to carbon: mg C = mg O₂ × (12/32) / PQ — PQ = photosynthetic quotient (mol O₂ evolved per mol CO₂ fixed), ≈1.0–1.2 (take 1.0 for nitrate-free assumptions unless stated).
¹⁴C method: PP (mg C m⁻³ per incubation) = (R\_s/R\_a) × ΣCO₂ ; rate = PP/t — R\_s = sample activity (DPM, dark-corrected), R\_a = activity added, ΣCO₂ = dissolved inorganic carbon (mg C m⁻³).
Chlorophyll a (Jeffrey & Humphrey trichromatic, 90% acetone, in µg mL⁻¹ of extract): C\_a = 11.85(E₆₆₄−E₇₅₀) − 1.54(E₆₄₇−E₇₅₀) − 0.08(E₆₃₀−E₇₅₀) ; then Chl a (mg m⁻³) = C\_a × v /(V × L) — v = extract volume (mL), V = volume filtered (L), L = cuvette path (cm).
Depth-integrated production: PP\_int = ∫₀^Z\_eu PP(z) dz, evaluated by the trapezoidal rule over discrete depths (mg C m⁻² h⁻¹ → × day length for m⁻² d⁻¹).
Redfield stoichiometry: 106 CO₂ + 16 HNO₃ + H₃PO₄ + 122 H₂O → (CH₂O)₁₀₆(NH₃)₁₆H₃PO₄ + 138 O₂ ; C:N:P = 106:16:1 (atoms), C:N ≈ 6.6 by atoms, ≈ 5.7 by mass.
f-ratio: f = new production / (new + regenerated production), dimensionless, 0–1.
Nutrient uptake (Michaelis–Menten): µ = µ\_max·S/(K\_s + S) — S = nutrient concentration (µmol L⁻¹), K\_s = half-saturation constant.
VGPM (Behrenfeld & Falkowski): PP\_eu = 0.66125 × P^B\_opt × [E₀/(E₀ + 4.1)] × Z\_eu × Chl\_sat × DL — P^B\_opt in mg C (mg Chl)⁻¹ h⁻¹, E₀ = surface daily PAR (mol photons m⁻² d⁻¹), DL = day length (h).
Trophic transfer (Ryther): P\_fish = PP × Eⁿ — E = transfer efficiency per step (≈0.1 in the open ocean, up to 0.15–0.2 in short upwelling chains), n = number of trophic steps.

Example 1 — light-and-dark bottle. Initial O₂ = 6.00 mg L⁻¹; after 4 h the light bottle reads 7.20 and the dark bottle 5.60 mg L⁻¹. Day length 12 h, PQ = 1.0.

NPP = 7.20 − 6.00 = 1.20 mg O₂ L⁻¹ per 4 h; R = 6.00 − 5.60 = 0.40; GPP = 7.20 − 5.60 = 1.60 mg O₂ L⁻¹ per 4 h.
GPP in carbon = 1.60 × 12/32 = 0.60 mg C L⁻¹ per 4 h = 0.15 mg C L⁻¹ h⁻¹ = 150 mg C m⁻³ h⁻¹.
Over a 12-h light day: 1,800 mg C m⁻³ d⁻¹ (1.8 g C m⁻³ d⁻¹) gross; net (light hours only) = 1.20 × 0.375 × 3 = 1,350 mg C m⁻³ d⁻¹. Over 24 h, respiration of 0.40 × 6 = 2.4 mg O₂ L⁻¹ exceeds nothing here, so the water is net autotrophic.

Example 2 — ¹⁴C production and depth integration (upwelling station off Kochi). 1.0 × 10⁶ DPM of NaH¹⁴CO₃ added, dark-corrected filter activity 3,500 DPM, ΣCO₂ = 24,000 mg C m⁻³, incubation 4 h.

PP = (3,500/1,000,000) × 24,000 = 84 mg C m⁻³ per 4 h = 21 mg C m⁻³ h⁻¹ at that depth.
Profile (mg C m⁻³ h⁻¹): 0 m = 10, 5 m = 15, 10 m = 12, 20 m = 5, 30 m = 1. Trapezoidal integration: (10+15)/2×5 = 62.5; (15+12)/2×5 = 67.5; (12+5)/2×10 = 85; (5+1)/2×10 = 30. Sum = 245 mg C m⁻² h⁻¹.
For a 10-h effective light day: 2,450 mg C m⁻² d⁻¹ = 2.45 g C m⁻² d⁻¹ — typical of active upwelling, about five times an oligotrophic Arabian Sea station.

Example 3 — chlorophyll a. 2.0 L filtered, extracted in 10 mL of 90% acetone, 1-cm cuvette; E₆₆₄ = 0.045, E₆₄₇ = 0.012, E₆₃₀ = 0.008, E₇₅₀ = 0.002.

Corrected: 0.043, 0.010, 0.006. C\_a = 11.85(0.043) − 1.54(0.010) − 0.08(0.006) = 0.50955 − 0.01540 − 0.00048 = 0.4937 µg mL⁻¹.
Chl a = 0.4937 × 10/(2.0 × 1) = 2.47 mg m⁻³ — a moderately productive coastal value.

Example 4 — Secchi, euphotic depth and Redfield drawdown. Secchi depth 4.0 m.

K\_d ≈ 1.7/4.0 = 0.425 m⁻¹; Z\_eu = 4.605/0.425 = 10.8 m. At 5 m, I\_z/I₀ = e^−0.425×5 = e^−2.125 = 0.119, i.e. 11.9% of surface PAR.
In a 20-m mixed layer nitrate falls from 8.0 to 1.0 µmol L⁻¹. ΔN = 7.0 mmol m⁻³ → C fixed = 7.0 × 106/16 = 46.4 mmol C m⁻³ = 46.4 × 12 = 556 mg C m⁻³; integrated over 20 m = 11.1 g C m⁻² of new production. Required P = 7.0/16 = 0.44 mmol m⁻³ (13.6 mg P m⁻³).

Example 5 — satellite model and fish yield. Chl\_sat = 0.5 mg m⁻³, Z\_eu = 40 m, P^B\_opt = 4.0, E₀ = 45 mol photons m⁻² d⁻¹, DL = 12 h.

0.66125 × 4.0 = 2.645; E₀/(E₀+4.1) = 45/49.1 = 0.9165; 2.645 × 0.9165 = 2.4242; × 40 = 96.97; × 0.5 = 48.48; × 12 = 582 mg C m⁻² d⁻¹ (0.58 g C m⁻² d⁻¹).
Shelf yield estimate: 2.45 g C m⁻² d⁻¹ (Example 2) over 120 upwelling days = 294 g C m⁻²; over 10,000 km² (1 × 10¹⁰ m²) = 2.94 × 10¹² g C = 2.94 Mt C. Three trophic steps at 10% each → 2.94 × 10⁶ × 10⁻³ = 2,940 t C of fish; at a wet-weight:carbon ratio of about 10 → ≈29,000 t potential landings. One fewer trophic step would multiply this by ten — which is why short diatom–copepod–sardine chains are so productive.

Feature	Diatoms	Dinoflagellates	Picocyanobacteria
Size	20–200 µm	10–200 µm	0.6–2 µm
Motility	Non-motile	Two flagella	Non-motile
Wall	Silica frustule	Organic theca or naked	Peptidoglycan
Preferred water	Cool, mixed, nutrient-rich	Warm, stratified, calm	Warm, oligotrophic
Nutrient affinity	Low (high K\_s), fast growth	Intermediate, often mixotrophic	Very high (low K\_s)
Fate of carbon	Sinks — high export	Blooms, often HABs	Microbial loop, low export
Indian example	Chaetoceros in Kerala upwelling	Noctiluca winter bloom, Arabian Sea	Prochlorococcus, central Arabian Sea

	Upwelling shelf (SW India, monsoon)	Open Arabian Sea (inter-monsoon)	Bay of Bengal (open)
Surface nitrate	High (>5 µmol L⁻¹)	Near zero	Near zero
Chlorophyll	2–10 mg m⁻³	0.1–0.3 mg m⁻³	0.1–0.3 mg m⁻³
Production	1–3 g C m⁻² d⁻¹	0.2–0.5 g C m⁻² d⁻¹	0.2–0.5 g C m⁻² d⁻¹
Dominant size class	Microplankton (diatoms)	Pico/nanoplankton	Pico/nanoplankton
f-ratio	High (0.5–0.8)	Low (0.1–0.2)	Low
Limiting control	Light/stability	Nitrogen	Stratification (barrier layer)

Where it is used

Fisheries forecasting and management: INCOIS Potential Fishing Zone and Ocean State Forecast advisories from satellite chlorophyll and SST; CMFRI stock assessment and the sardine–upwelling link; FSI resource surveys — roles as fishery scientist, remote-sensing analyst, data scientist.
Research and survey cruises: CSIR-NIO, CMLRE, NCPOR, NIOT and university departments run productivity, pigment and zooplankton programmes on FORV *Sagar Sampada*, RV *Sindhu Sadhana* and *Sagar Nidhi*; BGC-Argo float data analysis under the Deep Ocean Mission.
Aquaculture and hatcheries: mass culture of *Nannochloropsis*, *Isochrysis*, *Chaetoceros*, *Tetraselmis* and of *Artemia* and rotifers as live feed in MPEDA/RGCA and CMFRI/CIFA hatcheries; pond productivity management and HAB/toxin watch for shrimp and bivalve farms.
Environmental consultancy and EIA: plankton, chlorophyll and productivity baselines for CRZ clearances for ports, desalination plants, outfalls, thermal power and offshore wind; monitoring under the National Water Quality Monitoring and coastal-pollution programmes.
Climate and carbon work: quantifying the biological pump, ocean-acidification impacts on pteropods and coccolithophores, Arabian Sea OMZ expansion and deoxygenation — inputs to India's climate assessments and blue-carbon accounting.
Industry: microalgal biotechnology (omega-3, astaxanthin, pigments, biofertiliser, algal biofuel pilots), ballast-water treatment testing and compliance, and diatomite/biostratigraphic work in petroleum exploration.

Interview questions

Q: Why do phytoplankton account for about half of global photosynthesis with a tiny fraction of global plant biomass? A: Their biomass turns over in days, so a very small standing crop supports a very large annual production — the sea stores little, but produces fast.
Q: Distinguish compensation depth from critical depth. A: Compensation depth is where an individual cell's photosynthesis equals its respiration; critical depth is where photosynthesis integrated over the whole mixed layer equals integrated respiration — a bloom needs the mixed layer shallower than the critical depth.
Q: What does the f-ratio tell you? A: The fraction of total production fuelled by new nitrogen; at steady state it approximates the fraction available for export to depth or to fisheries.
Q: Why is the Bay of Bengal less productive than the Arabian Sea despite enormous river input? A: River freshwater creates a strong low-salinity barrier layer that suppresses vertical mixing, so subsurface nitrate rarely reaches the euphotic zone; turbidity also limits light.
Q: What causes the two annual Arabian Sea blooms? A: South-west monsoon winds drive Ekman upwelling (June–September), and dry cold north-east monsoon air drives convective mixed-layer deepening and nutrient entrainment (January–February).
Q: Why do diatoms dominate upwelling and picocyanobacteria the gyres? A: Diatoms have low nutrient affinity but very fast growth and exploit nutrient pulses; picoplankton have high surface-to-volume ratios and low half-saturation constants, so they win when nutrients are scarce.
Q: What is the microbial loop and why does it matter for carbon export? A: DOC taken up by bacteria and passed through flagellates and ciliates is largely respired back to CO₂ in the surface layer, so systems dominated by the loop export little carbon despite substantial production.
Q: How would you measure GPP and respiration at sea with no radioisotope licence? A: Paired light and dark BOD bottles incubated in situ with Winkler or optode oxygen measurement — light minus dark gives GPP, initial minus dark gives respiration.
Q: Why can satellite chlorophyll mislead in Indian coastal waters? A: These are optically complex Case-2 waters where suspended sediment and coloured dissolved organic matter also absorb and scatter, so standard blue-green band-ratio algorithms overestimate chlorophyll unless regionally tuned.
Q: What is an HNLC region, and what did LOHAFEX show? A: A region with ample macronutrients but low chlorophyll because iron limits growth; the 2009 Indo-German LOHAFEX iron addition in the Southern Ocean produced a bloom that was largely grazed by copepods, so little carbon was exported.
Q: Why is the Arabian Sea OMZ so intense? A: High surface production supplies abundant sinking organic matter whose remineralisation consumes oxygen, while sluggish, poorly ventilated intermediate circulation in a landlocked northern basin fails to replenish it.
Q: A plankton sample has a C:N atomic ratio of 12. What do you conclude? A: It is well above the Redfield 6.6, indicating nitrogen-deficient cells accumulating carbohydrate or lipid — typically a bloom that has drawn nutrients down, or senescent material.
Q: Why is diel vertical migration important beyond predator avoidance? A: Migrators feed at night in the surface and respire and defecate at depth, providing an active carbon flux that can rival passive sinking.

Common mistakes

Reporting chlorophyll or cell counts as "productivity" — biomass is a stock, production is a rate per unit time.
Writing light-bottle change as GPP: the light bottle gives net production; GPP needs the dark bottle subtracted.
Forgetting the PQ and the 12/32 factor when converting oxygen to carbon, or mixing mg L⁻¹ with mg m⁻³ (1 mg L⁻¹ = 1,000 mg m⁻³).
Multiplying an hourly rate by 24 instead of by the effective light day, and not integrating over the euphotic depth before quoting an areal rate.
Treating compensation depth and critical depth as the same thing, or quoting the euphotic zone as a fixed 100 m regardless of turbidity.
Assuming nitrogen limits everywhere — iron limits HNLC waters, silicate limits diatoms after a bloom, and phosphorus can limit some coastal and estuarine systems.
Assuming every dense bloom is harmful, or that every red tide produces toxins — many are harmless, and some toxic blooms occur at low cell densities.
Applying Redfield ratios to individual samples as if fixed; they vary with species, nutrient status and light.
In the lab: delayed filtration, exposing extracts to light, ignoring the 750 nm turbidity blank, or omitting phaeopigment correction, all of which bias chlorophyll results.