primary production (chapter 2 in the marine ecology book)

  • primary production → it is the formation of organic matter through the trapping of light energy and assimilation of inorganic elements

  • compensation depth → it is the depth at which the gross photosynthetic carbon assimilation by phytoplankton equals teh respiratory carbon losses, or when the net photosynthesis is 0

  • critical depth → same as compensation depth for for th ewhole phytoplankton population in the water column at a specific depth, usually lower than compensation depth

what are phytoplankton?

  • phytoplankton are photosynthetic, drifting algae or cyanobacteria

  • they occur across multiple size classes

    • microplankton (20-200 µm): many dinoflagellates and dioatoms (identifiable by light microscopy)

    • nanoplankton (2-20µm): dinoflagellates, haptophytes (often require electron microscopy)

    • picoplankton (0.2-2 µm): includes cyanobacteria and tiny green algae

  • phytoplankton are responsible for 50% of earth’s primary productivity

nutrition (trophic modes) in microalgae

  • autotrophy: CO2 + light → organic carbon (photosynthesis)

  • Auxotrophy: autrophic but require some organic molecules

  • heterotrophy: use organic carbon

    • phagotrophy (ingests particles)

    • osmotrophy (absorb DOM)

  • mixotrophy: combine photosynthesis + heterotrophy

photosyntehsis and primary production

photosynthesis basics:

  • 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

  • the calvin cycle occurs in the dark reaction and uses RUBISCO, the most abundant enzyme on earth

chemolithotrophic primary production

  • some bacteria use chemical energy instead of light

definition of primary production

  • formation of organic matter from inorganic molecules via photosynthesis or chemosynthesis

  • marine phytoplankton account for 90-96% of ocean primary production

  • despite low biomass, phytoplankton has very high productivity, unlike land plants with high biomass but lower turnover

environemtnal control on productivity

light

  • light decreases exponentially with depth due to:

    • absorption/scattering by water

    • particles

    • dissolved organic matter (DOM)

  • irradiance (E) =2,718 totoal radiant flux on a surface

  • euphotic zone: to 1% of surface light; supports algal growth

pigments

  • chlorophyll a = main pigment

  • accessory pigments broaden usable wavelengths:

    • chlorphyll b and c

    • carotenoids

    • phycobiliproteins

  • these create absorption spectra adapted to underwater light

photosynthesis-irradiance curves (P-E curves)

  • key parameters:

    • Pmax: max photosynthetic rate

    • Ek: saturation irradiance

    • Ec: compensation irradiance (P = R)

compensation depth and critical depth

  • compensation depth → depth where photosynthesis = respiration

  • critical depth → bloom occurs when mixed layer is lower than the critical depth, allowing net production

  • spring bloom initiation depends on:

    • increasing light

    • shallowing of mixed layer

    • stratification due to warming

nutrients and phytoplankton growth

nutrient requirement

  • major biomass elements; CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorous)

  • macronutrients: N, P, Si, S, K, Na

  • micronutrients: Fe, Zn, Cu, Mn, Se, plus vitamins

  • limiting nutrients:

    • most marine systems → nitrogen (N) is limiting

    • sometimes P or Si (especially diatom-rich systems)

    • Fe can be limiting in HNLC regions

carbon system

  • DIC forms:

    • CO2

    • HCO3 - (bicarbonate)

    • CO3 2-

  • CO2 or HCO3 - is used for photosynthesis

  • carbon rarely limits marine phytoplankton

nitrogen cycle

  • major processes:

    • nitrogen fixation

    • nitrification

    • denitrification

    • Anammox

    • DNRA

phosphorous cycle

  • phosphorous occurs mainly as HPO4 2- and cycles through remineralisation and uptake

eutrophication

  • excess nutrients → excessive algal growth

  • associated with redfield ratio (C:N:P = 106:16:1)

from photosynthesis to growth

  • growth = photosynthetic assimilation minus losses:

    • respiration

    • grazing

    • sinking

    • exudation

  • population growth usually exponential, described by:

    • r = μ − λ

    • where μ is growth rate and λ is loss rate

seasonal cycles of phytoplankton

  • winter: low light, depe mixing → low biomass

  • spring: stratification + nutrients + light → spring bloom

  • summer: nutrient depletion → lower biomass, smaller cells

  • autumn: mixing returns nutrients → autumn bloom

climate change impacts

  • key impacts on phytoplankton

    • higher temperatures

    • melting ice → more open water in polar areas

    • stronger stratification → lessmixing → nutrient limitation in tropics

    • increased precipitation→ coasstal browning (less light)

    • higher CO2 → lower pH

    • shifts in species composition

  • global patterns:

    • tropics/subtropics: reduced productivity due to stratification

    • polar regions: increased productivity due to ice melt and laigth availability