Biology of mixed layer Primary production Irradiance Phytoplankton Physical mixing processes Nutrients Primary production by Phytoplankton - small drifting organisms that photosynthesize Competition and limits on production

Photosynthesis (P) Carbon dioxide (C,O) + Water (H,O) Requires chloroplasts Plants, algae Carbon dioxide (C,O) + Water (H,O) + Nutrients (N,P) + Light energy Oxygen (O) + Organic matter (C,H,O,N,P) proteins fats carbohydrates nucleic acids

Every living thing respires (plants too!) Respiration (R) Every living thing respires (plants too!) Carbon dioxide (C,O) + Water (H,O) + Nutrients (N,P) Oxygen (O) + Organic matter (C,H,O,N,P) proteins fats carbohydrates nucleic acids

Primary Production - Definitions Gross Primary Production (GPP) = rate of carbon fixation by photosynthesis units = [Mass / Area / Time], e.g. [g C m-2 y-1] Respiration (R) = rate of carbon (CO2) loss through metabolism Net Primary Production (NPP) = GPP - ΣR Need GPP>ΣR for net growth!

Production ≠ Biomass Production is a rate e.g. [g C m-2 y-1] Biomass is a concentration e.g. [g C m-2]

“Paradox of the Plankton” There are many species of phytoplankton, despite few limiting resources and lots of mixing. Phytoplankton (single-celled primary producers) have various competitive strategies that enable coexistence.

Four major players Cyanobacteria Diatoms Coccolithophores Dinoflagellates Small (<1 μm) or Large (0.5-4 mm) Nitrogen fixers Large (2-200 μm) Have silica frustules Small (2-25 μm) Have CaCO3 tests Large (5-2000 μm) Have unique life cycle & blooms

What limits production? Nutrients Light Intensity Spectrum Temperature Grazing by zooplankton Write the general equation on the board here

The environment varies in space and time The environment varies in space and time. Different phytoplankton grow well under different conditions. High Low Nutrients Low High Light Intensity Deep water / Winter Shallow water / Summer Narrow Broad Light Spectrum Low High Temperature

Nutrients N, P, Si, Fe Nitrogen is most often limiting in ocean Bioavailable forms of inorganic N: Nitrate (NO3-) Ammonium (NH4+) Nitrite (NO2-)

Growth rate varies with [nutrient] μ = Specific growth rate (d-1) Curve “saturates” Specific growth rate is defined as the increase in cell mass per unit cell mass per unit time N = [Nutrient]

Growth rate varies with [nutrient] μ = Specific growth rate (d-1) Ks μmax/2 μmax Specific growth rate is defined as the increase in cell mass per unit cell mass per unit time “half-saturation constant” N = [Nutrient]

Growth rate varies with [nutrient] μ = Specific growth rate (d-1) Ks μmax/2 μmax Michaelis-Menten Kinetics Specific growth rate is defined as the increase in cell mass per unit cell mass per unit time “half-saturation constant” N = [Nutrient]

At low nutrient concentrations, smaller phytoplankton tend to grow faster Assume cell is a sphere. Surface area: Volume: Surface area to volume ratio: Smaller cells have relatively more surface area for taking up nutrients.

Different strategies of nutrient use Coccolithophores Low μmax Low Ks Diatoms High μmax High Ks High or variable nutrients High mixing, upwelling Low average irradiance High turbulence Chronically low nutrients Stratified conditions High average irradiance Low turbulence

Light Intensity / Irradiance Larger plankton (diatoms and dinoflagellates) most adapted to high-nutrient conditions. High Low Nutrients Low High Light Intensity / Irradiance Deep water / Winter Shallow water / Summer Narrow Broad Light Spectrum Low High Temperature

PAR = photosynthetically active radiation (visible light wavelengths) Irradiance = power of electromagnetic radiation per unit area of ocean’s surface (e.g. W m-2) - or - energy per area per time (e.g. mol photons m-2 s-1).

Light (PAR) attenuates with depth Iz Z0 Iz = irradiance at depth z Units of [Watts m-2] or [mol photons m-2 s-1] Light attenuates as it is absorbed and scattered by particles in the water. Z

Average Primary Production saturates at high PAR (photosynthetically active radiation) Inside Fe patch Outside Fe patch Hiscock et al. 2008

Species adapt to different light levels Irradiance 1 Photo-inhibition at high light levels Ryther 1956 Too much light damages cells and reduces photosynthesis (photo-inhibition)

Diatoms most adapted to low-light conditions High Low Nutrients Low High Light Intensity / Irradiance Deep water / Winter Shallow water / Summer Narrow Broad Light Spectrum Low High Temperature

Attenuation varies with wavelength. More wavelengths are available near the surface. Plankton use colored pigments to harvest light at different wavelengths. violet red

Different color pigments absorb different wavelengths of light Pigments (colored molecules) Phytoplankton with different pigments Chlorophyll*

Phytoplankton with multiple pigments capture more wavelengths All phytoplankton have chlorophyll Coccolithophores and diatoms have carotenoids Cyanobacteria have phycoerythrin, phycocyanin

Light Intensity / Irradiance Coccolithophores and Cyanobacteria most adapted to broad spectrum of light found in shallower mixed layer High Low Nutrients Low High Light Intensity / Irradiance Deep water / Winter Shallow water / Summer Narrow Broad Light Spectrum Low High Temperature

Growth-temperature curves vary among species but share upper limit (Eppley 1972) Growth rate Temperature oC

Diatoms grow fastest at low temperatures Flagellates Divisions per day Temperature oC

Diatoms most adapted to colder temperatures High Low Nutrients Low High Light Intensity / Irradiance Deep water / Winter Shallow water / Summer Narrow Broad Light Spectrum Low High Temperature

Can you explain why production peaks where it does? Production is a rate e.g. [g C m-2 y-1] Biomass is a concentration e.g. [g C m-2]