OC211(OA211) Phytoplankton & Primary Production

1. OC211(OA211) Phytoplankton & Primary Production LECTURE 6 Week 6 (i) Photosynthesis & Light (ii) Critical Depth Theory Dr Purdie SOC (566/18) email: DAP1@soc.soton.ac.uk

2. The Photosynthesis Light Curve Instantaneous rates of photosynthesis are controlled by external factors: • Temperature • Irradiance • CO2 and O2 concentrations • The Photosynthesis – Irradiance response (P vs E) can be divided into three regions.(i) light limited region(ii) light saturated region(ii) photoinhibited region Fig7.2; Falkowski & Raven

3. The Photosynthesis Light Curve (i) At low light levels photosynthesis rates are linearly proportional to irradiance ie double light level doubles photosynthesis rate. In this part of curve (light limited part of curve) rate of photon absorption determines rate of steady state electron transport from water to CO2. • If measure photosynthesis using O2 changes then at low irradiance the rate of O2 consumption will be greater than O2 production hence net O2 evolution is negative. • The light level where photosynthetic production of O2 balances consumption of O2 by respiration is the Compensation Irradiance (Ec). (in natural populations of microalgae 5-30 mmol/m2/s) Fig7.2; Falkowski & Raven

4. The Photosynthesis Light Curve • The initial slope of the P vs E curve is proportional to the maximum quantum yield. • It is often given symbol a • This is a function of the “Light Reactions” of photosynthesis. • Usually photosynthesis is normalised to chlorophyll biomass (ie rate of production divided by chlorophyll) and if so a superscript B is added to indicate this normalization aB

5. The Photosynthesis Light Curve (ii) As irradiance increases photosynthetic rates become increasingly non linear and rise to a saturation level or Pmax . • When the photosynthesis rate is chlorophyll normalized (ie divided by chlorophyll concentration express as PBmax or ‘Assimilation number’ • units mgC.mgchl-1h-1 • At saturation the rate of photon absorption exceeds the rate of steady state electron transport from water to CO2. • Pmax is thus a function of the “Dark Reactions” or “Light Independent” reactions of photosynthesis and is thus influenced by environmental conditions influencing enzyme reactions eg temperature.

6. The Photosynthesis Light Curve • The intersection of a and Pmax is given the symbol Ek or light saturation parameter. It represents an optimum on the P vs E curve. • Ek = Pmax/a • Ek is independent of whether Pmax or a are normalized to chlorophyll, cell volume, cell carbon etc. • This does not allow for photoinhibition and more complex mathematical formulations used to include inhibition effects. Fig7.2; Falkowski & Raven

7. The Photosynthesis Light Curve • (iii) Further increases beyond light saturation can lead to a reduction in photosynthesis rate from maximum saturation level. This is photoinhibition and is dependant on intensity of light and duration of exposure. • A number of models – mathematical formulations used to fit data to a line. The hyperbolic tangent function often used • P = Pmax .tanh(a.E/Pmax) • This does not allow for photoinhibition and more complex mathematical formulations are used to include inhibition effects.

8. The Photosynthesis Light Curve • Pvs E curves are usually measured over a few hours under artificial light conditions. • This provides an indication of the physiological adaptive state of the phytoplankton population • The P vs E response can indicate light stress at high light levels if the population is “shade adapted” ie sampled from deep chlorophyll maximum.

9. The Critical Depth Theory 22 • Photosynthesis is driven by light and therefore restricted to upper parts of ocean • Photosynthesis rates decrease with depth. • Calculation of production per unit area mgC m-2 h-1 • note: volume units must be in m-3 mgCm-3h-1 (5 x 15) + ((22-15) x 5)/2) (7 x 15) + ((15-11) x 7)/2) (5 x 15) + ((11-5) x 5)/2) (5 x 15) + ((5-0) x 5)/2) Total = 389 mgC m-2 h-1 0m 5m 15 12m 11 17m 5 22m 0

10. The Critical Depth Theory • The surface mixed layer of the ocean indicated by temperature structure • A single cell can photosynthesise in surface water but if mixed down the light is too low in intensity and respiration uses up carbon and energy . Therefore no carbon accumulation or no net positive primary production. • This gave rise to the Critical Depth concept. • Gran and Braarud (1935) first suggested. • Sverdrup (1953) developed a mathematical model

11. The Critical Depth Theory • COMPENSATION DEPTH (Dc) where photosynthesis of a cell equals its respiration Pc=Rc • Compensation irradiance (Ec) = (5-10 mmols m-2s-1) • Therefore: above Ec: Pc > Rc • below Ec: Pc < Rc (therefore a net loss) • Phytoplankton are continually mixed above and below the compensation depth therefore experience an average irradiance. The euphotic zone is the portion of the water column supporting net primary production and the base of the euphotic zone is the compensation depth. • Above the compensation depth net daily photosynthesis is positive, below this depth it is negative.

12. Figure 9.3: Falkowski & Raven

13. The Critical Depth Theory CRITICAL DEPTH (Dcr) defined as "the depth to which phytoplankton can be mixed and at which the total photosynthesis for the water column is equal to total respiration (of the primary producers)" At the Critical Depth, Photosynthesis throughout the water column will equal respiration: Pw =Rw units same mgC m-2 h-1 (NB area units) Also at the critical depth the average irradiance for the water column equals the compensation irradiance. E = Ec

14. The Critical Depth Theory Fig 41. Parsons, Takahashi & Hargrave

15. The Critical Depth Theory The model relates Ec to Dc Ec =Eoe-kDc Dc = ln(Eo)- ln (Ec) k Phytoplankton are mixed up and down in the surface layers of the water column, it is useful to know the average amount of light (ED) in the euphotic zone. This is given by the expression: ED = Eo. (1 - e-kD) kD where: Eo = surface irradiance k = extinction coefficient D = depth over which irradiance averaged

16. The Critical Depth Theory How far down can a population of phytoplankton be mixed to balance photosynthetic production and respiration consumption of carbon? i.e. Critical Depth Rearrange above equation and substitute Ec for ED we get the following expression to calculate the critical depth: Dcr = Eo. (1 - e-kDcr) k.Ec If k.Dcr is large then this is simplified to: Dcr = Eo k.Ec

17. The Critical Depth Theory • Where the amount of phytoplankton carbon respired is matched by area of carbon gained by photosynthesis then this is Critical Depth. • If cells are mixed downward below this depth there will be no net photosynthesis. • If depth of mixing is above critical depth positive net photosynthesis can occur. • Therefore we can use a simple equation linking • 1) surface irradiance, • 2) diffuse attenuation coefficient and • 3) a known compensation irradiance To estimate when the spring bloom starts in temperate latitudes. Model assumes: (i) plants uniformly distributed with depth in mixed layer (ii) plant nutrients non limiting (iii) extinction coefficient in water column is constant (iv) production of plant material is proportional to amount of radiation (v) respiration is constant with depth

18. The Critical Depth Theory Fig 3.11 Mann & Lazier