PHYTOPLANKTON ECOLOGY and NUTRITION, part 1


I. Mechanisms to reduce sinking
    A. Essential requirement of phytoplankton is to remain in suspension

        1. At some depth called the compensation point, there will be insufficient light for growth
                (point at which photosynthesis = respiration)
        2. Phytoplankton are generally heavier than water so they sink
        3. Define excess density = (r’- r)
            i. r’ = density of organism
            ii. r = density of water
            iii. Densities of components of phytoplankton
                a) Proteins                      ~1.3 g cm-3
                b) Carbohydrates            ~1.5 g cm-3 (cellulose)
                c) Nucleic acids              ~1.7 g cm-3
                d) SiO2                           ~2.6 g cm-3 (diatom walls)
                e) Lipids                          ~0.86 g cm-3
            iv. Range of densities of phytoplankton ~0.999-1.26 g cm-3
        4. So most phytoplankton will naturally tend to sink; BUT, because of their need for light, one of their main requirements is
                to remain in suspension

    B. Stoke’s Law (applies to Reynold’s numbers less than 500)
        1. Basic rule
            a. For a spherical object of density r’ sinking in water of density r, the sinking rate is a balance between gravitational effects
                on the density difference between the object and water, and the resistance (drag)
            b. Stoke's Law
                i. Vs = terminal velocity of the sphere
                ii. g = acceleration of gravity
                iii. r = radius
                iv. m = viscosity
                v. (r’- r) = excess density
        2. size and shape
            a. most algae are not spherical; elongation for example increases surface area, increases drag and decreases
                sinking velocity – this is accounted for by a term called form resistance (fr = coefficient of form resistance)
            b.Stoke's Law including a term for 'form resistance'
                i. colony (filament) formation increases form resistance
                ii. spines and protrusions on some algae in calm waters
            c. Why aren’t cells as large as possible to increase drag and decrease resistance to sinking?
                i. The radiance would increase also – that would tend to increase sinking rate
                ii. The volume also would increase – problems for molecular diffusion, nutrient exchange
            . Live phytoplankton tend to sink at lower rates than do senescent/dying phytoplankton of the same size and shape -- implies phytoplankton must have other mechanisms to actively reduce sinking rates
        3. lipid accumulation
            a. lipids are generally 2-20% of algal dry weight
            b. usually a small effect on sinking (except for Botryococcus)
        4. mucilage secretion
            a. polysaccharide (holds a lot of water)
            b. increasing the radius with a neutrally buoyant substance that may decrease the overall density, but increases the radius
            c. cells with lots of mucilage sink faster
            d. also the surface area is decreased relative to volume and diffusion of nutrients is decreased
        5. gas vacuoles - blue-green algae that form surface scum
        6. ionic regulation
            a. exchange heavy ions for light ions
            b. more important in marine algae than in freshwater algae
        7. swimming with flagella

II. Temperature and Growth
    A. Stenotherms – have narrow temperature optima
        1. may be cold (e.g., in arctic) or warm (e.g., in tropics)
stenotherms have high growth at a narrow range of temperatures and low growth (or death) outside of that small temperature range
    B. Eurytherms – have broad temperature optima
Eurytherms have high growth rates at a wide range of temperatures
    C. The growth curves reflect enzyme optima
    D. In phytoplankton, the temperature is strongly related to respiration, but more weakly to photosynthesis
III. Light and Growth
    A. Photosynthesis versus irradiance (P versus I)
        1. Response of phytoplankton to changes in irradiance
Typical photosynthesis rate versus irradiance rate for phytoplankton
        2. Photosynthesis decreases at high irradiance – photoinhibition
        3. Photosynthetic rate with depth

Typical photosynthetic rate with depth -- photoinhibition at the surface, high rates below that, and then decreasing photosynthetic rates as light decreases exponentially
    B. Interaction with temperature
        temperature is most important at high photosynthetic rates (limited by the dark reactions)
Temperature effects on photosynthesis -- decreasing temperatures tend to decrease the maximum rates of photosynthesis

    C. Critical mixing depth
        1. The compensation point is generally at ~1% of incident light
        2. Area 1 = growth of phytoplankton possible
        3. Area 2 respiration greater than photosynthesis
1% incident light levels -- relationship between phytoplankton growth and respiration with depth

        4. 4 cases:
            i. epilimnion is well above the 1% incident light level – always enough light for growth
            ii. deeper epilimnion – algae are mixed below the 1% light level, but still net positive growth (as long as epilimnion is above the
                critical mixing depth, there will be algal production)
           iii. Critical mixing depth:
                        area with growth = area without growth
                        no net algal growth
            iv. Epilimnion > critical mixing depth (negative net growth)
    D. Implications of light effects on phytoplankton populations
        1. Blooms of algae under the ice
        2. Light is often a factor in causing a spring bloom
        3. Blue greens that are buoyant due to gas vacuoles can shade other algae
        4. Different groups and species of algae have different light tolerances
 
 

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