The Role of Light Flashcards
Why does solar radiation vary with latitude and seasons
Sun Angle: Varies with latitude, time of year and time of day
Very high reflectance when sun angle is below 5 degrees
Length of day
Atmospheric Conditions
Scattered Light
Light reflects or bounces off particles/molecules so we can see them
Absorbed Light
Absorption by molecules of water to heat (Infrared)
What is the impact of absorbed and scattered light
Absorbed light used for photosynthesis
Scattering and absorption effects the quantity/quality of light available
Solar Energy Percentages Ocean
50% absorbed, reflected and scattered in the atmosphere
50% absorbed/scattered in the first ~10m – UV scattered and IR absorbed
50% penetrates (visible light) to about 1000m and decays exponentially with depth
Coastal vs Open Ocean Light Profiles
More scattering and absorption when more particulates in coastal waters. Less in open ocean
Coastal waters have green shifted light profiles from sediments
1% surface light at 10m in Coastal and 100m in open ocean
Light Exponential Decay
ID = IOe^(-kD)
ID = irradiance at depth (micromol photons/ms)
IO = irradiance at the surface (micromol photons/ms)
k = light extinction coefficient 1/m
D = depth
Light Extinction Coefficient
Describes how rapidly light is attenuated
k is higher in coastal waters
Composed of 3 terms: kw (water), kp (particulates) and kd (dissolved)
Two Ways to Measure k
PAR Sensor: Record light intensity directly at surface and at depth (k = ln(Io) - ln(ID) / D)
Secchi Disk: White circular disk lowered until not visible, that depth is Ds (k = 1.7/Ds)
Main Characters of Phytoplankton
Unicellular microscopic photoautotrophs
Very small in size
Most have a hard outer covering
Many are eukaryotic
Both prokaryotic and eukaryotic forms have thylakoids
Role of Phytoplankton in Marine Ecosystems
Primary producers in marine ecosystems
Form base of virtually all food webs
Represent available energy for transfer up the trophic pyramid
Form extensive blooms, can be toxic
Influence atmospheric and aquatic chemistry (produce O2, CO2 drawdown and C export, cloud formation)
Oil and sediment deposits
Major impact on global climate
Four Major Groups
Cyanobacteria
Diatoms
Coccolithophores
Dinoflagellates
Cyanobacteria
Two Main Groups
Prokaryotic
Coccoid: Discovered first because of bright orange phycoerythrin fluorescence
Prochlorophytes: Discovered later because of its dim red fluorescence
These two groups make up a major fraction of phytoplankton in ocean
Trichodesmium
N2 fixer and colonial or free-living filaments
Form large blooms and surface aggregationsDia
Diatoms
Have silica dioxide frustule
Single cells or chains of cells
Two Groups of Diatoms
Centric: Radial symmetry and pelagic
Pennate: Bilateral symmetry and benthic
What is a frustule made of
Made of silica and two valves (epitheca and hypotheca)
Raphe: elongated groove in frustule – excretes sticky stuff
Where are diatoms most productive
In areas of upwelling
Diatom sediment
They make silica sediment called diatomaceous earth found on sea bottom
Psuedonitzschia
This is a diatom that forms large toxic blooms off our shores
Domoic acid
Coccolithophores
Unicellular or colonail
External body scales (coccolith) CaCO3
May have flagella
CO2 produced in calcification is used in photosynthesis
Coccolithophore vs Diatom Bloom
Diatom blooms appear very greenish-brown, whereas Coccolithophore blooms are milky white and blue
Coccolithophore affect on climate
They produce dimethyl sulphide (DMS) which helps to form clouds and reduce solar radiation and cool planet
Coccolithophore sediment
Produce white chalky sediment
Calcareous sediments
Dinoflagellates
Unicellular ovoid shape
Can have 2 flagella rotary swimming motion
May have theca made of cellulose plates
Coral symbionts
Nutritional Modes
Obligate Heterotrophs
Facultative Heterotrophs
Obligate Heterotrophs
Protoperidinium
Dinoflagellate with a Sticky veil called pallium to digest prey
Noctiluca scintillans
Naked dinoflagellate uses tentacle to capture prey
Harmful Algal Blooms
Large aggregations of dinoflagellates
Some produce saxitoxin that is harmful to animals
Cause PSP
Bioluminescence
Some dinoflagellates flash in response to mechanical, temperature or chemical stimulation
Burglar Alarm Hypothesis
Dinoflagellates get eaten by Mysids, but when they flash their light they attract cuttlefish which come and eat Mysids
Chl a Absorbtion
Peaks at 450 and 660 nm
How do phytoplankton absorb more wavelenghts
Use accessory pigments to broaden the absorption, but all energy is still directed to Chl a to be used in photosynthesis
Cyanobacteria has most accessory pigments
All phytoplankton will have Chl a
Methods for Measuring Chl a
In Vivo and In Vitro fluorescence
Remote sensing with satellites
In Vivo Fluorescence
Flow through fluorometer towed behind ship, emits blue light, causes Chl a to emit red light and this intensity is measured
In Vitro Fluorescence
Sample seawater from various depths, filter known volume and extract Chl a. Measure in fluorometer
Remote Sensing with Satellites
Convert images of ocean colour into estimates of Chl a
Depth of 5-25 m
Photo-Inhibition
As irradiance increases so does production up to a point then it decreases
Light Saturation
As irradiance increases so does production up to a point then it levels off
Compensation Irradiance
The irradiance where photosynthetic rate is equal to respiration rate
Euphotic Zone
Where there is just enough light for photosynthesis to balance out the respiration rate, also known as the compensation depth
Compensation Light Intensity
Amount of light at the base of euphotic zone
What causes variability in light quality in the ocean? And how do phytoplankton deal with it ?
So as light is absorbed and scattered different wavelengths are left behind. This happens with depth or due to particles or organs. Phytoplankton use accessory pigments to absorb many wavelengths across the PAR.
Station A had k = 0.28 and station B had k = 0.14 with a surface irradiance of 1900 at both. How deep is euphotic zone at each, and why does this make sense.
A: 16.5m
B: 32.93m
Station A is near the coast with lots of FW. More sediment and particles that cause light to attenuate faster
What colour of light are phytoplankton not absorbing? What might this tell you about the colour they appear to our eyes.
They are not absorbing greens and yellows
They reflect these colours so that is why they appear green, yellow or brown to our eyes.
At what time of year would you expect the depth of the surface mixed layer to be the greatest in the middle of the pacific? Why?
In the winter. Because the depth of the mixed layer is controlled by the stability of salinity, temperature and density of the water.
In the winter there is no large FW input to alter the salinity and temperature a great deal, and the ocean temperature is fairly consistent with the lack of sunlight.
Explain the difference between compensation depth and critical depth
The compensation depth is the depth where the amount of light (compensation light intensity) is just enough for photosynthesis to balance respiration. This is the base of the euphotic zone.
The critical depth is where the average light intensity above it is equal to the compensation light intensity
While out on a research cruise in the Subtropical North Atlantic, you conduct a TD cast that includes measurements of light intensity through the water column. You measure a surface irradiance of 450 uE m-2 s-1. From the vertical profile of temperature, you see that the mixed layer depth is 120 meters.
You use the vertical profile of light intensity to calculate a light extinction coefficient of 0.12 m-1 and from bottle incubations for primary productivity determine that the compensation light intensity is 45 umol photons m-2 s-1. Based on these measurements, do you expect there to be a phytoplankton bloom at your sampling location?
We know:
IO = 450
IC = 45
k = 0.12
Dcr = IO/(k*IC)
Dcr = 83.3
The mixed layer is 120 m and the Dcr is 83.3 m so there will be no bloom.
A student in Biological Oceanography went on a field trip to Haro Strait on a cloudy day in early fall. When they arrived on station H3 in Haro Strait, they deployed the CTD and did a full profile of the water column to 150 m. After downloading the data, it showed a surface irradiance of 650 mol photons m2 s1, 50 umol photons m2 s1 at 23 m, and a distinct thermocline at 21 m.
The student completed their sampling of chlorophyll-a biomass and nutrients from depths in the upper 40 m of the water column (0, 5, 10, 20, 30 and 40 m), and then returned to the lab to analyze their samples. Before measuring the chlorophyll concentration, the student remembered reading a paper that provided a range of values for compensation depth in Haro Strait phytoplankton assemblages at the same time of year, with an average of 17.5 m.
1. Explain how you would solve this problem.
2. Should the student expect to see evidence of a bloom in their chlorophyll-a samples?
Justify your answer numerically.
3. Extension: Would there be any differences in biomass between depths? Explain.
What we know:
Io = 650
I23 = 50
Mixed Layer = 21 m
1) Solve for k = ln(Io) - ln(ID) / D
k = 0.11151953728
2) Solve for IC
IC = IO / e^Dc*k
IC = 92.4
3) Solve for Dcr
Dcr = IO / k*IC
Dcr = 63
The MLD is 21m and the Dcr is 63m so we would see a bloom.
