1. PRIMARY PRODUCTIVITY
Productivity is the rate of biomass formation
Primary productivity (photosynthesis) of phytoplankton can be measured directly by O2 production
Photosynthesis can be summarized as
6CO H20  ---->  C6H12O6 + 6O2 + 6H2O
Respiration is the reverse reaction
The most common techniques for PP measurement are:
the light-dark bottles technique for O2 production
14C uptake measurements of CO2 assimilation
harvest method
Whole water column O2 or CO2 flux

2. Primary Producers in Aquatic Ecosystems
Cyanobacteria, Protista (microalgae, macroalgae), Aquatic plants
These groups are all found in both standing and flowing water
In standing water the two main primary producer communities are:
a.) the phytoplankton
(suspended cells or colonies of microalgae or cyanobacteria)
b.) the littoral community
(benthic micro- or macroalgae or cyanobacteria,
epiphytic microalgae, and
macophytes (macroalgae or plants)-emergent/submerged
In running water the two main primary producer communities are:
a.) benthic algae attached to rocks (generally in fast current)
b.) submerged macrophytes & associated epiphytes—in areas of slow current

3. Afternoon
Nightime
Depth m 2 4 6 8 10
mg/L O2 5
Depth m 2 4 6 8 10
mg/L O2 5
Average [O2]
Epilimnion =
9.8 mg/L
Average [O2]
Epilimnion =
6.9 mg/L

4. Van Dorn sampler obtains samples of water from a desired depth

5. A Yellow Springs Instrument DO meter being calibrated

6. Changes in Dissolved Oxygen
Water samples from various depths are enclosed in light (transparent) and dark (completely opaque) bottles,
For each depth initial readings of dissolved O2 are taken (IB) and
the light (LB) and dark (DB) samples are incubated for a period long enough to produce measurable changes in O2.
During the incubation, we expect that the initial DO concentration (IB) at a given depth will decrease to a lower concentration in the dark bottles (DB) due to respiration of phytoplankton.
Conversely, we expect light bottle (LB) should increase from their initial values (IB).

8. For this we use the photosynthetic quotient (PQ)
Productivity measurements are usually expressed in terms of carbon not oxygen
Therefore we need to convert the changes in oxygen concentration to corresponding changes in carbon.
For this we use the photosynthetic quotient (PQ)
and the respiratory quotient (RQ), which are ratios describing the relative amounts of oxygen and carbon involved in photosynthesis and respiration. :
PQ = (molecules of oxygen liberated during photosynthesis) / (molecules of CO2 assimilated)= usually around 1.2
      
RQ = (molecules of CO2 liberated during respiration) / (molecules of oxygen consumed) = usually very close to 1.0
     

9. Calculations
Gross photosynthesis = [(LB - DB) * 1000 * 0.375] / (PQ *D t)
Net photosynthesis = [(LB - IB) * 1000 * 0.375] / (PQ * D t)
Respiration = [(IB - DB) * RQ * 1000 * 0.375] / D t
Gross and net photosynthesis and respiration are expressed as mg C/m3/t
LB, DB, and IB are dissolved oxygen concentrations in mg/L
D t is the incubation period eg. hours
1000 converts L to m3 (1 L = 1000 cm3)
0.375 converts mass of oxygen to mass of carbon and is a ratio of moles of carbon to moles of oxygen (12 mg C/32 mg O2 = 0.375)

10. Because we are using bottles incubated at various depths in the photic zone to measure primary productivity in situ, and the measurements vary with depth, we need to obtain a profile of primary productivity with depth.
% of surface irradiance
Primary productivity
(mgC/m3/d)
Depth (z)
Let us say that we wish to incubate samples at the 75% , 50%, 25%, 10% and 1% light levels. How do we decide at what depths to incubate samples at?

11. The Secchi disk—a simple way to estimate light extinction

12. Light extinction --Light enters from above and its intensity (I) is sharply attenuated with depth (z)—absorption by water or solute molecules or scattered by particles
Section 10.6 Iz
z 50%
Photic
zone
z 10%
z 1% z
Page 144 in text

13. For a lake with Secchi disk transparency of 3m k=0.57 (1.7/3)
%light
ln(%light/100)
z=-ln (%light/100)/k (k=0.57)
0.75
-0.29
0.50 m
0.5
-0.69
1.22 m
0.25
-1.39
2.43 m
0.1
-2.30
4.04 m
0.01
-4.61
8.08 m

14. If we place dark and light bottles at each of these depths, and calculate the Gross Primary productivity at each
GPPz
gC/m3/d
0.5
1.0
1.5
z 75% x
z 50% x
Photic
zone
Find the area under the GPP vertical profile
Its units will be gC/m2/d, Why? x
z 10% x m
z 1% x z

15. If we place dark and light bottles at each of these depths, and calculate the Gross Primary productivity at each
Find the area under the GPP vertical profile
Its units will be gC/m2/d, Why?
GPPz x
1.0
0.5
1.5
gC/m3/d m
3.0
5.0
7.0
0.45 sq in B
Photic
zone
Area B represents
0.5 gC/m3/d * 2 m = 1.0 gC/m2/d A
Area A represents
[4.10/0.45] *1.0 gC/m2/d=9.1 gC/m2/d
Average productivity per unit volume
9.1gC/m2/d / 9.1 m = 1.0 gC/m3/d
If you average all the GPP estimates
1.2,1.65,1.70, 1.1,0.22,
=1.174 gC/m3/d x 9.1 m = 10.7 gC/m2/d
4.10 sq in z
Averaging the readings leads to a nearly 20% overestimate, Why?

16. Primary producers differ in their photosynthetic responses to light intensity

17. A B Comparing the Photosynthethis/irradiance curves among species C
GPP/t
per unit of biomass A B C
Light intensity
10%
25%
50%

18. Assuming that A, B and C are similar in all respects other than their P/I curves, which of these species would you expect to perform best in the well mixed water column of a deep lake (25% light level at 10 m, max depth 100 m)
a) A
b) B
c) C
d) A and B would do equally well
e) A and C would do equally well
GPP/t
per unit of biomass A B C
Light intensity
10%
25%
50%

19. Assuming that A and B are equal in all respects other than their P/I curves, under what conditions would you expect B to outperform A?
a) in the epilimnion of a clear stratified lake, (Assume 25% light level at the thermocline)
b) in the well-mixed water column of a deep lake (Assume 25% light level at 10m, max depth 100 m)
c) in the hypolimnion of a clear stratified lake (Assume 25% light level at the thermocline)
d) growing on the substrate near shore in the littoral zone of a clear lake (assume 25 % light level at the outer boundary of the littoral)
e) both a and d
f) both b and c
GPP/t
per unit of biomass A B C
Light intensity
10%
25%
50%
Could C ever be expected to dominate?

20. The seasonal dyanamics of the phytoplankton in lakes
Temperature adaptations of different algal groups
Thermal stratification, sinking rates and nutrient dynamics
Food-web interaction—effects of grazing zooplankton
mid-summer low biomass
community shifts to inedible forms

21. Early spring—diatoms dominate--under cold temperatures and low light conditions
plenty of nutrients in the well mixed water column
Summer—lake warms up, thermocline forms diatoms fall out of the mixing layer—low viscosity and low mixing depth
Asterionella the only diatom that can still hang in.
Mid-summer—nutrients lost from mixed layer (sedimentation of algae), warm temperatures favour green algae, and zooplankton herbivory is high favouring fast growing small species eg Chlorella
Late summer—herbivores eliminate edible species, large colonial cyanobacteria dominate eg. Microcystis
Fall—water cooling, thermocline breaks up, mixing depth increases, nutrients increase, diatoms dominate
Winter—low light and cold temp low biomass

22. Cyclotella:centric diatom (around 20 microns) and Stephanodiscus (around 50 microns)
Usually abundant in spring plankton

23. Asterionella
colonial diatom—elongate cells joined at the base to form stellate colonies
Fairly large for planktonic diatoms, each cell microns
Commonly found in dense blooms during May, prior to the onset of thermal stratification

24. Chlorella: a unicellular non-flagellated green algae

25. Lake Zooplankton
Rotifers
Copepods
Cladocera
Copepod
Larvae--Nauplius

26. Anabaena: cells in large colonies (filaments) with no gelatinous matrix, coiled or straight, heterocysts and akinetes usually present, cells 3-5 microns

27. Microcystis: cells in large colonies irregularly arranged within a gelatinous matrix
Colony of Microcystis
A common bloom forming cynabacterium that sometimes can be highly toxic
Colonial growth pattern—cells embedded in a gelatinous matrix
(3b)

28. Adaptations of benthic diatoms to life in streams
They can live either as solitary cells, usually raphed—capable of slow gliding movement on rocks
or in chains joined at the valvular surface, attached to substrate either by mucilage pad or by a stalk

29. Why is this life cycle be advantageous for life in streams?
Achnanthes longipes: alternation between a motile, solitary phase and a stalked sessile phase
When inoculated into fresh media, the cells are at first motile (I), then become sessile and produce a stalk that anchors them to the substratum (II). This stalk continues to be synthesized as the cell is pushed away from the substratum (III), and eventually mitosis occurs producing a row of cells stacked one upon the other (IV). These cells eventually detach from one another (I) and begin the cycle again.
Why is this life cycle be advantageous for life in streams?

30. Daily fluctuations in O2 provides and integrated measure of community metabolism over a reach.

31. O2 mg/L A B C
Day
Night
Which one of these curves is from a treated sewage effluent and which is from an untreated sewage effluent?

32. You find that C outperforms both A and B during the summer months but not in the early spring. Assuming that all three species have similar temperature optima and nutrient uptake affinities, which of the following explanations is most plausible?
C is the least palatable species to herbivorous zooplankton
b) B does best at low light intensity, and A does best at high light intensity, but C does best under fluctuating light intensities
c) C has the most eccentric shape
d) a and b are both plausible
e) a and c are both plausible
GPP/t
per unit of biomass A B C
Light intensity
10%
25%
50%