1. Light and Primary Production in Shallow, Turbid Tributaries
John Klinck, Richard Zimmerman,
Victoria Hill and Mike Dinniman
Old Dominion University
OUTLINE
GrassLight BioOptical SAV model
Optical-Primary Production model
Upper Chester River simulation

2. GrassLight and Submerged Aquatic Vegetation (SAV)

3. Predicting SAV Distributions: Grasslight
[CO2]
Temperature
Leaf Area Index as
Function of Depth
LAI = (z) 2
(z) R = 4 6 8 10 12 14 16 18 20 40
lai (m m -2 )
Depth (m)
E(l,z)
Water Quality:
Ed(l,z) =exp[-Kd(l)z]
Kd(l) = f(aCDOM,[Chl a],TSM)
+ Bathymetry
Light Limited Distribution

4. Applying GrassLight to the Chester River
Mesohaline tributary
Highly turbid
TSM » 30 mg L-1
Eutrophic
Chl a » 20 mg m-3
Gridded 30 m bathymetry
Potential SAV habitat (< 3 m depth) fringing the shore

5. Applying GrassLight to the Chester River
GrassLight prediction of SAV density based on average WQ data is consistent with VIMS field observations
TSM = 30 mg L-1
Chl a = 20 mg m-3
zE(22%) = 0.2 m
zE(13%) = 0.3 m
zE(1%) = 0.8 m

6. Applying GrassLight to the Chester River
Improving water quality to average for Sandy Point
TSM = 10 mg L-1
Chl a = 10 mg m-3
zE(22%) = 0.7 m
zE(13%) = 0.9 m
SAV distribution expands
Still below ‘historic” distribution limit of 3 m
Euphotic depth zE(1%) = 2 m
So, what about the phytoplankton?

7. Turbidity has a greater effect on the euphotic depth than Chl a

8. Effect of water transparency on phytoplankton productivity

9. Light Controlled Primary Production (z-t Model)

10. Basic Bio-optical Interactions
Spectral light based on GrassLight (1 nm resolution)
Gregg and Carder (1990) atmosphere light
Single species primary producer with internal pools of carbon, nitrogen and phosphorus
Nitrate, Ammonia and Phosphate nutrients
Light controls primary production ( with nutrient limitation)
Nutrient absorption by Redfield ratios to chlorophyll production
Respiration > fixation releases nutrients
Non-cohesive sediment (suspended and benthic)

11. Modeling photosynthesis
PBg(z) is controlled by light availability:
f P – quantum yield of photosynthesis (=1/8)
A *f (l ) – spectral phytoplankton absorptance
[Chl a] – biomass, to scale absorptance
E(l,t,z) – wavelength, time and depth-dependent irradiance
- light limited photosynthesis m m

12. 4 day simulation Initial values: Chla = 10 mg/m3 NO3 = 5 m mol/m3
NH4 = 10 m mol/m3
PO4 = 1 m mol/m3
TSS = 20 g/m3
PUR = photosynthetically utilized radiation

13. Near surface growth (above 1 m)
Vertical mixing provides nutrients
Chla removed at night by mortality (grazing)
Night respiration releases nutrients

14. Nutrients Growth uses NH4 Night respiration releases some nutrients
Surface PO4 and NH4 limit growth

15. Physical Effect of Optical Characteristics
Shallow water allows subsurface heating by short wave
T increase balances S effect on density
Vertical mixing on day 3 due to subsurface heating

16. Primary Production in the turbid Chester River

17. Upper Chester River
25 m grid spacing

18. Chester River Depth extracted from 25 m DEM
Initial constant T, linearly declining S
Imposed (constant) temperature and salinity at entrance
Winds and surface heat flux
No precipitation or runoff
M2 tide variation
4 day simulation

19. Solution Display Location

20. Solution at mid-river
(near Corsica River)

21. Surface and Bottom Chlorophyll (mg/m3)
Shallow tributary (Corsica River) is productive
Shallow river flanks are productive
Vertical and lateral nutrient fluxes fuel primary production

22. Conclusions
Turbidity (mainly) and chlorophyll (somewhat) control light in turbid tributaries
Light (mainly) controls primary production and nutrient uptake in tributaries
Turbidity reduction may be more important than reduction in nutrient loading to control eutrophication

23. Thanks Are there any questions?

24. Basic Bio-optical Interactions