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

GrassLight and Submerged Aquatic Vegetation (SAV)

Predicting SAV Distributions: Grasslight [CO2] Temperature Leaf Area Index as Function of Depth LAI = 0.0286(z) 2 - 1.8768 (z) + 30.151 R = 0.9999 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

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

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

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?

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

Effect of water transparency on phytoplankton productivity

Light Controlled Primary Production (z-t Model)

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)

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

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

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

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

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

Primary Production in the turbid Chester River

Upper Chester River 25 m grid spacing

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

Solution Display Location

Solution at mid-river (near Corsica River)

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

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

Thanks Are there any questions?

Basic Bio-optical Interactions