Chesapeake Bay Environmental Model Package

• A coupled system of watershed, hydrodynamic and eutrophication models

• The same package used for the 2002 load allocations

• The same package used for the 2004 native oyster study

RegionalAtmosphericDeposition

Model

BenthosComponent

WatershedModel

HydrodynamicModel

EutrophicationModel

SAVComponentThe CBEMP circa

1999

Particulate Organic Matter

Dissolved Nutrients

Dissolved Oxygen

FilterFeeders

Particulate Organic Matter

DepositFeeders

Dissolved Nutrients

OxygenDemand

respiration

filt

rati

on

se

ttli

ng

bio

de

po

sit

s

fee

din

g

se

dim

en

t-w

ate

r

e

xc

ha

ng

e

excretion

se

dim

en

t-o

xy

ge

n

de

ma

nd

respiration

excretion

diagenesis

diagenesis

WaterColumn

Sediments

Diagenesis Model with Benthos

Key Assumptions and Properties

• The model is run for 10 years, 1985-1994, on a grid of 3000 surface elements (~4 km2) using time steps of 15 minutes

• Oysters are restricted to their historical spatial distribution

• The model is parameterized for Chesapeake Bay native oysters

Key Assumptions and Properties

• A spatially-uniform mortality rate is specified that combines effects of predation, disease, and harvest

• Oyster biomass is dynamically computed based on local conditions including food availability, salinity, dissolved oxygen, and suspended solids

Fundamental Equation

FFhmrFFFFrFFPOCFrtd

FFd 2

FF = filter feeder biomass (mg C m-2)α = assimilation efficiency (0 < α < 1)Fr = filtration rate (m3 mg-1 filter feeder carbon d-1)POC = particulate organic carbon in overlying water (mg m-3)r = specific respiration rate (d-1)β = predation rate (m2 mg-1 filter feeder C d-1)hmr = mortality rate due to hypoxia (d-1)t = time (d)

Modeled Effect of Temperature on Filtration

Modeled Effect of Solids on Filtration

From Jordan

Model

Particulate Carbon Budget

Dissolved Oxygen Budget

Particulate Nitrogen Budget

Dissolved Nitrogen Budget

Particulate Phosphorus Budget

Dissolved Phosphorus Budget

Filtration Rates

Filtration Rate Source Commentsm3/g oyster C/d

0.22 Jordan's thesis mean value T > 20C. 0.26 Newell&Koch Average of measures at 20 and 25 C

0.027 to 0.33 Epifanio & Ewart For algal suspension greater than 1 mg C/L0.27 Riisgard Computed for a 2.1 g DW oyster at 27 to 29 C

0.455 Model, CB4 Summer average0.238 Model, Choptank Deep Summer average0.202 Model, Choptank Shallow Summer average0.238 Model, ET9 Summer average

Carbon Deposition

Carbon Deposition Source Commentsg C/g Oyster C/d

0.099 Jordan's thesis mean value T > 20C. 0.03 Haven & Morales see havens_ingestion spreadsheet

0.002 to 0.012 Tenore & Dunstan depends on C concentration, range is 0.1 to 0.7 mg/L0.050 Model, CB4 Summer average0.088 Model, Choptank Deep Summer average0.118 Model, Choptank Shallow Summer average0.096 Model, ET9 Summer average

Respiration

Respiration Source Commentsg DO/g oyster C/d

0.03 to 0.06 Boucher & Boucher-Rodini spring-summer rates, N excretion includes urea0.017 Dame, Spurrier, Zingmark Annual average0.02 Dame (ecological efficiencies) 1 g DW oyster at 20 to 30 C

0.042 Model, CB4 Summer average0.040 Model, Choptank Deep Summer average0.041 Model, Choptank Shallow Summer average0.041 Model, ET9 Summer average

Ammonium Excretion

NH4 excretion Source Commentsmg N/g oyster C/d

< 0.1 Hammen et al. NH4 + urea2.8 to 3.88 Boucher & Boucher-Rodini spring-summer rates, N excretion includes urea

0.8 Srna & Baggaley NH4 only, 1 g oyster at 20 C4.8 to 7.9 Magni et al Ruditapes and Musculista

2.20 Model, CB4 Summer average1.43 Model, Choptank Deep Summer average1.52 Model, Choptank Shallow Summer average2.05 Model, ET9 Summer average

Seasonal Variation in Oyster Density

Annual Variation in Autumn Oyster Density

Mg C/sq m

Conclusions

• Our results are consistent with alternate investigations including Officer et al. 1982, Gerritsen et al. 1994, and Newell&Koch 2004.

• The greatest ecosystem service of feasible oyster restoration appears to be SAV restoration.

• Other ecosystem services provided by oysters include nitrogen removal and dissolved oxygen enhancement.

Conclusions

• Oysters have larger impact on their local environment than system-wide

• We recommend restoration target specific areas with suitable environments. Look for improvements on similar scales.

Criteria for benthic control of phytoplankton (Officer et al. 1982)

• Shallow water depths (2 to 10m)

• A large and widespread population as opposed to more localized regions

• Partially-enclosed regions with poor hydrodynamic exchange with adjacent water bodies

Suspension-feeding bivalve model … applied to Chesapeake Bay (Gerritsen et al. 1994)

• Existing bivalves consume more than 50% of primary production in shallow freshwater and oligohaline reaches

• In deep mesohaline portions, bivalves consume only 10% of primary production

• Use of bivalves to improve water quality of large estuaries will be limited by the depth and width of the estuary

Modeling seagrass density in response to … bivalve filtration (Newell & Koch)

• The presence of modest levels of oysters (< 12 g C/sq m) reduced suspended sediment concentrations by an order of magnitude