A biogeochemical model for the northwestern Atlantic continental shelf Katja Fennel & John Wilkin Thanks also to: Hernan Arrango, Julia Levin, Dale Haidvogel, Sybil Seitzinger Institute of Marine and Coastal Sciences Rutgers University NASA IDS PARADIGM

Objectives examine biogeochemical cycling of carbon, nitrogen, oxygen and other essential elements at ocean margins and the land-ocean interfaceexamine biogeochemical cycling of carbon, nitrogen, oxygen and other essential elements at ocean margins and the land-ocean interface investigate the fate of inorganic and organic matter inputs from landinvestigate the fate of inorganic and organic matter inputs from land assess the effects of anthropogenic perturbations and natural variability on the coastal ecosystemsassess the effects of anthropogenic perturbations and natural variability on the coastal ecosystems

Continental shelves High productivityHigh productivity Most burial on shelves and slopeMost burial on shelves and slope Basis for world’s fisheriesBasis for world’s fisheries Vulnerable to anthropogenic nutrient loadingsVulnerable to anthropogenic nutrient loadings Shelf processes widely ignored in basin-wide and global modelsShelf processes widely ignored in basin-wide and global models

Nested physical- biological model

North Atlantic ROMS Climatological heat/freshwater fluxes 3-day average NCEP winds SSH (m)

Initial and boundary conditions for biological variables: Nitrate: NODC data from 30 o – 50 o N and 60 o – 80 o WNODC data from 30 o – 50 o N and 60 o – 80 o W Temperature and Salinity approximations for water masses of Shelf and Slope/Open Ocean separatelyTemperature and Salinity approximations for water masses of Shelf and Slope/Open Ocean separatelyOthers: Small positive values of ~0.1 mmol N m -3Small positive values of ~0.1 mmol N m -3 Slope/openocean Shelf water NODC nitrate data in T-S diagram

Initial and boundary conditions for biological variables: Nitrate: NODC data from 30 o – 50 o N and 60 o – 80 o WNODC data from 30 o – 50 o N and 60 o – 80 o W Temperature approximations for water masses of Shelf and Slope/Open Ocean separatelyTemperature approximations for water masses of Shelf and Slope/Open Ocean separatelyOthers: Small positive values of ~0.1 mmol N m -3Small positive values of ~0.1 mmol N m -3 Slope/openocean Shelf water

NO 3 Chlorophyll Large detritus Organic matter N2N2 NH 4 NO 3 Water column Sediment Phytoplankton NH 4 Mineralization Uptake Nitrification Grazing Mortality Zooplankton Susp. particles Aerobic mineralization Denitrification

Sediment remineralization Org. matter reaching the bottom is remineralized instantaneouslyOrg. matter reaching the bottom is remineralized instantaneously Remineralization is distributed between oxic and anoxic pathwayRemineralization is distributed between oxic and anoxic pathway Denitrification is related to carbon oxidation (measured as sediment oxygen uptake)Denitrification is related to carbon oxidation (measured as sediment oxygen uptake) 1 mol org. matter ~ 4 mol NH 4, 6 mol N 21 mol org. matter ~ 4 mol NH 4, 6 mol N 2 From Seitzinger & Giblin (1996)

Mean annual PP in MAB: 200 gC /m 2 /y 200 gC /m 2 /y Observational estimates: 290 gC /m 2 /y in New York Bight (Malone et al. 1983) 290 gC /m 2 /y in New York Bight (Malone et al. 1983) 310 gC /m 2 /y in MAB 310 gC /m 2 /y in MAB (O’Reilly and Busch 1984) Boundary effect

Middle Atlantic Bight (MAB) Cape Hatteras Hudson Delaware Chesapeake Georges Bank Nantucket Shoals 50, 100, 200, m isobaths: dashed lines

Rundenitrification riverine nutrients/PON Anono Byesno Cyesyes 2-yr simulations, starting from same initial condition in January 1994 after a 1-yr spinup

A 227 gC /m2 /y B 184 gC /m2 /y 20% reduction from A C 200 gC /m2 /y 10% increase from B no DNF; no river N no river N

Mean annual denitrification flux in MAB: 1.1 mmol N /m2 /d Observational estimates for North Atlantic Shelves: 0.7 mmol N /m2 /d Nova Scotia to Cape Hatteras: 1.4 mmol N /m2 /d (Seitzinger and Giblin 1996)

DNF: 5.3 TN Rivers: 1.8 TN denitrification removes 90% of all N entering MABdenitrification removes 90% of all N entering MAB 0.4 TN 4.2 TN x mol N y -1 Fennel et al. (submitted to Global Biogeochemical Cycles)

3.3 DIN 0.9 PON 2.9 PON 2.5 DIN DNF: 5.3 TN Rivers: 1.8 TN cross-isobath export of PON, import of DINcross-isobath export of PON, import of DIN x mol N y TN 4.2 TN denitrification removes 90% of all N entering MAB denitrification removes 90% of all N entering MAB Fennel et al. (submitted to Global Biogeochemical Cycles)

From Seitzinger and Giblin (1996)

N* = N – 16 P (Gruber & Sarmiento 1997) N = N 2 fixation Denitrification

Extrapolation of DNF rate to North Atlantic shelf area: 2.3 x10 12 mol N y x10 12 mol N y -1 North Atlantic Nitrogen fixation: 3.7 – 6.4 x10 12 mol N y -1 (Michaels et al. 1996) 3.7 – 6.4 x10 12 mol N y -1 (Michaels et al. 1996) 2.3 x10 12 mol N y -1 (Gruber & Sarmiento 1997) 0.3 x10 12 mol N y -1 (Hansell et al. 2004) Estimated shelf denitrification flux is 35 – 700% of estimated N 2 fixation flux. Is Denitrification on Continental Shelves Significant in North Atlantic N-cycle?

Assume: N supply to MAB is matched by C supply in Redfield ratio; N loss due to denitrification is matched by outgassing of CO 2 to atmosphere 5.3 x10 10 mol N y -1 ~= 4.2 x10 12 gC y -1 Extrapolated to North Atlantic shelf area: 0.18 PgC y -1 30% of the North Atlantic C uptake -0.6 PgC y -1 (Takahashi et al. 2002) Implications for C-cycle

Outlook Also available:Also available: –Carbon and Oxygen chemistry –More complex ecosystem (Lima and Doney 2004) In the future:In the future: –Dynamic sediment component –Data assimilation

Courtesy: Jay O’Reilly

SST SST Span Span ( Annual Range in monthly mean SST) Courtesy: Jay O’Reilly

Taylor, K.E. JGR 2001

Total time-space statistics for test field p and reference field q:

Spatial annual statistics:

Spatial-monthly statistics with skill metric: Statistics like spatial-annual but for each month Skill metric (0<=S<=1, S→1 as p approaches q)

Log(SeaWiFS Chl) mean and variance for the MAB