Benthic Nutrient Cycling in Boston Harbor and Massachusetts Bay

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1 Benthic Nutrient Cycling in Boston Harbor and Massachusetts Bay Anne Giblin, Charles Hopkinson & Jane Tucker The Ecosystems Center, Marine Biological Laboratory Woods Hole, MA 2543 September 22, 1999

2 Boston Harbor Goals Determine the role of the sediments in nutrient cycling Amount of nutrients released relative to NPP needs Ratios of nutrients released (e.g. N/Si) Size of sink if appropriate (denitrification; N,P, and Si burial) Determine the role of the sediments in oxygen dynamics Importance as an oxygen sink Storage of reduced endproducts Determine how role of the sediments has changed over time Sludge disposal ceased in 1991 Treatment being upgraded to secondary Relocation of outfall to offshore

3 Inflow Concerns Ecological Nutrients Contaminants Organic Material Food Chain Community Structure Living Resources Human Health Contaminants Bacteria Viruses Bioaccumulation Gas Exchange N 2, O 2, CO 2 ATMOSPHERE WATER COLUMN Light Outflow Sources Rivers Boundary Nonpoint Effluents Dissolved Particulate Microbes Planktivorous Fish Phytoplankton Detritus Zooplankton Mammals Piscivorous Fish Demersal Fish Regeneration N, P, Si, O 2, CO, CO 2 Microbes Epibenthos SEDIMENT Infauna

4 Nitrogen Cycle in Coastal Waters

5 Boston Harbor Stations for Benthic Fluxes N $# BH2 BH3 QB1 BH8A 2 4 km

6 Sediment Oxygen Demand - Northern Harbor mmol O 2 m 2 d a. BH2 mmol O2 m 2 d -1 Jan b. BH3 Jan-

7 Sediment Oxygen Demand - Southern Harbor mmol O 2 m 2 d c. BH8A Apr-95 Oct-95 Apr-96 Oct-96 Apr-97 Oct-97 Apr-98 Oct-98 Apr-99 mmol O 2 m 2 d -1 Oct-99 Jan d. QB1 Apr-95 Oct-95 Apr-96 Oct-96 Apr-97 Oct-97 Apr-98 Oct-98 Apr-99 Oct-99 Jan-

8 DIN Fluxes - Northern Harbor mmol N m 2 d a. BH2 NO3 NH4 mmol N m 2 d -1 Jan b. BH3 NO3 NH4-5 Jan-

9 DIN Fluxes - Southern Harbor Apr-95 Oct-95 Apr-96 Oct-96 Apr-97 Oct-97 Apr-98 Oct-98 Apr-99 Oct-99 Jan- mmol N m 2 d -1 c. BH8A NO3 NH4 25 mmol N m 2 d d. QB1 NO3 NH4-5 Apr-95 Oct-95 Apr-96 Oct-96 Apr-97 Oct-97 Apr-98 Oct-98 Apr-99 Oct-99 Jan-

10 Denitrification Losses mmol N m 2 d a. BH2 mmol N m 2 d b. BH3

11 Denitrification - Methods Comparisons a. BH2 Stoichiometry 1. b. BH3 Stoichiometry mmol N m 2 d GC DGA mmol N m 2 d GC DGA May Jun Jul Aug Sep Oct. May Jun Jul Aug Sep Oct c. BH8A Stoichiometry 1. d. QB1 Stoichiometry mmol N m 2 d DGA mmol N m 2 d DGA May Jun Jul Aug Sep Oct May Jun Jul Aug Sep Oct

12 Phosphate Fluxes - Northern Harbor 4. mmol P m -2 d a. BH mmol Pm -2 d b. BH3 4.

13 Phosphate Fluxes - Southern Harbor 4. mmol P m -2 d c. BH8A -1. mmol P m -2 d d. QB Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- 99

14 Flux Ratios a. DIC vs O 2 b. DIC vs DIN DIC (mmol m -2 d -1 ) DIC/O2=1 BH2 BH3 BH8 QB1 DIC (mmol m -2 d -1 ) DIC/DIN = 6.6 BH2 BH3 BH8A QB O 2 (mmol m -2 d -1 ) DIN (mmol m -2 d -1 ) DIN (mmol m -2 d -1 ) DIN/DIP=16 BH2 BH3 BH8A QB1 c. DIN vs DIP DIP (mmol m -2 d -1 ) DIN (mmol m -2 d -1 ) d. DIN vs Si DIN/Si=1 BH2 BH3 BH8A QB Si (mmol m -2 d -1 )

15 Dissolved Sulfides in Porewaters H 2 S (mm) H 2S (mm) a. BH2-2 b. BH3 Depth (cm) July Aug Depth (cm) July Aug Depth (cm) H 2 S (mm) c. BH8A -2 July -4 Aug -6-8 Depth (cm) H 2S (mm) d. QB1-2 July -4 Aug

16 Harbor Role The current four sites are representative of depositional and reworked areas in the Harbor and therefore may over-represent the role of the Harbor sediments somewhat. Based upon these four stations, we estimate that the sediments could supply 35% of the N and 58% of the P required for primary production. However, new inputs from the current Deer Island outfall contribute much more than needed for NPP. The importance of sedimentary recycled nutrients in supporting NPP will increase when the outfall moves offshore.

17 Harbor Role (cont d) Harbor sediments are an active site of denitrification and more than half of the nitrogen mineralized in the sediments is subsequently denitrified and lost from the ecosystem. Although the proportion of nitrogen lost from the sediments is high, it is typical of marine sediments. However, because most of the nitrogen entering Boston Harbor are not cycled through the sediments only a relatively minor percentage of the N inputs to Boston Harbor from sewage and other sources is lost by denitrification. Hence, moving the outfall should not have a large effect on the N budget of Massachusetts Bay as a whole.

18 Harbor Role (cont d) The ratio of N/Si is greater than 1. at most stations at most times of the year, therefore sediments are releasing nutrients with an N/Si ratio favorable to diatoms. Interannual oxygen uptake rates continue to be variable at most stations, however, the extremely high rates observed in the early part of the study ( ) have not been repeated. Highest oxygen uptake rates are usually associated with a dense cover of tube building amphipods. Although the amphipods continue to be present the lower rates we are now observing near the Long Island sludge disposal site suggest some mining of sediments organic stores may have taken place.

19 Massachusetts Bay Goals Determine the role of the sediments in nutrient cycling Amount of nutrients released relative to NPP needs Ratios of nutrients released (e.g. N/Si) Size of sink if appropriate (denitrification; N,P, and Si burial) Determine the role of the sediments in oxygen dynamics Importance as an oxygen sink Storage of reduced endproducts Determine patterns of annual and interannual variability Determine how the role of the sediments changes with outfall relocation

20 Benthic Flux Stations 71 ' 7 56' 7 52' 7 48' 7 44' 7 4' MB1 MB2 MB5 BH2 BH3 MB3 QB1 BH8A 42 16' 42 28' 42 32' 42 32' 42 28' 42 24' 42 24' 42 2' 42 2' 42 16' Kilometer s 42 12' LEGEND Benthic Flux Stations N 42 12' 71 ' 7 56' 7 52' 7 48' 7 44' 7 4'

21 Sediment Oxygen Demand 35 O2 (mmol m -2 d -1 ) MB1 Jan- 35 O2 (mmol m -2 d -1 ) MB2 Jan-

22 Sediment Oxygen Demand O2 (mmol m -2 d -1 ) MB3 O2 (mmol m -2 d -1 ) Jan MB5 Jan-

23 Bay Role Sediment fluxes were not measured in Massachusetts Bay during Previous measurements had shown that benthic respiration rates exhibited low interannual variability, less than 2%. This suggested that any change due to the outfall relocation would be readily detectable. Benthic respiration rates measured in 1999, however,have been higher than average, and may reflect greater carbon loading to the sediments from an unusually large diatom bloom, and warmer than usual bottom water temperatures. October rates will be needed to determine if this year s rates would have fallen outside what was considered normal based upon the data.