Benthic Nutrient Cycling in Boston Harbor and Massachusetts Bay Anne Giblin, Charles Hopkinson & Jane Tucker The Ecosystems Center, Marine Biological Laboratory Woods Hole, MA 02543 September 22, 1999
Nutrient CyclingBoston 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
Concerns Gas Exchange N2, |O2, CO2 Human Health Contaminants Bacteria Viruses Bioaccumulation Ecological Nutrients Contaminants Organic Material Food Chain Community Structure Living Resources ATMOSPHERE Light WATER COLUMN Outflow Inflow Mammals Planktivorous Fish Sources Rivers Boundary Nonpoint Effluents Piscivorous Fish Phytoplankton Dissolved Zooplankton Demersal Fish Microbes Particulate Detritus Regeneration Epibenthos Microbes N, P, Si, O2, CO2 SEDIMENT Infauna
Nutrient Cycling DIN Fluxes - Southern Harbor 25 d. QB01 NO3 20 -1 d NH4 2 15 10 mmol N m 5 0 -5 Jul-96 Jul-97 Jul-98 Jul-95 Jul-99 Apr-98 Apr-95 Apr-96 Apr-97 Apr-99 Jan-95 Jan-96 Oct-96 Jan-98 Jan-99 Oct-99 Jan-00 Oct-95 Jan-97 Oct-97 Oct-98
Nutrient Cycling 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.
Nutrient Cycling 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.
Nutrient Cycling Harbor Role (cont’d) The ratio of N/Si is greater than 1.0 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 (1993-1995) 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.
Nutrient Cycling 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
Nutrient Cycling Bay Role Sediment fluxes were not measured in Massachusetts Bay during 1998. Previous measurements had shown that benthic respiration rates exhibited low interannual variability, less than 20%. 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 1992-1997 data.