Benthic Nutrient Cycling in Boston Harbor and ...OMSAP Public Meeting September 1999 Benthic...

OMSAP Public MeetingSeptember 1999

Benthic Nutrient Cycling in BostonHarbor and Massachusetts Bay

Anne Giblin,Charles Hopkinson& Jane TuckerThe Ecosystems Center,Marine Biological LaboratoryWoods Hole, MA 02543

September 22, 1999

OMSAP Public MeetingSeptember 1999

Nutrient Cycling 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

InflowOutflow

Light

ConcernsEcologicalEcological NutrientsNutrients Contaminants Organic MaterialOrganic Material Food Chain Community Structure Living Resources

Human Health Contaminants Bacteria Viruses Bioaccumulation

SEDIMENT

Mammals

Infauna

PiscivorousFish

Zooplankton

Phytoplankton

PlanktivorousFish

Epibenthos

DemersalFish

RegenerationRegeneration

DetritusDetritusParticulateParticulate

Microbes

Dissolved

WATER COLUMN

SourcesSources Rivers BoundaryBoundary Nonpoint EffluentsEffluents

GasExchangeExchangeN2, | O2, CO2

ATMOSPHERE

N, P, N, P, SiSi, O, O22, CO, CO22 MicrobesMicrobes

OMSAP Public MeetingSeptember 1999

Nutrient Cycling Nitrogen Cycle in Coastal Waters

OMSAP Public MeetingSeptember 1999

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QB01 BH08A

Nutrient Cycling Boston Harbor Stations forBenthic Fluxes

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Sediment Oxygen Demand -Northern Harbor

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Sediment Oxygen Demand -Southern Harbor

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling DIN Fluxes - Northern Harbor

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling DIN Fluxes - Southern Harbor

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Denitrification Losses

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Denitrification - Methods Comparisons

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Phosphate Fluxes - NorthernHarbor

OMSAP Public MeetingSeptember 1999

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OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Flux Ratios

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Dissolved Sulfides in Porewaters

OMSAP Public MeetingSeptember 1999

The current four sites are representative of depositionaland reworked areas in the Harbor and therefore mayover-represent the role of the Harbor sedimentssomewhat. Based upon these four stations, we estimatethat the sediments could supply 35% of the N and 58%of the P required for primary production. However, newinputs from the current Deer Island outfall contributemuch more than needed for NPP. The importance ofsedimentary recycled nutrients in supporting NPP willincrease when the outfall moves offshore.

Nutrient Cycling Harbor Role

OMSAP Public MeetingSeptember 1999

Harbor sediments are an active site of denitrification andmore than half of the nitrogen mineralized in thesediments is subsequently denitrified and lost from theecosystem. Although the proportion of nitrogen lost fromthe sediments is high, it is typical of marine sediments.However, because most of the nitrogen entering BostonHarbor are not cycled through the sediments only arelatively minor percentage of the N inputs to BostonHarbor from sewage and other sources is lost bydenitrification. Hence, moving the outfall should nothave a large effect on the N budget of MassachusettsBay as a whole.

Nutrient Cycling Harbor Role (cont’d)

OMSAP Public MeetingSeptember 1999

The ratio of N/Si is greater than 1.0 at most stations atmost times of the year, therefore sediments are releasingnutrients with an N/Si ratio favorable to diatoms.

Interannual oxygen uptake rates continue to be variableat most stations, however, the extremely high ratesobserved in the early part of the study (1993-1995) havenot been repeated. Highest oxygen uptake rates areusually associated with a dense cover of tube buildingamphipods. Although the amphipods continue to bepresent the lower rates we are now observing near theLong Island sludge disposal site suggest some “mining”of sediments organic stores may have taken place.

Nutrient Cycling Harbor Role (cont’d)

OMSAP Public MeetingSeptember 1999

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 withoutfall relocation

Nutrient Cycling Massachusetts Bay – Goals

OMSAP Public MeetingSeptember 1999

Nutrient Cycling Benthic Flux Stations

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Ben thic Flux Stat ionsl

LEGEND

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Sediment Oxygen Demand

OMSAP Public MeetingSeptember 1999

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Nutrient Cycling Sediment Oxygen Demand

OMSAP Public MeetingSeptember 1999

Sediment fluxes were not measured in MassachusettsBay during 1998. Previous measurements had shownthat benthic respiration rates exhibited low interannualvariability, less than 20%. This suggested that anychange due to the outfall relocation would be readilydetectable. Benthic respiration rates measured in 1999,however,have been higher than average, and may reflectgreater carbon loading to the sediments from anunusually large diatom bloom, and warmer than usualbottom water temperatures. October rates will beneeded to determine if this year’s rates would have fallenoutside what was considered normal based upon the1992-1997 data.

Nutrient Cycling Bay Role