James T. Hollibaugh

Coastal metabolism and the oceanic organic carbon balance

Net organic metabolism (that is, the difference between primary production and respiration of organic matter) in the coastal ocean may be a significant term in the oceanic carbon budget. Historical change in the rate of this net metabolism determines the importance of the coastal ocean relative to anthropogenic perturbations of the global carbon cycle. Consideration of long-term rates of river loading of organic carbon, organic burial, chemical reactivity of land-derived organic matter, and rates of community metabolism in the coastal zone leads us to estimate that the coastal zone oxidizes about 7 × 1012 moles C/yr. The open ocean is apparently also a site of net organic oxidation (∼16 × 1012 moles C/yr). Thus organic metabolism in the ocean appears to be a source of CO2 release to the atmosphere rather than being a sink for atmospheric carbon dioxide. The small area of the coastal ocean accounts for about 30% of the net oceanic oxidation. Oxidation in the coastal zone (especially in bays and estuaries) takes on particular importance, because the input rate is likely to have been altered substantially by human activities on land.

Influence of Sulfide Inhibition of Nitrification on Nitrogen Regeneration in Sediments

Nitrification, a central process in the nitrogen cycle, converts ammonium to nitrite or nitrate. In experiments with estuarine sediment, addition of 60 and 100 μM hydrogen sulfide (HS−) reduced nitrification by 50 and 100 percent, respectively. Aerobic incubation of ammonium-enriched sediment slurries showed that previous HS− exposure reduced nitrification for at least 24 hours; nitrification rates recovered slowly after one-time HS− exposure. Sulfide inhibition of nitrification could limit nitrogen loss through coupled nitrification-denitrification and may contribute to the previously observed difference in net nitrogen cycling between freshwater and marine sediments. This interaction could also exacerbate eutrophication in coastal environments.

ANNUAL CYCLE AND INTERANNUAL VARIABILITY OF ECOSYSTEM METABOLISM IN A TEMPERATE CLIMATE EMBAYMENT

We have studied the net and gross metabolism of Tomales Bay, a temperate climate estuary in northern California. Tomales Bay has proved to be heterotrophic, implying that the bay oxidizes a subsidy of organic carbon from outside the system, in excess of inorganic nutrients supplied to it from outside and in addition to material cycling within it. Net organic oxidation releases dissolved inorganic nutrients, and the system exports these dissolved inorganic products. Dissolved inorganic phosphorus is exported to the ocean via mixing and constitutes the most direct record of net ecosystem production (NEP). Excess dissolved inorganic nitrogen is lost to denitrification. Excess dissolved inorganic carbon largely results in alkalinity elevation and hydrographic export of alkalinity due to sulfate reduction. The negative NEP of this system results in little release of CO2 to the atmosphere, because of this alkalinity elevation. A major purpose of the study was to ascertain the relative importance of various sour...

Tomales bay metabolism: CNP stoichiometry and ecosystem heterotrophy at the land-sea interface

Bays and estuaries receive inputs from the adjacent land and exchange materials with the coastal ocean. Net system metabolism records the role of such land-sea interface ecosystems in altering the chemical state of essential plant nutrients. Water and salt budgets have been constructed for Tomales Bay, California, in order to estimate water advective and mixing exchange rates between the bay and coastal ocean over 2 years. Net non-conservative fluxes of dissolved P, N and C were calculated. The bay is a net source for dissolved P, C and total alkalinity, and is a net sink for dissolved fixed N. Stoichiometric analysis can be used to interpret the non-conservative nutrient fluxes. The dissolved P source is interpreted to be the result of net oxidation of approximately 10 mmol organic C m−2 day−1. CO2 released by this oxidation largely escapes to the atmosphere. This net respiration is about 12% of the total system respiration. Most dissolved inorganic N liberated during the oxidation is not exported hydrographically; rather, the bay is a net sink of dissolved fixed N. This N is assumed to be lost from the water via denitrification (3·2 mmol m−2 day−1). Sulphate reduction (5·1 mmol m−2 day−1) is also an important component of respiration in this system. Denitrification is a minor component of gross C metabolism, but is the major sink of N in this system. The organic matter driving this net heterotrophy may be of either terrestrial or marine origin.

Estimating denitrification rates in estuarine sediments : A comparison of stoichiometric and acetylene based methods

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrient stoichiometry in the subtidal sediments of Tomales Bay, California (USA). By amending whole cores with acetylene and saturating nitrate concentrations, we obtained potential denitrification rates, which ranged between 4 and 30 mmol N m−2 d−1. We determined the apparent Michaelis constant (Kapp) and the maximum potential rate (Vmp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the Kapp and Vmp of the denitrifying community exhibited significant variation over both depth in the sediment column and time of sampling.Estimates ofin situ denitrification obtained using our ‘kinetic-fix’ adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m−1 d−1. Denitrification rates obtained using benthic flux stoichiometry ranged between 0.7 and 4.1 mmol N m−2 d−1. Average denitrification rates obtained using the ‘kinetic-fix’ acetylene block approach exceeded those obtained from net benthic flux stoichiometry; however, these differences were not significant. We conclude that our ‘kinetic-fix’ adaptation of the acetylene block technique provides realistic estimates of denitrification in sediments, even when pore water nitrate concentrations are low and nitrification and denitrification are closely coupled.

Stoichiometry of C, N, P, and Si fluxes in a temperate-climate embayment

The research was sponsored by NSF Grant Nos. OCE 85-00679 (awarded to SVS) and OCE 82-14837 (awarded to JTH).

Importance of terrestrially-derived, particulate phosphorus to phosphorus dynamics in a west coast estuary

Allochthonous inputs of suspended particulate matter from freshwater environments to estuaries influence nutrient cycling and ecosystem metabolism. Contributions of different biogeochemical reactions to phosphorus dynamics in Tomales Bay, California, were determined by measuring dissolved inorganic phosphorus exchange between water and suspended particulate matter in response to changes in salinity, pH, and sediment redox. In serum bottle incubations of suspended particulate matter collected from the major tributary to the bay, dissolved inorganic phosphorus release increased with salinity during the initial 8 h; between 1–3 d, however, rates of release were similar among treatments of 0 psu, 16 psu, 24 psu, and 32 psu. Release was variable over the pH range 4–8.5, but dissolved inorganic phosphorus releases from sediments incubated for 24 h at the pH of fresh water (7.3) and seawater (8.1) were similarly small. Under oxidizing conditions, dissolved inorganic phosphorus release was small or dissolved inorganic phosphorus was taken up by particulate matter with total P content <50 μmoles P g−1; release was greater from suspended particulate matter with total phosphorus content >50 μmoles P g−1. In contrast, under reducing conditions maintained by addition of free sulfide (HS−), dissolved inorganic phosphorus was released from particles at all concentrations of total phosphorus in suspended particulate matter, presumably from the reduction of iron oxides. Since extrapolated dissolved inorganic phosphorus release from this abiotic source can account for only 12.5% of the total dissolved inorganic phosphorus flux from Tomales Bay sediments, we conclude most release from particles is due to organic matter oxidation that occurs after estuarine deposition. The abiotic, sedimentary flux of dissolved inorganic phosphorus, however, could contribute up to 30% of the observed net export of dissolved inorganic phosphorus from the entire estuary.

Dissolved and particulate nutrient transport through a coastal watershed-estuary system

Abstract Tomales Bay and its adjacent watershed are the location of integrated research on the CNPSi biogeochemical coupling between the land and coastal ocean and cycling of these materials within the bay. In the present paper, budgets have been constructed to describe the rainfall delivery of dissolved nutrients to the watershed and export of dissolved and particulate nutrients from the watershed, mostly in runoff. The quantity of dissolved materials, especially dissolved organic materials, delivered to the watershed by rainfall is about the same as the export. Suspended load transport represents the major net removal of C, N, and P from the watershed, and this flux shows large interannual variation. Runoff adjusted particle flux from the watershed is small at present in comparison with estimates based on sedimentation rate in the bay over the past 130 years. This difference apparently cannot be explained by natural or managed interannual variation in runoff or by other obvious aspects of water management. We believe that changes in agricultural land use have led to recent decreases in erosion and removal of particulate materials from the watershed. Even though the watershed has been disturbed by agricultural practices over the past 130 years, the system as a whole appears largely to have recovered to steady-state conditions.

Sulfate reduction and sediment metabolism in Tomales Bay, California

Sulfate reduction rates (SRR) in subtidal sediments of Tomales Bay, California, were variable by sediment type, season and depth. Higher rates were measured in near-surface muds during summer (up to 45 nmol cm-3 h-1), with lower rates in sandy sediments, in winter and deeper in the sediment. Calculations of annual, average SRR throughout the upper 20 cm of muddy subtidal sediments (about 30 mmol S m-2 d-1) were much larger than previously reported net estimates of SRR derived from both benthic alkalinity flux measurements and bay wide, budget stoichiometry (3.5 and 2.6 mmol m-2 d-1, respectively), indicating that most reduced sulfur in these upper, well-mixed sediments is re-oxidized. A portion of the net alkalinity flux across the sediment surface may be derived from sulfate reduction in deeper sediments, estimated from sulfate depletion profiles at 1.5 mmol m-2 d-1. A small net flux of CO2 measured in benthic chambers despite a large SRR suggests that sediment sinks for CO2 must also exist (e.g., benthic microalgae).

Tomales Bay, California

The Land-Sea Interface (LSI) is a region of particular importance to human endeavor. The region includes the exposed outer coast, coastal waters, bays, estuaries, and deltas. Its area is small in comparison with that of the adjacent open ocean or land, but the LSI is at once a region of high biological productivity and complexity and the portion of the ocean most heavily impacted by human activities. The LSI also has a profound effect on the transport of materials between the land and the sea. Chemically reactive materials transported from the land to the sea undergo intense biogeochemical cycling and net transformations within the LSI. There are, for example, important sorption reactions associated with the transition from freshwater to saltwater. These reactions alter the distribution and bioavailibility of metals and may affect their toxicity and bioaccumulation.