Biogeochemical transformation of a nutrient subsidy: salmon, streams, and nitrification

Migratory animals can alter ecosystem function via the provision of nutrient subsidies. These subsidies are heterogeneous in space and time, which may create hot spots or hot moments in biogeochemical transformations, in turn altering the ecosystem effect of the subsidy by changing the form of the nutrients. Annual migrations of Pacific salmon (Oncorhynchus spp.) transport nutrients from the marine environment to their natal freshwater ecosystems. Salmon subsidies provide high quality nutrients (e.g., nitrogen, phosphorus, carbon) that may also be large in quantity where salmon migrations are near historic levels. We hypothesized that the nutrient subsidy provided via the excretion of ammonium (NH₄ ⁺) by live salmon would stimulate microbially mediated nitrification rates in stream sediments and increase streamwater nitrate (NO₃ ^?) concentrations. We quantified sediment nitrification in seven streams in Southeast Alaska before and during the salmon run in 2007 and 2008. Nitrification rates increased 3-fold from before to during the salmon run (mean ± SE = 0.07 ± 0.01 to 0.24 ± 0.02 mgN gAFDM^?1 d^?1, respectively). The variation in nitrification was explained by both streamwater and exchangeable NH₄ ⁺ concentrations (R ² = 0.50 and 0.71, respectively), which were low before salmon and increased relative to the size of the salmon run. To experimentally test the effect of salmon subsidies on nitrification rates, we staked senesced salmon carcasses on stream sediments for 3 weeks during the salmon run and then measured nitrification rates directly under the carcasses. Sediment nitrification was 2–5 times higher under the carcasses compared to nearby sediments without the direct carcass influence. Our results confirm that biogeochemical transformations alter the form of salmon-derived nitrogen, representing an overlooked aspect in the dynamics of this subsidy. Therefore, animal-derived nutrient subsidies are not passively retained or exported in recipient ecosystems, but also transformed, thereby influencing the form and incorporation of these nutrient subsidies.     

Benthic nutrient fluxes in a eutrophic,polymictic lake

Sediment release rates of soluble reactive phosphorus (SRP) and ammonium (NH₄) were determined seasonally at three sites (water depth 7, 14 and 20 m) in Lake Rotorua using in situ benthic chamber incubations. Rates of release of SRP ranged from 2.2 to 85.6 mg P m⁻² d⁻¹ and were largely independent of dissolved oxygen (DO) concentration. Two phases of NH₄ release were observed in the chamber incubations; high initial rates of up to 2,200 mg N m⁻² d⁻¹ in the first 12 h of deployment followed by lower rates of up to 270 mg N m⁻² d⁻¹ in the remaining 36 h of deployment. Releases of SRP and NH₄ were highest in summer and at the deepest of the three sites. High organic matter supply rates to the sediments may be important for sustaining high rates of sediment nutrient release. A nutrient budget of Lake Rotorua indicates that internal nutrient sources derived from benthic fluxes are more important than external nutrient sources to the lake.     

Fluff deposition on intertidal sediments: effects on benthic biota, ammonium fluxes and nitrification rates

The effects of fluff deposit on benthic biota,NH₄ ⁺ fluxes and nitrification was studied in thelaboratory using waterlogged and reflooded intertidal sediments fromMarennes-Oléron Bay, France. The fluff deposit was enriched inNH₄ ⁺ compared to underlying sediments, and promotedchanges of the sediment pH, E_h, C:N ratio, C:chla ratio and the NH₄ ⁺ efflux tooverlying water. Statistical analysis showed that pore waterNH₄ ⁺ concentrations were strongly influenced byinteractions between fluff, drying, depth and bioturbation. The fluff depositresulted in anoxia in the top sediments and moved the nitrification zone tosurface layers in fluff. However, the NH₄ ⁺ enrichment influff did not significantly change actual nitrification rates (range 0–1mmol m^–2 d^–1) or potentialnitrification rates (range 3–11 mmolNO₃ ^– m^–2d^–1).     

Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O₂ m^?2 h^?1), ammonium (NH₄ ⁺), nitrate (NO₃ ^?), dissolved organic nitrogen, dinitrogen gas (N₂), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH₄ ⁺), NO₃ ^? and N₂ fluxes (annual means +14.6, +15.9 and 44.6 μmol N m^?2 h^?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O₂ m^?2 h^?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH₄ ⁺ flux which may have masked reductions in NH₄ ⁺ flux associated with BMA assimilation during the light. Peaks in net N₂ fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO₃ ^? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O₂ m^?2 h^?1.     

Effects of Atrazine,Metolachlor, Carbaryl and Chlorothalonil on Benthic Microbes and Their Nutrient Dynamics

Atrazine, metolachlor, carbaryl, and chlorothalonil are detected in streams throughout the U.S. at concentrations that may have adverse effects on benthic microbes. Sediment samples were exposed to these pesticides to quantify responses of ammonium, nitrate, and phosphate uptake by the benthic microbial community. Control uptake rates of sediments had net remineralization of nitrate (−1.58 NO₃ µg gdm⁻¹ h⁻¹), and net assimilation of phosphate (1.34 PO₄ µg gdm⁻¹ h⁻¹) and ammonium (0.03 NH₄ µg gdm⁻¹ h⁻¹). Metolachlor decreased ammonium and phosphate uptake. Chlorothalonil decreased nitrate remineralization and phosphate uptake. Nitrate, ammonium, and phosphate uptake rates are more pronounced in the presence of these pesticides due to microbial adaptations to toxicants. Our interpretation of pesticide availability based on their water/solid affinities supports no effects for atrazine and carbaryl, decreasing nitrate remineralization, and phosphate assimilation in response to chlorothalonil. Further, decreased ammonium and phosphate uptake in response to metolachlor is likely due to affinity. Because atrazine target autotrophs, and carbaryl synaptic activity, effects on benthic microbes were not hypothesized, consistent with results. Metolachlor and chlorothalonil (non-specific modes of action) had significant effects on sediment microbial nutrient dynamics. Thus, pesticides with a higher affinity to sediments and/or broad modes of action are likely to affect sediment microbes'' nutrient dynamics than pesticides dissolved in water or specific modes of action. Predicted nutrient uptake rates were calculated at mean and peak concentrations of metolachlor and chlorothalonil in freshwaters using polynomial equations generated in this experiment. We concluded that in natural ecosystems, peak chlorothalonil and metolachlor concentrations could affect phosphate and ammonium by decreasing net assimilation, and nitrate uptake rates by decreasing remineralization, relative to mean concentrations of metolachlor and chlorothalonil. Our regression equations can complement models of nitrogen and phosphorus availability in streams to predict potential changes in nutrient dynamics in response to pesticides in freshwaters.     

Biosensor Determination of the Microscale Distribution of Nitrate, Nitrate Assimilation, Nitrification, and Denitrification in a Diatom-Inhabited Freshwater Sediment

High-resolution NO₃⁻ profiles in freshwater sediment covered with benthic diatoms were obtained with a new microscale NO₃⁻ biosensor characterized by absence of interference from chemical species other than NO₂⁻ and N₂O. Analysis of the microprofiles obtained indicated no nitrification during darkness, high rates of nitrification and a tight coupling between nitrification and denitrification during illumination, and substantial rates of NO₃⁻ assimilation during illumination. Nitrification during darkness could be induced by purging the bulk water with O₂ gas, indicating that the stimulatory effect on nitrification by illumination was caused by algal production of O₂. NH₄⁺ addition did not stimulate nitrification during darkness when O₂ was restricted to the upper 1-mm layer, and there was thus a low nitrification potential in the permanently oxic top 1 mm of the sediment.     

Effects of macrophyte-associated nitrogen cycling bacteria on denitrification in the sediments of the eutrophic Gonghu Bay, Taihu Lake

Integrated Elodea nuttallii-immobilized nitrogen cycling bacteria (INCB) technology was used for ecological restoration in the eutrophic Gonghu Bay, Taihu Lake. Sediment denitrification was investigated through microcosm incubations with four different treatments: bare sediment core as control without restoration, sediment + E. nuttallii, sediment + E. nuttallii + INCB, and sediment + INCB. The sediments with E. nuttallii-INCB assemblage (E-INCB) had the highest denitrification rates among all the treatments, and the E-INCB increased the denitrification rate by 162% in the sediments. The presence of macrophytes yielded a penetration depth of O₂ to more than 20 mm below the sediment–water interface (SWI), while the depth was only 4 mm in the sediments without macrophytes. The quantity of denitrifier in E-INCB sediments (within ~2 cm below the SWI) showed a significant increasing trend during one-month incubation, which was one order of magnitudes higher than that in the sediments without INCB. Macrophytes caused deeper O₂ penetration and increased oxic-anoxic interface, which could stimulate the coupled nitrification–denitrification. The high denitrification rate of the E-INCB treatment may result from the increased inorganic nitrogen content in the vicinity of the SWI, causing more nitrate to reach the anoxic denitrification zone. The results showed that E-INCB assemblage could increase benthic N removal by stimulating denitrification via combined O₂ penetration and enhanced microbial N cycling processes. E-INCB might be used as a potential restoration method for controlling fresh water system eutrophication.     

Seasonal distribution of nitrifying bacteria and rates of nitrification in coastal marine sediments

The distribution of nitrification potential (NP) with depth in sediment and season was investigated in a shallow sandy sediment (0.5 m water) and a deeper muddy sediment (17m water). In both sediments, nitrifying bacteria were present in the anoxic strata (oxygen penetration was 5 mm below the surface). The NP at 6–8 cm depth in the sediment was 50% and 10% of the surface NP at the sandy and muddy sediment, respectively. It is suggested that bioturbation and physical disturbance of the sediment were the most likely reasons for this distribution. The NP increased as sediment temperature decreased. This effect was less marked in the muddy sediment. It is concluded that during the summer, the numbers or specific activity of nitrifying bacteria diminished for the following reasons: There was decreased O₂ penetration into the sediment and increased competition for O₂ by heterotrophs; there was increased competition for NH₄ ⁺ and there was inhibition by H₂S. These effects counteracted the potentially higher growth rates and increased rates of NH₄ ⁺ production at the elevated summer temperatures. The potential nitrification rates in the upper 1 cm, which were measured at 22°C, were converted to calculated rates at the in situ temperature (Q₁₀=2.5) and in situ oxygen penetration. These calculated rates were shown to closely resemble the measured in situ rates of nitrification. The relationship between the in situ rates of nitrification and the nitrification potential is discussed.     

Seasonal effects of zebra mussels on littoral nitrogen transformation rates in Gull Lake, Michigan, U.S.A.

• 1 Zebra mussels (Dreissena polymorpha) are successful colonisers of lake littoral habitats and they interact strongly with littoral benthos. Previous research suggests that localised areas colonised by zebra mussels may be hotspots of nitrogen (N) cycling.
• 2 The effects of zebra mussels on nitrification and denitrification rates were examined approximately every other month for 1 year in Gull Lake, Michigan, U.S.A. Littoral sediment was collected from an area free of zebra mussels and distributed into shallow trays; rocks colonised with zebra mussels were placed in half of the trays, while uncolonised rocks were placed in the remaining trays. After an incubation period of 6–8 weeks in the lake, sediment and zebra mussels were collected from the trays, replaced with new sediment and zebra mussels, and placed in the lake for the next interval. In the laboratory, sediment nitrification and denitrification rates were measured for each tray.
• 3 Sediment nitrification rates did not increase in the presence of zebra mussels; instead nitrification rates were sensitive to changes in water temperature and increased with increasing exchangeable sediment ammonium. In contrast, denitrification rates increased in sediment trays with zebra mussels in the winter when nitrate (NO₃⁻) availability was high and when Chara did not grow in the trays.
• 4 Sediment denitrification was NO₃⁻‐limited in all seasons, regardless of zebra mussel treatment. However, sediment in the presence of zebra mussels responded less to NO₃⁻ addition, suggesting that NO₃⁻ limitation of denitrification can be reduced by zebra mussel activity. Zebra mussels have a seasonally variable impact on sediment denitrification rates, and this may translate into altered seasonal patterns of N cycling in localised areas of lakes where they are particularly abundant.

     

Seasonal oxygen,nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

The regulatory roles of temperature, eutrophication and oxygen availability on benthic nitrogen (N) cycling and the stoichiometry of regenerated nitrogen and phosphorus (P) were explored along a Baltic Sea estuary affected by treated sewage discharge. Rates of sediment denitrification, anammox, dissimilatory nitrate reduction to ammonium (DNRA), nutrient exchange, oxygen (O₂) uptake and penetration were measured seasonally. Sediments not affected by the nutrient plume released by the sewage treatment plant (STP) showed a strong seasonality in rates of O₂ uptake and coupled nitrification–denitrification, with anammox never accounting for more than 20 % of the total dinitrogen (N₂) production. N cycling in sediments close to the STP was highly dependent on oxygen availability, which masked temperature-related effects. These sediments switched from low N loss and high ammonium (NH₄ ⁺) efflux under hypoxic conditions in the fall, to a major N loss system in the winter when the sediment surface was oxidized. In the fall DNRA outcompeted denitrification as the main nitrate (NO₃ ^?) reduction pathway, resulting in N recycling and potential spreading of eutrophication. A comparison with historical records of nutrient discharge and denitrification indicated that the total N loss in the estuary has been tightly coupled to the total amount of nutrient discharge from the STP. Changes in dissolved inorganic nitrogen (DIN) released from the STP agreed well with variations in sedimentary N₂ removal. This indicates that denitrification and anammox efficiently counterbalance N loading in the estuary across the range of historical and present-day anthropogenic nutrient discharge. Overall low N/P ratios of the regenerated nutrient fluxes impose strong N limitation for the pelagic system and generate a high potential for nuisance cyanobacterial blooms.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

The response of planktonic and microbenthic algal assemblages to nutrient enrichment in shallow coastal waters,southwest Sweden

Field and laboratory nutrient (nitrogen and phosphorus) enrichment experiments were performed using natural phytoplankton and microphytobenthic assemblages from the brackish water Öresund, S.W. Sweden. The response of algae from a low-nutrient area (Falsterbo Canal) was compared to that of algae from a polluted, nutrient-rich area (Lomma Bay).The biomass (measured as chlorophyll a) of both phytoplankton and microphytobenthos from the Falsterbo Canal increased after the addition of nitrogen. Phytoplankton growth was stimulated by the addition of phosphorus to the nitrogen-rich water of the polluted Lomma Bay. Sediment chlorophyll a showed no significant increase after the addition of nutrients in the Lomma Bay. In containers without sediment, phytoplankton uptake was calculated to account for ≈ 90% of the disappearance of inorganic fixed nitrogen from the water. In the sediment containers the microphytobenthos was estimated to account for ≈20% of the nitrogen uptake. The rest was presumably lost mainly through denitrification.When containers with microphytobenthos from Lomma Bay were kept in the dark, phosphorus was released at a rate of up to ≈ 180 μM · m^?2 · day^?1. We suggest that by producing oxygen microbenthic algae keep the sediment surface oxygenated thereby decreasing phosphorus transport from the sediment to the overlying water.     

Net sediment N2 fluxes in a southern New England estuary: variations in space and time

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N₂ across the sediment–water interface over an annual cycle using the N₂/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N₂–N m^?2 h^?1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (?5 to ?650 μmol N₂–N m^?2 h^?1) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.     

Hypolimnetic phosphorus and nitrogen dynamics in a small,eutrophic lake with a seasonally anoxic hypolimnion

In situ estimates of sediment nutrient flux are necessary to understand seasonal variations in internal loading in lakes. We investigated the sources and sinks of nutrients in the hypolimnion of a small (0.33 km²), relatively shallow (18 m max. depth), eutrophic lake (Lake Okaro, New Zealand) in order to determine changes in sediment nutrient fluxes resulting from a whole lake sediment capping trial using a modified zeolite phosphorus inactivation agent (Z2G1). Sediment nutrient fluxes in the hypolimnion were estimated as the residual term in a nutrient budget model that accounted for mineralisation of organic nutrients, nutrient uptake by phytoplankton and mixing, nitrification, adsorption/desorption and diffusion of dissolved nutrients at the thermocline. Of the total hypolimnetic phosphate and ammonium fluxes during one period of seasonal stratification (2007–08), up to 60 and 50%, respectively, were derived from the bottom sediments, 18 and 24% were due to mineralisation of organic species, 36 and 28% were due to phytoplankton uptake and 9 and 6% were from diffusion across the thermocline. Adsorption/desorption of phosphate to suspended solids and nitrification were of minor (<8%) importance to the total fluxes. Any reduction in sediment nutrient release by Z2G1 was small compared with both the total sediment nutrient flux and the sum of other hypolimnetic fluxes. Uneven sediment coverage of Z2G1 may have been responsible for the limited effect of the sediment capping layer formed by Z2G1.     

Nitrification, denitrification, and nitrate ammonification in sediments of two coastal lagoons in Southern France

Summary Seasonal and diurnal variations in sediment-water fluxes of O₂, NO ₃ ^– , and NH ₄ ⁺ as well as rates of nitrification, denitrification, and nitrate ammonification were determined in two different coastal lagoons of southern France: The seagrass (Zostera noltii) dominated tidal Bassin d'Arcachon and the dystrophic Etang du Prévost. Overall, denitrification rates in both Bassin d'Arcachon (<0.4 mmol m^–2 d^–1) and Etang du Prévost (<1 mmol m^–2 d^–1) were low. This was mainly caused by a combination of low NO ₃ ^– concentrations in the water column and a low nitrification activity within the sediment. In both Bassin d'Arcachon and Etang du Prévost, rates of nitrate ammonification were quantitatively as important as denitrification.Denitrification played a minor role as a nitrogen sink in both systems. In the tidal influenced Bassin d'Arcachon, Z. noltii was quantitatively more important than denitrification as a nitrogen sink due to the high assimilation rates of the plants. Throughout the year, Z. noltii stabilized the mudflats of the bay by its well- developed root matrix and controlled the nitrogen cycle due to its high uptake rates. In contrast, the lack of rooted macrophytes, and dominance of floating macroalgae, made nitrogen cycling in Etang du Prévost more unstable and unpredictable. Inhibition of nitrification and denitrification during the dystrophic crisis in the summer time increased the inorganic nitrogen flux from the sediment to the water column and thus increased the degree of benthic-pelagic coupling within this bay. During winter, however, benthic microalgae colonizing the sediment surface changed the sediment in the lagoon from being a nitrogen source to the over lying water to being a sink due to their high assimilation rates. It is likely, however, that this assimilated nitrogen is liberated to the water column at the onset of summer thereby fueling the extensive growth of the floating macroalgae, Ulva sp. The combination of a high nitrogen coupling between sediment and water column, little water exchange and low denitrification rates resulted in an unstable system with fast growing algal species such as phytoplankton and floating algae.     

Nitrogen dynamics at the sediment–water interface in shallow, sub-tropical Florida Bay: why denitrification efficiency may decrease with increased eutrophication

Nitrogen (N) dynamics at the sediment–water interface were examined in four regions of Florida Bay to provide mechanistic information on the fate and effects of increased N inputs to shallow, subtropical, coastal environments. Dissimilatory nitrate (NO₃ ^?) reduction to ammonium (DNRA) was hypothesized to be a significant mechanism retaining bioreactive N in this warm, saline coastal ecosystem. Nitrogen dynamics, phosphorus (P) fluxes, and sediment oxygen demand (SOD) were measured in north-central (Rankin Key; eutrophic), north-eastern (Duck Key; high N to P seston ratios), north-western (Murray Key; low N to P ratios), and central (Rabbit Key; typical central site) Florida Bay in August 2004, January 2005, and November 2006. Site water was passed over intact sediment cores, and changes in oxygen (O₂), phosphate (o-PO₄ ^3?), ammonium (NH₄ ⁺), NO₃ ^?, nitrite (NO₂ ^?), and N₂ concentrations were measured, without and with addition of excess ¹⁵NO₃ ^? or ¹⁵NH₄ ⁺ to inflow water. These incubations provided estimates of SOD, nutrient fluxes, N₂ production, and potential DNRA rates. Denitrification rates were lowest in summer, when SOD was highest. DNRA rates and NH₄ ⁺ fluxes were high in summer at the eutrophic Rankin site, when denitrification rates were low and almost no N₂ came from added ¹⁵NO₃ ^?. Highest ¹⁵NH₄ ⁺ accumulation, resulting from DNRA, occurred at Rabbit Key during a picocyanobacteria bloom in November. ¹⁵NH₄ ⁺ accumulation rates among the stations correlated with SOD in August and January, but not in November during the algal bloom. These mechanistic results help explain why bioreactive N supply rates are sometimes high in Florida Bay and why denitrification efficiency may decrease with increased NO₃ ^? inputs in sub-tropical coastal environments.     

Spatial and temporal variability in benthic processes along a mangrove-seagrass transect near the Bangrong Mangrove,Thailand

Benthic primary production and nutrient dynamics were examined along a transect in the Bangrong mangrove forest in Thailand. Six stations were established extending from a high-intertidal site within the mangrove forest to low-intertidal flats and seagrass beds in front of the mangrove forest. Benthic processes (O₂ and CO₂ fluxes) and nutrient dynamics (mineralization, sediment-water fluxes, pore water and sediment pools) were measured under light and dark conditions during wet and dry seasons over a 2-yr period. The sediments were mostly autotrophic, only the mangrove forest sites were net heterotrophic during the wet season. Maximum daily net primary production was found at the non-vegetated tidal flats (40–75 mmol O₂ m^-2d^-1), where light and nutrient availability were highest. The variation in benthic mineralization along the transect was minor (1.6–4.3 mmol CO₂ m^-2h^-1) and did not reflect the large changes inorganic matter content (organic carbon: 0.7–4.2% DW) and quality (C:N ratio varied from 25 to 100), suggesting that the mineralizable pool of organic matter was of similar magnitude at all sites. There was only minor seasonal variation in rates of mineralization. The net primary production showed more variation with lower rates in the mangrove forest (reduced with 74%) and higher rates at the tidal flats (increased with 172%) and in the seagrass beds (increased with 228%) during the wet season. The nutrient pools and fluxes across the sediment-water interface were generally low along the transect, and the sediments were efficient in retaining nitrogen in the nutrient limited mangrove/seagrass environment. Pools and fluxes of phosphorus were generally very low suggesting that benthic primary production was phosphorus limited along the transect. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Estuarine Sediment pH and Oxidation-Reduction Potential on Microbial Hydrocarbon Degradation

Microbial mineralization rates of two petroleum hydrocarbons, as affected by pH and oxidation-reduction potential, were determined in a Barataria Bay, Louisiana, sediment using ¹⁴C-labeled hydrocarbons. Hydrocarbon mineralization rates were inferred from the activity of respired ¹⁴CO₂. Sediment pH and oxidation-reduction potential were important factors in governing the population of hydrocarbon-degrading microorganisms in the sediment and subsequent mineralization rates. Highest mineralization rates occurred at pH 8.0, and the lowest occurred at pH 5.0. At all pH levels mineralization decreased with decreasing oxidation-reduction potential (i.e., increasing sediment anaerobiosis). Generally, mineralization rates for octadecane were greater than those for naphthalene. Aerobic microorganisms in the oxidized sediment were more capable of degrading hydrocarbons than anaerobic microorganisms in reduced sediment of the same pH.     

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 (K_app) and the maximum potential rate (V_mp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the K_app and V_mp 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.     

Potential rates of nitrification and denitrification in an oligotrophic freshwater sediment system

Potential rates of nitrification and denitrification were measured in an oligotrophic sediment system. Nitrification potential was estimated using the CO oxidation technique, and potential denitrification was measured by the acetylene blockage technique. The sediments demonstrated both nitrifying and denitrifying activity. E_h, O₂, and organic C profiles showed two distinct types of sediment. One type was low in organic C, had high O₂ and E_h, and had rates of denitrification 1,000 times lower than the other which had high organic C, low O₂, and low E_h. Potential nitrification and denitrification rates were negatively correlated with E_h. This suggests that environmental heterogeneity in denitrifier and nitrifier populations in oligotrophic sediment systems may be assessed using E_h before sampling protocols for nitrification or denitrification rates are established. There was no correlation between denitrification and nitrification rates or between either of these processes and NH₄ ⁺ or NO₃ ^– concentrations. The maximum rate of denitrification was 0.969 nmole N cm^–3 hour^–1, and the maximum rate of nitrification was 23.6 nmole cm^–3 hour^–1, suggesting nitrification does not limit denitrification in these oligotrophic sediments. Some sediment cores had mean concentrations of 6.0 mg O₂/liter and still showed both nitrification and denitrification activity.