Sediment Oxygen Consumption Rates Research Articles

Annual biogeochemical cycling in intertidal sediments of a restored estuary reveals dependence of N, P, C and Si cycles to temperature and water column properties

Estuarine intertidal sediments are important centres for organic matter remineralization and nutrients recycling. Nevertheless, there is limited understanding regarding how these processes occur along the salinity gradient and their seasonality. Here, we report on the seasonal biogeochemical cycles from three types of intertidal sedimentary habitats (freshwater, brackish and marine) located in the Western Scheldt estuary (The Netherlands and Belgium). A full year of solute fluxes, porewater nutrient and sediment pigment concentrations at a monthly resolution revealed clear differences in the biogeochemistry of the three sites, indicating that environmental conditions determined the local nutrient dynamics. Temperature controlled sediment oxygen consumption rates and nutrient fluxes, but also affected pore water nutrient concentrations up to 14 cm deep. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (−1.62 mmol m−2 d−1 and -2.84 mmol m−2 d−1, respectively), while only the freshwater sediments showed a net influx of phosphate (−0.07 mmol m−2 d−1). We estimated that intertidal sediments remineralized a total of 10,000 t C y−1, with 97% of mineralization occurring in the brackish and marine parts. Overall, sediments removed 11% (1500 t N y−1) and 15% (∼200 t P y−1) of the total nitrogen and phosphorus entering the estuary from riverine input. Moreover, observations revealed the historical improvement of water quality resulting from water treatment policies. This spatiotemporal study of OM remineralization and early diagenesis in estuarine systems highlights the importance of intertidal sediments for estuarine systems. Our observations can be used in models to predict estuarine biogeochemistry or assess climate change scenarios.

Winter mixing accelerates decomposition of sedimentary organic carbon in seasonally hypoxic coastal seas

Deltaic systems are characterized by the highest sedimentation rates in the globe. Meanwhile, sedimentary organic matter therein can be efficiently decomposed so that these depositional systems may deviate substantially from the oft-quoted correlation between net sediment accumulation and preservation of organic matter. The exact mechanisms that cause such a deviation in any given case, however, remain poorly understood. In this study, we utilize a novel 224Ra/228Th disequilibrium method to examine sediment oxygen consumption and the release of diagenetic products of organic matter along the major mud wedge system in the inner shelf of the East China Sea. Our sampling campaign was carried out in two contrasting seasons: the summer when seasonal hypoxia was at its peak and physical conditions were relatively quiescent, and the winter when the water column was well oxygenated by intense winter mixing and underlying deposits were subjected to reworking. Unexpectedly, during summer 2017 when the seafloor received the annual maximum supply of organic matter, sediment oxygen consumption rates and benthic fluxes of NH4+ were relatively low, ranging from 6 to 59 mmol O2 m−2 d−1 and from 1.6 to 13 mmol N m−2 d−1, respectively. In contrast, during winter 2018 sediment oxygen consumption rates and benthic fluxes of NH4+ surged to 44–690 mmol O2 m−2 d−1 and 22–58 mmol N m−2 d−1, respectively. We have also identified an exponential relationship between amplification factor of sediment surface area and oxygen concentration in the bottom water. This relationship suggests that kinetic energy dissipation in the water column not only controlled air-sea exchange and seawater mixing, but also intensified sediment–water interaction. Importantly, sediment oxygen consumption rates (FO2) in the mud wedge can be empirically described using a modified form of Michaelis-Menten kinetics, suggesting that FO2and the associated benthic consumption and production of chemicals are controlled by both the transport and reaction processes. We have further demonstrated that a large portion of the organic matter deposited over the seafloor in summer is likely decomposed in winter. Overall, this study highlights intense winter mixing as an important mechanism that causes the highly efficient decomposition of sedimentary organic matter in coastal seas.

Constraining release of pollutants from anoxic bottom sediment via water-lifting aeration in a source water reservoir, East China

Seasonal hypoxia in water bodies can increase levels of reduced chemical species in the hypolimnion as they are released from anoxic bottom sediments. Water-lifting aeration (WLA) is a technology known to solve this problem by elevating near-sediment dissolved oxygen (DO) and increasing mixing in the water column in a canyon reservoir with deep water (>20 m). The influence of WLA in reservoirs with large surface area and small water depth has not been adequately evaluated. For this purpose, the efficiency of an innovative WLA system was reviewed in Zhoucun Reservoir (ZCR), a mildly eutrophic reservoir with a mean depth of 15 m. Three years of field experiment was performed in ZCR. Water and sediment samples were collected before and after the operation of the WLA system to explore the causes for the deterioration of water quality in ZCR, and the efficiency of the innovative WLA system installed in 2015. With the use of a microporous aerator and a kinetic energy dissipator, the new-style WLA system is different from WLA studied before. As a result of thermal stratification and the long duration of anaerobic environment in the hypolimnion, the sediment oxygen consumption rate in ZCR was calculated to be 16.14 mg m−2 h−1, which was much higher than other reservoirs, and the main pollutants in ZCR were ammonia, phosphorus, manganese and sulphide. After the operation of the new WLA system, the anaerobic environment in the bottom water was completely reversed and the DO concentration in the hypolimnion water has been maintained at more than 6 mg L−1. The release of pollutants from sediments, such as ammonia, phosphorus, manganese and sulphide, was inhibited significantly. Ammonia, phosphorus, manganese and sulphide in water decreased by 89%, 84%, 97% and 87%, respectively. The results of this study demonstrate that employing WLA systems to oxygenate and mix stagnant water bodies is a viable and potentially favourable management strategy for reservoirs with large surface area and relatively small water depth.

가막만 빈산소 해역의 퇴적물 산소소모율과 용존산소 고갈에 의한 황화수소 용출

가막만 빈산소 해역에서 챔버 실험과 지화학적 분석을 통하여 퇴적물 산소소모율과 퇴적물 특성을 파악하였다. 가막만 표층퇴적물의 유기탄소 함량은 만의 안쪽이 높고 바깥쪽으로 갈수록 감소하는 경향을 보였다. 만의 안쪽 퇴적물에는 바깥쪽에 비해 상대적으로 육성기인 유기물이 더 많이 포함되어 있다. 빈산소가 빈번하게 발생하는 가막만 북부 내만의 시추퇴적물에 기록된 탄산칼슘 함량의 분포는 이 지역에서 과거 백 년 이전부터 빈산소가 발생하였음을 지시한다. 2010년과 2011년 2월, 5월, 8월 및 11월에 가막만 빈산소 해역에서 퇴적물 챔버 실험을 하였는데, 북부 내만에 위치한 정점 C3과 남부 외해 쪽에 위치한 정점 C17에서 퇴적물 산소소모율은 각각 <TEX>$3.98-12.43mmol\;m^{-2}d^{-1}$</TEX>, <TEX>$3.28-8.18mmol\;m^{-2}d^{-1}$</TEX> 범위였다. 퇴적물 산소소모율은 수온에 가장 큰 영향을 받는 것으로 보인다. 정점 C3과 정점 C17 에서 산소가 모두 고갈되어 저층이 무산소로 바뀌면 유독성의 황화수소 플럭스는 각각 <TEX>$1.38mmol\;m^{-2}d^{-1}$</TEX>, <TEX>$1.3mmol\;m^{-2}d^{-1}$</TEX>로 수층으로 용출되어 나왔다. This study investigated sediment oxygen consumption rates and geochemical characteristics of sediment in hypoxic area of the Gamak Bay based on the chamber experiments and geochemical analyses. The organic carbon contents of surface sediment in the Gamak Bay showed that the inner bay area has higher organic carbon content than those of the outer bay. They toward the outer bay, contents dropped off. The vertical profiles of calcium carbonate (<TEX>$CaCO_3$</TEX>) content at piston core sediment assumed that the hypoxia have been frequently occurred during past century in the northern inner bay. The benthic chamber experiments were conducted in February, May, August and November 2010, 2011 in the hypoxic area of the Gamak Bay. In the sediment incubation experiment with chamber at site C3 in the northern inner bay and site C17 in the southern outer bay, the sediment oxygen consumption rate ranged from <TEX>$3.98mmol\;m^{-2}d^{-1}$</TEX> to <TEX>$12.43mmol\;m^{-2}d^{-1}$</TEX> and <TEX>$3.28mmol\;m^{-2}d^{-1}$</TEX> to <TEX>$8.18mmol\;m^{-2}d^{-1}$</TEX>, respectively. When the oxygen was completely depleted, the toxic hydrogen sulfide was released with <TEX>$1.38mmol\;m^{-2}d^{-1}$</TEX> and <TEX>$1.3mmol\;m^{-2}d^{-1}$</TEX>, respectively.

A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

The Louisiana shelf (LA shelf) in the northern Gulf of Mexico experiences hypoxic conditions every summer due to the combination of eutrophication and strong water column stratification. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on hypoxia generation on the LA shelf. The model assumes a constant oxygen utilization rate in the water column and a sediment oxygen consumption rate that depends on the bottom water oxygen concentration and temperature. Despite its simplicity, the model reproduces the observed variability of dissolved oxygen and hypoxia on the LA shelf, highlighting the importance of physical processes. Model results demonstrate that both river discharge and wind forcing have a strong effect on the distribution of the river plume and stratification, and thereby on bottom dissolved oxygen concentrations and hypoxia formation on the LA shelf. The seasonal cycle of hypoxia is relatively insensitive to the seasonal variability in river discharge, but the time-integrated hypoxic area is very sensitive to the overall magnitude of river discharge. Changes in wind speed have the greatest effect on the simulated seasonal cycle of hypoxia and hypoxic duration, while changes in wind direction strongly influence the geographic distribution of hypoxia. Given that our simple oxygen model essentially reproduces the evolution of hypoxia simulated with a full biogeochemical model and that physical processes largely determine the magnitude and distribution of hypoxia, a full biogeochemical model might not be necessary for short-term hypoxia forecasting.

Sediment oxygen consumption and benthic organic carbon mineralization on the continental shelves of the East China Sea and the Yellow Sea

Sediment oxygen consumption (SOC) rates on the shelves of the East China Sea (ECS) and the Yellow Sea (YS) were measured during five field studies in the period April 2010 to May 2011, using the batch intact core incubation technique. The measured SOC rates ranged from 1.73 to 17.56mmol O2m−2d−1, which are comparable to values reported for other typical continental shelves worldwide. For the shelves of both the ECS and the YS there was a remarkable spatial and temporal variation in SOC rates. SOC was used as a proxy to assess the benthic organic carbon mineralization in the ECS and the YS, and to prepare simple organic carbon budget. For the YS it was found that on average approximately 90% of the organic carbon derived from primary productivity was decomposed in the water column and 8% was decomposed in the sediment, resulting in <4% being permanently buried in the YS sediment. In contrast, approximately 70% of the organic carbon derived from primary productivity was decomposed in the water column in the ECS and 17% was decomposed in the sediment, suggesting that the permanently buried percentage (14%, relative to the primary productivity) was higher in the ECS. The contrasting contributions of benthic mineralization to the organic carbon cycle in the ECS and the YS indicated that there is tight coupling between pelagic and benthic biogeochemical processes in the ECS, but in the YS, the regenerated production and external non-marine nutrients regulated pelagic primary production.

Benthic ecosystem functioning beneath fish farms in different hydrodynamic environments

To quantify how fish farming modifies short‐term benthic carbon cycling in fjord environments and the role of hydrodynamics in modifying effects on the benthos, we assessed benthic ecosystem structure and respiration and used isotope labeled algae as a tracer to quantify C flow over 48 h through macrofauna and bacteria in sediments collected from beneath fish farm sites in (1) high water‐flow areas, (2) low water‐flow areas, and (3) two appropriate control sites situated downstream from the farms. Bacterial biomass was significantly greater in sediments collected from the fish farm sites relative to the controls. This was also true for sediment oxygen consumption (SOC) rates averaged over each 48 h investigation, which were significantly correlated with total benthic (macrofauna and bacteria) biomass. Short‐term C‐uptake rates by macrofauna were elevated in both fish farm treatments compared with bacterial C uptake and were significantly higher in sediments from the low flow fish farm site relative to both controls. While SOC rates were significantly higher in experiments using sediments from the low flow fish farm site; faunal abundance, biomass uptake, C uptake, bacterial biomass, and metabolism in sediments from both fish farm treatments were not significantly different from one another. Fish farming can dramatically alter benthic ecosystem functioning, and significant effects can occur around fish farms irrespective of the water‐flow regime the farms are moored in.

The influence of pore‐water advection, benthic photosynthesis, and respiration on calcium carbonate dynamics in reef sands

To investigate diel calcium carbonate (CaCO3) dynamics in permeable coral reef sands, we measured pore‐water profiles and fluxes of oxygen (O2), nutrients, pH, calcium (Ca2+), and alkalinity (TA) across the sediment‐water interface in sands of different permeability at Heron Reef, Australia. Background flushing rates were high, most likely as a result of infaunal burrow irrigation, but flux chamber stirring enhanced pore‐water exchange. Light and pore‐water advection fueled high rates of benthic primary production and calcification in sunlit surface sediments. In the light, benthic photosynthesis and calcification induced surface minima in Ca2+ and TA and peaks in pH and O2. Oxygen penetration depth in coarse sands decreased from ∼ 1.2 cm during the day to ∼ 0.6 cm at night. Total oxygen uptake (TOU) in dark chambers was three to fourteen times greater than diffusive uptake and showed a direct effect of pore‐water advection. Greater sediment oxygen consumption rates were observed in higher permeability sands. In the dark, TA release was not stimulated by increasing TOU because of a damping effect of pore‐water advection on metabolic CaCO3 dissolution efficiency. On a daily basis, CaCO3 undergoes net dissolution in Heron Reef sands. However, pore‐water advection can reverse the CaCO3 budget and promote CaCO3 preservation under the most energetic conditions.

Benthic nutrient fluxes at longline sea squirt and oyster aquaculture farms and their role in coastal ecosystems

To assess the impact of aquaculture activities, we measured the primary production, total sediment oxygen consumption, and benthic nutrient flux at two aquaculture farms (sea squirt and oyster) and one reference site. The estimated primary production in the water column ranged from 19–169 mmol C m−2 d−1. Total sediment oxygen consumption rates ranged from 58 to 328 mmol m−2 d−1, 1.1 to 40 mmol m−2 d−1 for nitrogen, 0.17 to 3.0 mmol m−2 d−1 for phosphate, and 7.3 to 74 mmol m−2 d−1 for silicate. The average total sediment oxygen consumption at the longline farms was >2.5 times higher than the reference site. Nitrate was significantly removed by denitrification at the longline farms in July and September and ranged from −5.4 to −0.09 mmol m−2 d−1, which is higher than for other coastal sediments. The benthic fluxes of nitrogen and phosphate at longline farms were up to 16 and six times higher than at the reference site, respectively. The average nitrogen requirements of the primary producers were 9.3 mmol m−2 d−1 at the sea squirt farm, 7.0 mmol m−2 d−1 at the oyster farm, and 13.5 mmol m−2 d−1 at the reference site, corresponding to 88, 316, and 27.2% of the nitrogen supplied by benthic fluxes, respectively. Our results suggest that benthic nutrient fluxes at longline farms are a major nutrient source for primary production in coastal waters.

Sediment and Lower Water Column Oxygen Consumption in the Seasonally Hypoxic Region of the Louisiana Continental Shelf

We report integrated measurements of sediment oxygen consumption (SOC) and bottom water plankton community respiration rates (WR) during eight cruises from 2003 to 2007 on the Louisiana continental shelf (LCS) where hypoxia develops annually. Averaged by cruise, SOC ranged from 3.9 to 25.8 mmol O2 m−2 day−1, whereas WR ranged from 4.1 to 10.8 mmol O2 m−3 day−1. Total below-pycnocline respiration rates ranged from 46.4 to 104.5 mmol O2 m−2 day−1. In general, below-pycnocline respiration showed low variability over a large geographic and temporal range, and exhibited no clear spatial or inter-annual patterns. SOC was strongly limited by dissolved oxygen (DO) in the overlying water; whereas, WR was insensitive to low DO, a relationship that may be useful for parameterizing future models. The component measures, WR and SOC, were similar to most prior measurements, both from the LCS and from other shallow estuarine and coastal environments. The contribution of SOC to total below-pycnocline respiration averaged 20 ± 4%, a finding that differs from several prior LCS studies, but one that was well supported from the broader estuarine and oceanic literature. The data reported here add substantially to those available for the LCS, thus helping to better understand oxygen dynamics on the LCS.

Impacts of exotic mangrove forests and mangrove deforestation on carbon remineralization and ecosystem functioning in marine sediments

Abstract. To evaluate how mangrove invasion and removal can modify short-term benthic carbon cycling and ecosystem functioning, we used stable-isotopically labeled algae as a deliberate tracer to quantify benthic respiration and C-flow over 48 h through macrofauna and bacteria in sediments collected from (1) an invasive mangrove forest, (2) deforested mangrove sites 2 and 6 years after removal of above-sediment mangrove biomass, and (3) two mangrove-free control sites in the Hawaiian coastal zone. Sediment oxygen consumption (SOC) rates averaged over each 48 h investigation were significantly greater in the mangrove and mangrove removal site experiments than in controls and were significantly correlated with total benthic (macrofauna and bacteria) biomass and sedimentary mangrove biomass (SMB). Bacteria dominated short-term C-processing of added microalgal-C and benthic biomass in sediments from the invasive mangrove forest habitat and in the 6-yr removal site. In contrast, macrofauna were the most important agents in the short-term processing of microalgal-C in sediments from the 2-yr mangrove removal site and control sites. However, mean faunal abundance and C-uptake rates in sediments from both removal sites were significantly higher than in control cores, which collectively suggest that community structure and short-term C-cycling dynamics of sediments in habitats where mangroves have been cleared can remain fundamentally different from un-invaded mudflat sediments for at least 6-yrs following above-sediment mangrove removal. In summary, invasion by mangroves can lead to dramatic shifts in benthic ecosystem function, with sediment metabolism, benthic community structure and short-term C-remineralization dynamics being affected for years following invader removal.

Variable sediment oxygen uptake in response to dynamic forcing

Seiche‐induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (JO2), diffusive boundary layer thickness (δDBL), and sediment oxic zone depth (zmax) in situ. High temporal‐resolution microprofiles across the sediment‐water interface and current velocity data within the bottom boundary layer in a medium‐sized mesotrophic lake were obtained during a 12‐h field study. We resolved the dynamic forcing of a full 8‐h seiche cycle and evaluated JO2 from both sides of the sediment‐water interface. Turbulence (characterized by the energy dissipation rate, ε), the vertical distribution of dissolved oxygen across the sediment‐water interface (characterized by δDBL and zmax), JO2, and the sediment oxygen consumption rate (RO2) are all strongly correlated in our freshwater system. Seiche‐induced turbulence shifted from relatively active (ε = 1.2 × 10−8 W kg−1) to inactive (ε = 7.8 × 10−12 W kg−1). In response to this dynamic forcing, δDBL increased from 1.0 mm to the point of becoming undefined, zmax decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and JO2 decreased from 7.0 to 1.1 mmol m−2 d−1 over a time span of hours. JO2 and oxygen consumption were found to be almost equivalent (within ~ 5% and thus close to steady state), with RO2 adjusting rapidly to changes in JO2. Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating JO2.

Effects of phytoplankton vertical migration on the formation of oxygen depleted water in a shallow coastal sea

In this paper, oxygen budget was estimated for the lower layer of water column in a semi-enclosed bay, Ago Bay, Japan. Benthic oxygen consumption rates were measured directly with an in situ measurement device from 13 July to 16 August 2004. Oxygen budget was calculated based on physical, chemical and biological processes using the observed data. Along with the change of the water column structure at the time of a hit of typhoon, dominant phytoplankton species shifted from the diatom Skeletonema costatum to the dinoflagellate Heterocapsa circularisquama. During the diatom-dominating period, oxygen supply rate in the lower layer due to photosynthesis was comparable to or slightly lower than the sediment oxygen consumption rate. In contrast, during the dominance of the dinoflagellate, net oxygen budget was significantly negative in the lower layer while it was positive in the upper layer. This could be attributed to the migration behavior of the dominant dinoflagellate H. circularisquama that swim up to the upper layer and produce oxygen in daytime, and swim down to the lower layer and consume oxygen in nighttime. The results of the present study suggest that phytoplankton migration behavior can enhance the development of oxygen depleted water mass in the lower layer of eutrophic shallow coastal seas.

The effects of infaunal biodiversity on biogeochemistry of coastal marine sediments

We tested the role of benthic infaunal functional diversity in regulating the biogeochemistry of nearshore sediments using laboratory microcosms. Single and multispecies assemblages of deposit‐feeding polychaetes (Clymenella torquata, Spio setosa, and Leitoscoloplos fragilis) were used, and fluxes of oxygen and phosphate, as well as profiles of oxygen and pH in the sediment pore water, were measured. Significant differences in flux rates were found among treatments that were unrelated to polychaete abundance or biomass alone. Multispecies assemblages had lower flux rates of both oxygen and phosphate than rates calculated from the single‐species treatments. Depth integrated oxygen and pH profiles also showed significant differences between multispecies assemblages and selected single‐species treatments. These differences were most likely from species‐specific feeding and burrowing behavior and species‐related interactions. Coefficients of variance for both oxygen and pH were highest for microcosms with no polychaetes and lowest for the assemblages, indicating a dampening effect of multispecies assemblages on porewater heterogeneity. When oxygen flux data was incorporated into a model of oxygen dynamics in Long Island Sound, results indicated that shifts in the benthic community composition could change sediment oxygen consumption rates sufficiently to disrupt the balance between the physical supply of oxygenated water and biological oxygen demand. The results of this study confirm the importance of benthic functional biodiversity to nearshore sedimentary processes and suggest that losses of functional diversity can have significant effects on ecosystem function.

Respiration patterns in the deep ocean

The biological pump affects atmospheric CO2 levels and fuels most heterotrophic activity in the deep ocean. The efficiency of this pump depends on the rate of carbon fixation, export out of the euphotic zone and the depth of respiration. Here we study the depth dependence of respiration patterns, hence particulate carbon flux, using a compiled data set of sediment oxygen consumption rates. We show that the depth relationship can best be described by a double exponential model. For the upper part of the ocean, our resulting equation is similar to previous flux‐depth relationships but predicted fluxes are significantly larger in deeper waters. This implies a more efficient biological pump. Total oceanic respiration below the shelf break (200 m) is estimated to be 827 Tmol O2 yr−1.

The effects of shrimp trawling on sediment oxygen consumption and the fluxes of trace metals and nutrients from estuarine sediments

The effects of shrimp trawling on sediment oxygen consumption and the sediment–water exchange fluxes of nutrients (ammonium, phosphate and silicate) and trace metals (Mn, Ni, Cu, Cd and Pb) were determined for two separate experiments conducted during July 1998 and May 1999. Oxygen microelectrode measurements showed that during the 1998 experiment, the surface sediments were oxic down to depths of approximately 15 mm suggesting that the study area had not been recently trawled. However, after passage of the trawl gear oxygen was present to depths of only 5 mm. This suggests resuspension of the upper 1 cm of sediment and resulted in ammonium and manganese fluxes increasing by a factor of 2–3, which was attributed to a similar mechanism, i.e. a strengthening of the diffusive gradient. The measured fluxes of oxygen, phosphate and the trace metals Ni, Cu and Pb were not affected by trawling, while the flux of Cd was affected. During the 1999 shrimp trawl experiment, pre- and post-trawl oxygen penetration was limited to the upper 1 mm of sediment suggesting the area had been recently trawled. The diffusive fluxes of oxygen were in agreement with directly measured oxygen fluxes, both prior to and after trawling; further indication that trawling had little to no effect on the exchange of oxygen across the sediment–water interface during this time. Macrofaunal abundance was significantly reduced during 1999 and consistent with sediment oxygen consumption rates, which were also lower during this time. This resulted in the fluxes of oxygen, which were enhanced by biological processes during 1998, becoming diffusion controlled during 1999. This resulted in pre- and post-trawl fluxes of oxygen, ammonium, silicate, Mn, Ni, Cu and Pb, when considering the measurement error, which did not differ significantly. Cd again seemed affected by shrimp trawling activities. An increase or decrease in a trace metal's turnover time (the time in days required to replenish the water column concentration) can have important implications towards the overall health of an estuary. In Galveston Bay, trace metal turnover times averaged 0.4±0.5, 52±32, 27±15, 22±23 and 5.3±3.0 d for Mn, Ni, Cu, Cd and Pb, respectively. These relatively short turnover times indicate that the sediments can be an important source of trace metals to the overlying water column, especially during low-flow periods of the Trinity River, which can sometimes last more than 1 yr. This, in turn, could possibly lead to the non-conservative mixing behaviors, for both nutrients and trace metals, previously reported for the Trinity Bay region of Galveston Bay, as conservative mixing is observed in the lower estuary. Our study shows that the effects of shrimp trawling are largely dependent on the prevailing sediment redox conditions due to the shallow penetration depth of both oxygen and the trawl gear. Therefore, it is unlikely that trawling activities adversely affect the overall health of Galveston Bay. However, repeated trawling with removal of the upper oxic sediment layers could trend surface sediments towards anoxia and ultimately lead to changes in benthic–pelagic coupling.

Primary production of benthic microalgae in two shallow coastal lagoons of different trophic status in the southern Baltic Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 178:179-191 (1999) - doi:10.3354/meps178179 Primary production of benthic microalgae in two shallow coastal lagoons of different trophic status in the southern Baltic Sea Jürgen Meyercordt*, Lutz-Arend Meyer-Reil Institut für Ñkologie, Ernst-Moritz-Arndt-Universität Greifswald, D-18565 Kloster/Hiddensee, Germany *E-mail: meyercor@rz.uni-greifswald.de ABSTRACT: Primary production of undisturbed sediments, taken from 2 shallow coastal lagoons in the southern Baltic Sea, was measured in situ and in the laboratory. In the highly eutrophicated and turbid water (mean attenuation coefficient = 2.97 m-1) of Kirr-Bucht (Darss-Zingster Bodden), chlorophyll a concentrations were about 8 times higher than at moderately eutrophicated (mean attenuation coefficient = 0.66 m-1) Rassower Strom (Nordrügensche Bodden). The difference between the 2 areas in terms of chlorophyll a concentrations of the sediment was smaller (only about 2.5 times higher at Kirr-Bucht). Sediment oxygen consumption rates were similar at both sampling stations, showing a marked seasonal trend. Hourly rates of gross production, measured by in situ incubations, revealed maximal values of 30.0 and 17.6 mg C m-2 h-1 for sediments of Kirr-Bucht and Rassower Strom, respectively. Agreement between production rates measured in situ and in laboratory incubations was acceptable (R2 = 0.833). Irradiance dependency of gross production could be well described by 2 parameters, α (slope of light-saturation curve at low irradiance) and Pm (photosynthetic capacity), of a photosynthesis model. Calculated annual rates for Kirr-Bucht (0.6 m water depth) were 60 g C m-2 yr-1 for gross production and 5 g C m-2 yr-1 for net production. For Rassower Strom (3.4 m water depth), 23 g C m-2 yr-1 for gross production and -31 g C m-2 yr-1 for net production were calculated. If production rates were extrapolated to irradiance conditions at comparable water depths at both sampling locations (e.g. 1 m), the detrimental effect of the eutrophicated water column of Kirr-Bucht on benthic primary production became evident in the gross production rates (Kirr-Bucht: 25 g C m-2 yr-1, Rassower Strom: 51 g C m-2 yr-1) and was even more significant for the net production rates (Kirr-Bucht: -30 g C m-2 yr-1, Rassower Strom: -3 g C m-2 yr-1). At a depth of 1 m, a much higher deficit in the carbon balance for the benthic system of Kirr-Bucht was calculated than for Rassower Strom. Therefore, benthic community respiration at the highly eutrophicated location could not by a large margin be balanced by authochthonous benthic primary production, whereas, at the moderately eutrophicated location, benthic respiration and production were nearly balanced. KEY WORDS: Primary production · Microphytobenthos · Irradiance · Eutrophication · Estuary · Baltic Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 178. Publication date: March 17, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consumption, macro- and meiofauna dynamics

Benthic activity was examined at three stations (18 m water depth) in the northwestern Adriatic Sea. Carbon mineralisation rates, as based on sediment oxygen consumption rates, ranged from 54 to 89 g C m −2 y −1. The relatively high carbon mineralisation rates, large macrofaunal biomass (9 to 16 g C m −2) and macrofaunal production (11 to 19 g C m −2 y −1) provide evidence of high organic-matter input and intense benthic–pelagic coupling. This is further supported by the high dominance of the suspension-feeding bivalve Corbula gibba, which accounts for 52 to 63% of the total annual macrofaunal biomass production. Although the infaunal distribution of total macrofauna showed a sharp decline in densities and biomass with depth into the sediment, different patterns within the dominant taxa were observed. Whilst the bivalve Corbula gibba and the amphipod Ampelisca sp. were restricted to the surface layer, other species such as the dominant bivalve Mysella sp. and the gastropod Hyala sp. were not confined to a specific depth level and the majority of the populations occurred deeper than 5 cm into the sediment. Bioturbation, based on the occurrence of macrofauna, extended to at least 20 cm. Nematodes and foraminifera together formed 80 to 90% of the meiofaunal community in the upper 5 cm of the sediment. Annual mean densities ranged from 3.40 to 6.07×10 6 ind. m −2. Maximum abundance of meiofauna was not encountered at the station where maximum macrofaunal activity was recorded, and this could reflect the negative effect of biological interaction on meiofaunal densities in areas that have a high food supply.

Mineralization and burial of organic carbon in sediments of the southern Weddell Sea (Antarctica)

Benthic fluxes of oxygen, alkalinity (AT), total carbonate (CT or ΣCO 2) and dissolved organic carbon (DOC) were measured during sediment-water incubations at 16 stations in the southern Weddell Sea (Antarctica) with water depths between 280 and 2514 m. The total sediment oxygen consumption rates (TSOC) were in general low (1.74-3.61 mmol m −2 day −1) and more comparable to measurements in slope and deep-sea sediments at a few thousand meters water depth. The decrease of TSOC with water depth was lower than that observed in many other seas. The mean carbon to nitrogen ratio (C/N) in the solid phase of surficial sediment was 8.3. Measured benthic fluxes of alkalinity, corrected for contributions from nitrification and denitrification, were quantitatively used to correct the fluxes of total carbonate for dissolution of solid phase carbonates. The ΣCO 2 fluxes, originating from carbonate dissolution (0.1661–1.77 mmol m −2 day −1 were 2.6–71 % of the ΣC0 2 fluxes (0.984–3.73 mmol m −2 day −1) resulting from organic carbon oxidation. Measured benthic fluxes of oxygen, ΣC0 2 and nitrate were, together with estimated denitrification rates and sediment C/N ratios, used to model respiration quotients (RQ) for organic carbon oxidation and estimate composition of the organic matter undergoing degradation. Modelled RQ varied roughly between 2/3 and 1 (mean 0.87). Measured fluxes of ΣC0 2 were 1.6–3.2 times higher than integrated organic C mineralization rates (measured during closed incubations of sieved, homogenized sediment), indicating macrofaunal (plus possibly meiofaunal) respiration to be important. However, low abundances of bioirrigating benthic macrofauna and small differences in benthic fluxes of oxygen, ΣCO 2 and alkalinity found between replicate sediment cores, suggested that macrofaunal respiration was quantitatively unimportant in these sediments. The higher measured fluxes of ΣC0 2 compared to the integrated mineralization rates, were therefore most likely caused by a large fraction of the respiration occurring directly on the sediment surface. This degradation of newly deposited organic matter was not reflected in the integrated organic C mineralization rates. Also, there was no obvious effect of this surficial degradation process on the pore water distributions of ΣCO 2. Benthic mass balances of carbon revealed that benthic fluxes of DOC were 3–147% of the corrected fluxes of ΣCO 2, and the recycling efficiencies (E) were up to 35% higher if the DOC fluxes were included in the calculations of E, rather than the inorganic ΣCO 2 flux alone. The recycling efficiencies, including the benthic flux of DOC, ranged between 57 and 88% (mean 78%). Measured rates of inorganic C accumulation (for most stations <0.3 mmol C m −2 day −1 were a factor of 6–7 lower than organic C accumulation rates (0.457–1.94 mmol C m −2 day −1).

Sediment-Water Oxygen and Nutrient Exchanges along the Longitudinal Axis of Chesapeake Bay: Seasonal Patterns, Controlling Factors and Ecological Significance

Sediment-water oxygen and nutrient (NH4 +, NO3 −+NO2 −, DON, PO4 3−, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l−1, apparently limiting SOC rates. Fluxes of NH4 + were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations ( 500 μmol N m−2 h−1) were observed. Nitrate + nitrite (NO3 −+NO2 −) exchanges were directed into sediments in areas where bottom water NO3 −+NO2 − concentrations were high (>18 μM N); sediment efflux of NO3 −+NO2 − occurred only in areas where bottom water NO3 −+NO2 − concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m−2 h−1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH4 −, PO4 3−, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.