Anders Tengberg

Resuspension and its effects on organic carbon recycling and nutrient exchange in coastal sediments: in situ measurements using new experimental technology

The effect of resuspension on sediment–water solute exchange was studied in the archipelago outside the city of Goteborg (Sweden) during the winter months 2001/2002 by a combination of benthic flux (employing both in situ and laboratory incubations), meteorological and hydrodynamic measurements. Resuspension was created in in situ incubation chambers to study the possible resuspension effect on the fluxes of nutrients, oxygen and total carbonate. In most cases, resuspension changed the flux rates. The fluxes of oxygen, total carbonate and phosphate decreased during the resuspension event. The oxygen fluxes before resuspension were between 15 and 23 mmol m−2 day−1 in the four chambers, and decreased during resuspension to between 8 and 18 mmol m−2 day−1. The fluxes of total carbonate in the chambers before resuspension were 12–35 mmol m−2 day−1 and decreased to 4–15 mmol m−2 day−1. The change for each flux during resuspension was statistically significant. The fluxes of nitrate–nitrite and silicic acid increased, whereas the fluxes of ammonium did not change significantly. Hydrodynamic and meteorological data showed that the natural created resuspension at the study site were not due to wind-induced waves, but to strong bottom currents that can be caused when water built-up against land during strong wind events. When the wind abates, the water is flushing back and creates strong currents high enough to cause resuspension of the bottom sediments. A major storm passing the area during the experimental period caused particulate material to suspend to a concentration of about 11 mg/l in the bottom water, which was similar to the suspended material concentration in one set of the incubation chambers.

Benthic biogeochemistry: state of the art technologies and guidelines for the future of in situ survey

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists. Our understanding of sediment biogeochemistry and exchange fluxes was revolutionized by the introduction of benthic chambers and in situ micro-electrode profilers that allow for the direct measurement of chemical fluxes between sediment and water at the sea floor and for porewater composition. Since then, rapid progress in the technology of in situ sensors and benthic chambers (such as the introduction of gel probes, voltammetric electrodes or one- and two-dimensional optodes) have yielded major breakthroughs in the scientific understanding of benthic biogeochemistry. This paper is a synthesis of discussions held during the workshop on sediment biogeochemistry at the “Benthic Dynamics: in situ surveillance of the sediment–water interface” international conference (Aberdeen, UK—March 25–29, 2002). We present a review of existing in situ technologies for the study of benthic biogeochemistry dynamics and related scientific applications. Limitations and possible improvement (e.g., technology coupling) of these technologies and future development of new sensors are discussed. There are countless important scientific and technical issues that lend themselves to investigation using in situ benthic biogeochemical assessment. While the increasing availability of these tools will lead research in yet unanticipated directions, a few emerging issues include greater insight into the controls on organic matter (OM) mineralization, better models for the understanding of benthic fluxes to reconcile microelectrode and larger-scale chamber measurements, insight into the impacts of redox changes on trace metal behavior, new insights into geochemical reaction pathways in surface sediments, and a better understanding of contaminant fate in nearshore sediments.

The benthic silica cycle in the northeast Atlantic: annual mass balance, seasonality, and importance of non-steady-state processes for the early diagenesis of biogenic opal in deep-sea sediments

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 μM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment–water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment–water interface and the annual DSi efflux was close to 0.057 mol Si m−2 yr−1. Biogenic silica accumulation was close to 0.008 mol Si m−2 yr−1 and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si m−2 yr−1. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si m−2 yr−1). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.

Factors influencing organic carbon recycling and burial in Skagerrak sediments

Different factors influencing recycling and burial rates of organic carbon (OC) were investigated in the continental margin sediments of the Skagerrak (NE North Sea). Two different areas, one in the southern and one in the northeastern part of the Skagerrak were visited shortly after a spring bloom (March 1999) and in late summer (August 2000). Results suggested that: (1) Organic carbon oxidation rates (C ox ) (2.2-18 mmol C m -2 d -1 ) were generally larger than the O 2 uptake rates (1.9-25 mmol m -2 d -1 ). Both rates were measured in situ using a benthic lander. A mean apparent respiration ratio (C ox :O 2coor ) of 1.3 ± 0.5 was found, indicating some long-term burial of reduced inorganic substances in these sediments. Measured O 2 fluxes increased linearly with increasing C ox rates during the late summer cruise but not on the early spring cruise, indicating a temporal uncoupling of anaerobic mineralization and reoxidation of reduced substances. (2) Dissolved organic carbon (DOC) fluxes (0.2-1.0 mmol C m -2 d -1 ) constituted 3-10% of the C ox rates and were positively correlated with the latter, implying that net DOC production rates were proportional to the overall sediment OC remineralization rates. (3) Chlorophyll a (Chl-a) concentrations in the sediment were significantly higher in early spring compared to late summer. The measured C ox rates, but not O 2 fluxes, showed a strong positive correlation with the Chl-a inventories in the top 3 cm of the sediment. (4) Although no relationship was found between the benthic fluxes and the macrofaunal biomass in the chambers, total in situ measured dissolved inorganic carbon (C T ) fluxes were 1-5.4 times higher than diffusive mediated C T fluxes, indicating that macrofauna have a significant impact on benthic exchange rates of OC remineralization products in Skagerrak sediments. (5) OC burial fluxes were generally higher in northeastern Skagerrak than in the southern part. The same pattern was observed for burial efficiencies, with annual means of ∼62% and ∼43% for the two areas respectively. (6) On a basin-wide scale, there was a significant positive linear correlation between the burial efficiencies and sediment accumulation rates. (7) The calculated particulate organic carbon (POC) deposition, from benthic flux and burial measurements, was only 24-78% of the sediment trap measured POC deposition, indicating a strong near-bottom lateral transport and resuspension of POC. (8) A larger fraction of the laterally advected material of lower quality seemed to seetle in the northeastern Skagerrak rather than in the southern Skagerrak. (9) Skagerrak sediments, especially in the northeastern part, act as an efficient net sink for organic carbon, even in a global continental margin context.

Imbalance in the carbonate budget of surficial sediments in the North Atlantic Ocean: variations over the last millenium?

Abstract Fluxes contributing to the particulate carbonate system in deep-sea sediments were investigated at the BENGAL site in the Porcupine Abyssal Plain (Northeast Atlantic). Deposition fluxes were estimated using sediment traps at a nominal depth of 3000 m and amounted to 0.37±0.1 mmol C m−2 d−1. Dissolution of carbonate was determined using flux of total alkalinity from in situ benthic chambers, is 0.4±0.1 mmol C m−2 d−1. Burial of carbonate was calculated from data on the carbonate content of the sediment and sedimentation rates from a model age based on 14C dating on foraminifera (0.66±0.1 mmol C m−2 d−1). Burial plus dissolution was three times larger than particle deposition flux which indicates that steady-state is not achieved in these sediments. Mass balances for other components (BSi, 210Pb), and calculations of the focusing factor using 230Th, show that lateral inputs play only a minor role in this imbalance. Decadal variations of annual particle fluxes are also within the uncertainty of our average. Long-term change in dissolution may contribute to the imbalance, but can not be the main reason because burial alone is greater than the input flux. The observed imbalance is thus the consequence of a large change of carbonate input flux which has occured in the recent past. A box model is used to check the response time of the solid carbonate system in these sediments and the time to reach a new steady-state is in the order of 3 kyr. Thus it is likely that the system has been perturbed recently and that large dissolution and burial rates reflect the previously larger particulate carbonate deposition rates. We estimate that particulate carbonate fluxes have certainly decreased by a factor of at least 3 and that this change has occurred during the last few centuries.

Lake Metabolism: Comparison of Lake Metabolic Rates Estimated from a Diel CO2- and the Common Diel O2-Technique.

Lake metabolism is a key factor for the understanding of turnover of energy and of organic and inorganic matter in lake ecosystems. Long-term time series on metabolic rates are commonly estimated from diel changes in dissolved oxygen. Here we present long-term data on metabolic rates based on diel changes in total dissolved inorganic carbon (DIC) utilizing an open-water diel CO2-technique. Metabolic rates estimated with this technique and the traditional diel O2-technique agree well in alkaline Lake Illmensee (pH of ~8.5), although the diel changes in molar CO2 concentrations are much smaller than those of the molar O2 concentrations. The open-water diel CO2- and diel O2-techniques provide independent measures of lake metabolic rates that differ in their sensitivity to transport processes. Hence, the combination of both techniques can help to constrain uncertainties arising from assumptions on vertical fluxes due to gas exchange and turbulent diffusion. This is particularly important for estimates of lake respiration rates because these are much more sensitive to assumptions on gradients in vertical fluxes of O2 or DIC than estimates of lake gross primary production. Our data suggest that it can be advantageous to estimate respiration rates assuming negligible gradients in vertical fluxes rather than including gas exchange with the atmosphere but neglecting vertical mixing in the water column. During two months in summer the average lake net production was close to zero suggesting at most slightly autotrophic conditions. However, the lake emitted O2 and CO2 during the entire time period suggesting that O2 and CO2 emissions from lakes can be decoupled from the metabolism in the near surface layer.

Versatile use of port and harbor MetOcean systems for safety, environmental monitoring, science and recreation

Online monitoring of meteorological and oceanographic parameters are done in many ports and harbors. Typically the collected data has been restricted to usage in the daily operation of the local port. Some recent ports and harbor installations, that will be presented here, have successfully combined the traditional navigational use of data with environmental monitoring and scientific investigations. The data has also been made available to the public over the Internet and on so called WAP servers (available through cellular phones).

Accurate oxygen monitoring made possible with optodes

To measure oxygen optically, with so called optodes, is today an established technique in aquatic science and monitoring. Since this technology was made commercially available almost three years ago it has proven itself through numerous applications. The accuracy and precision is comparable to Winkler titrations(Winkler L.W (1888) the overall standard in measuring dissolved oxygen since 1888) and the sensors are stable for years. Because of the high accuracy, the long term stability, a low sensitivity to fouling and a low and predictable pressure behavior this technology has opened new possibilities in aquatic science and monitoring. A further development of this technique can also be used to asses oxygen distributions in two dimensions at the sediments-water interface.

Studies of fluxes of dissolved iron and manganese in the Gulf of Finland

Metal fluxes (total dissolved iron and manganese) across the water-sediment interface were studied in situ in the Gulf of Finland in June 2002 and 2003 using an autonomous benthic lander. Distribution of these metals in the sediment and the porewater was also investigated. Focus was given to understand the behavior of the studied metals at various redox conditions and the influence when sediment resuspension is occurring. Our data indicate the importance of the redox conditions for the iron fluxes with no fluxes at oxic, intermediate at anoxic and high fluxes at suboxic conditions. The fluxes of manganese do not seem to have any correlation with the oxygen levels. Both metals were significantly affected by resuspension, which led to a rapid release.