Frede Østergaard Andersen

Determination of organic carbon in marine sediments: a comparison of two CHN-analyzer methods

Abstract Two commonly used methods for the separation of organic (OC) and inorganic carbon (IC) have been compared and discussed. Measurement of OC in marine sediments by automated CHN-analysis is dependent on precise and fast methods for the separation of OC and IC. The difference-on-ignition (DOI) and the HCl-acidification (HCl) methods produced similar measures of OC and C: N ratio on sediment samples of different OC and IC composition. A clear separation of sedimentary OC and IC was observed in a loss-on-ignition (LOI) temperature scan from 100 to 800 °C, confirming the applicability of the DOI method. We recommend the DOI method for routine measurements of OC in marine sediments, because the more elaborate HCl method produces high and unstable baselines on the recorded CHN-chromatographs and is liable to corrode the interior of CHN-analyzers.

Colonization of submerged macrophytes in shallow fish manipulated Lake Væng: impact of sediment composition and waterfowl grazing

Abstract The impact of sediment composition and waterfowl grazing on colonization of submerged macrophytes in shallow biomanipulated Lake Vaeng was studied by means of growth experiments: curly leaved pondweed ( Potamogeton crispus L.) was transplanted to separate pots containing each type of sediment found in Lake Vaeng, and the pots were planted at two locations, one sheltered and one exposed. Half of macrophyte pots were behind a fence to protect them against grazing by water- fowl (mainly coots, Fulica atra L.). Sediment composition had some impact on macrophyte growth, being highest on mud from the centre of the lake. The fenced-in macrophytes became up to 6.5 times longer than the unfenced macrophytes. It was concluded that macrophytes were able to grow on all substrates present in Lake Vaeng, but waterfowl grazing probably controlled and delayed macrophyte colonization.

Chemical Lake Restoration Products: Sediment Stability and Phosphorus Dynamics

Laboratory experiments with sediments from three shallow Danish lakes were conducted to evaluate the effects of chemical lake restoration products during resuspension. Phosphorus (P) removal, sediment stability, sediment consolidation and color reduction were studied over time. The investigated products were aluminum (Al), Phoslock (a commercial bentonite product coated with lanthanum) and a combination of Al covered with bentonite (Al/Ben). All treatments effectively reduced the P concentration in the water. However, the treatments containing Al reduced the P concentration immediately after resuspension, whereas Phoslock required several days after resuspension to reduce the P concentration. Especially Phoslock, but also Al/Ben, increased the sediment stability threshold by 265% and 101%, respectively, whereas Al had no stabilizing effect. The fresh Al floc was resuspended 5x easier than untreated sediment. The largest consolidation of the sediment occurred with addition of Phoslock, followed by Al/Ben, while Al alone had no effect. Enhanced consolidation may be of importance for macrophyte colonisation of organic sediment. Phoslock improved the light climate moderately by removing color, whereas Al was very effective in removing color. Ben/Al showed intermediate effects on color reduction. These findings are important when decisions are made on restoration method for a specific lake, which may be more or less wind exposed.

Effects of aluminum, iron, oxygen and nitrate additions on phosphorus release from the sediment of a Danish softwater lake

The effects of oxygen, aluminum, iron and nitrate additions on phosphate release from the sediment were evaluated in the softwater Lake Vedsted, Denmark, by a 34-day experiment with undisturbed sediment cores. Six treatments were applied: (1) Control - O2 (0–20% saturation), (2) O2 (100% saturation) (3) Al3+ – O2, (4) Fe3+ + O2, (5) Fe3+ – O2, and (6) NO3− – O2. Al2(SO4)3*18 H2O and FeCl3*4H2O were added in amounts that theoretically should immobilize the exchangeable P-pool in the top 5 cm of the sediment, while sodium nitrate concentrations were increased to 5 mg N l−1. The four treatments with metals or NO3− reduced the P efflux from the sediment significantly as compared to the suboxic control treatment. Mean accumulated P-release rates for suboxic treatments with Al3+, Fe3+, and NO3− were: –0.27 mmol m−2 (st. dev = 0.02 mmol m−2, N = 5), 0.58 mmol m−2 (st. dev = 0.30 mmol m−2, N = 5) and 1.40 mmol m−2 (st. dev = 0.14 mmol m−2, N = 5), respectively. The oxic treatment with Fe3+ had a P efflux of 0.36 mmol m−2 (st. dev = 0.08 mmol m−2, N = 5). The two highest P-release rates were observed in the control treatment and the treatment with O2 (14.50 mmol m−2 (st. dev = 3.90 mmol m−2, N = 5) and 2.31 mmol m−2 (st. dev = 0.80 mmol m−2, N = 5), respectively). In order to identify changes in the P and Fe binding sites in the sediment as caused by the treatments, a sequential P extraction procedure was applied on the sediment before and after the efflux experiment. Addition of O2, Fe3+ and NO3− to the sediment increased the amounts of oxidized Fe3+ and P→BD. Al3+ addition resulted in a lower fraction of P→BD but a correspondingly higher fraction of Al-bound P. Addition of Al3+ decreased the Fe-efflux from the suboxic sediment as well as the amount of oxidized Fe3+ in the sediment. This questions the use of Al compounds that contain sulfate because of the possible formation of FeS, which will restrict upward migration of Fe2+ and the formation of new Fe-oxides in the surface sediment. Instead, we suggest the use of AlCl3 for lake restoration purposes.

Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water

Effects of pH, alkalinity and conductivity on the adsorption of soluble reactive phosphorus (SRP) onto lanthanum (La) modified bentonite clay (Phoslock(®)) were investigated in laboratory experiments using eight different types of filtered water representing freshwater with low and normal alkalinity and brackish water with high alkalinity. Different dose ratios (0-200; w/w) of Phoslock(®):P were applied to determine the maximum P binding capacity of Phoslock(®) at SRP concentrations typical of those of sediment pore water. The 100:1 Phoslock(®:)P dose ratio, recommended by the manufacturer, was tested with 12 days exposure time and generally found to be insufficient at binding whole target SRP pool. The ratio performed best in the soft water from Danish Lake Hampen and less good in the hard water from Danish Lake Langesø and in brackish water. The explanation may be an observed negative relationship between alkalinity and the SRP binding capacity of Phoslock(®). A comparative study of Lake Hampen and Lake Langesø suggested that the recorded differences in P adsorption between the two lakes could be attributed to a more pronounced dispersion of Phoslock(®) in the soft water of Lake Hampen, leading to higher fractions of dissolved (<0.2 μm) La and of La in fine particles. In the same two lakes, pH affected the SRP binding of Phoslock(®) negatively at a pH level above 8.1, the effect being reversible, however. The negative pH effect was most significant in hard water Lake Langesø, most likely because of higher [Formula: see text] concentrations.

Occurrence of orthophosphate monoesters in lake sediments: significance of myo- and scyllo-inositol hexakisphosphate.

Orthophosphate monoesters often constitute a significant fraction of total phosphorus in lake sediments. The knowledge on the specific composition and recalcitrance of these compounds is however limited. The main aim was therefore to identify and quantify specific orthophosphate monoesters in sediment from 15 Danish lakes by solution (31)P NMR spectroscopy. The four most quantitatively important orthophosphate monoesters were myo-inositol hexakisphosphate (myo-IP(6)), scyllo-inositol hexakisphosphate (scyllo-IP(6)) α-glycerophosphate (α-GP) and β-glycerophosphate (β-GP). The compounds were identified in 9, 4, 8 and in all 15 lakes, respectively. In total these four components made up 46-100% of the orthophosphate monoester pool. The glycerophosphates (GPs) are most likely degradation products of phospholipids, created as an artifact by the alkaline extraction procedure used for (31)P NMR spectroscopy, while the inositol hexakisphosphates (IPs) are naturally occurring compounds. There was a significant positive correlation between myo-IP(6) and total aluminium in the sediment and a negative correlation between myo-IP(6) and lake water pH, suggesting that myo-IP(6) is stabilized in the sediment by adsorption at slightly acidic or neutral conditions. In three lakes, the depth distribution of the orthophosphate monoesters was investigated. The content of scyllo-IP(6) and myo-IP(6) was constant with sediment depth in two of the lakes while the content of myo-IP(6) decreased with depth in one of the lakes. In all cases the IPs seem to be preserved with sediment depth to a higher extent than the orthophosphate diesters and especially the GPs suggesting that IPs can be a sink for phosphorus in the lake ecosystem or at least delay P-recycling for years.

Phosphorus dynamics during decomposition of mangrove (Rhizophora apiculata) leaves in sediments

Abstract Rhizophora apiculata leaf litter decomposition and the influence of this process on phosphorus (P) dynamics were studied in mangrove and sand flat sediments at the Bangrong mangrove forest, Phuket, Thailand. The remaining P in the mangrove leaf litter increased with time of decomposition to 174% and 220% of the initial amount in the litter in sand flat and mangrove sediment, respectively, although about 50% of the dry weight had been lost. The incorporation of P into the litter was probably associated with humic acids and metal bridging, especially caused by iron (Fe), which also accumulated in considerable amounts in the litter (5–10 times initial concentration). The addition of leaves to the sediment caused increased concentrations of dissolved reactive phosphate (DRP) in the porewater, especially in sand flat sediment. The DRP probably originated from Fe-bound P in the sediment, because decomposition of buried leaf litter caused increased respiration and reduced the redox potential (Eh) in the sediments. Binding of P to refractory organic material and oxidized Fe at the sediment–water interface explains the low release of DRP from the sediment. This mechanism also explains the generally low DRP concentration in the mangrove porewater, the low nutrient content of the R. apiculata leaves, but also the higher total sediment P concentration of the mangrove sediment as compared to sediments outside the mangrove. Both the low release rates for DRP from the sediment and the accumulation of P associated with leaf litter decomposition tend to preserve P in the sediments.

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.

Plant Bound Nutrient Transport. Mass Transport In Estuaries And Lagoons

In many regions of the world external nutrient loading to estuaries has recently been decreasing. These estuaries are only beginning to adjust to this change. The change is visible as better water quality. The period where the phytoplankton is growth limited has become longer, resulting in a better light climate at the bottom with the potential for recolonization of benthic macrophytes. This phase of recovery often shows a nonlinear behaviour and can be longer than expected (Borum 1996, 1997). National monitoring programs have been developed with the purpose of following the expected recovery phase. Nutrient loading, water quality and the export of nutrients from these systems are the main focus of this monitoring. For this reason, increasing attention has been paid to nutrient mass balances at the outer boundary of lagoons and estuaries. The measurements of estuarine mass balances have traditionally been limited to dissolved inorganic nutrients (ammonia, nitrite, nitrate, phosphate) and fine particulate matter fraction trapped on filters (e.g. glass fiber filters). Often no distinction is made between living and detrital particulate matter. This chapter will show that most of these nutrient mass balances are incomplete. The reason is that in shallow productive microand meso-tidal estuaries plant bound nutrient transport is essential for the nutrient mass balance (Flindt et al. 1997a, 1999, Salomonsen et al. 1997, 1999). Why is plant bound nutrient transport not included in the mass balances? A part of the explanation is that the plant matter accumulates at the bottom – and the transport afterward takes place as bedload transport. Although Odum et al. (1979) pointed out that import/export studies have failed to account for transport of particulate loads on or near the bed of estuaries, this transport is still often neglected. Another explanation is that when a piece of a macroalgae is infrequently trapped in a water sample of a few litres, this piece will usually be removed, because the sample is considered non-representative and would introduce a high variability among the samples. The inclusion of a 50 g wwt piece of Ulva in a 5 l water sample may increase the total nitrogen and total phosphorus concentration by a factor of 50. The problem with monitoring nutrient import/export from estuaries is primarily related to the sampling strategy. Instead of only filtrating 1-5 l water samples through a glass fiber filter, we also need to filter the water column horizontally using nets with a mesh size of 0.5-1.0 cm in diameter, and the filtered amount has to be between 100 to 10,000 m. The sampling strategy should also reflect the different depths in which the plants are transported in the water column.

Can artificial plant beds be used to enhance macroinvertebrate food resources for perch (Perca fluviatilis L.) during the initial phase of lake restoration by cyprinid removal

Restoration of shallow lakes to a clear-water state, often characterized by high submerged macrophyte cover and a high proportion of piscivores such as perch, Perca fluviatilis L., frequently involves removal of a large proportion of the zoobenthivorous fish, such as bream, Abramis brama L., and roach, Rutilus rutilus L. (i.e. biomanipulation). However, establishment of submerged macrophytes is often delayed following fish removal. This is unfortunate because plant beds typically host high densities of the macroinvertebrates constituting the diet of small perch and thus help perch to go through the bottleneck from feeding on macroinvertebrates to feeding on fish. Establishment of artificial plant beds may be a useful tool to enhance macroinvertebrate population growth and thus food resources for small perch until the natural plants have established. To investigate this restoration option, we studied during two growing seasons (June–October) the composition and abundance of the macroinvertebrate community in artificial plant beds installed in shallow Lake Væng (Denmark) comprising the initial phase of a biomanipulation effort by fish removal. Lake areas with artificial plant beds exhibited substantially higher macroinvertebrate densities than the lake bottom. This suggests that artificial plant beds may be used as feeding grounds for small perch, similarly to the well-known refuge effect for zooplankton against fish predation. In this way, artificial plant beds could help maintain a clear-water state during the transient period when natural submerged vegetation is not yet established in the lake.