Resuspension, ephemeral mud blankets and nitrogen cycling in Laholmsbukten, south east Kattegat

Experimental studies of intact cores from the Baltic Sea were conducted to determine the response of sediment nutrient recycling processes to varied inputs of organic matter. A 2 mo enrichment experiment was carried out in the laboratory on sediment cores held at 4°C using a flow-through system where overlying waters were continuously replaced at a rate of 1 d.' The experiments were designed to simulate the deposition of organic matter that occurs during a typical spring diatom bloom ( l x ) and under enriched conditions with eutrophicatlon at approximately 3 times (3x) a normal spring bloom utilizing added organic matter from a natural phytoplankton assemblage collected in a eutrophic coastal fjord during the spring diatom bloom. Low and constant sediment-water fluxes were observed throughout the duration of the experiment in control cores with no added organic matter. In all cases an immediate response was noted when a single pulsed addition of algal material was added to the sediment surface. Sediment-water fluxes of ammonium (NHdt), and dissolved inorganic phosphate (DIP) increased significantly (ANOVA, p < 0.01). For nitrite + nitrate (NO; + NO3-) and dissolved silicate (DSi) sediment-water fluxes, differences were initially observed; however, only the NO2-+ NO3-fluxes were significantly different over time (ANOVA, p < 0.01). Fluxes of NOz-+ NO3-were into the sediment for 3 to 10 d after addition of organic material, followed by small fluxes out of the sediment. The addition of algal material proportionate to a normal spring bloom ( l x ) had only a minor effect on porewater nutrient concentrations, whereas the 3x treatment substantially modified both the short-and long-term response of sediments. A greater proportion of anaerobic decomposition products, e.g. NH,' and DIP, were observed with an expansion of more reducing conditions resulting from the addition of organic matter. The percentage of Si remineralized decreased as the flux of n~aterial to the sediment increased. Deposition rates similar to a typical spring bloom did not have long-term effects on the nutrient recycling processes; however, increases in the present level of deposition (as simulated in this study), which are forecasted with further eutrophication in the Baltic Sea, may have a significant impact on nutrient biogeochernical cycles.

Section: Characterization Of Added Algal Materialsmentioning

confidence: 99%

Section: Extrapolation To the Baltic Seamentioning

confidence: 99%

Biogeochemistry of N, P and Si in Baltic Sea sediments:response to a simulated deposition of a spring diatom bloom

Dj¹,

Johnstone²

1995

“…Muddy sediment was characterized by a rather high organic content (around 4 % org. C), a high oxygen consumption, probably an ephemeral nature of the muddy layer (Floderus & HBkanson 1989), and absence of bioturbation. In darkness, such a sediment would only be oxic in the top few millimeters (Jerrgensen & Revsbech 1985), and only there would NH4+ oxidation take place.…”

Section: Nutrient Fluxes and Sediment Typementioning

confidence: 99%

Influence of sublittoral microphytobenthos on the oxygen and nutrient flux between sediment and water. A laboratory continuous-flow study

Sundbäck¹,

Enoksson²,

Granéli³

et al. 1991

166

Influence of sublittoral microphytobenthos on the flux of oxygen and inorganic nutrients ( N , P, Si) at the sediment-water interface was studied using undisturbed cores of sandy and muddy sediment incubated in a laboratory continuous-flow system, either in darkness or with a 16/8 h L/D cycle at in situ light level during summer. Sediment was collected In July at 15 m depth in a non-tidal, stratified bay in SE Kattegat. To test whether the higher content of inorganic nutrients below the halocline, compared with surface waters, could stimulate microphytobenthic growth, 2 levels of nutrient concentrations were used. Diel variations were found In L/D cores, but not in darkened cores, for oxygen, dissolved inorganic nitrogen and phosphorus content in the water overlying the sediment. The flux of NH4+, NO3-and ~0~~-out of the sediment decreased during light periods and occasionally a net uptake was recorded. Light-induced O2 production, and correlations between A fluxes (differences between day and night fluxes of 0, and nutrients), chlorophyll a content and algal cell numbers in the sediment, indicate that the decreased outflux of IN and POA3-was mediated by photosynthetic organisms. Diel variations were not studied for silicon, but a significantly lower outflux, or even an uptake, of Si(OH), from L/D cores supports this conclusion. This suggests that diatoms play a major role in the nutrient flux between sediment and water. Also, the differences in pore-water nutrient gradients between LID and dark cores point to the importance of sediment-associated organisms. Daily (24 h) net fluxes of nutrients were primarily out of the sediment, but the magnitude depended on both light conditions and sediment type. Daily net outflux was significantly lower in U D cores than in darkened cores for all nutrients except NO3-in muddy cores and NO2-in sandy cores. Net uptake in L/D cores was recorded for Si(OH)4 and NO3-In sandy sediment. Outflux of nutrients was significantly higher from muddy sediments in comparison w~t h sandy sediments (except NO3-), especially in permanent darkness. No significant effect of nutrient enrichment on the abundance of sublittoral benthic microalgae could be shown. Results suggest that m~crophytobenthos can influence sediment-water exchange of inorganic nutrients even at sublittoral depths, and when measuring nutrient flux in permanently darkened cores from depths around 15 m in the Kattegat, summer flux rates will be overestimated by a factor varying between 2 and 6, depending on sediment type.

“…Equally, deposition of algal phytodetritus provokes sediment nutrient exchange by microbial remineralisation and gradientdriven diffusion. While these findings are valuable to coastal resource managers, relatively few workers have considered the potential hydrodynamical role in sediment nutrient processing (Floderus & Håkanson 1989, Vorosmarty & Loder 1994, Nielsen et al 1995, Sloth et al 1996, Asmus et al 1998. Laboratory studies by Christiansen et al (1997) showed that resuspension changes the diffusive sediment water fluxes of nutrients and oxygen consumption.…”

Section: Introductionmentioning

confidence: 99%

Physico-biogeochemical controls on benthic-pelagic coupling of nutrient fluxes and recycling in a coastal upwelling system

Dale¹,

Prego²

2002

-1 ), probably as a result of the bed sediment resuspension engendered by upwelling and stirring of phytodetrital fluff held in suspension as neutrally buoyant material above the sediment surface. We hypothesise that hydrodynamical processes play an important role in determining the quantity of nutrients remineralised in the Pontevedra ria and, in the case of nitrogen, the rate of denitrification at the benthic boundary layer.