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

Abstract

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.

Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon

“…Trends in NH 4 into the water column during hypoxia corroborate the benthic measurements performed at Portita (Fig. 24) and demonstrate that sediments underlying seasonally hypoxic waters are important sites of preferential nutrient regeneration and return to the water column, as previous investigators have shown (Conley and Johnstone, 1995;Cowan and Boynton, 1996;Rozan et al, 2002;Viktorsson et al, 2013). Importantly, the results show that diffusive losses of NH + 4 and PO 3− 4 are important under hypoxic conditions, whereas irrigation fluxes are more important under the oxygenated conditions found in winter.…”

Black spots produced by buried macroalgae in intertidal sandy sediments of the Wadden Sea: Effects on the meiobenthos

“…Eutrophication in estuaries and coastal waters is well documented and may explain the increased growth of certain species of epibenthic macroalgae that take advantage of such conditions (Schories & Reise, 1993;. Increasing degradable organic matter may strongly affect the biogeochemical cycling and nutrient processes (Conley & Johnstone, 1995) and lead to oxygen depletion (Kelly & Nixon, 1984), as well as to accumulation of toxic sulphide (Holmer & Kristensen, 1994;Osinga et aL, 1995) even at the surface sediment layer. Increasing organic matter may also cause long-term changes in the zoobenthic community (Beukema, 1991(Beukema, , 1992Michaelis & Reise, 1994) and even massive death of benthic fauna (Reise, 1983;Schories, 1991;Heip, 1995).…”

Enhanced benthic activity in sandy sublittoral sediments: Evidence from13C tracer experiments

“…In the northern temperate latitudes, large phytoplankton blooms occur in spring (Reid et al 1990), where sand is the predominant sediment type on the shelf. Due to the shallowness of shelf seas, up to 50% of this biomass can settle to the sea floor (Jørgensen et al 1990), building up the bulk of the food supply for the benthic community (Conley & Johnstone 1995). Most of the deposited material is directly mineralized in these sediments (Berner 1982;Jahnke et al 2000), causing low organic carbon contents and emphasizing the prominent role of the continental shelf in the marine carbon cycle.…”