MEPS 122:265-276 (1995)  -  doi:10.3354/meps122265

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

Conley DJ, Johnstone RW

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-1. The experiments were designed to simulate the deposition of organic matter that occurs during a typical spring diatom bloom (1x) and under enriched conditions with eutrophication 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 (NH4+), and dissolved inorganic phosphate (DIP) increased significantly (ANOVA, p < 0.01). For nitrite + nitrate (NO2- + 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 NO2- + 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 (1x) 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. NH4+ 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 material 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 biogeochemical cycles.


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