Using a n outdoor flow-through experimental setup consisting of twelve 30 1 containers, effect of accumulation of drifting filamentous macroalgae on a shallow-water sediment system was studied for 3 w k after the addition of 0.9 (low dose) and 1.8 k g fresh wt m-' (high dose) of filamentous red algae. Estimates of structural changes were based on relationships between numbers and biomass of bacteria, autotrophic microflora, ciliates and meiofauna and their qualitative composition. Effects on the functional level were assessed by measuring primary productivity, changes in carbon pools, as well as oxygen and nutrient flux. The low-dose treatment did not significantly alter the composition or patterns of primary productivity and nutrient fluxes when compared with the control (no addition). The high-dose addit~on decreased the abundance of microalgae, cil~ates and me~ofauna, whereas no clear trend was seen for bacteria relative to the control. From the oxygen flux values it was apparent that the systems in control and low-dose containers were autotrophic (P > R), whereas in the high-dose treatments the oxygen concentration fell sharply, exhibiting a net oxygen consumption most of the time due to fast m~neralization of the macroalgal biomass. The ammonium concentration increased significantly (maximum mean rate 1 4 mm01 m-' d-l) and concomitantly with low oxygen val.ues. The content of particulate organic carbon (POC) in the top 5 mm sediment increased by 2 g m-' in both control and low-dose containers due to increased meiofaunal biomass, while in the high-dose treatments the POC content decreased by 2.3 g m-' From the results it appears that the influence of accumulating macroalgae on the sediment system depends on the amount and the physiological status of the macroalgae. With a high load of drifting macroalgae in a stagnant situation, the structure and function of the sediment community are strongly affected. No significant flow of organic material from the macroalgal mat to the sediment system could be proven. The macroalgal mat apparently constitutes a n independent habitat, which influences the sediment community by shading and, when mineralization is fast, by creating unfavourable conditions via low oxygen values.

An outdoor experimental system was used to investigate the effect of an increased load of inorganic nitrogen and phosphorus on the lower trophic levels of the food web of a shallow-water sandy sediment Estimates of structural changes were based on relationships between the biomass of autotrophs, heterotrophic bacteria and meiofauna and their qualitative composition. Effects on the functions of the sediment community were assessed by measuring primary and bacterial productivity and meiofaunal grazing rates using radiolabelling, as well as by measuring changes in oxygen and nutrient fluxes and carbon pools. The sediment system responded within 2 to 3 wk to the nutrient enrichment Meiofauna biomass increased, resulting in higher relative importance of oligochaetes and harpacticoid copepods Primary productivity increased faster than meiofaunal grazing, resulting in an increase of microalgal biomass by a factor of 4 Diatoms and filamentous cyanobacteria were favoured by the increased nutnent levels The stimulated photosynthetic acbvity had a negative feedback on the producing sediment layer, which was lifted off by oxygen bubbles entrapped in the mucus-rich top most layer. Stimulated growth of the filamentous green alga Enteroniorpha clathrata resulted In a biomass of ca 2 g C m-' after 4 wk, which was more than 2 times the biomass of microautotrophs living in the sediment. Bacterial productivity responded only weakly to the nutrient additions and the grazing pressure on bacteria was high during the whole 4 wk experiment. Meiofauna removed on average about 4 % of the microalgal biomass and 12 % of the bacterial biomass per day in treatments with increased nutrient levels, and about 10 % of autotrophs and 7 "/o of bacteria in treatments with ambient nutrient levels Although autotrophic organisms were more important carbon sources than bacteria for meiofauna, the impact of grazing on microautotrophs was small, whereas grazing may b e important in regulating bacterial growth. Measurements of ammonium, n~t r a t e , phosphate and silicate flux showed that shallow-water sediment with a high primary productivity in the top layer functions as a sink for inorgamc nutrients In terms of biomass, the addition of nutrients led to a dominance of meiofauna. However in terms of productivity, autotrophs predominated and thus, during the initlal phase of increasing nutnent levels, the entire sediment system was dominated by autotrophic processes.

No abstract

No abstract

No abstract