Impact of accumulating drifting macroalgae on a shallow-water sediment system: an experimental study

Abstract

Using an outdoor flow-through experimental set-up consisting of twelve 30 1 containers, effect of accumulation of drifting filamentous macroalgae on a shallow-water sediment system was studied for 3 wk after the addition of 0.9 (low dose) and 1.8 kg 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 an independent habitat, which influences the sediment community by shading and, when mineralization is fast, by creating unfavourable conditions via low oxygen values.