Resumen de Macroalgal mats in a eutrophic lagoon: dynamics and control mechanisms
One of the most striking phenomena of eutrophication in shallow coastal waters is the mass accumulation of macroalgae. These macroalgal blooms have a negative effect on the functioning of the ecosystem, fisheries activities and tourism. This thesis focuses on the regulation of seasonal and spatial dynamics of blooms of Ulva cf. scandinavica Bliding in the Veerse Meer, a eutrophic, brackish lake in the SW Netherlands. Studies have been performed on biomass dynamics, winter survival, taxonomy, auto-ecology and dynamics and regulation of macroalgal mats. The annual dynamics of Ulva biomass development in the Veerse Meer is described in conceptual model. Four phases are distinguished in the bloom: a dormant phase, a build-up phase, a stationary phase and a degradation phase. Algal fragments survive through the winter in the sediment. Probably, wind-induced currents cause the release of the algae from the sediment, thereby triggering the transition from the dormant to the build-up phase, however, further research is required. Light is the main factor regulating growth during the build-up phase. Only after a prolonged period of clear weather, nitrogen becomes growth limiting. Self-shading in the mats increases during the build-up phase, eventually causing the shift of the build-up phase to a more or less stationary phase. A high biomass can be maintained during this phase due to nutrient (mainly nitrogen) recycling in the mat. Finally, the degradation phase starts, during which biomass again decreases to winter levels. Management measures, aiming to decrease the macroalgal biomass development in the Veerse Meer, should be primarily directed to a decrease in nitrogen load. For a further recovery of the ecosystem of the lake, a more natural tidal regime is requested. Additionally, harvesting of algal mats during summer and of fragments in the sediment during winter will bring some relief
