Bathymetric control of Holocene spit migration in a lacustrine environment

Wave processes are well known for developing spit systems in large or elongated lakes by inducing longshore sediment transport for spit migration, while bathymetric interaction is less studied. In this study, we investigate the combined effects of wave processes, antecedent topography and lake level changes on the development of Holocene spit systems in the Danish lake Mossø. Wave climate prediction and a digital elevation model were used to provide a conceptual model for the development of the spits system, while optically stimulated luminescence (OSL) dating of spit sand and antecedent topography analyzed via boreholes and seismic survey data were used to evaluate effects on spit migration other than wave setting. We found that spit migration stagnated during the Holocene when reaching areas of deeper waters, but continued following shallowing after deep basin infilling with lacustrine sediments. During periods of bathymetrically induced stagnation of prominent spits, less stable or more slowly migrating spits became prominent in the development of the spit system. No clear effects on spit migration caused by lake level fluctuations could be demonstrated. However, such fluctuations may have been important for the stabilization of spits and subsequent development of a major barrier shoreline.

Changed cycling of P, N, Si, and DOC in Danish Lake Nordborg after aluminum treatment

Loading, retention, and in-lake cycling of phosphorus (P), nitrogen, silica, and dissolved organic carbon (DOC) were studied 1 year before and 3 years after P-inactivation by aluminum (Al) hydroxide in Danish Lake Nordborg in 2006. Simultaneously, external P loading was reduced by 40% via establishment of precipitation ponds in two inlets. After Al treatment, the internal P loading (sediment P release) during summer declined 90%–94%, owing to adsorption to aluminum hydroxide. Also, silicate regeneration from the sediment was reduced by 69%–76%, and sediment oxygen uptake as well as ammonium release declined markedly. Consequently, lake water total P, dissolved inorganic P, silicate, and DOC decreased by 73%, 97%, 87%, and 46%, respectively. The Secchi depth increased in the summer period during the first post-treatment year, but declined afterwards to pre-treatment levels, even though the mean lake-water total P concentration during summer was reduced from ∼240 µg·L–1 before treatment to 26–65 µg·L–1 in the first three post-treatment years. We conclude that a further reduction in external P loading is needed to obtain the full effect of the Al treatment in Lake Nordborg.