Stratigraphy of fossil pigments and Cladophora and its relationship with deposition of tephra in Lake Mývatn,Iceland

Fossil plant pigments and Cladophora fragments were analyzed in a 6.53 m long sediment profile from Lake Mývatn, Iceland, covering most of its history of about 2300 years. A decrease in myxoxanthophyll (produced by Cyanobacteria) with time and an increase in the benthic Cladophora reflects a gradual shift from planktonic to benthic primary production as water depth is reduced (to 3.15 m at the core site) because of sediment accumulation.Two periods of relatively high concentrations of myxoxanthophyll coincided with relatively frequent deposition of tephra (volcanic ash) but did neither conform with tectonic activity which might have changed the water level nor the available climatic record. Sediment depth, tephra content and percent undegraded chlorophyll (an indicator of pigment preservation) together could explain 56.7% of the variation in myxoxanthophyll. It is hypothesized that temporary increases in myxoxanthophyll resulted from periodic nutrient enrichment by fresh tephra deposited in the watershed or because tephra increased the erosion of organic soil in the water catchment area.Fluctuations in Cladophora show an inverse relationship with myxoxanthophyll in the uppermost 4 m of the core, and may result from a shading effect of planktonic Cyanobacteria on the phytobenthos or competition for nutrients released by the bottom sediments.