Accumulation of algal pigments and live planktonic diatoms in aphotic sediments during the spring bloom in the transition zone of the north and Baltic Seas

Abstract

The sediment contents of algal pigments and live planktonic diatoms were measured in cores sampled at 6 stations with aphotic sediments at depths between 27 and 55 m located in the transition zone between the Baltic Sea and the Skagerrak. Five of the stations were sampled before and after the spring phytoplankton bloom in 2001. Within the area, there was a highly significant increase after the bloom in the sediment content of viable planktonic diatoms, quantified by the dilution extinction method, and in the sediment content of the pigments chlorophyll a and fucoxanthin. The composition of algal pigments as well as the number of germinable diatoms suggested that live pelagic diatoms were the source of almost the entire pool of chlorophyll a in the sediment both before and after the spring bloom. In the northern Kattegat, Great Belt, Femer Belt and in the Arkona Sea, the pools of diatoms increased 10- to 100-fold during the spring bloom. In total, the sediment received between 0.3 and 4 million germinable units of pelagic diatoms cm-2. In terms of organic nitrogen, the enrichment of live diatoms in the sediment corresponded to on average 202% of the total spring bloom production, calculated from the pool of nitrate consumed in the water column during the spring bloom. A similar calculation based on the enrichment of the pigments chlorophyll a and fucoxanthin indicated input corresponding to 24 and 64 % respectively of the potential production during the spring bloom. There was considerable variation among stations; in the central Kattegat there was no detectable sedimentation of diatoms or pigments. Here, the pools of pigments and diatoms decreased during the spring bloom and the shape of the pigment profiles also indicated that these pools did not originate from a recent input. Other stations received twice as much diatom biomass than could be produced from the pool of inorganic nitrogen in the mixed layer before the bloom. It is hypothesized that the unequal distribution of the spring bloom sedimentation is a result of episodic sedimentation events in combination with advection of the water masses. The potentials of using sediment profiles of pigments to quantify spring bloom sedimentation are discussed.