Characteristics of oligotrophic hardwater lakes in a postglacial land-rise area in mid-Sweden

1. We describe some remarkable ephemeral, oligotrophic hardwater lakes formed because of land rise in the coastal areas of the Baltic Sea, that are unique in Sweden and probably also worldwide. Two younger, coastal lakes were studied by regular sampling for 1 year and compared with an older (i.e. greater altitude) lake, that passed through the oligotrophic hardwater stage some 3-4000 years ago. 2. Despite some differences in composition of the catchment, the two younger lakes were similar with regard to water chemistry and plankton community composition. The concentration of phosphorus was low while nitrogen was high, resulting in very high N/P quotients (101 and 131). Although water colour was moderate, the concentration of organic carbon was extremely high (average values of greater than or equal to 20 mug TOC L-1), consisting mainly of dissolved compounds (DOC). 3. While the plankton was poorly developed, sediments in both lakes were covered by a layer of photosynthesising micro-organisms. This substantial `microbial mat', which has not been described in detail before, was up to 15 cm thick and dominated by cyanobacteria and purple sulphur bacteria. The concentration of sediment phosphorus was extremely low (352 mug g(-1) dw) in one of the lakes and dominated by organic-bound (residual) phosphorus. 4. Deep sediments in the older lake, representing its oligotrophic hardwater period, differed in phosphorus composition from the currently oligotrophic hardwater lakes by having a strong dominance of HCl-extractable (Ca-bound) phosphorus. This indicates that phosphorus, initially organic-bound within the microbial mat, is subsequently bound to calcium. We hypothesise that this is promoted by the environmental conditions created by the benthic photosynthetic activity, in combination with the prevailing hardwater conditions. 5. The rich and flourishing microbial community on the sediments may also explain the high concentration of DOC in the lake.