Skaftslamkrypa (Elatine hexandra) i Bolmen, utbredning, tillväxt, reproduktion och styrande faktorer

The plants in the genus Elatine are characteristic for oligo and mesotrophic lakes. This study focuses on Elatine hexandra one of the four species in Sweden. It is stated as vulnerable by the National Environment Agency and a conservation plan with guidelines to preserve the plant in Sweden is under preparation. There have been a few previous studies investigating this plant in other countries but this is the first one for Sweden. The aim of this study was to investigate the distribution, growth and reproduction of E. hexandra in the lake Bolmen, and to investigate what the factors that could influence them. Factors that where included in this study were water depth, wave exposure, amount of epiphytes and sediment characteristics.   An analysis of emerging seedlings from soil seed banks revealed a maximum density of 2200 seeds /m2 to 10 cm depth. A linear regression showed that the size of the seed bank was negatively related to wave exposure (F1,3= 36,71 P=0,009, R2=0,96).   In August, the depth distribution of the plants was described in 5 transects at 7 sites. All plants were found to grow submerged, the depth of the plants ranged between 3-74 cm. The mean depth of the plants was significantly more shallow then expected from an random distribution in 2 of the 7 sites (F6,55= 10,47, P<0,05, ANOVA, Tukey, P<0,05). Further the mean depth differed between the localities and it was positively related to the water content of the surface sediment in shallow water (0-15 cm, linear regression, F1, 5=14, 43, P=0, 02, R2 = 0, 88), but not with water content of the surface sediment in deep water (15-30 cm). Mean plant depth was not correlated with wave exposure or epiphytic biomass suggesting that local sediment factors were more important in this lake. Furthermore, a comparison with 7 randomly located sites without Elatine showed the water content of the surface sediment was higher in sites without Elatine. This supports the supposition that soft sediments are poor growth habitats for this species.   In October, when the water level had dropped to its annual minimum, plant size and seed production were examined in emerged and submersed plants at 5 sites. Plant length was smaller in emerged (25.1±7.6mm, mean±95%conf.int) than in submerged positions (40.3±8.3mm, F1, 40=14.9, P<0, 05, ANOVA). Similarly, the number of root per node was smaller in emerged (2.2±0.2) than in submerged positions (2.8±0.2, F1, 40=14.9, P<0, 05, ANOVA). No consistent difference was found with respect to seed production per plant, 34.4±22.5 and 61.4±24.2 seeds per plant in emerged and submerged positions, respectively (Depth: F1,40=1.8, P>0.05, ANOVA). In this case, the effect of depth was dependent on site (Depth X Site: F4, 40= 2.7, P<0, 05, ANOVA).   Annual and interannual variations in water regime in relation to distribution and size of the seed bank are proposed to be of key importance for conservation of E. hexandra. Further studies on the ability to germinate above water in Sweden may shed light on the finding that no plant occurred above water level in the present study. An investigation of the seed banks in former habitats is proposed to answer if the species have abandoned the habitat or if the habitat no longer is suitable while there are still seeds resting in the seed bank.