A long term lake-salinity record and its relationships to Daphnia populations

Abstract

Sediment cores from the shallow, presently saline, volcanic maar lake of Tower Hill were collected in March 2000, dated using 210Pb, and analysed for diatoms to infer changing salinity levels. Diatom inferred salinity was used to assess the hatching response of incorporated Daphnia ephippia. The early growth rate of Daphnia from different core depths, in media with two different salinities (0.5 g/L and 4 g/L) was also measured. Sediments have steadily accumulated in the Tower Hill basin over the past 50 years since the lake was last reported to have dried. The diatom record indicates a fairly stable salinity regime, with salt concentrations fluctuating between 1 and 2 g/L until 1990, followed by an exponential increase in salt concentrations peaking in the year 2000 at around 13 g/L. There were significant fluctuations in ephippia densities, with at least two large peaks and several periods of low ephippial accumulation. Ephippial densities were only weakly correlated with salinity changes. Maximum hatching of ephippia, all of the one species, Daphnia thomsoni, occurred in the top 7 cm of the cores, corresponding to the past 10 years. The oldest eggs to hatch were from sediments dated around 37 years old. The body length after six days growth of 19 clones, from 3 core sections (0–2cm; 2–4cm; >4cm) was measured under conditions of low salinity (0.5 g/L) and high salinity (4 g/L). Surprisingly, all but one clone reached a larger size at high salinity. However, clones from the top 2cm, which were produced during the lake's recent period of high salinity showed the strongest growth response at 4 g/L. Analysis of allozymes at three polymorphic loci did not find any evidence of genetic differentiation based on allelic frequencies. In order to assess possible reasons for differences in hatching patterns observed in cores taken in 2000, two additional cores were collected in September 2002. One showed a similar hatching pattern to the 2000 cores, and the other exhibited a more even pattern of hatching over its length.