Eutrophication increases cladoceran dormant egg production and sexual activity: Implications for geospatial dispersal and hybridization
Cultural eutrophication usually increases primary and secondary productivity, but what are the consequences on ‘dormant egg” production in resident and regional populations? Here we utilize carefully dated sediment core layers, retrieved remains, and rearing lab experiments to examine the abundance and flux of cladoceran resting eggs at individual sites. Moreover, because both carapaces and ephippia of the genus Bosmina are well-preserved in sediments, the ratio of the two items provides an important check on per capita production of ephippia, i.e. increases in sexual activity through time (pre-clearance vs post-clearance cultural horizons). Enhanced propagule production and sexual activity could have several regional metapopulation consequences: 1) more rapid geographic dispersal (increased exchange vector), 2) greater turnover of species at sites (due to accelerated colonization, extinction rates), and 3) enhanced regional hybridization or introgression.
Eutrophication greatly increases production of over-wintering 'dormant' eggs by aquatic organisms. Flux determinations for Daphnia and Bosmina ephippia from sediment cores indicate that cultural eutrophication increases ephippial production 10- to 35-fold. Most of the increase comes from larger population size. Increased ratios of ephippia production are also common-place in culturally disturbed modern sediments relative to pre-cultural horizons (3.5X increase). Enhanced levels of sexuality could result from density-dependent effects, chemical interactions (endocrine disrupting compounds), or from selection for increased sexual activity (bet-hedging). Relative to the “bet-hedging” option, Bosmina reared in the laboratory show high clonal variability in production of males, ephippial females, and dormant eggs, indicating at least a heritable basis for selection to modify ratios of sexual activity.