Tests of autotrophic picoplankton as early indicators of nutrient enrichment in an ultra-oligotrophic lake

1. The potential of picocyanobacteria as early indicators of changes in nut rient loading and trophic status was examined in an ultra-oligotrophic lake . 2. The study consisted of in situ, short-term, nutrient-addition bioassays and surveys of picocyanobacterial variables and other indices of trophic st ate at six sites selected to include a range of localized nutrient loadings to the lake from its catchment. The bioassays and surveys were conducted s ix times throughout 1 year. 3. Experimental additions of small amounts of ammonium-N and phosphate-P di d not stimulate picocyanobacterial growth, and phosphate additions often re duced picocyanobacterial growth rates. 4. A correlation analysis using pooled data from all sites and sampling dat es showed that the abundance of aggregated picocyanobacteria was strongly c orrelated with nutrient concentration and ratios. Other variables, includin g the concentration of single-cell picocyanobacteria, chlorophyll a and pri mary production, were poorly correlated with nutrients. 5. The results show that picocyanobacteria in oligotrophic lakes are sensit ive to extremely small changes in nutrient availability and that they can r espond in complex ways. The mechanisms by which they respond to such change s require further study before they can be used as bio-indicators of nutrie nt enrichment.