4. DISCUSSION
We assessed and monitored genetic diversity within and between populations of brown trout over 40 years in a total of 27 small alpine lakes in ten geographically separate systems in central Sweden. We used genetic data to identify genetically separate populations and detected 29 such populations that were stable over time. Individual populations occurred in 1-4 alpine lakes within the same water system and at least 12 of the lakes harbored more than one population.
We quantified the genetic diversity within and between populations and applied recently proposed indicators to track genetic diversity over time, and we propose that these indicators can aid in biodiversity monitoring. All populations were located in protected areas where maintaining biodiversity is a common goal. While we found no general trend with respect to decrease or increase of genetic diversity among the 29 populations of the ten water systems we did observe considerably lower levels of genetic diversity in the two above-tree-line systems. In one of these systems we also found a decrease of genetic diversity at a magnitude exceeding the proposed limiting values for within population diversity change (ΔH indicator).
In contrast, increased genetic diversity was observed in several separate populations as well as in three population systems (Figure 7). The effective population size (N e) was generally low; 20 out of 29 local populations and five out of eight metapopulations revealed N e<500 which is below the threshold value proposed for this indicator. The between population indicator ΔF ST showed significant increase of divergence, potentially suggesting reduced gene flow in the system where the ΔH and N e indicators gave warning signals (Skåarnja 2; Figure 7b), although the change was within the proposed limiting values. In two systems ΔF STshowed a significant reduction, potentially indicating increased gene flow, but here too within the proposed limiting values (cf. Figure 3). Applying the CBD indicators to the present data provides conclusions consistent with the national indicators but with less detail (Figure 7c).