Deep chlorophyll maxima in small boreal forest lakes after experimental catchment and shoreline logging

In situ profiles of chlorophyll fluorescence combined with depth-specific water chemistry and biotic analysis were used to assess deep chlorophyll maxima (DCM) in four boreal shield lakes that were subjected to different levels of watershed disturbance by clear-cut logging. Phytoplankton communities within the DCM varied among lakes, but were generally comprised of chrysophytes, diatoms, and cryptophytes. One lake also had deep cyanobacterial populations. Relative in situ fluorescence (absolute fluorescence at each peak minus background fluorescence) at the fluorescence peaks was significantly correlated with the level of chlorophyll a at the peak (r2 = 0.81, p < 0.025) for all lakes. In general, DCM peak heights were consistent in each lake over the four-year post-logging sampling period, whereas DCM depths declined slightly in the most heavily logged lakes associated with increases in dissolved organic carbon. Therefore, on the basis of DCM data, only mild effects on lake water quality were apparent after the logging activity.