Environmental factors and aquatic macrophyte response in regulated lakes - a statistical approach

Abstract

In regulated lakes, it is necessary to distinguish between depth and relative elevation. Depth is the height of water overlaying the plants at any time, thus the instantaneous value may be nil. The optimal “depth” and elevation datum is shown to be the median water level. However, depth in its time-averaged sense is not equivalent to relative elevation when the water level fluctuates throughout time. The magnitude of the discrepancy is generally large in shallow waters. Environmental stress factors, e.g. ice-scour, influence aquatics over a much larger part of the “depth” gradient with a variable water level. Factors operating through threshold statistics, e.g. subsurface irradiance, are most adversely influenced by a time-varying water level. In general, regulation impact results in a compressed vertical niche. Simultaneously, the potential niche is shifted into deeper waters. Vegetation data from ninne oligotrophic, regulated Norwegian lakes show significant correlation to the reduced vertical niche defined by ice-scour and threshold irradiance. Water-level schedules determine the actual vegetational response; thus the total probability distribution of water levels should replace the regulation height or mean annual range of water level variation in an analysis of response features.