The lakes do not always respond immediately to a nutrient loading reduction; persistence of eutrophica-tion may be attributed to continuously high external loading preventing a shift to a clearer state. However, even when the P loading has been sufficiently reduced, resistance against improvements occurs. This resistance may be 'chemical': the P concentrations remain high due to release of P from nutrient pool accumulated in the sediment at high loading (Figure 1). It typically takes 10-15 years, but sometimes longer, before this surplus pool of P in the lake sediment is released or permanently buried, the duration depending both on the thickness of the nutrient enriched sediment layer, the nature of P binding sites in the sediment, and on the flushing rate in summer when P accumulates in the lake water.
Biological resistance also affects internal P loading and the physicochemical environment. Particularly planktivorous and benthivorous fish contribute to biological resistance. Continuously high fish predation prevents both the appearance of large herbivorous zooplankton, and thus a higher grazing on phytoplankton, and diminishes the number of benthic animals that stabilize and oxidize the sediment. However, recent studies have shown surprisingly fast (within less than 5-10 years) reductions in fish biomass and increases in the proportion of piscivores when both external and internal nutrient loading is efficiently controlled. Grazing by herbivorous waterfowl as coot (Fulica atra) and mute swan (Cygnus olor) or lack of plant seeds or turions may also create resistance by delaying the recol-onization of submerged macrophytes.
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