Vegetation abundance in lowland flood plan lakes determined by surface area, age and connectivity

Abstract

SUMMARY 1. We analysed the vegetation structure of 215 lakes in the flood plain of the river Lower Rhine in relation to environmental variables related to hydrological connectivity, lake morphometry, lake age and land use on adjacent land. 2. The frequency distribution of the cover of submerged macrophytes was not normal, implying that submerged macrophytes in any one lake were either scarce or abundant. 3. We observed clear water lakes with submerged macrophyte dominance over a wide range of total P concentration (0.020–0.40 mg total P L−1). 4. Multiple logistic regression indicated that the probability of dominance by submerged macrophytes decreased markedly with the surface area, depth and age of the lakes. The surface area effect occurred independently of the depth. Further, there was a negative relationship between submerged macrophyte dominance and the long‐term annual duration of inundation by the river. 5. Nymphaeid cover showed a distinct optimum with respect to mean lake depth, being almost absent in lakes shallower than 0.5 m. In contrast to what was found for submerged plants, the probability of occurrence of nymphaeids increased with lake age. 6. The probability of helophyte occurrence increased with lake age, and decreased with the presence of trees, cattle grazing, surface area, use of manure and mean lake depth. 7. In all cases the critical level of one factor (e.g. mean lake depth) depended on other factors (e.g. surface area or age of lake). Thus, in the present study, small lakes tended to remain dominated by submerged macrophytes up to a greater depth than large lakes, and helophytes colonised smaller lakes in an earlier phase. 8. The effect of inundation by the river was modest. This could be because most of our lakes are rarely inundated during the growing season and experience only moderate current velocities while flooded. 9. The results have practical implications for future management of flood plains for conservation purposes. In new water bodies, macrophyte domination will be promoted if many small shallow lakes, rather than few large deep ones, are excavated.

Shallow lakes theory revisited: various alternative regimes driven by climate, nutrients, depth and lake size

“…All of these lakes are inundated about once a year by the eutrophic river; so nutrient loading varies relatively little among the lakes. In accordance with the theory, most of these lakes were either nearly devoid of vegetation or in a contrasting state with a rich plant community, but the likelihood of the richly vegetated state was higher in smaller lakes (other factors such as depth being equal) (Van Geest et al, 2003). These small vegetated lakes also supported low densities of benthivorous bream (Abramis brama) (Grift et al, 2001), the key fish species promoting the opposite poorlyvegetated, turbid state in shallow Dutch lakes (Scheffer, 1998).…”

“…Small lakes appear to have a higher chance to be in a vegetated clear state. Although various factors may explain this correlation (Van Geest et al, 2003), suppression of fish in smaller lakes may be a common factor. Fish are rare in small, isolated lakes, probably as a result of a higher likelihood of fish kills combined with their low colonizing ability (Tonn & Magnuson, 1982;Dahl, 1990;Magnuson et al, 1998;Hershey et al, 1999).…”

“…However, an opposite effect of low-water extremes is probably more common. The risk of fish kills due to anoxic conditions in summer or winter may become higher if the water level is low, leading to clear water conditions that promote subsequent vegetation dominance (Van Geest et al, 2003).…”

“…Two recent studies exemplify this. An analysis of 215 shallow lakes situated in the Dutch floodplain of the lower River Rhine (Van Geest et al, 2003) revealed particularly clear relationships between lake size, the abundance of fish and the presence of vegetation. All of these lakes are inundated about once a year by the eutrophic river; so nutrient loading varies relatively little among the lakes.…”

Water-level fluctuations affect macrophyte richness in floodplain lakes

“…Several studies stress the effects of timing and duration of flooding for macrophyte composition and succession in floodplain lakes Henry et al, 1994;Van den Brink, 1994;Bornette et al, 1998;Van Geest et al, 2003). However, far less attention has been paid to the effects of low river water levels on macrophyte composition and succession.…”