Light availability had a stronger effect on growth rate and
plant morphology than temperature for all species. RGR of all
three species increased approximately three-fold from low to high
light availability, but varied little with temperature, suggesting a
rather flat response to temperature with a growth optimum in the
range 20–30 ◦C. However, for L. major there was a great decline
at 30 ◦C both in growth rate and photosynthetic rate, indicating
stress responses to summer mean temperatures above 25 ◦C. In
a microcosm experiment, McKee et al. (2002) also found that L.
major growth favored summer temperature around 25 ◦C. The relative
growth rates were within the range of previous studies on the
three species (Table 3). Photosynthetic rates were the only parameter
for which temperature had an equal or stronger effect than
light availability. We found that as mean summer water temperature
increased from 20 ◦C to 30 ◦C there was a general increase in
photosynthetic rates for E. densa and E. canadensis and for L. major
between 20 and 25 ◦C. This is a typical response following effects of
temperature on enzymatic processes in the photosynthetic process
(Berry and Raison, 1981).
Although there was an overall correspondence in temperature
effect on photosynthetic rate and growth rate, the photosynthetic
rates were much more sensitive to increasing temperature. The
optimal temperature for plant growth is usually lower than for
photosynthesis as photosynthesis is only one of several factors
determining growth (Körner, 1991). Hence, accelerating losses due
to faster increase in dark respiration relative to photosynthesis
at high temperature will lower the diurnal production and hence
contribute to the lower growth rates. But also elevated leaking of
dissolved organic matter from roots, faster leaf senescence and
changes in carbon allocation may be involved. Root allocation,
however, was highest at low temperatures and therefore did not
contribute to the measured differences in growth and photosynthetic
rates.