1 We investigated morphological responses of the submerged macrophyte Myrio
phyllum spicatum L. to water depth and wave exposure when grown in the same
substrate at two sites in two eutrophic lakes. Periphyton production was 4
-8 times higher at sheltered than at wave-exposed sites and its influence w
as further investigated in a glasshouse experiment. Morphological responses
in both experiments were compared by allometric analyses, with shoot weigh
t as covariate.
2 In the field study, plants shoots exhibited similar responses (increased
plant height and branch length, and decreased branch number) to sheltered c
onditions as to deep water. The partitioning between above- and below-groun
d biomass however, differed, with below-ground decreasing with an increasin
g water depth, but increasing or remaining unaffected at sheltered compared
with exposed conditions.
3 In the glasshouse experiment, plant responses to water depth were similar
to those in the field study. Furthermore, plant height increased when plan
ts were overgrown with periphyton.
4 High production of periphytic algae at sheltered sites appears to cause l
ight limitation of macrophytes. However, other factors such as nutrient upt
ake also appears to determine morphological responses. At sheltered sites,
where leaf nutrient uptake is reduced by abundant periphyton and thick boun
dary layers, plants allocate more biomass to roots. At deep and wave-expose
d sites, the absence of periphyton allows plants to take up nutrients throu
gh their leaves and allocation of biomass to shoots increases photosynthesi
5 Overall, relative allocation to shoot and root biomass appears to be prim
arily controlled by nutrient availability, whereas allocation of available
shoot biomass to particular structures is controlled by light availability.