Seasonal variations and the effect of reproductive development on resource acquisition by two intertidal fucoid species,
the iteroparous
Fucus serratus L. and the semelparous
Himanthalia elongata (L.) S. F. Gray were examined. The oxygen-exchange characteristics of vegetative apical tissue of both non-fertile and fertile
plants and receptacle tissue were compared at monthly intervals throughout reproductive development. Respiratory rates in
non-fertile
F. serratus varied seasonally between 1.5 and 8.0 μmol g
−1 fresh wt h
−1; in fertile plants the receptacle had a significantly lower respiratory rate than the vegetative tissue. The respiratory
rate of the vegetative button of fertile
H. elongata displayed less seasonal variation and was lower than that of the receptacle, which varied from a maximum of 9.5 μmol g
−1 fresh wt h
−1 at receptacle initiation in October to a minimum of 2.0 μmol g
−1 fresh wt h
−1 in February. The maximum photosynthetic rate (
P
max) of non-fertile plants of both species did not vary in a distinct seasonal manner (∼60 μmol g
−1 fresh wt h
−1 for
F. serratus and ∼12 μmol g
−1 fresh wt h
−1 for
H. elongata). In fertile plants, the
P
max of the receptacle tissue was (∼50% lower in
F. serratus, and at its peak three times higher in
H. elongata, than that of vegetative tissue. The stable carbon-isotope ratio (δ
13C) did not differ between different tissue types in
F. serratus, but values did vary seasonally, being less negative in the summer than in the winter (−13.5‰ compared to −18‰). The receptacle
tissue of
H. elongata also displayed a distinct seasonal variation in δ
13C values (−12‰ in summer, −16‰ in winter), whilst the δ
13C of the vegetative button did not vary seasonally. The rate of uptake of inorganic nitrogen by the vegetative thallus was
lower in
H. elongata than in
F. serratus. The receptacle tissue of
F. serratus had lower uptake rates than the vegetative tissue, whilst the uptake rate by
H. elongata receptacle tissue was higher than that of the vegetative button.
Received: 14 March 1997 / Accepted: 22 April 1997
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