Plants at all sites developed well during winter and early spring
but seasonal growth of Sargassum muticum during 2008 was far
from homogenous between the three study sites (2-way ANOVA:
p < 0.001, SNK Moyrus < Cashel < Letterard). Plants in the tide pool
remained shorter than on the open shore throughout the year
(Fig. 3a). Differences between the sheltered (Cashel) and exposed
site (Letterard) became evident during April/May of both 2007 and
2008, when thalli at Cashel started showing signs of weakness and
shed fronds before they reached full maturity. In 2007, all plants
had died back to the perennial holdfast by June. During 2008,
development was similar, except that senescence set in about one
month later. Adults of the native Himanthalia elongata exhibited
uninterrupted seasonal (reproductive) growth at Cashel (Fig. 3b). At
the exposed site and in the rock pool S. muticum thalli developed to
full maturity in both years investigated (Fig. 3a).
Considerable differences between different sites also existed for
frond size distributions (Fig. 4): at Letterard, 42% of plants had
grown to 150e200 cm by July 2007, by which time 58% of the plants
at Cashel had already died back to 0e20 cm, and only 25% of the
plants in the tide pool had reached their maximum length
(30e40 cm; Fig. 4).
The early senescence at the sheltered location was reflected by
the low number of fertile plants and slow receptacle development.
Receptacle development differed significantly between sites and
months, and interaction between sites and months was also significant
(2-way ANOVA: p < 0.001, SNK Moyrus < Cashel < Letterard).
Although reproductive development was slower in the tide pool
compared to Letterard, at both locations all plants monitored were
fertile by August (Fig. 5). At Cashel only a few plants developed
receptacles: no fertile plants were found in April and only 10% of the
plants were fertile in May. After this time, most plants shed their
fronds rapidly and monitoring for fertility was abandoned, as only
a few thalli remained available.