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surface water temperature averaged 9°C for 2 months (Komatsu et al. 2003). In
Croatia, the biomass of C. taxifolia was reduced after cold winters (9.5-10.5°C).
In addition to suggesting a sharp lower limit for survival, the results suggest that
C. taxifolia naturally has a wide temperature tolerance. These results, in turn,
refute early suggestions that the Mediterranean clone had become increasingly
cold-adapted while propagated and dispersed through the aquarium industry.
In the Mediterranean and NSW, C. taxifolia is considered pseudoperennial
because individuals dieback in winter months (Fig. 15.2). In both locations,
researchers have documented that C. taxifolia reached maximal size at the end of
the summer months and then decreased in dimensions, especially blade height, dur-
ing cold weather (Meinesz et al. 1995; Ceccherelli and Cinelli 1999b; Glasby et al.
2005a). Temporary diebacks in shallow areas with only knots of stolons and a few
bleached blades were often observed during winter when temperatures reached as
low as 11°C (Wright and Davis 2006). In Italian waters, percent cover went from
2.7% in April to 100% in October (Balata et al. 2004). Williams and Schroeder
(2004) found that chloroplasts were translocated to buried portions of tissue when
fragments were either heat or cold-shocked.
West and West (2007) simultaneously compared the impact of six salinities
(range: 15-30 ppt) and four temperatures (15-30°C) on C. taxifolia from Lake
Conjola, NSW. Blades, rhizomes, and thalli (= rhizome plus one blade) had similar
responses in the lab trial. Some fragments in all morphological categories doubled
in size over the week-long trial by producing new stolons and fronds; the maximum
growth rate was 174 mm/week (West and West 2007). Fragments grew well at
salinities
20°C, while mortality approached 100% at
lower salinities and temperatures. The result was especially interesting in light of
the changes to some NSW waterways. In 2001, the entrance to Lake Conjola was
manipulated to keep the lake permanently open to the sea (West and West 2007).
The current salinity in Lake Conjola is always above 30 ppt. Prior to 2001, Lake
Conjola often was less than 17 ppt for extended periods of time. Entrance manipula-
tion may have improved the success of invasive C. taxifolia .
22.5 ppt and temperatures
15.3.4
Blade Lengths, Depths, and Densities
The ability to vary photosynthetic capacity (e.g., blade length, pigment concentra-
tion) when spatial competition occurs and along a depth gradient promotes inva-
siveness of marine macrophytes. Dumay et al. (2002) found an increase in blade
length in response to competition between C. taxifolia and the seagrass Posidonia
oceanica . In Mediterranean waters, blades of aquarium strain C. taxifolia as long
as 60 cm were recorded in water up to 100-m deep (Meinesz et al. 1995; Belsher
and Meinesz 1995), while the average range in shallower waters was 4-20 cm
(Ceccherelli and Cinelli 1997, 1998). In Croatia, frond lengths ranged from 10 to
18 cm (Ivesa et al. 2006). In California, the maximum frond length was 24.6 cm,
with a mean of 10.4 cm (Williams and Grosholz 2002). In Australia, Wright and
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