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Flowering events opment of Posidonia meadows and what additional
of Posidonia oceanica meadows conservation measures should be undertaken.
PROPOSED MONITORING PROCEDURE
Another key parameter to monitor could be the occur- The presence/absence of reproductive structures
rence of Posidonia flowering and seed production. The (buds, inflorescences and inflorescence peduncles),
Mediterranean seagrass Posidonia oceanica exhibits shoot density and flowering intensity (per square metre
both vegetative and sexual reproduction. Sexual repro- and per shoot) should be measured annually (in Oc-
duction in this species is considered a rare and sporadic tober–November) along transects at a given depth, in
phenomenon, although some episodic flowering has one or more meadows of an MPA. Whenever possible,
been observed associated with extreme summer temper- flowering and shoot intensity should be measured at
atures (Diaz-Almela et al., 2007). Flowering of Posidonia different depths in large meadows, from the shallowest
meadows takes place in autumn, in September–October limit to near the deepest limit.
in shallow meadows, and November–December in deep
ones. Widespread flowering events also seem to occur Shoot density and mortality are particularly important
periodically every 10–11 years in correlation with the solar variables to measure as they can provide an indica-
activity cycle (Pergent et al., 1989). tion of the net population growth of the meadow. Net
changes in shoot density can be measured within per-
Posidonia oceanica is highly vulnerable to warming, as manent plots established in the meadow, and shoots
rising seawater temperatures can induce declines in within those permanent plots can be marked with
shoot abundance, affecting the overall stability of the sea- cable ties. This procedure can reveal shoot mortality
grass meadows and the balance between shoot mortality and recruitment, in addition to net changes in shoot
and recruitment (Marbá and Duarte, 2010). Similarly, the density.
induction of flowering could be a response mechanism by
the plant to thermal stress and its prevalence and intensity It is particularly useful to follow up changes in shoot
may increase with the amplitude of the thermal anomaly. density and/or mortality as this can provide a direct
indication of the effects of climate change on the health
Recent studies seem to indicate that the intensity of flow- of this key habitat. Flowering prevalence and intensity
ering correlates with an increase in shoot mortality and are variables that are easy to add into an existing moni-
negative net growth of the Posidonia meadows (Diaz- toring programme, especially if it is performed annu-
Almela unpublished data). Monitoring the occurrence of ally between September and December. Temperature
Posidonia flowering and fruiting in protected areas to- sensors placed in the same localities can provide ad-
gether with net changes in shoot density may provide the ditional data for potential correlation with the observa-
key to understanding possible trends in the future devel- tions in the meadows.

Flowering Posidonia oceanica. Photo: E. Diaz

MEDITERRANEAN MARINE PROTECTED AREAS AND CLIMATE CHANGE: A GUIDE TO REGIONAL MONITORING AND ADAPTATION OPPORTUNITIES 33