Effects of thermal stress on seagrass populations
Patterns of thermal performance in P. oceanica varied among
populations and deviated from expectations with respect to range
position, highlighting the challenges of extrapolating climate change
vulnerability predictions from local observations. Among cool, central
and warm-edge populations, only measures of new leaf production
supported models of local-adaptation/acclimatization and no metrics
consistently supported expectations of thermal-niche conservatism
between populations. The positive relationship between new-leaf
production and thermal anomalies indicates higher rates of leaf turnover
(i.e., shorter lifespan) under warmer conditions, potentially reflecting
a coping strategy for thermal stress within fragments. Cool-edge and
warm-edge populations did display similar rates of growth and survival
under warm-edge (29ºC) conditions, suggestive of niche conservatism,
however central populations exhibited comparatively poor performance, in
contrast to expectations. Moreover, patterns of poor thermal performance
by central populations at the warm edge were consistent with results
among centre-centre procedural controls, which experienced similar
absolute temperatures to warm-edge treatments. Growth patterns from the
natural meadows over the same period further support these findings,
showing lower growth rates and higher new-leaf production in
centre-populations relative to cool and warm-edge meadows following the
thermal stress period, reinforcing that experimental findings were not
an experimental artefact.
Unlike centre-populations, the high resilience of cool-edge P.
oceanica to thermal stress was consistent with documented responses of
congeneric Posidonia species to a natural heatwave of similar
magnitude. In 2011, Western Australia experienced the most intense
marine heatwave on record (Hobday et al. 2016) andPosidonia australis displayed relatively minor impacts despite
experiencing thermal anomalies in excess of 5ºC and absolute
temperatures reaching 30ºC (Strydom et al. 2020). In contrast toP. australis, other seagrasses (i.e. Amphibolis
antarctica ) experienced severe losses (Fraser et al. 2014) and
kelp forests (Ecklonia radiata ) suffered a 100 km range
contraction (Wernberg et al. 2016) in response to the 2011
heatwave. While the response by P. oceanica to large anomalies in
the current study was relative to cool-edge (c.f. warm-edge in P.
australis ) conditions, the similarity between species to survive
temperatures up to 30ºC highlights a remarkable latent capacity to
withstand thermal stress in these edge populations. Latent tolerance to
extreme events potentially reflects the warm evolutionary origins of the
genus and the high resilience Posidonia has displayed to climatic
upheaval over the past 60 mya (Por 2009; Bianchi et al. 2012).
The idea of P . oceanica as resilient to heat stress may
appear at odds with the prevailing literature, which has demonstrated
(Diaz-Almela et al. 2009; Marba & Duarte 2010) and projected
(Jordà et al. 2012; Chefaoui et al. 2018) high sensitivity
of P. oceanica to warming. However, the results of the current
study are consistent with these previous findings and help place them in
a broader geographical context. Central populations of P.
oceanica were severely affected by thermal anomalies up to 1.5ºC above
long term summer maxima in the current study. Anomalies of this
magnitude are slightly cooler than those experienced during the 2003
Mediterranean heatwave, which caused a steep increase in shoot mortality
in central populations (Marba & Duarte 2010). Our findings therefore
support previous evidence that P. oceanica in central populations
are living close to their thermal limits but suggest that populations
elsewhere may indeed have greater tolerance of high temperatures. The
reason why central populations of P. oceanica are more severely
affected by high temperatures than cool and warm-edge populations is
unclear. The genetic structure of P. oceanica is characterised by
strong separation between the eastern and western basins, consistent
with a vicariance event during the last glacial maxima (Arnaud‐Haondet al. 2007). Within the western basin, genetic connectivity is
relatively low for both the cool-edge and central experimental
collection locations, but both these locations are directly connected to
the same central node within the regional metapopulation (Rozenfeldet al. 2008). Central and cool-edge populations, therefore, share
more genetic and climatological similarities over geological timescales
than cool-edge and warm-edge populations (Chefaoui et al. 2017),
whereas central and warm-edge populations share more contemporary
similarities in terms of selection pressure on upper thermal limits.
Despite this, central populations exhibit the greatest sensitivity to
temperature, across the distribution of P. oceanica forcing a
re-evaluation of the vulnerability and management of this species in
response to climate change.
While field-based evidence of performance could not separate cool-edge
and warm-edge populations, the maximum realised temperature in our
experiment was 29.3ºC. Therefore, we cannot rule out the possibility
that differences in performance between populations occur at higher
temperatures. Indeed, previous laboratory-based experiments have
suggested that warm-edge populations can survive temperature up to 36ºC,
whereas cool-edge populations and central populations of P.
oceanica display upper thermal limits around 30ºC (Bennett et
al. 2021). Such experimental evidence would suggest that
cool-edge populations were close to their upper threshold during the
current experiment whereas warm-edge populations retained a larger
thermal buffer. Previous laboratory experiments, however, did not
document the subsequent recovery of P. oceanica following thermal
stress. While cool-warm and cool-centre treatments clearly showed signs
of thermal stress in the current study, our results suggest that they
have greater capacity to endure and recover from heat stress than
central-populations.