Mediterranean warming triggers seagrass (Posidonia oceanica) shoot mortality

Abstract

Rapid warming of the Mediterranean Sea threatens marine biodiversity, particularly key ecosystems already stressed by other impacts such as Posidonia oceanica meadows. A 6-year monitoring of seawater temperature and annual P. oceanica shoot demography at Cabrera Archipelago National Park (Balearic Islands, Western Mediterranean) allowed us to determine if warming influenced shoot mortality and recruitment rates of seagrasses growing in relative pristine environments. The average annual maximum temperature for 2002-2006 was 1 1C above temperatures recorded in 1988-1999 (26.6 1C), two heat waves impacted the region (with seawater warming up to 28.83 1C in 2003 and to 28.54 1C in 2006) and the cumulative temperature anomaly, above the 1988-1999 mean annual maximum temperature, during the growing season (i.e. degree-days) ranged between 0 1C in 2002 and 70 1C in 2003. Median annual P. oceanica shoot mortality rates varied from 0.067 year À1 in 2002 to 0.123 year À1 in 2003, and exceeded recruitment rates in all stations and years except in shallow stations for year 2004.Interannual fluctuations in shoot recruitment were independent of seawater warming (P40.05). P. oceanica meadows experienced a decline throughout the study period at an average rate of À0.050 AE 0.020 year À1 . Interannual variability in P. oceanica shoot mortality was coupled (R 2 40.40) to seawater warming variability and increasing water depth: shoot mortality rates increased by 0.022 year À1 (i.e. an additional 2% year À1 ) for each additional degree of annual maximum temperature and by 0.001 year À1 (i.e. 0.1% year À1 ) for each accumulated degree water temperature remained above 26.6 1C during the growing season. These results demonstrate that P. oceanica meadows are highly vulnerable to warming, which can induce steep declines in shoot abundance as well indicating that climate change poses a significant threat to this important habitat.

Inter-annual variation of shoot density and biomass, nitrogen and phosphorus content of the leaves, and epiphyte load of the seagrass <i>Posidonia oceanica</i> (L.) Delile off Mallorca, western Mediterranean

“…The intensive labour required by a yearly shoot (Marbà et al, 2005) and because plagiotropic, horizontally-growing rhizomes produce many more new shoots than orthotropic, vertically-growing rhizomes (González-Correa et al, 2007), census data might reflect actual net shoot balances at the meadow scale only if the permanent plots used to obtain them sample the density of plagiotropic rhizomes in the meadow accurately. Although further research should be done to resolve this contradiction, the extremely warm summer of 2003 seems to have increased the mortality rate of P. oceanica shoots (Marbà and Duarte, 2010 The disparity of the inter-annual trends detected by different studies (Table 3) suggests that local conditions and processes may have a large influence on the inter-annual variability of shoot density. Indeed, Posidonia oceanica meadows with no evidence of anthropogenic disturbances show predominantly stable shoot density, while those where anthropogenic influences are clear show predominantly a reduction in shoot density (Ruiz-Fernández et al, 2009).…”

“…Changes in shoot density between years of the same magnitude have also been found in Catalonia (Sánchez-Rosas and Prats-Olivella, 2009), Murcia (Ruiz-Fernández et al, 2009) and the Balearic Islands (Álvarez et al, 2009). Additionally, the shoot density of permanent P. oceanica plots situated at depths of 10 m or less off Cabrera Island (Marbà and Duarte, 2010) The available evidence for the Balearic Islands (Ál-varez et al, 2009; this study) suggests that shoot density of Posidonia oceanica meadows was either stable or increasing during the 2003-2008 period, which contrasts with results from other studies that found a net reduction in shoot density on permanent plots where P. oceanica shoots are tagged and counted yearly (Marbà and Duarte, 2010). Annual monitoring of shoot density may not be sensitive enough to detect shoot density changes unless a large number of replicated counts are done (Marbà et al, 2005), but by averaging shoot density values from a large meadow surface the results might better reflect net shoot balances at the meadow scale.…”

“…Shoot density and biomass of P. oceanica tended to increase in the Bay of Calvi (Corsica, France) from 1993 to 1999, while epiphyte load did not show a definite inter-annual trend (Gobert, 2002). Tagging of P. oceanica shoots in permanent plots and yearly censuses of the surviving and new shoots has shown inter-annual variability of shoot mortality between 2002 and2006 off Cabrera Island (Mallorca, Spain) and that the highest mortality rates occurred in years with the warmest summers (Marbà and Duarte, 2010).…”

Footprints of climate change on Mediterranean Sea biota

“…The 2003 heat wave event was reported to result in mortality of benthic organisms, including gorgonians, sponges, molluscs, bryozoos (e.g., Garrabou et al, 2009;Bensoussan et al, 2010) and seagrass (Posidonia oceanica, Díaz-Almela et al, 2009;Marbà and Duarte, 2010) as well as mass flowering of P. oceanica (Díaz-Almela et al, 2007). These observations provide indications of the impacts that may be expected with future climate change in the Mediterranean, where the projected increased in the frequency and magnitude of heat waves will compromise the most vulnerable ecosystems, such as coral and P. oceanica meadows (e.g., Jordà et al, 2012).…”

Detection and Attribution of Observed Impacts

“…Poleward expansion of mangrove forests, consistent with expected behavior under climate change, has been observed in the Gulf of Mexico (Perry and Mendelssohn, 2009;Comeaux et al, 2012;Raabe et al, 2012) and New Zealand (Stokes et al, 2010). High temperatures have impacted seagrass biomass in the Atlantic Ocean (Reusch et al, 2005;Díez et al, 2012;Lamela-Silvarrey et al, 2012), the Mediterranean Sea (Marbà and Duarte, 2010), and Australian waters (Rasheed and Unsworth, 2011). Extreme weather events also contributed to the overall degradation of seagrass meadows in a Portuguese estuary (Cardoso et al, 2008).…”