Marine Macrophytes as Carbon Sinks: Comparison Between Seagrasses and the Non-native Alga Halimeda incrassata in the Western Mediterranean (Mallorca)

Abstract

Seagrass species play a critical role in the mitigation of climate change by acting as valuable carbon sinks and storage sites. Another important ecosystem service of this coastal vegetation is nutrient removal. However, coastal ecosystems are under increasing pressure of global warming and associated establishment of invasive species. To elucidate the respective contributions of seagrass species Posidonia oceanica and Cymodocea nodosa and the non-native macroalga Halimeda incrassata as primary producers and nutrient sinks in coastal habitats we conducted in-situ incubations in the North-western Mediterranean Sea. Measured metabolic activity and nutrient removal as well as calcification rates in these habitats over a 24 h period in spring and summer confirmed that the endemic seagrass P. oceanica represents a valuable ecosystem with high O2 production and considerable carbon capture. The documented regression of P. oceanica meadows with higher temperatures and decline in autotrophy as measured here causes concern for the continuity of ecosystem services rendered by this habitat throughout the Mediterranean Sea with progressing climate warming. In contrast, the enhanced performance of C. nodosa and the calcifying alga H. incrassata with increasing temperatures, under expected rates of future warming is uncertain to mitigate loss of productivity in case of a potential shift in marine vegetation. This could ultimately lead to a decline in ecosystem services, decreased carbon storage and mitigation of climate change. Furthermore, this study provides a first estimate for the growth rate of H. incrassata in the Mediterranean Sea, supporting evidence for the mechanism of its rapid extension.

Increasing spread rates of tropical non‐native macrophytes in the Mediterranean Sea

“…For example, Hypnea cornuta , a non‐native macroalga in the Mar Piccolo of Taranto, experiences increased biomass during high summer seawater temperatures, often reaching around 30°C, followed by a period of winter dormancy marked by the development of over‐wintering organs (Cecere et al., 2016). Likewise, the tropical non‐native macroalga Halimeda incrassata forms dense populations with green thalli in the Balearic Islands, exhibiting increased population growth and photosynthesis in summer (Marx et al., 2021), yet its activity diminishes, and thalli turn white with lower winter SSTs (Ballesteros et al., 2020). On the contrary, Grateloupia turuturu , which is a cosmopolitan macroalga confined to areas where seawater temperatures range between 4 and 26–28°C (Koerich et al., 2020; Petrocelli et al., 2020) displays maximum growth and size of thalli in the coldest winter month in the Lagoon of Venice and in the Mar Piccolo and in summer, when seawater temperatures ranged between 25 and 30°C, the population decreases (Cecere et al., 2011; Petrocelli et al., 2020).…”

“…Non‐native thermophilic macrophytes, whose native ranges are warmer than their non‐native environments, possess an advantage to spread faster with ocean warming. This advantage may be attributed to two mechanisms: first, the increase of minimum SST allows them to survive the winters in the Mediterranean Sea (Dimitriadis et al., 2020; Raitsos et al., 2010; Wesselmann et al., 2021) and second, the increasing maximum SST enhances their performance due to their higher thermal tolerance (Cecere et al., 2016; Marx et al., 2021), whereas it decreases the performance of temperate species (Cecere et al., 2011; Koerich et al., 2020; Petrocelli et al., 2020).…”

Undisturbed Posidonia oceanica meadows maintain the epiphytic bacterial community in different environments

“…Its three-dimensional structure creates spawning grounds, nurseries, or permanent habitats for many species, supporting a complex community, which colonises the above- and below-ground plant compartment (including matte ) (Bellan-Santini et al 1986 ; Borg et al 2006 ). Furthermore, P. oceanica meadows act as a carbon sink (blue carbon) mitigating climate change (Pedersen et al 2011 ; Pergent-Martini et al 2021 ), one of the most valuable ecosystem services for our times (Apostolaki et al 2011 ; Marx et al 2021 ). However, P. oceanica is sensitive to increasing temperatures, low and high salinities (Boudouresque et al 2006 ; Jordà et al 2012 ), pollution, and other anthropic pressures (Boudouresque et al 2006 ; Jordà et al 2012 ; Pazzaglia et al 2020 ).…”