MEPS 710:43-56 (2023)  -  DOI: https://doi.org/10.3354/meps14298

ABSTRACT: The Mediterranean Sea is warming more rapidly than the global average, leading to dire consequences for its inhabiting organisms. Seagrasses are key structural elements in coastal ecosystems, and studying how temperature affects these species is crucial to anticipate the implications of global warming. In this work, we use an empirically based numerical model to study the combined dynamics of Posidonia oceanica and Cymodocea nodosa and their resilience to sea warming. The model is parametrised using seagrass growth rates measured in the Western Mediterranean Sea. Under favourable growth conditions, our simulations predict the emergence of a coexistence region at the front between monospecific meadows. This region can be characterised by its width and local shoot densities, which depend on the coupling parameter between P. oceanica and C. nodosa. Such regions have been empirically observed in Ses Olles de Son Saura (Balearic Islands). A comparison between the field measurements at the study site with the model predictions was used to fit the value of the coupling parameter. Field data also relate the width of the coexistence region to the average length of P. oceanica leaves at the front. Remarkably, a linear relationship was found between the coupling parameter and leaf length. In the presence of sea warming, the model predicts an exponential decay in the population of P. oceanica, which is highly sensitive to temperature. This behaviour is a direct consequence of the clonal nature of the plant and can be characterised by the model parameters. Considering a scenario of high greenhouse emissions, our model forecasts that P. oceanica meadows will experience a 70% population decline by the year 2050. C. nodosa, with higher thermal resilience, acts as an opportunistic species and will colonise the space left by the degraded P. oceanica.

KEY WORDS: Western Mediterranean seagrasses · Sea warming · Predictions of population decay · Empirical agent-based model · Posidonia oceanica · Cymodocea nodosa · Coexistence region · Field measurements