Seagrass spatiotemporal dynamics with a time-dependent mortality

Abstract

[cat] La Posidonia Oceanica ´es una part vital dels ecosistemes costaners del Mediterrani, amb efectes positius des de proveir aliments i recer a altres organismes marins a previndre l’erosi´o de les costes i mitigar el canvi clim`atic. L’activitat humana est`a danyant aquestes praderies mil·len`aries a un ritme alarmant, en part degut al canvi clim`atic, ja que l’increment de temperatura de l’aigua augmenta la mortalitat d’aquesta planta marina. L’objectiu d’aquest treball ´es estudiar els possibles canvis en les din`amiques espaciotemporals de la P. Oceanica, centrant-nos en l’efecte d’una mortalitat dependent del temps. Com volem investigar els efectes de l’heterogene¨ıtat espacials i els efectes col·lectius de la P. Oceanica, farem servir un model que incloga aquests factors. El model emprat inclou termes espacials i no linears arran de tindre en compte interacciones col·lectives i no locals, i captura fen`omens complexos com la formaci´o de patrons, biestabilitat i punts de no retorn. Dos escenaris te`orics s’estudiaran: Un increment lineal amb el temps de la mortalitat i un increment abrupte d’aquesta. Al primer cas, un llindar en el ritme d’increment de la mortalitat s’ha trobat per damunt del qual els patrons no es poden formar i aleshores la vida d’una praderia cau r`apidament, ja que aquest mecanisme de resili`encia es perd. Quan la mortalitat es canvia abruptament, un arrelentiment cr´ıtic s’observa i, per mortalitats just damunt el punt de no retorn, regions de la praderia desapareixen independentment unes de les altres. En general, queda de manifest la import`ancia de tindre en compte l’estrucutra i din`amica espacial per a comprendre completament la situaci´o i evoluci´o de la praderia. S’ha tractat de fer prediccions de l’evoluci´o de les praderies a causa de l’escalfament global basant-se en dades de camp. Tot i que les incerteses a les dades dificulta la presentaci´o de conclusions quantitatives firmes, les ferramentes emprades per a estudiar aquestes dades incorporant una major part de la complexitat d’aquestes praderies s’ha explorat, servint com un pas endavant en l’enteniment del dest´ı de les praderies de P. Oceanica.

[eng] Posidonia Oceanica is a vital part of the Mediterranean coastal ecosystem, with positive effects ranging from providing food and shelter to other marine organisms to preventing coastal erosion and mitigating climate change. Human activity is damaging these millenary meadows at an alarming rate, partly through climate change, since water warming increases this seagrass’ mortality. The aim of this work is to study the possible changes in the spatiotemporal dynamics of P. Oceanica, focusing on the effect of a time-dependent mortality. Since we want to investigate the effects of spatial heterogeneity and collective effects, we will use a model that accounts for these factors. The model used includes spatial and non-linear terms accounting for collective and non-local interactions, and it captures complex phenomena such as pattern formation, bi-stability and tipping points. Two theoretical scenarios will be studied: A linear increase of mortality with time and an abrupt increase of mortality. In the first case, a threshold in the mortality increase rate is found over which patterns are not formed and hence the lifespan of a meadow decreases abruptly since this mechanism of resilience is lost. When the mortality is changed abruptly, critical slowing down is observed and, for mortalities just above the tipping point, patches of meadow disappear independently of each other. In general, the importance of taking the spatial structure and dynamics into account to fully understand the situation and evolution of a meadow is made clear. An attempt to make predictions of the evolution of meadows with an increase of mortality due to global warming based on field data is made. Although uncertainties in the data and the model hinder the capacity of presenting firm quantitative conclusions, the tools to study this data incorporating more of the meadow’s complexity is explored, serving as a step forward in the understanding of the fate of P. Oceanica meadows.