Demographics and population ecology of the range-expanding zooxanthellate coral Oculina patagonica in the Mediterranean Sea

Abstract

Shallow-water rocky ecosystems in temperate areas are dominated by macroalgae, whilst zooxanthellate corals are extremely rare. In an era of global change, sea warming plays a crucial role in the widespread phase shifts from coral- to algal-dominance in tropical coral reef ecosystems, and in the poleward spread and increase in abundance of some tropical and subtropical zooxanthellate corals into algal-dominated temperate areas. In the Mediterranean Sea, the zooxanthellate coral Oculina patagonica (Scleractinia, Oculinidae) is experiencing a range-expanding process, providing a good case study to examine how zooxanthellate corals can affect the structure of temperate ecosystems. In this PhD Thesis, different field studies were performed along the Iberian coast in the western Mediterranean to examine the demographics and population ecology of O. patagonica and contribute understanding the interaction among the main intrinsic and extrinsic factors in modulating the species’ dynamics. First, we document that O. patagonica is experiencing an invasive behavior that challenges the current conceptual framework of shallow-water rocky ecosystems, by providing evidence that the species is able to: i) form encrusting bioconstructions and drive a phase shift from macroalgal- to coral-dominated states, and ii) experience an invasive behavior at both population outbreak and geographical distribution range levels. Second, over a ~4-yr field experiment we document that a severe and recurrent pattern of partial mortality on O. patagonica colonies occurs under low seawater temperature (ST) conditions during winter, which appears to play a crucial role in constraining the growth dynamics of the species at the high-latitude area in the north-eastern Iberian coast. Third, we document the spatial variation in the demographics of O. patagonica (occurrence, density, cover, colony partial mortality and size-structure) by examining 314 locations along ~1300 km Iberian coast that provides a detailed baseline quantitative dataset. We found that the colony size-structure of coral populations in natural locations were unimodal and positively skewed, indicative of non-stable and growing populations. However, the species’ demographics showed a marked ‘abundant-center’ pattern that is not only related to the time of establishment but also to the relevant role of differences in coral population growth that correlate with key environmental variables. Finally, we layered our broad spatial scale observations on the demographics of O. patagonica, together with our and previous empirical and experimental studies on the interaction between biological traits and key environmental factors modulating coral performance, to identify the potential causes of the geographic range structure and invasion behavior of this species within the Iberian coast. The success of O. patagonica colonies and populations on natural locations appear to be generally constrained by low ST and light conditions, with thresholds that limit coral growth of mean annual photosynthetic active radiation at 3 m depth <30 mol photons m-2 d-1, mean ST <19ºC, percentile 10th-ST <14ºC and percentile 90th-ST <25ºC, but without severe high-ST periods (percentile 90th-ST >27ºC). In addition, the high availability of open space for O. patagonica to colonize, and the positive effects of sand scouring and sea urchin herbivory on coral population growth, indicate that open space availability is a crucial factor enhancing the proliferation of the species within the Iberian coast. We also found that artificial reefs foster the population growth and the expansion of the distributional range of this coral species. The broad spread of O. patagonica across the Mediterranean Sea, and its invasive behavior at both distributional range shift and population outbreak levels, able to drive phase shifts from macroalgae to coral dominance in natural and artificial reefs, is consistent with the ongoing process of zooxanthellate coral-mediated tropicalization of shallow-water rocky ecosystems documented in other subtropical and temperate areas under current global change.