Recruitment dynamics in a marine long-lived species: the Mediterranean deepwater alga Cystoseira zosteroides C.Agardh, 1820

Abstract

Knowledge about key demographic processes is required to understand and to predict the response of populations to environmental change. Recruitment is an essential process regulating population abundance and dynamics of many marine species. From the initial settlement of propagules to the first appearance in the population, algae are likely to experience substantial mortality. Notwithstanding, there is a poor knowledge about factors controlling their recruitment success. Cystoseria zosteroides C.Agardh, 1820 is an emblematic species of deep-water Mediterranean forests, thriving on rocky bottoms between 15 to 80 m depth. The perennial character of this species has a marked influence on these communities, which show slow dynamics and harbour a high biodiversity of sessile invertebrates and encrusting algae. However, a global decline of Cystoseira assemblages throughout the Mediterranean has been observed during the last decades. To date, few studies have focused on C. zosteroides populations, showing that they are dominated by large long-lived organisms, with low recruitment and growth rates. Nevertheless, the lack of knowledge about the factors controlling recruitment success difficult the understanding of the long-term viability of how deep Cystoseira forests face to rising human and climate-induced disturbances. Hence, the main objective of this study was to quantify recruitment and mortality rates of the early life stages of this species and its influence on the population dynamics. We annually monitored five populations of C. zosteroides located at Columbretes Islands, Medes Islands and Cap de Creus by using permanent transects over three-four years. Our results showed that disturbed and non-disturbed populations displayed contrasting population dynamics, especially regarding to recruitment rates. A negative correlation was found between adults and juvenile individuals. Non-disturbed populations showed fewer fluctuations in density, recruitment was also nil and were mostly dominated by large individuals, which showed lower annual mortality rates. In contrast,populations exposed to large disturbances showed an important decrease of adult density and experienced a substantial increase in recruitment, displaying high annual variation in density and higher annual mortality rates. In general, adults displayed lower mortality rates (~12%) than younger individuals (~40%). Our findings agree with the expected results for a long-lived species where under low frequency of disturbances C. zosteroides populations display few fluctuations over time because they are poorly influenced by the dynamic of early life stages. Conversely, disturbances have a great influence on population dynamics, decreasing the effects of density-dependent processes through the removal of adults, and hence enhancing recruitment rates. Nonetheless, the high natural mortality of early life stages observed in this study may limit the recovery potential of this species. This highlights the vulnerability of C. zosteroides to the increase of natural and human-induced observed in the in Mediterranean Sea