Succession patterns of turf algal communities in a fringing coral reef

Abstract

In scope of worldwide reef degradation due to anthropogenic environmental changes there is an urgent need to understand the different factors driving the processes within the coral reef ecosystem. Thus, the investigation of macroalgae becomes important, as they are observed to spread over the degraded reefs. Next to some conspicuous species, like Lobophora variegata and Dictyota spp., other smaller fialemntous algae, commonly known as algal turf, are observed. Algal turf has hardly been investigated to species level and has mainly been treated as functional group in many ecological studies. The need for studies at the species level is crucial, as certain species specific algal-coral interactions are reported to play an important role in the maintenance of the coral reefs. In order to increase the knowledge of turf algal communities in coral reefs ecosystems, the aim of the present study was to identify the species composition and to investigate the environmental factors potentially shaping the turf algal communities in a Carribean coral reef in the SW coast of Curacao. There were 101 different species identified, including 15 species of Chlorophyta, 10 species of Phaeophyceae, 61 species of Rhodophyta and 15 species of Cyanobacteria. To ease the investigation of these diverse assemblages, these species were grouped according to the lowest taxonomic level possible (preferably at genus level) for an adequate investigation under the dissection microscope (12 times magnification). Thus a total of 39 different taxa were distinguished. To understand the dynamics shaping these assemblages, the algal turf succession was followed over a period of 500 days, and the distribution of several turf algal species, growing on settlement tiles and as epiphytes on the dominant macrophyte L. variegata was investigated over a depth range from 2 to 40 m. Furthermore, in order to investigate the responses of turf algal communities towards changing environmental conditions, different successional stages were exposed in six multifactorial experimental studies to different environmental factors, including UV radiation, substrate quality, change in depth, eutrophication, annual variation, and grazing pressure. The findings of the present study confirmed that algal turf is not only comprises a diverse assemblage, but a highly complex community that is shaped and driven by specific responses to different environmental factors and interspecific relations. Furthermore, turf algal communities serve as habitat for different benthic and demersal fauna, provide a refuge for unicellular organisms, and may play a role as a bank of microstages for several bigger macroalgae. The communities responded to different environmental conditions, whereas the response depended on the species present and the different succesional stages. While the early stages were more affected by certain environmental stressors, e.g. grazing pressure, the later stages showed to be more tolerant, which were partly linked to the high observed tolerance of certain Cyanobacteria taxa, e.g. Dichothrix, which forms dense tufts, and might provide shelter for more sensitive organisms. The present study not only provides information about the composition and distribution of turf algal species in the research area, but also about community and species/taxa specific responses to different environmental stressors. This information is crucial as it allows for the development of a turf algal based assessment to predict environmental conditions, which can be used as monitoring device for the reef health. Overall the present study contributes to increase the knowledge of a long time understudied but important integrative part of the coral reef ecosystem. In the process of anthropogenic driven environmental change, further investigation of these complex communities will become crucial as they continue to spread over the reefs in the future.