Articles | Volume 13, issue 11
Research article
15 Jun 2016
Research article |  | 15 Jun 2016

Long-term macrobioerosion in the Mediterranean Sea assessed by micro-computed tomography

Claudia Färber, Jürgen Titschack, Christine Hanna Lydia Schönberg, Karsten Ehrig, Karin Boos, Daniel Baum, Bernhard Illerhaus, Ulla Asgaard, Richard Granville Bromley, André Freiwald, and Max Wisshak

Abstract. Biological erosion is a key process for the recycling of carbonate and the formation of calcareous sediments in the oceans. Experimental studies showed that bioerosion is subject to distinct temporal variability, but previous long-term studies were restricted to tropical waters. Here, we present results from a 14-year bioerosion experiment that was carried out along the rocky limestone coast of the island of Rhodes, Greece, in the Eastern Mediterranean Sea, in order to monitor the pace at which bioerosion affects carbonate substrate and the sequence of colonisation by bioeroding organisms. Internal macrobioerosion was visualised and quantified by micro-computed tomography and computer-algorithm-based segmentation procedures. Analysis of internal macrobioerosion traces revealed a dominance of bioeroding sponges producing eight types of characteristic Entobia cavity networks, which were matched to five different clionaid sponges by spicule identification in extracted tissue. The morphology of the entobians strongly varied depending on the species of the producing sponge, its ontogenetic stage, available space, and competition by other bioeroders. An early community developed during the first 5 years of exposure with initially very low macrobioerosion rates and was followed by an intermediate stage when sponges formed large and more diverse entobians and bioerosion rates increased. After 14 years, 30 % of the block volumes were occupied by boring sponges, yielding maximum bioerosion rates of 900 g m−2 yr−1. A high spatial variability in macrobioerosion prohibited clear conclusions about the onset of macrobioerosion equilibrium conditions. This highlights the necessity of even longer experimental exposures and higher replication at various factor levels in order to better understand and quantify temporal patterns of macrobioerosion in marine carbonate environments.

Short summary
In this study we present results from the first long-term bioerosion experiment (1–14 years of exposure) outside the tropical realm. A novel micro-CT approach was used to visualise and to quantify the development of macrobioerosion traces. After 14 years, 30 % of the original substrate volume was excavated chiefly by sponges. High spatio-temporal variability prohibited clear conclusions about the onset of macrobioerosion equilibrium conditions, calling for further long-term experiments.
Final-revised paper