Integrating Short- and Long-Term Bioerosion Processes (Greece)
Final Report Abstract
Bioerosion – the biologically driven corrosion and abrasion of hard substrates – is a key process during the (re)cycling of carbonate and the formation of calcareous sediments. Quantitative and qualitative assessments of bioerosion are important for carbonate budget calculations and palaeoenvironmental evaluations. This is particularly relevant since bioerosion is considered to increase with ongoing ocean acidification, with potentially detrimental effects on carbonate environments. The main objectives of the project were to analyse the spatio-temporal variability of bioerosion in the Eastern Mediterranean Sea, its contribution to the carbonate budget, and to set the results in a latitudinal and longitudinal context. This was based on three experiments: (1) a carbonate cycling experiment with settlement platforms deployed for a summer, a winter, and one year of exposure along a transect from 15 to 250 m water depth off the Peloponnese Peninsula, (2) a longterm experiment with marble blocks deployed for 1 to 14 years in shallow waters around the island of Rhodes, and (3) a coastal bioerosion experiment with substrates mounted for 1 and 2 years in the inter- to supratidal zones at Rhodes. The first two experiments were carried out prior to, but analysed during the present project, whereas the third one was started during the project, with some substrates remaining in place for a long-term exposure. Short-term bioerosion rates and traces were analysed by weight measurements and SEM of epoxy-resin casts, while long-term bioerosion rates and traces were assessed in a novel approach via micro-computed tomography (µCT). In total, the analysis of bioerosion traces recorded 44 ichnotaxa, produced by bioeroding cyanobacteria, chlorophytes, fungi, foraminifera, bacteria, sponges, polychaetes, bivalves, gastropods, diatoms, echinoids, and chitons. The carbonate cycling experiments revealed that the distribution and boring intensity of microbioerosion agents in the Mediterranean Sea is mostly controlled by light availability, but also subject to a distinct seasonal variability. The highest bioerosion activity was recorded in 15 m up-facing substrates in the shallow euphotic zone, largely driven by phototrophic cyanobacteria, while towards the chlorophyte-dominated deep euphotic to dysphotic zones and the organotroph-dominated aphotic zone the intensity and diversity of bioerosion strongly decreased. During summer, the bioeroding activity was significantly enhanced by elevated water temperatures, higher absolute light intensities, and lower water turbidity. The observed patterns were mirrored by the calculated carbonate budget with bioerosion rates exceeding carbonate accretion rates in the shallow subtidal zone. Towards and within the supratidal zone, the coastal experiment revealed a distinct decrease of microbioerosion activity and ichnodiversity. A higher range of borings on wind-and wave-exposed substrates indicated sea water supply as one of the limiting factors of coastal bioerosion. Analysis of macrobioerosion traces from the long-term experiment revealed a dominance of boring sponges. Early community development was observed during the first 5 years of exposure, where first excavating sponges settle, but yield low rates of macrobioerosion, followed by an intermediate stage when sponges formed large and more diverse borings. After 14 years, 30 % per volume was occupied by boring sponges, yielding maximum bioerosion rates of 900 g m-2 yr-1. High spatial variability of macroborers and low numbers of replicates, however, prohibited clear conclusions about the onset of macrobioerosion equilibrium conditions. In summary, this project followed an interdisciplinary approach to study the spatio-temporal variability of bioerosion patterns in the marine realm. The findings highlight the relevance of bioerosion for the carbonate budget of the Eastern Mediterranean Sea and underline the need for long-term experiments in order to better quantify macrobioerosion patterns.
Publications
- (2015) Effects of Water Depth, Seasonal Exposure, and Substrate Orientation on Microbial Bioerosion in the Ionian Sea (Eastern Mediterranean). Plos One 10: e0126495
Färber C, Wisshak M, Pyko I, Bellou N, Freiwald A
(See online at https://doi.org/10.1371/journal.pone.0126495) - (2016) Long-term macrobioerosion in the Mediterranean Sea assessed by micro-computed tomography. Biogeosciences Discussions
Färber C, Titschack J, Schönberg CHL, Ehrig K, Boos K, Baum D, Illerhaus B, Asgaard U, Bromley RG, Freiwald A, Wisshak M
(See online at https://doi.org/10.5194/bg-13-3461-2016)
