Bioerosion rates on shore and shallow water rocky bottoms along Maltese coasts

Abstract

Biological factors are often overlooked when viewing the morphodynamics of rocky coasts. Bioerosion occurs when, due to the effect of bioeroding organisms, material is broken down and fractionated. This subsequently alters the habitat in which these organisms live, possibly affecting the entire ecosystem. The most common bioerosive organisms found on Maltese rocky shores, namely limpets, chitons, top shells and sea urchins, were considered during this study. Limpet home scars were measured to study the effects of rock type (Globigerina Limestone and Coralline Limestone) and exposure on limpet scar volumes. The interaction of rock type and exposure was deemed as not statistically significant. The same could be said regarding the individual effect of both rock type and exposure. Correlation between the rock type and exposure of an area and the population density of bioeroding organisms present was also examined using transect data collected from 16 sites. The areas examined showed no correlation between the rock type and exposure and the resultant population density of the shore for any of the species. Quantitative measures of the bioerosion rate of the different species were taken and related to the exposure and rock type of the sites sampled. This was done by fasting the collected organisms for 24 hours and collecting the excreted faecal matter. These faecal pellets were than tested via a back-titration to gather estimates of CaCO3 in the faeces. Results obtained showed that there was significant influence of exposure or rock type on some species and not others. Arbacia lixula’s percentage CaCO3 in faecal pellets was affected by both exposure and rock type. Rock type also influenced the percentage CaCO3 in faecal pellets of the species Paracentrotus lividus, Patella caerulea and Lepidochitona caprearum. Further statistical tests were done on the mass of CaCO3 per unit body mass. It was discovered that Patella rustica was affected by changes in both exposure and rock type whilst Paracentrotus lividus was affected by a difference in rock type. The mass of CaCO3 per unit body mass indicated that different species having varied bioerosive abilities. Paracentrotus lividus had the highest, followed by Lepidochitona caprearum, Patella caerulea, Arbacia lixula, Patella rustica and Phorcus turbinatus with similar bioerosive abilities. The remaining species, Patella ulyssiponensis, Phorcus articulatus and Melarhaphe neritoides had a much lower mass of CaCO3 per unit body mass. Using the bioerosion rate and the population densities extrapolated from this study, the total bioerosive rate of each shore was estimated and compared. When compared statistically the different shore types produced no statistically significant difference between them.