Terrestrial discharge influences microbioerosion and microbioeroder community structure in coral reefs

Abstract

Microbioerosion rates and microbioeroder community structure were studied in four Kenyan protected coral-reef lagoons using shell fragments of Tridacna giant clams to determine their response to the influence of terrestrial run-off. Fourteen different microbioeroder traces from seven cyanobacteria, three green algae and four fungi species were identified. The river discharge-impacted reef and ‘pristine’ reef showed similar composition but higher microbioeroder abundance and total cyanobacteria- and chlorophyte-bioeroded areas when compared with the other study reefs. Cyanobacteria dominated during the north-east monsoon (NEM) relative to the south-east monsoon (SEM) season, with algae and cyanobacteria being major microbioeroders in the river-impacted and pristine reefs. The rate of microbioerosion varied between 4.3 g CaCO₃ m⁻² y⁻¹ (SEM) and 134.7 g CaCO₃ m⁻² y⁻¹ (NEM), and was highest in the river-impacted reef (127.6 g CaCO₃ m⁻² y⁻¹), which was almost double that in the pristine reef (69.5 g CaCO₃ m⁻² y⁻¹) and the mangrove-fringed reef (56.2 g CaCO₃ m⁻² y⁻¹). The microbioerosion rates measured in this study may not be high enough to cause concern with regard to the health and net carbonate production of Kenya’s coral reefs. Nevertheless, predicted increases in the frequency and severity of stresses related to global climate change (e.g. increased sea surface temperature, acidification), as well as interactions with local disturbances and their influence on bioerosion, may be increasingly important in the future.

Keywords: carbonate substrates, climate change, coralline algae, East Africa, microbial euendoliths, nutrients, river run-off, sediments