Increasing benthic dominance of the phototrophic sponge Lamellodysidea herbacea on a sedimented reef within the Coral Triangle

Abstract

Coral abundance on tropical reefs is declining due to climatic change and other direct anthropogenic impacts. The long-term consequences of coral ‘regime-shifts’ are not fully understood, but they are expected to impact ecosystem services (e.g., ecotourism and fisheries). Within the Wakatobi region of Indonesia a coral-to-sponge regime-shift has occurred on a sedimented reef (Sampela). The dominant sponge species, Lamellodysidea herbacea, is a phototrophic sponge that appears to be proliferating through effective sediment clearance mechanisms and rapid photoacclimation to turbid conditions. L. herbacea monitoring is required to determine whether this ecosystem is still transitioning, stable or whether the regime-shift is transient. This study (2013–2015) assessed the percentage cover and abundance of L. herbacea, along with the quantity of settled sediment, at Sampela, Pak Kasim’s (a nearby reef) and multiple reefs in the region. High growth rates for individual L. herbacea, but low recruitment rates were recorded at Sampela. Increases in the proportion of L. herbacea that decreased in areal coverage between sampling periods indicate short periods of positive growth, with this effect being greater in deeper, more sedimented zones. The percentage cover and abundance of L. herbacea, and the quantity of settled sediment, was low at all surrounding reefs and remained stable at Pak Kasim’s. However, the percentage cover of L. herbacea at Sampela was high and increased, in conjunction with a stable, comparatively high quantity of settled sediment. These results are the first to document a reef in the process of transitioning towards greater sponge dominance and indicate that efforts to reverse this regime-shift would likely require a reduced level of sedimentation.