Effect of suspended sediments on the sponge holobiont

Dredging can cause high suspended sediment concentrations (SSC) in the water column, posing risks to filter feeding organisms like sponges as sediment may clog their aquiferous systems and reduce feeding. In order to provide pressure:response values for sponges to SSC and tease apart the cause:effect pathways of dredging pressures, five heterotrophic and phototrophic species were experimentally exposed to a range of dredging-relevant SSC of up to 100 mg L-1, with light compensation across treatments to ensure that SSC was the primary physical parameter. This study shows that some sponge species exposed to high SSC (=23 mg L-1) for extended periods (28 d) have lower survival, increased necrosis, and reduced feeding ability with an associated depletion of energy reserves. In contrast, SSC of =10 mg L-1 causes few if any negative effects for most species and is thus suggested as a reasonable sub-lethal threshold for sponges. Microbial communities did not change significantly among SSC treatments, although nutritional shift from mixotrophy towards increased phototrophy was detected for some sponge species exposed to high SSC. However, it is expected that the combined effect of SSC with low light availability and sediment smothering will increase the negative effects on sponges.