Abstract
Dredging can cause high suspended sediment concentrations (SSC) in the water column, posing a hazard 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 depletion of energy reserves. In contrast, SSC of ≤10 mg L−1 caused few, if any, negative effects and is thus suggested as a prudent sub-lethal threshold for sponges. Microbial communities did not change significantly among SSC treatments, although a nutritional shift from mixotrophy towards increased phototrophy was detected for some sponge species exposed to high SSC. Importantly however, it is expected that the combined effect of SSC with low light availability and sediment smothering as occurs during dredging operations will increase the negative effects on sponges.
Highlights
Dredging is an essential part of all port operations and the need for dredging is likely to increase with the current trend towards larger ships with deeper draft requirements[1]
Most sponges appeared visibly healthy at the end of the experiment, with the exception of all Carteriospongia foliascens, a few individuals of Coscinoderma matthewsi, and Cliona orientalis exposed to 100 mg L−1
The suspended sediment concentrations (SSC) used in this study were designed to bracket SSCs measured during a recent large scale capital dredging project in NW Australia
Summary
Dredging is an essential part of all port operations and the need for dredging is likely to increase with the current trend towards larger ships with deeper draft requirements[1]. Sponges are sessile filter-feeding organisms playing important roles in many marine ecosystems, including substrate consolidation, habitat provision, seawater filtration, and bentho-pelagic energy transfer[11, 13]. Sponge symbionts tend to be highly host specific, but are generally stable across broad geographic and environmental gradients[29], with the stable host-microbe consortium often referred to as the ‘sponge holobiont’[28] Depending on their degree of nutritional dependence from symbiont primary production, the sponge holobiont can be described as either ‘phototrophic’, ‘mixotrophic’ or ‘heterotrophic’. The different modes of nutrition have important implications for understanding the environmental effects of dredging activities on sponges Due to their filter feeding activity, sponges may be affected by elevated suspended sediment concentrations (SSCs) and long term exposure to high SSCs can lead to clogging of their aquiferous systems, reducing the flow of oxygenated seawater to the sponge mesohyl and compromising heterotrophic feeding[32]. Periods of low light levels associated with dredging can cause photoacclimation of the photosymbionts[32, 42, 43], but extended periods of low light can result in dissociation of the holobiont (bleaching) and subsequent mortality in some phototrophic species[44]
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