Effects of combined dredging-related stressors on sponges: a laboratory approach using realistic scenarios

Abstract

Dredging can cause increased suspended sediment concentrations (SSCs), light attenuation and sedimentation in marine communities. In order to determine the combined effects of dredging-related pressures on adult sponges, three species spanning different nutritional modes and morphologies were exposed to 5 treatment levels representing realistic dredging scenarios. Most sponges survived under low to moderate turbidity scenarios (SSCs of ≤ 33 mg L −1 , and a daily light integral of ≥0.5 mol photons m −2 d −1 ) for up to 28 d. However, under the highest turbidity scenario (76 mg L −1 , 0.1 mol photons m −2 d −1 ) there was 20% and 90% mortality of the phototrophic sponges Cliona orientalis and Carteriospongia foliascens respectively, and tissue regression in the heterotrophic Ianthella basta. All three sponge species exhibited mechanisms to effectively tolerate dredging-related pressures in the short term (e.g. oscula closure, mucus production and tissue regression), although reduced lipids and deterioration of sponge health suggest that longer term exposure to similar conditions is likely to result in higher mortality. These results suggest that the combination of high SSCs and low light availability can accelerate mortality, increasing the probability of biological effects, although there is considerable interspecies variability in how adult sponges respond to dredging pressures.Sediments released into the water column by natural resuspension, river runoff and human activities such as dredging pose a potential risk to sensitive ecosystems such as coral reefs, seagrass meadows and sponge gardens 1-4 . Sediments in suspension, or settling back out of suspension (i.e. sedimentation), can affect epi-benthic organisms in a number of ways, including clogging of the feeding and filtering mechanisms. Water turbidity (cloudiness) can also temporarily reduce or extinguish benthic light 4-6 . These stressors can act alone but more often in combination, making impact prediction particularly difficult. Sponges are sessile filter-feeding organisms that play important roles in marine ecosystems, including substrate consolidation, habitat provision, seawater filtration and bentho-pelagic energy transfer 7-9 . Despite their abundance and ecological importance, our understanding of how sponges respond to turbidity is still very basic 10, 11 . This knowledge gap poses significant challenges to their effective management, especially for anthropogenic turbidity-generating activities such as dredging, that can at least be subject to some regulation and control 12 .Most sponges obtain energy heterotrophically, by filtering seawater through an aquiferous system or internal canal network 13,14 . However, many sponges can also obtain energy autotrophically from photosymbionts, of which Cyanobacteria are the most common 15,16 . Some bioeroding sponge species also harbour dinoflagellates of the genus Symbiodinium as photosymbionts 17,18 . The photosymbionts can provide >50% of the energy requirement of the host for some tropical sponge spec...

Effect of suspended sediments on the pumping rates of three species of glass sponge in situ

“…Our estimates that extended periods of reduced excurrent flow rates could be fatal are sup ported by mesocosm experiments in demosponges (e.g. Pineda et al 2017, Scanes et al 2018. Geodia barretti held in tanks with suspended sediment was found to reduce respiration rate (Kutti et al 2015), but for longer-term experiments, large sponges are difficult to keep in high enough numbers that they are not affected by other aspects of husbandry.…”

“…A similar study on explants of warm-water species found that 'tolerance' to suspended sediment varied among species (Pineda et al 2017). Most work has inferred sponge 'health' from respiration rates and tissue condition (Pineda et al 2017, Scanes et al 2018).…”

“…Sponges are sensitive to minute changes in water quality and respond to changes in seasonal characteristics of the water column (Reiswig 1971), food availability (Reis wig 1975, Kahn et al 2012, ocean acidification (Bates & Bell 2018) and suspended sediments (Gerro dette & Flechsig 1979, Carballo 2006, Tompkins-MacDonald & Leys 2008. Sediments may smother and clog a sponge (Airoldi 2003, Tompkins-MacDonald & Leys 2008, Strehlow et al 2017, and smaller grain sizes can fit through the incurrent pores on the sponge surface (< 20 µm) (Kilian 1952, Reiswig 1974 potentially damaging their aquiferous system. To deal with sediments, most demosponges contract to prevent entry of sediment or to expel already ingested sediments (Prosser et al 1962, Elliott 2004, Elliott & Leys 2007; others secrete mucus to slough sediments off their surface (Kowalke 2000).…”

Contraction Dynamics and Respiration of Small Single-Osculum Explants of the Demosponge Halichondria panicea

“…Even though severe reductions in the ambient oxygen level may cause changes in the contraction rate of demosponges such as T. wilhelma, which changes its normal contraction behavior at oxygen concentrations below 4% air saturation (Mills et al, 2018), periodic osculum closure of our explants seems unlikely to be stress-related within the much higher investigated oxygen concentrations of 28-100% air saturation. Furthermore, there is evidence that environmental disturbances, such as resuspended sediment during storms or dredging, can also stimulate contractile behavior (Reiswig, 1971a;Pineda et al, 2017), which may include protection and cleaning of the filtration apparatus (Elliott and Leys, 2007;Tompkins-MacDonald and Leys, 2008). This was unlikely, however, in our experiments as the explants were maintained in (0.2 µm) filtered seawater throughout the experimental period.…”

A meta‐analysis reveals global patterns of sediment effects on marine biodiversity

“…Despite ample emphasis on light attenuation, much of the literature has focused on the quantification of biotic responses to increased suspended sediment concentrations and sedimentation, with only little attention being paid to combined effects (e.g., Pineda et al, 2017). Suspended sediment in the pelagic domain can lead to serious physical damage associated with sediment abrasion, clogging of feeding mechanisms, tissue injuries and necrosis (Erftemeijer, Riegl, Hoeksema, & Todd, 2012;Fabricius, 2005;Rogers, 1990).…”