Responsiveness of Glass Sponges to Suspended Sediments at Canada’s Glass Sponge Reefs

• Author / Creator

  Grant, Nathan

• Glass sponge reefs are globally unique ecosystems on Canada’s western continental shelf that are susceptible to harm from fishing (e.g., bottom-contact trawling). In 2017, a marine protected area (MPA) was created to protect four of these reefs in Hecate Strait and Queen Charlotte Sound. Three sponge species form the reefs, Aphrocallistes vastus, Heterochone calyx and Farrea occa. Glass sponges are filter feeders, pumping water through their bodies to remove bacteria and oxygen and excrete wastes. Previous laboratory studies had shown glass sponges arrest their filtration in response to suspended sediment; this has never been studied in situ. Therefore the aim of this thesis was to determine whether glass sponge filtration is affected by resuspended sediments at the sponge reefs. The work was carried out at two reefs, one in the Strait of Georgia, BC and the other at the Hecate Strait and Queen Charlotte Sound Marine Protected Area. Using a remotely operated vehicle (ROV), current profilers and turbidity meters were deployed beside the glass sponge Aphrocallistes vastus on Fraser Ridge reef in the Strait of Georgia, BC, which is currently closed to bottom-contact fishing but which is not a MPA. Custom flowmeters were positioned into sponge oscula to record changes in filtration and changes in suspended sediment concentrations (SSC) were measured by changes in transmissivity using optical backscatter sensors. Sediment disturbances were created by the ROV. Increases in SSCs to 10 – 80 mg l-1 were correlated with decreased excurrent flow rate (r = -0.83 to -0.92). Both single arrests, lasting only 5 min in duration, and longer ‘coughing’ arrests lasting > 30 min were recorded, and together reduced feeding by up to 70%. A. vastus dominates Fraser Ridge reef, whereas two other species, Heterochone calyx and Farrea occa, comprise large portions of reef in the more northern Hecate Strait MPA. To determine whether species- or habitat-specific variations in the physiology of arrests in glass sponges, the responses of H. calyx, F. occa as well as a non-reef forming species, Rhabdocalyptus dawsoni, were studied in the MPA. Small sediment disturbances (< 5 – 10 mg l-1) generated by the ROV triggered single arrests in both H. calyx and R. dawsoni. H. calyx appeared to clog following two hours of exposure to SSCs of 3.2 mg l-1 and R. dawsoni arrested all pumping activity following extend exposure to SSCs < 1 mg l-1 above natural levels of turbidity (2.71 mg l-1). No arrests were recorded from F. occa but the low pumping rate of this sponge made it difficult to record flow out of this species. The distance suspended sediments could be transported was estimated using sediment collected from the reefs, and a settlement model. The model showed that a plume of sediment with <20 µm grain size has a range greater than the narrower portions of the MPA boundaries and would remain at high enough SSCs to induce arrests within the species studied. This study confirmed that different glass sponge species arrest filtration in response to elevated SSCs in situ. The thresholds for response varies between A. vastus, H. calyx and R. dawsoni, but all are significantly lower than those known to be generated by trawling (~ 100 – 500 mg l-1). These results provide threshold and response data for use in current and future conservation of Canada’s glass sponge reefs.

• Subjects / Keywords

  • Sediment
  • Sponge
  • Ecology

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3K64B85V

• License

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