Changing benthic communities on Saba Bank Is Saba Bank becoming a ‘Sponge reef’?

Summary 

Abstract - Due to multiple stressors, many coral reefs are degrading. Shifts from a coral
dominated system to an alternate phase are observed. For coral reefs, the shift described most
in the scientific literature is a shift to an algae dominated system. However, shifts to other
dominant species are observed as well, including sponge or cyanobacteria dominance. It is
thought that sponges may become dominant when macroalgae or turf algae cover increases.
Algae are producers of dissolved organic matter (DOM), a food source for sponges.
Cyanobacteria are producers of DOM as well, although it remains unclear whether sponges
can use this. Furthermore, the decrease in spongivore fish due to overfishing, and the increase
in picoplankton can result in increasing sponge cover. In this study, benthic cover of eleven
sites at Saba Bank in 2013 and 2015 is assessed. Corals and sponges were identified to species
level, to get insight into assemblages. Using the program CPCe, photo quadrats (N=10) of two
transects per site were analyzed. The same photo quadrats were used for analysis of sponge
diversity, all sponges larger than 4 cm were identified to species level. Last, in 2015 92
specimens of sponges were sampled and identified using DNA barcoding and morphological
analysis. Close-up photos of these specimens were made, so that sponge identification guide
of Saba Bank can be developed. A shift from turf algae dominance to cyanobacterial
dominance among the years 2013 and 2015 was observed. Possibly, Saba Bank experiences
some influence from the nearby islands Saba, St. Eustatius, and St. Kitts and Nevis.
Macroalgae cover was found to be higher in the northern and northeastern parts of the bank
(closer to the islands), whereas coral cover was lower in these parts. The coral with highest
cover in the benthic survey was Montastraea faveolata (34.7% and 45.5% of total coral cover
in 2013 and 2015). Using multivariate analysis, year and water depth had a significant effect
on coral composition, position of the site had no significant effect suggesting connectivity
between sites for corals. The species Xestospongia muta and Agelas sventres contributed most
to total sponge cover (X. muta: 11% and 12.9%; A. sventres: 10.4% and 15.4% of total sponge
cover in 2013 and 2015). Water depth, northing and non living cover had a significant effect
on sponge composition. Altogether, sponge cover was not high on Saba Bank and therefore it
is not (yet) becoming a sponge reef. In 2015, Saba Bank was dominated by cyanobacteria, this
may be beneficial to sponges, since cyanobacteria are producers of DOM.