Spatio–temporal variation in stable isotope signatures (δ13C and δ15N) of sponges on the Saba Bank
Sponges are ubiquitous on coral reefs, mostly long lived and therefore adaptive to changing environmental conditions. They feed on organic matter withdrawn from the passing water and they may harbor microorganisms (endosymbionts), which contribute to their nutrition. Their diets and stable isotope (SI) fractionation determine the SI signature of the sponge holobiont. Little is known of spatio–temporal variations in SI signatures of d13C and d15N in tropical sponges and whether they reflect variations in the environment. We investigated the SI signatures of seven common sponge species with different functional traits and their potential food sources between 15 and 32 m depth along the S-SE and E-NE side of the Saba Bank, Eastern Caribbean, in October 2011 and October 2013. SI signatures differed significantly between most sponge species, both in mean values and in variation, indicating different food preferences and/or fractionation, inferring sponge species-specific isotopic niche spaces. In 2011, all sponge species at the S-SE side were enriched in d13C compared to the E-NE side. In 2013, SI signatures of sponges did not differ between the two sides and were overall lighter in d13C andd15N than in 2011. Observed spatio–temporal changes in SI in sponges could not be attributed to changes in the SI signatures of their potential food sources, which remained stable with different SI signatures of pelagic (particulate organic matter (POM): d13C -24.9‰, d15N +4.3‰) and benthic-derived food (macroalgae: d13C-15.4‰, d15N +0.8‰). Enriched d13C signatures in sponges at the S-SE side in 2011 are proposed to be attributed to predominantly feeding on benthic-derived C.
This interpretation was supported by significant differences in water mass constituents between sides in October 2011. Elevated NO3 and dissolved organic matter concentrations point toward a stronger reef signal in reef overlying water at the S-SE than N-NE side of the Bank in 2011. The depletions of d13C and d15N in sponges in October 2013 compared to October 2011 concurred with significantly elevated POM concentrations. The contemporaneous decrease in d15N suggests that sponges obtain their N mostly from benthic-derived food with a lower d15N than pelagic food. Average proportional feeding on available sources varied between sponge species and ranged from 20% to 50% for benthic and 50% to 80% for pelagic-derived food, assuming trophic enrichment factors of 0.5‰ ± sd 0.5 for d13C and 3‰ ± sd 0.5 for d15N for sponges. We suggest that observed variation of SI in sponges between sides and years were the result of shifts in the proportion of ingested benthic- and pelagic-derived organic matter driven by environmental changes. We show that sponge SI signatures reflect environmental variability in space and time on the Saba Bank and that SI of sponges irrespective of their species-specific traits move in a similar direction in response to these environmental changes.