Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

Abstract

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 They feed on organic matter withdrawn from the passing water and they may harbor microorganisms (endosymbionts), which contribute to their nutrition.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 δ13C and δ15N 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 δ13C and δ15N 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): δ13C −24.9‰, δ15N +4.3‰) and benthic-derived food (macroalgae: δ13C −15.4‰, δ15N +0.8‰). Enriched δ13C 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 δ13C and δ15N in sponges in October 2013 compared to October 2011 concurred with significantly elevated POM concentrations. The contemporaneous decrease in δ15N suggests that sponges obtain their N mostly from benthic-derived food with a lower δ15N 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 δ13C and 3‰ ± sd 0.5 for δ15N 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.

Depth‐dependent detritus production in the sponge, Halisarca caerulea

“…5). Compared to previous work on H. caerulea from the same location, the δ 15 N tissue values are similar (van Duyl et al 2011), while the δ 13 C stable isotope values for sponge tissues from this study are enriched in 13 C by $ 2.5 relative to DOM suggesting the broad use of DOM as a food source (de Goeij et al 2008b), and macroalgal-derived DOM in particular. Additionally, van Duyl et al (2011) used the same mixing model and estimated that 74.8% of the diet of H. caerulea originates from coral-derived mucus in shallow, cryptic environments, from which up to 80% enters the DOM pool (Wild et al 2004).…”

“…Compared to previous work on H. caerulea from the same location, the δ 15 N tissue values are similar (van Duyl et al 2011), while the δ 13 C stable isotope values for sponge tissues from this study are enriched in 13 C by $ 2.5 relative to DOM suggesting the broad use of DOM as a food source (de Goeij et al 2008b), and macroalgal-derived DOM in particular. Additionally, van Duyl et al (2011) used the same mixing model and estimated that 74.8% of the diet of H. caerulea originates from coral-derived mucus in shallow, cryptic environments, from which up to 80% enters the DOM pool (Wild et al 2004). However, van Duyl et al (2011) did not include macroalgal-derived DOM sensu stricto (i.e., crustose coralline algae were used as a proxy) in their analysis, which may contribute a significant amount of the available DOM on open coral reefs, and sponges have been shown to utilize it as a food source (van Duyl et al 2018).…”

“…Additionally, van Duyl et al (2011) used the same mixing model and estimated that 74.8% of the diet of H. caerulea originates from coral-derived mucus in shallow, cryptic environments, from which up to 80% enters the DOM pool (Wild et al 2004). However, van Duyl et al (2011) did not include macroalgal-derived DOM sensu stricto (i.e., crustose coralline algae were used as a proxy) in their analysis, which may contribute a significant amount of the available DOM on open coral reefs, and sponges have been shown to utilize it as a food source (van Duyl et al 2018). The δ 13 C stable isotope values of sponge-derived detritus signatures, however, were enriched in 13 C by $ 2.0-4.0‰ relative to POM suggesting that the isotopic value of sponge-derived detritus is influenced by the consumption of POM.…”

“…The analytical precision of the instrument was AE 0.1‰, and the mean precision of sample replicates for δ 13 C was AE 0.4 ‰, and for δ 15 N was AE 0.2 ‰. The δ 13 C and δ 15 N tissue data were also used in a mixing model using the R package SIAR as described by van Duyl et al (2018) to determine the proportional contribution of algal vs. coral DOM, as well as POM and detritus for sponges in each treatment. We used data from different sites in the Caribbean that are consistent with the known variation in these values.…”

“…These inputs into the model included δ 13 C and δ 15 N values for algal DOM (δ 13 C −15.42 ‰; δ 15 N 0.80, average of Dictyota sp. and Lobophora sp., table 3 in van Duyl et al 2018), and based on the assumption that autotrophic algal DOM is a reflection of isotopic value of its source), coral DOM (δ 13 C −17.19‰; δ 15 N 0.26, average of all coral species table 2 in van Duyl et al 2011), and based on the assumption that autotrophic coral DOM is a reflection of the isotopic value of its source, POM (δ 13 C −24.91 ‰; δ 15 N 5.62, van Duyl et al 2018) and detritus (δ 13 C −19.54‰; δ 15 N 2.56, from 10 and 50 m controls [no sponges]). A trophic enrichment factor of 0.5 AE 0.5% (SD) for δ 13 C and 3.0 AE 0.5% (SD) for δ 15 N was used (van Duyl et al 2018).…”