Quantification of chemical and mechanical bioerosion rates of six Caribbean excavating sponge species found on the coral reefs of Curaçao

Abstract

Excavating sponges are among the most important macro-eroders of carbonate substrates in marine systems. Their capacity to remove substantial amounts of limestone makes these animals significant players that can unbalance the reef carbonate budget of tropical coral reefs. Nevertheless, excavating sponges are currently rarely incorporated in standardized surveys and experimental work is often restricted to a few species. Here were provide chemical and mechanical bioerosion rates for the six excavating sponge species most commonly found on the shallow reef of Curaçao (southern Caribbean): Cliona caribbaea, C. aprica, C. delitrix, C. amplicavata, Siphonodictyon brevitubulatum and Suberea flavolivescens. Chemical, mechanical and total bioerosion rates were estimated based on various experimental approaches applied to sponge infested limestone cores. Conventional standing incubation techniques were shown to strongly influence the chemical dissolution signal. Final rates, based on the change in alkalinity of the incubation water, declined significantly as a function of incubation time. This effect was mitigated by the use of a flow-through incubation system. Additionally, we found that mechanically removed carbonate fragments collected in the flow-through chamber (1 h) as well as a long-term collection method (1 wk) generally yielded comparable estimates for the capacity of these sponges to mechanically remove substratum. Observed interspecific variation could evidently be linked to the adopted boring strategy (i.e. gallery-forming, cavity-forming or network-working) and presence or absence of symbiotic zooxanthellae. Notably, a clear diurnal pattern was found only in species that harbour a dense photosymbiotic community. In these species chemical erosion was substantially higher during the day. Overall, the sum of individually acquired chemical and mechanical erosion using flow-through incubations was comparable to rates obtained gravimetrically. Such consistency is a first in this field of research. These findings support the much needed confirmation that, depending on the scientific demand, the different approaches presented here can be implemented concurrently as standardized methods.

Fig 1. In situ close-up of the six studied excavating sponge species.

A. Cliona amplicavata (α-stage) B. Cliona aprica (α-stage) C. Cliona caribbaea (β-stage) D. Cliona delitrix (α-stage) E. Siphonodictyon brevitubulatum (α-stage), please note that the yellow fistules of S. brevitubulatum are surrounded by a different sponge species (pink coloured) F. Suberea flavolivescens (α-stage). Black arrows in A, E and F point out oscula and ostia of the studied sponges.

Fig 2. Flow-through set-up.

A. Overview of the flow-through incubation set-up including the peristaltic pump with adjustable flow-speed continuously pumping fresh sea water through the incubation chambers. The excess water subsequently overflows into collection vessels for nutrient and total alkalinity analyses. Chambers are partly submerged in a holding tank to prevent temperature fluctuation. B. Close-up of a flow-through incubation chamber (500 mL) holding two sponge infested limestone cores.

Fig 3. Modelled effect of incubation time on sponge chemical bioerosion rates.

Black regression lines represent the modelled (GAM) mean dissolution in mg cm^-2 h^-1 by Cliona caribbaea (left panel) and Cliona delitrix (right panel) in standing incubations. 95% confidence limits are indicated by the grey band. Solid dots show the calculated mean dissolution rates at T₁, T₂, T₄ and T₆ (ST). Open dots represent rates derived from running flow-through incubations at T₁, T₂, T₃ and T₄, dotted lines here indicate 95% confidence limits. All rates are obtained using alkalinity anomaly techniques.

Fig 4. Chemical bioerosion.

Hourly rates of chemical dissolution (mg CaCO₃ cm^-2 d^-1) are given for the six studied sponge species (top). Both day (light grey) and night (dark grey) erosion rates are included. Rates are derived from alkalinity change in flow-through incubation. Error bars indicate the 95% confidence limits.

Fig 5. Total sponge bioerosion.

Final rates of total erosion by the six sponge species (top) acquired through flow-through incubation (FT) and buoyant weight loss over a three week period (BW). The different bioerosive components (FT) are based on a 12 h day/night diurnal cycle. Mechanical bioerosion rates based on long-term collection are provided separately (LT). Here, a distinction is made between actual sponge chips and other CaCO₃ fragments that have been dislodged through sponge bioeroding activity. Rates are given in mg CaCO₃ cm^-2 d^-1 and kg CaCO₃ m^-2 y^-1. Error bars indicate the 95% confidence intervals for each individual element. Species and boring strategies are provided at the top of each panel, CF: cavity-forming, GF: gallery-forming; NF: network-forming.

Fig 6. Scanning Electron Microscopy (SEM) image of material produced by C. caribbaea.

The provided image shows only a fraction of the total material produced over the 7-day collection experiment. Black arrows: some characteristic sponge chips, white arrows: other CaCO₃ fragments. Scale is provided in lower-right corner.