Jasper Bleijenberg

Abstract 

Before its mass-mortality in the 1980’s, Diadema antillarum was the most important benthic herbivore on Caribbean reefs. Since then, reefs have experienced an increase in turf/macroalgae abundance. Despite the approximate 40 years that have past, natural recovery is slow and variable. Therefore efforts are taken to boost D. antillarum recovery artificially. Several bottlenecks for recovery have been found, but in the last decade a new potential problem has risen: massive influxes of floating Sargassum species. Studies have shown that algae or its biofilm can induce sea urchin settlement to a certain extent, but no in vivo experiments have been conducted for D. antillarum. Here we investigated if these Sargassum floats interfere with the recruitment of D. antillarum by measuring the abundance of D. antillarum settlers in submerged and floating, rinsed and unrinsed Sargassum units and comparing values with a positive control. While the amount of data collected with the low sample sizes did not suffice to answer the research question in any statistically significant manner, hypothesis could be formulated.   

D. antillarum certainly is attracted to sargassum patches, whether as post-settler only or also as settler is however still debatable. Cues by conspecifics could possibly play a role in increased settlement in Sargassum, blurring the effect of Sargassum solely. A large variation in D. antillarum abundance was found for the unrinsed submerged treatment, which was possibly the result of migration from nearby existing ecosystems with older D. antillarum into the experimental units, or it was an artefact of the low sample size. To confidently distinguish signal from noise in a similar study, some alterations to the research design need to be made and drifting sargassum patches need to be sampled to get a sense of D. antillarum settler abundance on naturally occurring Sargassum. 

Abstract:

Knowledge gaps remain regarding the drivers of active coral restoration success that may impede our ability to effectively restore coral reef communities. Publications about ecological development of Acropora outplants on natural reefs reveal that long term survival is low and that growth and survivorship is negatively correlated with increased density, competition by other species and sedimentation. Freshly deployed artificial reefs have the potential to relieve corals from some of the stressors by facilitating a clean, competitor free environment. Especially the relieve from the prominent Dictyota macro-algae species wich we assumed to be a main competitor, might benefit the corals. Here, we conducted a field experiment of one year to investigate growth and survival of nursery-raised Acropora cervicornis corals outplants for two types of reefs as substrate: natural patch reefs and originally clean algae-free artificial reefs, and simultaneously investigated the effects of depth and position of attachment to the reefs on these variables.

Significant differences in survival between reef types up to 53 days after outplanting indicated that corals did benefit from clean artificial reef substrate in the early stages. However, after that time, survival of corals on both reef types converged so that no significant differentiation could be made. Final survival after a year was 50% ± 10.20% on natural patch reefs, while the survival on the artificial reefs plots decreased to 49.17% ± 13.86. Initial cleanliness of the substrate seemed to be not important to survival in the long run in the current study. However, in combination with other survival improving practices, overcoming initial losses of outplanted corals by using clean artificial substrates has potential to improve survival in the long run.

Instantaneous growth rates of Acropora cervicornis were .00379 day-1 (sd = .00129 day-1 ) and .00636 day-1 (sd = .00100 day-1 ) on artificial reefs and natural patch reefs respectively. These rates were affected by all variables under scrutiny. Place of attachment within the artificial reefs had the strongest effect, followed by the reef type for which confidence intervals were wider. Depth was found to have an effect but only very minimal over the small range of 15 to 18.6 meters. Acropora cervicornis grew faster on the natural patch reefs. The hypothesis that the absence of an established benthic community on artificial reefs might have a positive effect on coral growth rate cannot be supported. Differential growth rates are possibly induced by differences in water flow rates within reefs and are possibly the combined result of differential flow rates and overall nutrient availability between reef types. More investigation into the drivers for growth is necessary to confidently deduce the most important factors for growth on the two reef types