Decadal (2006-2018) dynamics of Southwestern Atlantic’s largest turbid zone reefs

Abstract

Tropical reefs are declining rapidly due to climate changes and local stressors such as water quality deterioration and overfishing. The so-called marginal reefs sustain significant coral cover and growth but are dominated by fewer species adapted to suboptimal conditions to most coral species. However, the dynamics of marginal systems may diverge from that of the archetypical oligotrophic tropical reefs, and it is unclear whether they are more or less susceptible to anthropogenic stress. Here, we present the largest (100 fixed quadrats at five reefs) and longest time series (13 years) of benthic cover data for Southwestern Atlantic turbid zone reefs, covering sites under contrasting anthropogenic and oceanographic forcing. Specifically, we addressed how benthic cover changed among habitats and sites, and possible dominance-shift trends. We found less temporal variation in offshore pinnacles’ tops than on nearshore ones and, conversely, higher temporal fluctuation on offshore pinnacles’ walls than on nearshore ones. In general, the Abrolhos reefs sustained a stable coral cover and we did not record regional-level dominance shifts favoring other organisms. However, coral decline was evidenced in one reef near a dredging disposal site. Relative abundances of longer-lived reef builders showed a high level of synchrony, which indicates that their dynamics fluctuate under similar drivers. Therefore, changes on those drivers could threaten the stability of these reefs. With the intensification of thermal anomalies and land-based stressors, it is unclear whether the Abrolhos reefs will keep providing key ecosystem services. It is paramount to restrain local stressors that contributed to coral reef deterioration in the last decades, once reversal and restoration tend to become increasingly difficult as coral reefs degrade further and climate changes escalate.

Benthic Cyanobacterial Diversity and Antagonistic Interactions in Abrolhos Bank: Allelopathy, Susceptibility to Herbivory, and Toxicity

“…Variable toxicity within the same BCMs that we observed (i.e., 3 out of 11 samples caused >50% mortality) should not imply a lack of chemical defense by cyanobacteria, as spatial and seasonal variability of chemical defenses against herbivores is expected due to metabolic responses to changes in the environment (Puglisi et al, 2019). The high spatial variability of Abrolhos reefs likely contributes to a context dependency of BCMs toxicity (Ribeiro et al, 2018;Teixeira et al, 2021). The large biomass of morbid sea slugs, Stilocheylus striatus, observed during cyanobacterial blooms in Abrolhos reefs (Ribeiro et al, 2017) is in line with the toxic potential of Abrolhos BCMs observed here.…”

“…Throughout the text samples, PLES and PAB will be referred to as PLES BCM and PAB BCM , respectively. The BCMs cover and long-term survey data are described in Teixeira et al (2021). An aliquot of each sample was rinsed with filtered (GF/F) seawater and snap-frozen or fixed in 1% glutaraldehyde.…”

“…Few studies have demonstrated that BCMs and the allelochemicals they produce can damage the coral's tissue and microbiome (e.g., Titlyanov and Titlyanova, 2009;Morrow et al, 2011). In the nutrified and turbid reef systems of Abrolhos (Leão et al, 2016), BCMs have been associated with long-term impacts on coral growth (Ribeiro et al, 2018) and altered states of benthic dynamics (Teixeira et al, 2021).…”

Integrating Different Types of Knowledge to Understand Temporal Changes in Reef Landscapes

“…Divers and reef scientists noticed coral declines more frequently than fishers probably because they are more likely to pay attention to the reef substrate and sessile invertebrates due to their recreational and scientific interest, respectively (Giglio et al, 2015). This perception is corroborated by the increase in coral mortality due to more frequent and intense thermal anomalies in addition to land-based stressors in Brazil (Teixeira et al, 2021). Reef scientists were also the only stakeholder group to notice a decline in sea urchin populations.…”

Unusual high coral cover in a Southwestern Atlantic subtropical reef

“…Reef ecosystems are one of the most diverse environments on Earth, commonly known to flourish in tropical areas (Spalding et al 2001). Although the development of coral reefs and, thus, the production of a biogenic carbonate framework can be limited by abiotic factors such as temperature and light penetration, coral communities still thrive in a range of environmental settings (Kleypas et al 1999;Perry and Larcombe 2003).…”