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.
Highlights
Tropical reefs are declining rapidly due to ocean warming, associated with mass coral bleaching and mortality [1], and by local stressors such as degrading water quality and overfishing [2, 3]
For pinnacles’ tops, the first principal component analysis (PCA) axis (PC1) evidenced a clear separation between offshore (PAB2 and PAB3) and nearshore reefs (PLES, Sebastião Gomes (SGOM) and TIMB) (Fig 4), the former associated with higher cover of corals, BCM, turf and sponges, and the latter with macroalgae and zoanthids
Habitat was the main source of community heterogeneity in the Abrolhos reefs, which sustained high coral cover over the studied decade, albeit with remarkable habitat and cross-shelf differences in benthic cover dynamics
Summary
Tropical reefs are declining rapidly due to ocean warming, associated with mass coral bleaching and mortality [1], and by local stressors such as degrading water quality and overfishing [2, 3]. In the last four decades, global coral cover declined between 50 and 75% [5] Such widespread degradation impairs biomineralization, reduces shoreline protection, fisheries production, and other ecosystem services, with profound impacts to the livelihoods of millions of people [6]. The temporally-persistent replacement of corals by non-building organisms, known as phase shift [10, 12], is associated with lower structural complexity and decreased community level-diversity [13], as well as to microbialization and diminished secondary production [14, 15]. The scarcity of baselines and time series [17], as well as major geographic data gaps, still impedes a thorough understanding of this phenomenon, its context-dependency [7, 18], and the multiplicative or antagonistic effects of global and local drivers upon coral decline [5, 19]. The overall complexity of reef systems and the wide array of environmental conditions under which they are able to persist adds to the complexity of understanding phase shifts
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