Abstract
The importance of macro-grazers in controlling macroalgal cover has long been recognized on tropical and temperate reefs, with fishes of primary importance on the former and sea urchins on the latter. However, the functional role of herbivorous urchins and fishes on subtropical marginal reefs remains poorly explored. To evaluate the relative importance of fishes and urchins on marginal subtropical reefs, this study used exclusion devices (excluding all grazers, fishes, or urchins) at two depths (1-2, 5-6 m) on Brazilian rocky reefs. Depth influenced responses within cages, with shallow sites changing from patchy barrens (dominated by crustose coralline algae) to epilithic algae-dominated within exclusion treatments, and sea urchins being the primary driver of benthic dynamics. In deeper water, the growth and senescence cycle of Sargassum species was associated with upwelling events during the spring and summer, and dominated benthic dynamics. No clear influence of herbivorous fishes was detected on benthic cover at either depth, despite biomasses similar to comparable tropical reefs where they do control macroalgal populations. Thus, abiotic factors seem to be a strong driver of benthic dynamics in the studied region, and top-down processes act only at shallow depths. Consequently, despite Brazilian subtropical communities being dominated by tropical species, the ecological drivers of these reefs may be more similar to temperate systems.
Highlights
Herbivory is a critical process controlling the distribution and diversity of macroalgal cover in many marine ecosystems (Lubchenco and Gaines, 1981; Bonaldo et al, 2014), and determines the spatial organization and dynamics of benthic assemblages (Horn, 1989; Bozec and Mumby, 2015)
The general composition of benthic cover oscillated through time and among treatments indicating some temporal trends in benthic composition (Figure 3)
Our exclusion experiment revealed that both biological and abiotic factors act in conjunction to structure benthic communities, with distinct effects related to depth on southwestern Atlantic rocky reefs
Summary
Herbivory is a critical process controlling the distribution and diversity of macroalgal cover in many marine ecosystems (Lubchenco and Gaines, 1981; Bonaldo et al, 2014), and determines the spatial organization and dynamics of benthic assemblages (Horn, 1989; Bozec and Mumby, 2015). On tropical and temperate reefs, herbivory is an important process modulating shallow marine benthic assemblages (Steneck et al, 2017). These effects are well demonstrated on Caribbean reefs, where herbivorous feeding limits algal growth while benefiting coral demographics, such as increasing larval settlement and increasing reef resilience (Burkepile and Hay, 2006; Bozec and Mumby, 2015). Urchin grazing can be significantly affected by declines in predator populations, which can induce high levels of grazing and bioerosion on benthic communities, leading to changes that can last for years, and having direct consequences on reef productivity and diversity (Sheppard-Brennand et al, 2017)
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