Severe coral loss shifts energetic dynamics on a coral reef

Abstract

Declining coral cover and loss of structural complexity are widely reported on today's coral reefs. While coral loss frequently triggers changes in coral reef fish assemblage structure, the ecosystem‐scale consequences of these changes are poorly known. Here we evaluate how four metrics of energy flow and storage that underscore a critical coral reef function, consumer biomass production, respond to severe coral loss on a coral reef in the northern Great Barrier Reef, Australia. We compared fish and benthic surveys at Lizard Island from 2003 to 2004 with surveys in 2018 using an individual‐level modelling approach that integrates growth and mortality coefficients to estimate community‐level standing biomass, productivity, consumed biomass and turnover. In the study period, coral cover declined by 72%–83% in forereef zones while turf cover increased by 18%–100% across all zones. Reef fish assemblages, in turn, responded with a 71% increase in standing biomass, 41% in productivity and 37% in consumed biomass, mainly driven by nominally‐herbivorous fishes (Labridae—Scarini, Acanthuridae and Siganidae). By contrast, biomass turnover rates declined by 19%. Our findings suggest that coral loss can drive energetic shifts on coral reefs, leading to more productive, but slower paced reef fish assemblages. Although the observed build‐up of biomass may appear positive, the decreased turnover rates indicate that the system is unable to maintain biomass replacement levels. This suggests that the enhanced productivity that accompanied coral loss may be driven by storage effects from the somatic growth of individuals already present, questioning the temporal stability of these changes to coral reef ecosystem functioning. A free Plain Language Summary can be found within the Supporting Information of this article.

Extreme environmental conditions reduce coral reef fish biodiversity and productivity

“…However, the lack of difference in benthic community structure observed between regions suggests that benthic structure was at least not a primary driver of the observed patterns. Although the loss of some specialist cryptobenthic species has been reported after substantial live coral cover loss 99,105 , previous studies have not detected substantial short-term changes in either small reef fish richness and abundance or in overarching ecosystem productivity 27,29,31,56,99 .…”

“…In the wake of losing coral habitat, communities of the most prominent reef consumers, teleost fishes, also decline or shift in composition 24 – 27 , which directly affects the provision of resources to people dependent on reef fisheries 28 . Although recent evidence suggests that some fish species will be able to cope with (or even benefit from) live coral loss, at least in the short term 28 – 31 , tropical reef fishes are typically adapted to a relatively narrow suite of environmental conditions. Thus, reef fishes may also be vulnerable to the direct effects of climate change on, for instance, sea surface temperatures 13 , 32 , 33 .…”

Variability in coral reef fish baseline and benchmark biomass in the central and western Indian Ocean provinces

“…Nevertheless, a study in the Seychelles suggested that catch rates of some groupers in Aldabra Atoll declined across a period of climate stress (Pistorius & Taylor, 2009). The biomass and productivity of herbivorous fish has been known to increase with declines in corals, however (McClanahan, Maina, & Pet‐Soede, 2002; Morais et al, 2020). In general, the loss of coral appears to have the largest impact on small‐bodied species that do not greatly influence the metric of fishable biomass (Graham et al, 2008; Graham et al, 2011).…”

“…In some cases, human impacts override the environmental drivers by influencing reef fish habitat (Williams et al, 2015). For instance, fishing is associated with changes in the benthic cover that could potentially influence both refuge from climate disturbances and fishery production (Robinson et al, 2017; Morais et al, 2020). Thus, there are likely to be various feedback mechanisms between human impacts on habitat and fish populations, making it challenging to understand cause and effect and outcomes on ecosystem functions, such as fishery production (Williams & Graham, 2019).…”

Metabolic Regulation, Oxygen Limitation and Heat Tolerance in a Subtidal Marine Gastropod Reveal the Complexity of Predicting Climate Change Vulnerability

“…Current models based on ‘fixed’ energy metabolism of individuals imply that survival at higher temperatures requires an increase in energy intake (food) to meet thermally elevated energy demands ( Brown et al, 2004 ; Van der Meer, 2006 ; Sinclair et al, 2016 ; Huey and Kingsolver, 2019 ). Such elevated energy demands occur at the same time that performance becomes suboptimal and food availability is likely limited by reduced production and generally a reduced energy flow through the ecosystem (see Morais et al, 2020 ). However, these energetic constraints can be negated or minimized in animals with flexible or temperature independent metabolic rates that allow the lowering of resting energy demands, as occurs in many gastropods, including T. bacillum .…”