Carbonate budgets as indicators of functional reef “health”: A critical review of data underpinning census-based methods and current knowledge gaps

Abstract

The carbonate budget of a reef describes the net rate of carbonate production resulting from various biologically-, physically-and chemically-driven production and erosion processes. Thus, budget state metrics can provide important information on a reef's growth potential and on the capacity of reefs to sustain key geo-ecological services such as habitat provision and coastal protection. Whilst various approaches for estimating carbonate budgets exist, census-based methods have gained recent interest because they quantify the contribution of different functional groups and taxa, and allow assessments of the links between changing reef ecology and budget states. However, the present paucity of supporting data on growth and erosion rates for the majority of coral species and reefassociated taxa represents a constraint on these budget estimates and limits meaningful between-site comparisons. In light of the growing interest in using carbonate budgets as a functional reef "health" assessment tool, this review thus considers our current state of knowledge regarding the geographic coverage of existing reef budget states and the availability of relevant supporting data. We use this to highlight current knowledge gaps, future challenges, and opportunities that emerging techniques may offer. The primary aim of this review is to encourage increased research efforts on budget states and underlying metrics in order to better constrain reef carbonate budget estimates from across a broad range of sites and environments.

Habitat‐specific biogenic production and erosion influences net framework and sediment coral reef carbonate budgets

“…Low contributions of these bioeroders to reef-scale CaCO 3 budgets have also been found elsewhere in the Indo-Pacific Van Woesik and Cacciapaglia 2018), but data remain scarce and inconclusive especially due to the difficulty of determining the abundance of endolithic organisms accurately and in detail (Lange et al 2020). Little beta-stage clionaid cover was observed across our sites (and almost entirely C. orientalis), with a maximum abundance observed on the protected reef slope.…”

Bleaching-driven reef community shifts drive pulses of increased reef sediment generation

“…Parrotfish bioerosion rates (kg CaCO 3 m −2 yr −1 ) were calculated for each individual greater than 10 cm using a model based on species, body length and life phase (as reported in [27]). The model was updated with bite rate and scar size data for the dominant central Indian Ocean parrotfish species [28]. Bioerosion rates of parrotfish equate to sediment generation rates because it is generally assumed that all ingested bioeroded substrate is excreted as sediment [13], and there is no evidence of intestinal dissolution of ingested bioeroded reef substrate occurring [29].…”

Two decades of carbonate budget change on shifted coral reef assemblages: are these reefs being locked into low net budget states?

“…The resultant three-dimensionality is a defining feature of most contemporary reefs and promotes high species diversity: tropical reefs are home to one-third of marine biodiversity and support the economy, safety and livelihoods of at least one-tenth of the world's human population who inhabit tropical coastal areas [2]. Sustaining these ecosystem services over time depends partly on reef growth potential, which is largely determined by the interaction between carbonate production and erosion rates [3], in addition to the processes of cementation, lithification and physical material export [4]. The minimum requirement for a coral reef to persist in the relatively short ecological term is for the resultant budgetary state of the reef (the balance between production and erosion/loss) to be at least neutral (i.e.…”