Abstract
A paramount challenge in coral reef ecology is to estimate the abundance and composition of the communities residing in such complex ecosystems. Traditional 2D projected surface cover estimates neglect the 3D structure of reefs and reef organisms, overlook communities residing in cryptic reef habitats (e.g., overhangs, cavities), and thus may fail to represent biomass estimates needed to assess trophic ecology and reef function. Here, we surveyed the 3D surface cover, biovolume, and biomass (i.e., ash-free dry weight) of all major benthic taxa on 12 coral reef stations on the island of Curaçao (Southern Caribbean) using structure-from-motion photogrammetry, coral point counts, in situ measurements, and elemental analysis. We then compared our 3D benthic community estimates to corresponding estimates of traditional 2D projected surface cover to explore the differences in benthic community composition using different metrics. Overall, 2D cover was dominated (52 ± 2%, mean ± SE) by non-calcifying phototrophs (macroalgae, turf algae, benthic cyanobacterial mats), but their contribution to total reef biomass was minor (3.2 ± 0.6%). In contrast, coral cover (32 ± 2%) more closely resembled coral biomass (27 ± 6%). The relative contribution of erect organisms, such as gorgonians and massive sponges, to 2D cover was twofold and 11-fold lower, respectively, than their contribution to reef biomass. Cryptic surface area (3.3 ± 0.2 m2 m−2planar reef) comprised half of the total reef substrate, rendering two thirds of coralline algae and almost all encrusting sponges (99.8%) undetected in traditional assessments. Yet, encrusting sponges dominated reef biomass (35 ± 18%). Based on our quantification of exposed and cryptic reef communities using different metrics, we suggest adjustments to current monitoring approaches and highlight ramifications for evaluating the ecological contributions of different taxa to overall reef function. To this end, our metric conversions can complement other benthic assessments to generate non-invasive estimates of the biovolume, biomass, and elemental composition (i.e., standing stocks of organic carbon and nitrogen) of Caribbean coral reef communities.
Highlights
Ecological models offer insights into complex community dynamics and biogeochemical cycling within ecosystems, but depend on accurate abundance estimates of taxa comprising communities (Diaz and Rutzler 2001; Van Oevelen et al 2006)
We found that surveys taking into account the three-dimensional nature of coral reefs and reef organisms, and using biomass rather than cover as an abundance metric, greatly altered the relative contribution of dominant benthic groups to overall community composition
Our results show that approximately half of the total reef surface area, biovolume, and biomass of the fringing reefs of Curacao resides in ‘hidden’, cryptic spaces (Figs. 1, 2, Table 2)
Summary
Ecological models offer insights into complex community dynamics and biogeochemical cycling within ecosystems, but depend on accurate abundance estimates (i.e., composition and biomass) of taxa comprising communities (Diaz and Rutzler 2001; Van Oevelen et al 2006). Structural parameters of reef surfaces and organisms were shown to be estimated with relatively high accuracy using photogrammetry in combination with underwater action cameras (Veal et al 2010; Guo et al 2016; but see Bryson et al 2017) that would be small enough to access and photograph cryptic reef surfaces While these parameters were used, for example, to better explain variation in the occurrence of mobile fish populations (Gratwicke and Speight 2005; Harborne et al 2012), attempts to implement photogrammetry to assess the overall abundance and composition of benthic reef communities, including cryptic habitats, are so far lacking
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