Fish and invertebrate communities show greater day/night partitioning on tropical than temperate reefs

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Diel partitioning of animals within ecological communities is widely acknowledged, yet rarely quantified. Investigation of most ecological patterns and processes involves convenient daylight sampling, with little consideration of the contributions of nocturnal taxa, particularly in marine environments. Here we assess diel partitioning of reef faunal assemblages at a continental scale utilizing paired day and night visual census across 54 shallow tropical and temperate reefs around Australia. Day/night differences were most pronounced in the tropics, with fishes and invertebrates displaying distinct and opposing diel occupancy on coral reefs. Tropical reefs in daytime were occupied primarily by fishes not observed at night (64% of all species sighted across day and night, and 71% of all individuals). By night, substantial emergence of invertebrates not otherwise detected during sunlit hours occurred (56% of all species, and 45% of individuals). Nocturnal emergence of tropical invertebrates corresponded with significant declines in the richness and biomass of predatory and herbivorous diurnal fishes. In contrast, relatively small diel changes in fishes active on temperate reefs corresponded to limited nocturnal emergence of temperate invertebrates. This reduced partitioning may, at least in part, be a result of strong top-down pressures from fishes on invertebrate communities, either by predation or competitive interference. For shallow reefs, the diel cycle triggers distinct emergence and retreat of faunal assemblages and associated trophic patterns and processes, which otherwise go unnoticed during hours of regular scientific monitoring. Improved understanding of reef ecology, and management of reef ecosystems, requires greater consideration of nocturnal interactions. Without explicit sampling of nocturnal patterns and processes, we may be missing up to half of the story when assessing ecological interactions.

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Statement: Study Sites Standardized underwater visual census surveys were conducted during the day and at night at 54 reef sites distributed throughout tropical (n = 30) and temperate (n = 24) regions spanning the Australian continent (Figure. 1). Paired day and night transects were undertaken at each site to enable comparisons of reef communities between day and night. Site selection was opportunistic, whilst prioritizing broad biogeographical representation, habitat variability, and logistics of site accessibility and safety. Survey methods Diel variation in reef faunal assemblages was assessed using the Reef Life Survey (RLS) methodology (Edgar et al. 2020), during day and night periods between November 2020 and May 2023. Day surveys were undertaken during sunlit periods at least 3 hours after sunrise and 3 hours before sunset, whilst night surveys commenced 2 hours after sunset to mitigate confounding influences of the crepuscular period. At each site, an experienced 2-person dive team categorized reef faunal assemblages along one to three 50-m-transect lines, which were set along particular depth contours (contours ranged 1.5 – 15 m depth). A total of 142 surveys (71 in each of day and nighttime periods) were conducted across the 54 sites, with means across multiple transects averaged at the site level (see number of paired transects in Figure 1). Fish assemblages were categorized within a 5 m belt, conducted either side of the transect line (500 m2 total). Observations were also constrained to 5 m above the seabed and 5 m below the transect line in the event of surveying a vertical reef wall. The species identities, abundances, and sizes of all fishes’ present within the belt were recorded, with the diver able to deviate anywhere within the 5 m belt to search for species concealed amongst complex reef structure (i.e., cave entrances, ledges, benthic vegetation). When classing fish sizes (used for biomass calculations), a diver estimated the total length of an individual (i.e., from snout to furthest extension of caudal fin) and allocated that individual to the nearest of the following size categories; 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 50.0, 62.5 and 75.0 cm. For individuals exceeding these size categories (i.e., sharks), size estimation was made to the nearest 12.5 cm (following Edgar et al 2004; see ‘Data manipulation and analysis’ for details). Following all fish counts, divers then returned down the transect line to quantify the identities, abundances and sizes of mobile invertebrate and cryptic fish species. Invertebrate observations were restricted to large (i.e., equal to or greater than 2.5 cm when mature) mobile mollusks, echinoderms, and crustaceans (Appendix S1: Table S1). Feather star species belonging to echinoderm family Crinoidea were recorded in field surveys but removed from analysis due to unreliable biomass estimates. Invertebrate sizing methods varied between taxa; crustacean species were measured by carapace length, urchins by test diameter (i.e., not including spines), sea stars by radial length (i.e., center point of body to tip of arm), and all other taxa by total length. Fish families categorized as ‘cryptic’ belonged to families listed in the Reef Life Survey Methods Manual (Appendix S1: Table S2). Observations of invertebrate and cryptic fish taxa were constrained to 1 m belt surveys either side of the transect line (total 100 m2), to ensure equal sampling effort and community representation across all taxa. With the diurnal survey complete, the transect line was left in place and marked by a surface float and GPS position to aid in site relocation for the night survey. Night surveys followed the same methodology as diurnal surveys, but using a LED white light dive torch (1,300 lumen, 120° beam, Big blue AL1300XWP) to locate and identify individuals within respective belts. Identification was performed in situ; any unidentifiable species were photographed and identified post-survey using appropriate field guides and consultation with experts.

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Credit
This work was financially supported by the Holsworth Wildlife Research Endowment – Ecological Society of Australia and the CSIRO via a Research Plus Indigenous Postgraduate Scholarship Grant