DOI: 10.5061/dryad.tx95x69sg
title: Data from: Overwintering tropical herbivores accelerate detritus production on temperate reefs

description: The tropicalization of temperate marine ecosystems can lead to increased
 herbivory rates, reducing the standing stock of seaweeds and potentially
 causing increases in detritus production. However, long-term studies
 analyzing these processes associated with the persistence of tropical
 herbivores in temperate reefs are lacking. We assessed the seasonal
 variation in abundances, macrophyte consumption, feeding modes and
 defecation rates of the range-extending tropical rabbitfish Siganus
 fuscescens and the temperate silver drummer Kyphosus sydneyanus and
 herring cale Olisthops cyanomelas on tropicalized reefs of Western
 Australia. Rabbitfish overwintered in temperate reefs, consumed more kelp
 and other macrophytes in all feeding modes and defecated more during both
 summer and winter than the temperate herbivores. Herbivory and defecation
 increased with rabbitfish abundance but this was dependent on temperature,
 with higher rates attained by big schools during summer and lower rates in
 winter. Still, rabbitfish surpassed temperate herbivores, leading to a
 five-fold acceleration in the transformation of macrophyte standing stock
 to detritus, a function usually attributed to sea urchins in kelp forests.
 Our results suggest that further warming and tropicalization will not only
 increase primary consumption and affect the habitat structure of temperate
 reefs but also increase detritus production, with the potential to modify
 energy pathways.
description: Herbivorous Fish Abundance and Rates of Herbivory
 Fish abundance was surveyed using Diver Operated Stereo-Video
 (S-DOV) before deploying bioassays of kelp filmed with Remote Underwater
 Videos (RUV). Three or four 25 × 5 m S-DOV transects were sampled along
 the ecotone between reef and seagrass, separating each transect by a
 minimum of ~10 m to ensure independence of replicates and ignored fish
 appearing from the back to avoid double counting (Goetze et al. 2019).
 Herbivory on kelp was assessed with tethered bioassays consisted on a
 cluster of at least nine individual ~15 cm long lateral blades of
 <i>Ecklonia radiata</i> attached to 0.5 m rods simulating kelp
 canopy, one cluster was deployed per reef in each sampling day,
 representing one independent sampling unit. Tethers were deployed within
 the typical feeding timeframe of diurnal herbivorous fish, from the
 morning until the afternoon (8:00 – 16:00 hrs) (Fox et al. 2009) and
 filmed for 3-4 hours with GoPro cameras to identify the species
 responsible for the consumption of kelp, their relative abundances (MaxN),
 their bite rates on kelp (bites h<sup>-1</sup>) and defecation
 rates (feces h<sup>-1</sup>). In addition to herbivory on
 tethered kelps, bites on other macrophytes attached to the substratum and
 drifting in the water column were also registered during RUV analyses
 (<i>e.g. </i>kelp, seagrass,<i>
 </i>S<i>argassum </i>spp, <i>Ulva</i> spp.,
 <i>Hypnea</i> spp.). We classified the herbivory modes as:
 browsing (substrate attached macrophytes, including kelp tethers), kelp
 browsing, drifting (non-attached macrophytes) and total herbivory
 (browsing + drifting consumption). Kelp lateral blades
 of bioassays were pressed between a white background and a perspex glass
 and photographed before and after deployment. The photographs of kelp were
 analyzed using the software ImageJ (rsb.info.nih.gov/ij/) to calculate the
 area consumed per time (cm<sup>2</sup>
 hr<sup>–1</sup>) (Vanderklift et al. 2009; Wernberg et al.
 2006; Zarco-Perello et al. 2017). This was transformed to biomass (g
 hr<sup>–1</sup>) using a linear area-weight regression.
 Herbivory and defecation rates were standardized for each fish species;
 bite rates estimates were multiplied by weight-specific bite sizes
 (cm<sup>2</sup>), calculated from bite size-weight regressions
 using bite measures from specimens of both species, while defecation rates
 were multiplied by the mean weight of each species calculated from the
 corresponding S-DOV carried out immediately before each herbivory assay
 (Shantz et al. 2015). Underwater temperature data for each reef was
 recorded <i>in situ</i> during the fish and herbivory surveys
 using hobo data loggers.