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Grazing halos on coral reefs: predation risk, herbivore density, and habitat size influence grazing patterns that are visible from space

In order to conduct a systematic analysis of patch reef and halo geometries, we manually digitized 757 patch reefs and their respective halos in the Florida Keys National Marine Sanctuary (FKNMS) and along a ~250 km reef tract of the South Water Caye Marine Reserve (SWCMR) in Belize. We divided the FKNMS into latitudinal segments running roughly perpendicular to the forereef. The frequency of the segments depended on patch reef density. For the middle and lower Florida Keys we digitized every patch reef encountered in every other 0.8 km band, while in the upper Florida Keys, we examined 0.4 km wide bands spaced 0.8 km apart. In Belize, we manually selected every patch reef visible in the high-resolution satellite scene, and randomly selected 100 patch reefs where we digitized the patch reef boundary and surrounding halo. Patch reefs are not uniform. In our landscape analysis we only digitized patch reefs that 1) were singular (i.e., not highly aggregate), 2) where the image was of high enough quality to assess the halo and patch reef boundaries (i.e. not obscured by clouds, turbidity, sun glint, or water depth) and 3) had halos that were distinct for more than ~ 70% of the patch perimeter. All patch reefs large enough to clearly see in the imagery (> 10 m), were surrounded by a halo, suggesting the ubiquity of halo formation in Florida and Belize. To determine halo width (i.e., the average distance from the patch reef edge to the halo boundary), one observer (BD) digitized the patch reef and the halo boundary. The halo boundary was estimated as the point of greatest transition in substrate reflectance surrounding the reef. For the 13 patch reefs where we conducted fieldwork, three untrained and one trained observer estimated the patch and halo boundaries to establish halo estimation error (see error bars in Fig. 3a in text). Using ArcGIS (version 10.4, ESRI 2017) and QGIS (version 2.18.13, QGIS Development Team 2017) software, we estimated halo extent as the average nearest neighbor distance between densified point clouds located along the halo boundary and patch edge.