“Biodiversidad bentónica de arrecifes rocosos de la costa patagónica: estado actual y predicciones ante futuros escenarios de cambio climático” [Biodiversity of epi-benthic communities along template rocky reefs in the Patagonian Atlantic Coast: current state and modelling under a climate change scenario]

Gonzalo Bravo, Juan Pablo Livore, Gregorio Bigatti, Nicolás Battini, Marianela Gastaldi, Daniel Lauretta, Martín Brogger, María Paula Raffo, Cristian Lagger

Seven rocky reefs, grouped in an area of about 11 km², were sampled off the coast of Punta Pardelas Bay inside Nuevo Gulf, Atlantic Patagonia (Fig. 1). The study area is part of the Natural Protected Area Península Valdés and this work was performed with the corresponding permit provided by the Subsecretaría de Conservación y Áreas Protegidas (DISPOSICIÓN N°076-SsCyAP/18) from Chubut Government. This latitude is considered as an ecotone of two marine biogeographic provinces (Argentinian and Magellanic), with both warm temperate and cold temperate species represented (Balech and Ehrlich 2008). The tidal regime is semi-diurnal with mean amplitudes of 3.8 m and spring tides of up to 5.7 m. Water temperature varies from 9-18°C. The first and only study of the subtidal benthic community at Punta Pardelas was performed by Olivier et al. (1966), but only down to 10 m depth. Our dataset presents species occurrences and species richness taken by underwater photoquadrats. Sessile species were recorded as percentage cover and mobile fauna as density. It is likely that some taxa, particularly mobile fauna, may present an avoidance behaviour and, thus, was not recorded. Hence, biodiversity estimation is likely underestimated through this methodology. However, we are confident that, for benthic reef biodiversity surveys, it is a cost-effective method and we encourage its use.

Figure 1.  

Study site, location and extension of all the rocky reefs sampled. Black lines represented the GPS track of the rocky reefs ledges.

Ledge borders were followed as underwater transects in all rocky reefs. Photoquadrats (25 x 25 cm), spaced at 2-5 m intervals, were taken by scuba diving (Fig. 2). Preliminary tests showed that a focal length of 50 cm, which, in turn, determined quadrat size, was the best to reduce the negative influence of water turbidity on the resolution of the image. The presence of cavities with a height of 1.5-3.0 m below the rocky ledges provided enough space to sample four different surface orientations (horizontal, vertical, overhang and cave floor). Rocky reefs were sampled at three different depths ranges 1-7 m: “shallow rocky reefs” (n = 2 reefs), 8-15 m: “mid-depth rocky reefs” (n = 3 reefs) and 16-25 m: “deep rocky reefs” (n = 2 reefs). Voucher samples were collected to confirm photo identification when necessary.

Figure 2.  

Diver with photoquadrat frame (25 x 25 cm) and GPS line. Photo: Yann Herrera Fuchs.

The major part of the financial support came from PICT-2018-0969 (ANPCyT- ARGENTINA). Minor funding was provided by a Rapid Ocean Conservation grant (ROC) from Waitt Foundation (https://www.waittfoundation.org/), Tides Foundation Grant Award TF2002-089196, Instituto de Conservación de Ballenas (ICB) with the Australis award granted to Gonzalo Bravo and ProyectoSub Foundation. All the authors are members of national institutions in Argentina.

This dataset presents species occurrences and species richness of underwater photoquadrats over rocky reefs in Punta Pardelas, Nuevo Gulf. Sessile species were recorded as percentage cover and mobile fauna as density. This is the first study of benthic communities in Punta Pardelas at three different depth levels: shallow rocky reefs (1-7 m), mid-depth rocky reefs (8-15 m) and deep rocky reefs (16-25 m).

Divers were equipped with a Canon 100D camera and two Ikelite DS-161 strobes, mounted on a stainless-steel structure with a 0.0625 m² quadrat (0.25 x 0.25 m). The camera had a 18-55 mm Canon lens and all the images were taken with the 18 mm setting, autofocus, ISO 400, Exposure 1/200 s at f/11 and flashes set on automatic TTL. A dive computer (Oceanic Geo2) was mounted on one side of the quadrat to register the depth and temperature of each photoquadrat. Divers carried a monofilament line that towed a surface buoy with a GPS loading a waypoint every 3 seconds (Bravo et al. 2021).

Species names were assigned when the photograph allowed us to observed the taxonomic diagnostic features of the organisms. When identification was inconclusive, only family or genus names were assigned and, in the case of filamentous algae or sponges, functional groups were assigned. Most of the species were identified by the project co-director Gonzalo Bravo who has extensive knowledge and observations of the local species in the field (see: https://www.inaturalist.org/lifelists/gonzalobravopatagonia). In some cases, extractive samples were collected for taxonomic confirmation or description of species not recorded in the area. The taxonomists who contributed to the identification of photoquadrats and extractive samples were Paula Raffo (Algae), Marianela Gastaldi (Porifera), Cristian Lagger (Tunicates), Martín Brogger (Echinodermata), Gregorio Bigatti (Mollusca) and Daniel Lauretta (Cnidaria: Actiniaria, Corallimorpharia). The taxonomic validity of the names was verified using the World Register of Marine Species (WoRMS; www.marinespecies.org). The geo-referencing of photoquadrats was recorded using a Garmin eTrex 10 GPS (WGS84 Datum) with a 5 m accuracy.

- GPS and underwater camera time were synchronised. This was done by aligning the camera clock with the GPS clock before each dive. The GPS was set on track mode recording one waypoint every 3 seconds.

- The portable GPS (Garmin Etrex 10) was placed in a dry bag on top of a Rescue Can buoy connected to the diver by a monofilament line using a diving reel. Divers maintained the monofilament line as tightly as possible to minimise angles between the buoy and the diver.

- Photoquadrat sampling.

- Photos were georeferenced using the function “Auto-tag photos” in Adobe Lightroom Classic version: 9.1.

- Percentage cover of algae and sessile invertebrates was calculated using a 100 point grid overlaid on each photo, using CoralNet software (Beijbom et al. 2015). On the same image, all the mobile fauna was counted to calculate density. All the photoquadrats are stored in a public CoralNet source: https://coralnet.ucsd.edu/source/1933/

Nuevo Gulf in Chubut Province, Argentina. We selected seven rocky reefs grouped in an area of almost 11 km² in Punta Pardelas.

-42.652 and -42.617 Latitude; -64.284 and -64.225 Longitude.

The database by Bravo et al. (2020b), described here, is based on photoquadrat taxonomic identification and is supported by local taxonomists. The goal of this study was to update the benthic community data from Punta Pardelas in Nuevo Gulf, Atlantic Patagonia. The taxonomic coverage (Fig. 3) includes twelve phyla: Porifera (17%), Mollusca (17%), Rhodophyta (17%), Chordata (14%), Cnidaria (12%), Ochrophyta (8%), Echinodermata (6%), Chlorophyta (4%), Annelida (2%), Arthropoda (1%), Brachiopoda (< 1%) and Phatyhelminthes (< 1%). The class Calcarea (Porifera), the genus Halcurias (Cnidaria: Actiniaria) and the species Darwinella rosacea (Porifera) represented first records for Nuevo Gulf and were identified by extractive samples by taxonomists.

Figure 3.  

Taxonomic distribution and coverage.