High-Resolution Drone Images Show That the Distribution of Mussels Depends on Microhabitat Features of Intertidal Rocky Shores

Abstract

In this study, we used orthomosaics and a digital surface model (DSM) generated from drone surveys to (1) characterize the distribution of mussel (Mytilus galloprovincialis) aggregations at high resolution (centimeters), and (2) evaluate the role of topographic features, intertidal height, slope, and orientation angle in determining mussel distribution on two rocky shores oriented differently on both sides of a beach on the French Brittany coast. We first developed and tested a mussel visualization index (MVI) for mapping mussel aggregations from drone images. Then, we analyzed mussel distribution on the two shores. The results showed a contrasted total mussel-occupied area between the two rocky shores, with a higher occupation rate and a clear pattern of distribution depending on topographic features on the rocky shore oriented to the west. Intertidal height, and its associated immersion time, was the main factor determining mussel distribution. An optimum intertidal height was found in the center of the distribution height range, at c.a. 4.5 m above the lowest astronomical tide (LAT), where individuals are under immersion phase on average 43% of the time. Within this optimum, the occupation rate of the mussels was significantly higher in microhabitats facing south and west, particularly at intermediate slope angles. These results demonstrate the role of microhabitat topographic features on the development of intertidal mussels and their final distribution. Furthermore, the results highlight the importance of mesoscale structures of habitats (e.g., 100 m), which seem to be responsible for the differences we observed between the two shores. Our methodological approach highlights the main advantage of using high-resolution drone images to address ecological processes in intertidal ecosystems. Indeed, drone imagery offers the possibility to assess small-scale interactions between individuals and habitat conditions over a wide area, which is technically infeasible from fieldwork approaches or by using satellite remote sensing due to their lower resolution. Scale integration and methodological complementarity are powerful approaches to correctly represent the processes governing the ecology of intertidal ecosystems. We suggest using this methodology to monitor long-term changes of sentinel sessile species.

Patterns of spatial and temporal dynamics of mixed Mytilus edulis and M. trossulus populations in a small subarctic inlet (Tyuva Inlet, Barents Sea)

“…We also considered transect-specific characteristics (the same for all localities along the transect): Distdistance (by the midline of the inlet) between the transect and the settlement closest to the river mouth (R+0.5), m; Widthdistance from the uppermost to the deepest sampling localities within the same transect, m (roughly proportional to the width of the mussel belt on a given stretch of shore) and Expshore exposure (north or south). These parameters are related to factors considered as the most important for mussels (Seed and Suchanek, 1992;Barbosa et al, 2022). Depth, Bottom, Kelp and Cov were assessed in parallel with mussel sampling in 2009-2010.…”

A Benchmarking of Commercial Small Fixed-Wing Electric UAVs and RGB Cameras for Photogrammetry Monitoring in Intertidal Multi-Regions

“…From each region, an orthophoto mosaic, a Digital Elevation Model (DEM), a terrain roughness index raster image and a classified mosaic based on the following land-cover classes: water, sand, rocks, mussels, algae and urban, were obtained. Rotatory-wing UAVs were also used for automating coral reef assessment [15], fauna monitoring and shark surveillance [16], sensitive marine habitat mapping and classification [17], underwater benthic monitoring and mapping [18], intertidal mudflat mapping [19], mussel distribution [20], and macroalgae monitoring [21][22][23], among others, but all of them follow the same pattern: either the resolution is low or the study area is small. Due to the great length of the Galician coastline (more than 1500 km [24]) and the short time of low tides for data collection, this work proposes the use of a small fixed-wind electric UAV for this type of operation.…”