Characterizing Geomorphology of Mesophotic Coral Reef Ecosystems in the Southwestern Gulf of Mexico: Implications for Conservation and Management

Abstract

Coral reefs are the most biodiverse ecosystems on earth and are presently experiencing severe declines globally. Shallow coral reef ecosystems (<30 m) have been studied extensively while mesophotic coral ecosystems (MCE) are poorly studied. As a result, MCE are rarely included in marine reserve design and management, despite their ecological importance and connectivity to shallow reefs. In this study, we assessed the fine-scale topographic complexity, a proxy for structural complexity, for a group of coastal coral reefs in a marine park in the southwestern Gulf of Mexico, in depths between 2 and 49 m. We conducted hydrographic surveys using a semi-portable multibeam echosounder system to produce 3D bathymetry digital terrain models (DTM) with a 2.5 m spatial resolution for three submerged bank reefs and two emerging reefs. From these models, descriptive terrain parameters were calculated for each reef, including slope, aspect, curvature, rugosity and ruggedness. Results show that all reefs are predominantly northeast-southwest oriented, with well-defined leeward and windward sides. For the three submerged bank reefs, structural complexity increased with depth. Estimated mean ruggedness and rugosity were highest at 20–40 m depth range on windward side slopes. Emerging reefs showed high structural complexity, particularly at the 25–40 m depth range. We identified a spur and groove zone with maximum ruggedness (0.26) and rugosity (3.17) values, and four channels with steep slopes (68°) and dispersed mounds. We found that at mesophotic depths (>30 m), southern reefs basements from two distinct reefs merge to form a continuous complex. This has important management implications since presently, only 28.7% of this reef complex (mostly shallow areas) are within the existing limits of the marine park’s core zone. Considering the newly recognized importance of MCE, we propose expanding and reshaping the core zone to include the entire reef complex which mostly encompasses MCE with high structural complexity. Our study illustrates the value of semi-portable MBES for marine planning in developing countries and remote poorly studied areas.

Structural Complexity of Coral Reefs in Guam, Mariana Islands

“…On average, the structural complexity was similar across nearly all surveyed sites. Rugosity, the most frequently used metric to quantify structural complexity [10,34,35], ranged from 1.49 to 4.39 and averaged 2.28 across all transects, which is higher than the rugosity values reported in recent studies in Australia, the Philippines, Puerto Rico, Hawaii, and the Gulf of Mexico [15,36,42,45,74]. The slope, which ranged from 35 to 49 degrees (average of 43 degrees), was similar to that of mesophotic reefs in the southwestern Gulf of Mexico [74] and was steeper than the values that have been reported from other shallow-water reefs in Puerto Rico and Australia [15,35].…”

“…Rugosity, the most frequently used metric to quantify structural complexity [10,34,35], ranged from 1.49 to 4.39 and averaged 2.28 across all transects, which is higher than the rugosity values reported in recent studies in Australia, the Philippines, Puerto Rico, Hawaii, and the Gulf of Mexico [15,36,42,45,74]. The slope, which ranged from 35 to 49 degrees (average of 43 degrees), was similar to that of mesophotic reefs in the southwestern Gulf of Mexico [74] and was steeper than the values that have been reported from other shallow-water reefs in Puerto Rico and Australia [15,35]. Overall, the high structural complexity measured in Guam's reefs could suggest that the reefs have been able to retain much of their complexity despite the significant decline in coral cover in recent years [61,62].…”

“…DSMs were first smoothed using a low-pass filter (3 × 3 neighborhood window) and clipped using a negative 1 m buffer from the edge to remove potential artifacts and outliers. Terrain metrics, which are frequently used to quantify structural complexity and shape biotic communities on reefs or indicate ecosystem health and stability, e.g., [17,38,40,[73][74][75], were derived from the DSMs using a 3 × 3 pixels (4 mm) neighborhood window (Table 1). The vector ruggedness measure (VRM) was also derived based on a 5 cm neighborhood window [37].…”

The reef fish assemblage of a coral reef system in the southwestern Gulf of Mexico

“…The ' N, 95º47'-96º14' W; Fig. 1) is composed by > 50 patch, fringing and platform type coral reefs that have formed during the Holocene, from depths up to 45 m (Mayorga-Martínez et al, 2021;Ortiz-Lozano et al, 2018;Tunnell, 2007). The reefs are geostructures with well-defined geomorphological zones including the fore reef (windward slope), the reef crest, the reef flat, and the back reef (leeward slope).…”

Upper mesophotic reef fish assemblages at Bahía de Banderas, Mexico

“…Our knowledge of fishes from mesophotic zones comes mostly from studies conducted in the last two decades. These studies have been conducted on coral reefs of Caribbean island systems ( Garcia-Sais 2010 , Gonzalez-Zapata et al 2018 ), the northern Gulf of Mexico ( Mayorga-Martínez et al 2021 ), Brazilian coasts and islands ( Meirelles et al 2015 , Rosa et al 2016 ), as well as the Hawaiian Islands ( Boland and Parrish 2005 , Fukunaga et al 2016 , Pyle et al 2016 , Fukunaga et al 2017 ) and islands of the South Pacific and Australia ( Abesamis et al 2018 , Coleman et al 2018 , Williams et al 2019 ; see Loya et al (2019) ).…”