No abstract

Submanne canyons can provide large quantities of food in aggregated form on the deepsea floor by acting as conduits for manne macrophyte production produced in the intertidal and shallow subtidal zone. Longshore transport dehvers substantial quantities of macrophyte detntus fronl surfgrass Phyllospadix lorreyi, kelps Macrocystis pyrifera and Egregia menziesii, and other macroalgae to the heads of Scripps and La Jolla Submarine Canyons. Strong tidal and gravity currents distribute this material throughout much of the canyon System, where it is utilized as food and habitat by benthic fauna. Video data taken from remotely operated vehicles and submarines were used to evaluate differences in detrital Cover and megafaunal abundance In the canyons, and at nearby reference Stations.Within the canyons dense mats of detntus were common down to 550 m, and M. pyrifera holdfasts were observed at 700 and 900 m. Virtually no dnft material was observed out of the canyons. Comparisons of megafaunal invertebrates in and out of the canyons revealed generally higher densities at noncanyon sites due to large numbers of urchins. Species richness of all megafauna and abundance of nonurchin megafauna were greater in the canyons than out. It is likely that urchin abundance in canyons is reduced through disturbance by currents and detrital flows in the canyons. Species nchness and abundance of fishes were greater in the canyons at all depths for which comparative data were available (100 to 500 m). From 150 to 200 m in Scripps Canyon, juvenile Pacific hake Merlucciusproductus were so abundant at tirnes that their bodies obscured visibility. Turbot Pleuronichfhys sp. and zoarcids Lycodes pacifica were also abundant in Scripps Canyon from 100 to 300 m. Data from this study support the hypotheses that macrophyte detntus Covers large areas of the La JoLla and Scripps Canyon axis, and that megafaunal abundance 1s associated with detritus at both large and small spatial scales

Submarine canyons are important sources of habitat heterogeneity on the slopes of continents and islands, but the study of canyon ecology has been largely restricted to continental margins. Here we use visual and video surveys from 36 submersible dives to evaluate the role of canyons as abundance and diversity hotspots for megafauna in the Hawaiian Archipelago, an island chain embedded in an oligotrophic ocean. We surveyed megafauna in canyon and slope settings at depths of 350–1500 m along the margins of four islands: the low ‘islands’ of Nihoa and Maro Reef, and the high islands of Oahu and Moloka’i. Megafaunal communities in canyons differed significantly from those in nearby slope habitats at all depths. Highly mobile fishes and invertebrates were consistently more abundant in canyons than on nearby slopes at the same depth off all islands, suggesting that canyons may be important sources of larvae for surrounding habitats. In the few cases where megafaunal abundances were similar or higher on the slope, the differences were typically driven by higher slope abundance of sessile suspension feeders or animals with limited mobility, i.e. by organisms which are likely to have difficulty with high currents and sediment transport in canyons. Megafaunal species richness and diversity generally trended higher within canyons, especially for the highly mobile taxa. Canyons contained 41 megafaunal species never observed on the slope, and increased estimated regional species richness by 25–30 species, indicating that canyons enhanced beta and gamma (regional) biodiversity. An expected trend of greater enhancement of diversity and abundance in canyons on the margins of high versus low oceanic islands was not observed, although megafauna were generally more abundant in both canyon and slope habitats on the high islands (Oahu and Moloka’i). We conclude that submarine canyons on both low and high islands in the Hawaiian Archipelago may provide keystone structures, enhancing megafaunal abundance, providing source populations for the open slope, and enhancing local and regional species diversity.

Hotspots of benthic production below their maximum reported depth, and probably use the habitat and its resident fauna at much greater depths in the canyon. SIR-Topographical features that accumulate organic debris, such as submarine canyons, are common along many coasts and can support hotspots of secondary production. The floor of the La Jolla submarine canyon (32° 52' N, 117° 15' W) is covered by a persistent mat of surfgrass and kelp detritus from a depth of 15 m to at least 300 m. The detritus is inhabited by a dense assemblage of amphipod (Orchomene limodes, Aoroides spina sus) and leptostracan (Nebalia spp.) crustaceans which at times achieve densities of more than 3 million individuals and bio-3.0 mass exceeding 1 kg (dry 'b weight) per square metre. In ~ monthly samples collected ~ 2. 0 from March 1992 to March E.