MEGAFAUNAL POPULATIONS IN LYDONIA CANYON, WITH NOTES ON THREE 

 OTHER NORTH ATLANTIC CANYONS 



Barbara Hecker 



Associate Research Scientist 



Lamont Doherty Geological Observatory 



Palisades, NY 10964 



Mcgafaunal populations in Lydonia Canyon were photographically 

 surveyed during 16 camera sled tov/s and 17 ALVIN dives conducted between 

 May 1979 and September 1982. A total of 114,742 m^ of the seafloor, between 

 130 and 2330 m depth, v/as analyzed for this study. Lydonia Canyon is a 

 relatively narrow canyon that incises Georges Bank approximately 11 miles 

 north of the shelf-slope break. It has a narrow^, sinuous sediment-covered 

 axis flanked by steep w^alls. The v/alls on both sides of the canyon exhibit 

 massive exposures of outcrop and steep, talus-strew^n slopes. 



Total megafaunal abundances in Lydonia Canyon are very high (up to 

 30 individuals per m^) at shallov/ depths, and intermediate at mid-slope (5 

 individuals per m^) and low^er-slope (7 to 8 individuals per m^) depths. 

 Megafaunal distribution patterns within the canyon are complex and faunal 

 similarities among areas w^ithin a depth range are low. These low faunal 

 similarities reflect; patchy distributions of several of the dominant shallow 

 water taxa, substrate heterogeneity on the canyon w^alls, and differing 

 physical regimes in various parts of the canyon. The megafauna inhabiting 

 the axis and lower walls is usually dominated by filter-feeding corals and 

 sponges, and these taxa frequently comprise assembleges that are quite 

 distinct from those inhabiting the upper walls and the nearby slope. 



Three other North Atlantic canyons (Oceanographer, Heezen, and 

 Corsair) were surveyed during 7 ALVIN dives conducted in September 1977. 

 Megafaunal populations differ substantially among the canyons, and between 

 the axis and wall within a canyon. While most of the taxa are widely 

 distributed, localized high abundances of individual taxa account for much of 

 the observed differences. This patchiness can only partialy be attributed to 

 differences in sustrate requirements, since several of the dominant taxa are 

 not restricted to hard substrate. However, as was the case in Lydonia 

 Canyon, the fauna in all three of these canyons is heavily dominated by 

 sessile filter feeders. 



While these filter feeders require threshold currents and suspended 

 particles for their livelihood, they may also be susceptible to stresses 

 associated with increased sediment loads such as: tissue abrasion in regions 

 of strong current flo^^^, clogging of filtering apparatuses in tranquil regions, 

 or decreased larval settling success. Additionally, concurrent physical studies 

 show enhanced current speeds and resuspension in the vicinity of canyons. 

 This indicates that the canyon system may w^ell serve a role in transporting 

 and concentrating pollutants in the vicinity of these communities. 



B-13 



