The influence of hydrodynamic regime on infaunal assemblages inhabiting carbonate sediments on central Pacific seamounts
We investigated the following hypotheses for deep seamounts in the central Pacific Ocean: (1) infaunal and microbial abundances are elevated in regions of current intensification, (2) infaunal lifestyles reflect variation in hydrodynamic conditions and (3) bioturbation is more intense in high-energy regimes. Our studies were carried out at three sites: the northwest perimeter of the Horizon Guyot sediment cap (1840 m), which is characterized by strong bottom currents and rippled foraminiferan sands, and the central summits of Horizon Guyot (1480 m) and Magellan Rise (3150 m), whose sediments are unrippled and finer grained. Contrary to our first hypothesis, the high-energy, Horizon perimeter sediments exhibited lower biological activity than the summit sites, as reflected in lower organic nitrogen (0.011% vs. 0.015-0.017%), higher C/N ratios (19 vs 11), lower bacterial counts (1.21 vs 2.03-2.15 × 10 8ml -1) and lower macrofaunal abundances (255 vs 388-829 m -2). Sediment organic carbon values (0.14-0.19%) and meiofaunal abundances (2866-5150 m -2) did not differ significantly among the three sites. Infaunal life habits varied among sites but sediment mixing did not. Macrofauna were found deeper in rippled perimeter sediments than in the cap sediments. Sessility and surface-feeding modes dominated among polychaetes at the higher-energy Horizon perimeter, while motility and subsurface feeding were common in the quieter, finer-grained regimes. Significant sediment mixing takes place on 100-year time scales a all three sites, probably a result of large, infaunal bioturbators at the cap sites and physical sediment instability at the perimeter site. Excess 210Pb exhibited moderately high inventories (38-59 dpm cm -2) and deep penetration (15 cm). Estimated mixing coefficients (D b) ranged from 0.6 to 3.0 cm 2y -1 at the three sites. Our findings indicate that hydrodynamic differences can lead to greater variation in sediment and faunal characteristics on a single seamount than are found in similar regimes on different seamounts. Comparison of the Horizon Guyot and Magellan Rise data to comparable data from eastern Pacific seamounts, reveals lower organic carbon content, microbial abundance, macrofaunal densities, and subsurface deposit feeder representation, in central than eastern Pacific seamount sediments.