The morphology, physiology, and relative abundances of two deep-water seaweed assemblages off Cape Fear, North Carolina, were examined, one at 30 m and the other at 42 m depth. Both sites are subtropical areas of low physical disturbance. In situ O2-exchange rates for abundant species were measured. Species showed net photosynthesis at ≤ 40μE · m−2 · s−1 (≈ 0.01 I0). Thin flat species, such as Dictyopteris hoytii Tayl., Dictyota cervicornis Kütz., and D. Divaricata Lamour., had higher photosynthetic capacity (> 2 × ) and greater pigment content than species with other thallus forms (calcified, coarsely branched, leathery); however, thin flat species are not biomass-abundant year-round at the high-diversity 30-m site. Rather, perennial species with coarsely branched or thick leathery thalli predominated. The annual species, Dictyota divaricata, dominated (> 85% cover) at the low-diversity 42-m site. Low tissue-N levels and relatively high CN̈ atom ratios suggested nutrient limitation. Macroalgae with calcified, coarsely branched, and leathery thalli, particularly Zonaria tournefortii (Lamour.) Mont., Galaxaura obtusata (Ellis et Soland.) Lamour., and Eucheuma isiforme (C. Ag.) J. Ag. had CN̈ ratios of ≈ 25, well above the Redfield ratio of 6.6 for balanced growth. A measured intrusion of cooler nutrient-rich bottom water, presumably of continental-slope origin, resulted in an almost three-fold increase in ambient N, which was usually < 0.4 μMNO3− + NO2− during summer. This source and N flux out of surrounding sediments are undoubtedly a critical source of nutrients, particularly during the summer, the period of highest growth. This study suggests that opposing selective pressures have influenced abundances in these deep-water low-disturbance habitats. Species with thin flat thalli have high photosynthetic performance and, possibly, increased N-uptake ability, but show restricted seasonality; perhaps seaweeds with lower productivity maintain year-round abundances due to a large investment of photosynthate into supportive tissue, and concomitant abilities of perennation and vegetative spread.
Algae (Macro, Turf and Crustose Coralline)
USA - Continental Atlantic
SCUBA (open-circuit or unspecified)