A comparison of nutrient‐limited productivity in Sargassum natans from neritic vs. oceanic waters of the western North Atlantic Ocean

Physiological studies and seawater nutrient analysis showed that the productivity of the macroalga Sargassum natans was significantly enhanced by higher N and P availability in neritic compared to oceanic waters of the western North Atlantic Ocean (17–40°N). The initial slope of the P vs. I curve ( α ), photosynthetic capacity ( P max ), dark respiration, and the light saturation irradiance ( I ) were all significantly greater in neritic compared to oceanic populations. The higher productivity of neritic S. natans correlated with higher levels of tissue N and P; C: N, C: P, and N: P ratios averaged 27.9, 347, and 10.2 in neritic populations compared to 49.4, 877, and 18.1 in oceanic populations. Lower alkaline phosphatase activity in neritic vs. oceanic populations corroborated the higher P limitation in oceanic waters. Experimental pulses with dissolved inorganic N, DIN (NO 3 − ), and soluble reactive phosphate, SRP (PO 4 3− ), significantly enhanced net P max and dark respiration of oceanic but not neritic S. natans, demonstrating that increased N and P availability enhances productivity of nutrient‐depleted S. natans. Higher DIN and SRP concentrations within Sargassum windrows along shelf fronts in neritic regions explained the higher productivity and suggest that chronic nutrient limitation in oceanic regions is related to highly patchy nutrient supply.