Seasonal nitrate physiology of Macrocystis integrifolia Bory

Ambient sea-water nitrate and tissue nitrogen (ethanol soluble nitrate and amino acids, as well as total nitrogen) of Macrocystis integrifolia Bory were monitored over a 2-yr period in Bamfield, Vancouver Island, British Columbia. Sea-water nitrate varied from a high of 12 μmol · 1^?1 (individual values as high as 23 μmol · 1^?1 were recorded) in late winter to below detection limits for most of the summer. Tissue nitrate and total nitrogen paralleled the ambient nitrate levels and showed summer minima and winter maxima (from 0 to 70 μmol · g fresh wt^?1 for nitrate and from 0.8 to 2.9% of dry wt for total N). The nitrate uptake capacity was inversely proportional to tissue nitrate concentration and, furthermore, was much higher for subapical surface blades (60–70 nmol · cm^?2 · h^?1) than for older, deeper blades (5–10 nmol · cm^?2 · h^?1). Nitrate uptake by subapical blade disks in summer is apparently higher in dark (1.0–1.7 μmol · g fresh wt^?1 · h^?1) than in light (0.6–1.3 μmol · g fresh wt^?1 · h^?1) and the data obtained in 36–108 h experiments indicate nitrate pool sizes of 30–90 μmol · g fresh wt^?1. These pools are $\text{2}\text{3}$ to nearly full in winter. Ammonium does not inhibit nitrate uptake. It is taken up and apparently utilized much faster than nitrate and it may well be an important source of nitrogen for marine macrophytes.