In situ rates of nitrate uptake by giant kelp,Macrocystis Pyrifera (L.) C. Agardh: Tissue differences,environmental effects,and predictions of nitrogen-limited growth

NO⁻₃ uptake rates were measured in situ for seven types of tissue of adult sporophytes of Macrocystis pyrifera (L.) C. Agardh. Uptake by mature blades of canopy fronds followed saturation kinetics. At near-saturation concentrations, mean rates of uptake by different tissues ranged from 0.1 to 2.1 μg-at.· g wet wt⁻¹ · h⁻¹ (7–68 ng-at. · cm⁻²· h⁻¹). Different tissues incubated under similar environmental conditions took up NO⁻₃ at different rates, indicating physiological differences. Uptake rates showed no apparent relation to tissue age or nutritional history, but were influenced by certain environmental factors: uptake was 44–48% slower in dark than at ambient mid-day light levels, and dark uptake was 40% slower by blades incubated at 12m depth than by blades incubated at the surface. These physiological and environmental differences resulted in generally rapid uptake by tissues located at or just below the surface and slower uptake by tissues deeper in the water column.Daily NO⁻₃ uptake by M. pyrifera populations was predicted using a model based on in situ NO⁻₃ uptake rates. According to predictions of the model, NO⁻₃ concentrations of 1–2 μM throughout the water column are required to support kelp growth at the 4% · day⁻¹ (wet wt) rate typical of inshore populations. Vertical stratification, with high NO⁻₃ concentrations only at near-bottom depths, would result in severe nitrogen-limitation of growth. Seasonal changes in frond size distribution do not greatly affect nitrogen-limited growth rate. The model was also used to evaluate potential fertilizing methods and problems in management of offshore kelp farms.