The growth of giant kelp, Macrocystis pyrifera wasstudied by field measurements of nutrients and other waterparameters. Gross primary production of the kelp bed atPoint Loma, California was calculated at 1.5 moles-O2/m2-dayor 14.6 g-C/m2-day. This represents one of the highestmeasured values of gross primary production. A rate ofproduction twice as high might be attained under optimalconditions.
The condition most limiting Macrocystis production wasthe low concentration of dissolved nutrients, especiallynitrogenous substances, near the surface. Kelp compensatedfor this limitation by translocating nitrogenous compoundsfrom depths where nutrient concentrations were higher.Summer dieoff of the surface canopy may be caused byinability of plants to translocate nutrients due to lowavailability of nutrients in deeper water.
Nutrient concentrations varied seasonally. Surfacenitrate concentrations were low for most of the year(usually less than 1 μM), higher during the winter. Nitrateconcentrations at 6 and 9 meters depth usually exceeded1 μM. They were highest during spring upwelling months,lowest during summer months.
Nutrient concentrations in the kelp bed were notdepleted by the kelp or enhanced by sediment nutrientregeneration, implying relatively fast exchange of water inthe bed with outside waters.
Nutrient concentrations varied at different longshorelocations. Surface nitrate concentrations were more likelyto be higher near the tip of Point Loma than 5 kilometersfarther north. Localized upwelling caused this.
In the shallow nearshore (depth<5 meters) the nutrientconcentrations were higher than those in the kelp bed atcomparable depths. The cause remains unknown.
