Nutrients and production of giant kelp, Macrocystis pyrifera, off southern California1

Concentrations of nutrients in the vicinity of the giant kelp ( Macrocystis pyrifera ) off San Diego, California, varied seasonally. Surface nitrate concentrations were low for most of the year (usually <1 µ M), higher during winter. Nitrate concentrations below 4.5‐m depth usually exceeded 1 µ M and were highest during spring upwelling, lowest during summer. Nutrients in the kelp bed were not depleted by the kelp nor enhanced by sediment regeneration, implying relatively fast exchange between water in the bed and outside waters. Nutrient concentrations varied at different longshore locations. The condition most limiting to M. pyrifera was probably the low concentration of dissolved nutrients, especially nitrogenous substances, near the sea surface. Translocation of nitrogenous compounds by the kelp plant from depths where concentrations are higher could compensate for this limitation. The summer die‐off of the surface canopy may be caused by the inability of plants to translocate nutrients due to low availability of nutrients in the deeper water. The maximum change in dissolved oxygen measured was 0.43 mol O 2 m −2 between 0900 and 1130 PST; extrapolated to 1 day this yields a production of 1.0 mol O 2 m −2 d −1 (9.5 g C m −2 d −1 ).