Physical pathways and utilization of nitrate supply to the giant kelp, Macrocystis pyrifera

Abstract

To determine the relative importance of different sources of nitrate to the annual nitrogen needs of the giant kelp Macrocystis pyrifera, we measured ambient nitrate concentrations at a kelp forest for 13 months and characterized nitrate delivery using water column thermal structure and flow data collected in the forest and at its offshore edge. The forest's monthly nitrate supply varied by a factor of 50, while measured net nitrogen acquisition varied only fivefold. Maximum net nitrogen acquisition rates for fronds in the forest interior were 0.18 mmol N g−1 month−1 during spring upwelling in 2005 and declined fourfold during autumn until upwelling resumed the following year. Modeled gross nitrogen uptake with consideration of Michaelis–Menten kinetics for nitrate and mass transfer limitation was higher than observed net acquisition except during the warm stratified summer and autumn months, when net acquisition exceeded modeled gross uptake. This shortfall indicates that the kelp forest received over half its nitrogen from sources other than nitrate such as ammonium from epibionts. Most of the nitrate in the forest was delivered as a result of upwelling‐favorable winds and convection. Internal waves and local streams contributed <9% of the nitrate delivered to the forest on an annual basis and 20% during stratified periods. Kelp used less than 5% of the nitrate supplied to the forest. Nitrate delivery to this modest sized kelp forest was roughly equivalent between alongshore (45%) and cross‐shore flows (55%), which distinguishes it from large kelp forests in which cross‐shore flows dominate exchange.