Ciencia

Limnol. Oceanogr., 53(4), 2008, 1589–1603 2008, by the American Society of Limnology and Oceanography, Inc.

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Physical pathways and utilization of nitrate supply to the giant kelp, Macrocystis pyrifera
Jonathan P. Fram1 and Hannah L. Stewart2
Marine Science Institute, University of California at Santa Barbara, Santa Barbara, California 93106

Mark A. Brzezinski
Marine ScienceInstitute, University of California at Santa Barbara, Santa Barbara, California 93106; Department of Ecology Evolution and Marine Biology, University of California at Santa Barbara, Santa Barbara, California 93106

Brian Gaylord
Bodega Marine Laboratory and Section of Evolution and Ecology, University of California at Davis, Bodega Bay, California 94923

Daniel C. Reed
Marine Science Institute,University of California at Santa Barbara, Santa Barbara, California 93106

Susan L. Williams
Bodega Marine Laboratory and Section of Evolution and Ecology, University of California at Davis, Bodega Bay, California 94923

Sally MacIntyre
Marine Science Institute, University of California at Santa Barbara, Santa Barbara, California 93106; Department of Ecology Evolution and Marine Biology,University of California at Santa Barbara, Santa Barbara, California 93106 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 forestand 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 g21 month21 during spring upwelling in 2005 and declined fourfold during autumn until upwelling resumed the following year. Modeled gross nitrogen uptake withconsideration 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 nitratein 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%) andcross-shore flows (55%), which distinguishes it from large kelp forests in which cross-shore flows dominate exchange.

1 Corresponding 2 Present

author (jfram@msi.ucsb.edu). address: Friday Harbor Labs, University of Washington, Friday Harbor, Washington 98250.

Acknowledgments We thank Brent Mardian, Clint Nelson, Shannon Harrer, Devin Wu, Matt Wright, Andrew Rassweiler, and John Ecker forassistance with logistics in the field; Janice Jones and Bill Clinton for assistance with the in situ nitrate analyzer; David Salazar for assistance with the current profilers; and Albert Carranza for determining the chemical composition of the kelp samples. Chad Helmle, Brice Loose, and Chris Gotschalk contributed toward development of the software for processing the time series temperature and currentmeter data. This manuscript was greatly improved by the suggestions from two reviewers. Funding was provided by University of California Marine Council grant 04-T-CEQI-08-0048 and National Science Foundation (NSF) grant ocean sciences (OCE) 02-41447 to B.G., NSF grants division of environmental biology (DEB) 01-08572, OCE 02-35238, and DEB 04-14659 to S.M., and NSF OCE 99-82105 and OCE 06-20276 to...