UPTAKE AND RELEASE OF NITROGEN BY THE MACROALGAE GRACILARIA VERMICULOPHYLLA (RHODOPHYTA) 1 Anna Christina Tyler 2 and Karen J. McGlathery Department of Environmental Sciences, University of Virginia, PO Box 400123, Charlottesville, Virginia 22904-4123, USA Macroalgae, often the dominant primary produc- ers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, active- ly growing macroalgae release N to the water col- umn; yet little is known about the quantity or nature of this release. Using 15 N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rho- dophyta, Gracilariales), a non-native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back-barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3  7.2 lmol N  g dw  1  d  1 ) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8  11.6 lmol N  g dw  1  d  1 ) was 67% of gross daily uptake, and also varied among sites and sea- sons (range 5 33%–99%). Release was highest when growth rates and nutrient availability were low, pos- sibly due to senescence during these periods. Dur- ing summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N  m  2  d  1 . Our results suggest that most esti- mates of macroalgal N uptake severely underesti- mate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours). Key index words: dissolved organic compounds; invasive species; lagoons; macroalgae; nitrogen; organic matter; Virginia Coast Abbreviations: BOD, biological oxygen demand; C, carbon; DIN, dissolved inorganic nitrogen; DO, dis- solved oxygen; DOC, dissolved organic carbon; DOM, dissolved organic matter; DON, dissolved organic nitrogen; LTER, Long Term Ecological Research; N, nitrogen In shallow estuaries, benthic macroalgae are impor- tant primary producers, particularly in estuaries impacted by nutrient enrichment. As non-native in- vaders, macroalgae can also have a strong influence on ecosystem function and detrimentally impact local bio- diversity (Ruiz et al. 1997, 1999). The uptake and storage of nutrients (and carbon) by macroalgae rep- resents a significant temporary sink where biomass is high (Smith 1981, Valiela et al. 1997, McGlathery et al. 2001). Following senescence, decomposition is rapid because most bloom-forming algae are highly labile (Buchsbaum et al. 1991, Enriquez et al. 1993). This rapid release of organic matter and nutrients following the collapse of a macroalgal bloom can dramatically impact ecosystem processes by increasing heterotro- phic oxygen consumption in the water column, which may in turn lead to anoxia, dystrophy, and overall ecosystem instability (Sfriso et al. 1992, Viaroli et al. 1995, Rysgaard et al. 1996). Less well understood is the quantity of dissolved in- organic nitrogen (DIN) and organic matter (DOM) that living macroalgae, like phytoplankton, release to the water column. The release of DOM by living macro- algae has been acknowledged, with estimates of dis- solved organic carbon (DOC) release ranging from 0.5% up to 40% of total C fixed by photosynthesis (Khailov and Burlakova 1969, Harlin and Craigie 1975, Brylinsky 1977, Penhale and Capone 1981, Carl- son and Carlson 1984). However, until recently, the concomitant release of dissolved organic nitrogen (DON) has largely been ignored. We have shown pre- viously in microcosm incubations that by releasing DON during active growth, the macroalga Ulva lactuca may substantially increase the total flux of DON from the benthos to the water column (Tyler et al. 2001). Much of the DON released by U. lactuca may be com- prised of combined amino acids (Tyler et al. 2003), as has been shown for phytoplankton (Flynn and Berry 1999). Macroalgae may also release some nitrogen as DIN and free amino acids (Naldi and Wheeler 2002). The release of N by growing macroalgae demon- strates that estimates of macroalgal N demand based solely on biomass and tissue N content significantly underestimate the actual quantity of N passing through the macroalgal pool. Because N is generally thought to limit primary production in temperate es- tuaries (Howarth 1988), an accurate estimate of uptake and release of N by living and senescing macroalgae is important in determining the overall impact of mac- 1 Received 10 October 2005. Accepted 28 February 2006. 2 Author for correspondence and current address: Department of Biology, 141 Lomb Memorial Drive, Rochester Institute of Technology, Rochester, NY 14623-5603. E-mail tyler@alumni.virginia.edu. 515 J. Phycol. 42, 515–525 (2006) r 2006 by the Phycological Society of America DOI: 10.1111/j.1529-8817.2006.00224.x