RESEARCH ARTICLE Brian D. Badgley Æ Fredric Lipschultz Kenneth P. Sebens Nitrate uptake by the reef coral Diploria strigosa : effects of concentration, water flow, and irradiance Received: 10 April 2003 / Accepted: 31 October 2005 / Published online: 30 November 2005 Ó Springer-Verlag 2005 Abstract The effects of several environmental variables on net nitrate uptake by the scleractinian coral Diploria strigosa were investigated under controlled flow condi- tions. D. strigosa exhibited nitrate uptake rates ranging from 1–5 nmol cm 2 h 1 at ambient concentrations of 0.1–0.3 lM that are typical of oligotrophic reefs such as Bermuda. Net uptake ceased at approximately 0.045 lM. The uptake was positively correlated with concentration up to a saturation concentration of approximately 3 lM. The uptake was also positively correlated with water velocity at 1 lM, but not at 6 lM, suggesting diffusional limitation at low concentrations and kinetic limitation at higher concentrations. Nitrate uptake by D. strigosa was not affected by light intensity or time of day, but was almost completely inhibited by 48 h exposure to ammonium levels found on many reefs. Introduction Tropical symbiotic corals maintain high gross produc- tivity in oligotrophic waters (Odum and Odum 1955), and the mechanisms that explain this apparent paradox have been extensively studied. Attention has been focused on the sources of requisite nutrients and recy- cling of these nutrients within the coral symbiosis (Muscatine and Porter 1977), and nitrogen has been the primary element of interest given its possible role as a limiting nutrient on coral reefs and in other oligotrophic ecosystems. There are several sources from which corals can obtain the nitrogen needed for growth, tissue repair, mucus production and reproduction including uptake of dissolved organic (Schlichter 1982; Ferrier 1991; Al-Moghrabi et al. 1993) and inorganic (Muscatine and D’Elia 1978; Webb and Wiebe 1978; Wilkerson and Trench 1986; Bythell 1990; Atkinson et al. 2001) com- pounds, as well as capture of zooplankton (Sebens et al. 1996, 1998; Heidelberg 1999) and other particulate matter (Anthony 1999; Mills 2000, Mills et al. 2004). Compared to the relatively well documented and seemingly universal ability of corals to take up ammo- nium (Kawaguti 1953; Muscatine and D’Elia 1978; Burris 1983; Wilkerson and Trench 1986; Thomas and Atkinson 1997), evidence for nitrate uptake is more limited. Results generally support the ability of sclerac- tinian corals to take up dissolved nitrate (Wilkerson and Trench 1986; Bythell 1990, Grover et al. 2003). How- ever, Badgley (2002) was unable to induce nitrate uptake in Madracis mirabilis. Evidence for nitrate uptake has also been equivocal and variable among related cnidaria, such as anemones (Wilkerson and Muscatine 1984) and scyphozoans (Muscatine and Marian 1982). Uptake studies have commonly used unnaturally high concentrations of nitrate and thus may have greatly al- tered the uptake rates. Initial experimental concentra- tions have typically ranged from 10 to 30 lM (D’Elia and Webb 1977; Muscatine and D’Elia 1978; Wilkerson and Trench 1986), while reef concentrations are typically much less than 1 lM (Muscatine 1980; D’Elia 1988), although values can reach higher than 2 lM (Szmant 1997). At best, these studies can claim to observe the potential for uptake, but do not provide realistic uptake rates that can predict the contribution of nitrate to col- ony growth under natural environmental conditions. Nitrate uptake has been reported at ambient concentra- tions on a Caribbean reef for a single species, Acropora palmata (Bythell 1990). Recently, Grover et al. (2003) Communicated by P.W. Sammarco, Chauvin B. D. Badgley (&) Biology Department, University of South Florida, Tampa, FL, USA E-mail: bbadgley@mail.usf.edu Tel.: +1-813-9744074 Fax: +1-813-9743263 F. Lipschultz Bermuda Biological Station for Research, St. George’s, Bermuda K. P. Sebens Friday Harbor Laboratories, University of Washington, Friday Harbor, WA, USA Marine Biology (2006) 149: 327–338 DOI 10.1007/s00227-005-0180-5