Epiphyte loads on seagrasses and microphytobenthos abundance are not reliable indicators of nutrient availability in oligotrophic coastal ecosystems James W. Fourqurean a,b, * , Meredith F. Muth c , Joseph N. Boyer d a Department of Biology, Southeast Environmental Research Center, Marine Science Program, Florida International University, North Miami, FL 33181, USA b Fairchild Tropical Botanic Garden, Coral Gables, FL 33156, USA c Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA d Southeast Environmental Research Center, Florida International University, Miami, FL 33199, USA article info Keywords: Indicator Water quality Florida Keys Eutrophication Thalassia testudinum abstract Despite marked gradients in nutrient availability that control the abundance and species composition of seagrasses in south Florida, and the importance of nutrient availability in controlling abundance and composition of epiphytes on seagrasses in other locations, we did not find that epiphyte load on the dom- inant seagrass, Thalassia testudinum, or that the relative contribution of algal epiphytes to the epiphyte community, was positively correlated with nutrient availability in the water column or the sediment in oligotrophic seagrass beds. Further, the abundance of microphytobenthos, as indicated by Chloro- phyll-a concentration in the sediments, was not directly correlated with concentrations of nutrients in the sediments. Our results suggest that epiphyte and microphytobenthos abundance are not unambigu- ous indicators of nutrient availability in relatively pristine seagrass environments, and therefore would make poor candidates for indicators of the status and trends of seagrass ecosystems in relatively low- nutrient environments like the Florida Keys. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Increases in resource availability shift the competitive balance from slow-growing to fast-growing organisms. In nutrient-limited aquatic and marine systems, increases in nutrient supply leads pre- dictably from dominance by slow-growing rooted macrophytes, to progressively faster-growing primary producers like free-living and epiphytic macroalgas, culminating in dominance by single- celled algae and light limitation at very high nutrient loads (see Duarte (1995) and Burkholder et al. (2007) for reviews). In seagrass beds, increases in the abundance of epiphytes – the plants and ani- mals growing on the surfaces of the slower-growing seagrass leaves (Borowitzka et al., 2006) – are stimulated by nutrient load- ing (e.g. Borum, 1985; Silberstein et al., 1986; Neckles et al., 1994; Balata et al., 2008) and these increases in abundance have been implicated as the cause for declines of seagrasses during eutrophi- cation (e.g. Orth and Moore, 1983; Cambridge et al., 1986). Concen- tration of nutrients in the water column may not increase throughout the eutrophication process until after the slow-grow- ing, rooted macrophytes disappear (Cambridge et al., 1986), so concentrations of nutrients in the water column are poor indica- tors of eutrophication in oligotrophic systems. Owing to the rela- tionships between nutrient availability and epiphyte abundance observed in these and other systems, abundance of epiphytes and other fast-growing primary producers may provide a useful indicator of the relative availability of nutrients in macrophyte- dominated coastal systems. Microphytobenthos are an additional, important primary pro- ducer in shallow coastal ecosystems, whose productivity can be substantial in both phytoplankton-dominated (e.g. Schreiber and Pennock, 1995) and macrophyte-dominated systems (e.g. Moncre- iff et al., 1992). In areas of high light availability, nutrient addition enhances the biomass of the microphytobenthos (Dizon and Yap, 1999; Hillebrand et al., 2000; Herbert and Fourqurean, 2008), so biomass of microphytobenthos may also be indicative of nutrient availability. There are few regions where seagrass beds are as extensive as found in south Florida (Fourqurean et al., 2002), and south Florida has a large and rapidly growing coastal human population. Across the shallow coastal marine ecosystems of south Florida, there are pronounced gradients in the relative availability of nitrogen (N) and phosphorus (P), and a qualitative change in the limiting nutri- ent (Fourqurean and Zieman, 2002). In general, enclosed estuarine waters in the region are exceedingly P-limited owing to the lack of P with respect to N in freshwater runoff and long water residence 0025-326X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2010.03.003 * Corresponding author at: Department of Biology, Southeast Environmental Research Center, Marine Science Program, Florida International University, North Miami, FL 33181, USA. Tel.: +1 305 348 4084; fax: +1 305 348 4096. E-mail address: jim.fourqurean@fiu.edu (J.W. Fourqurean). Marine Pollution Bulletin 60 (2010) 971–983 Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul