Hydrobiologia 450: 187–196, 2001. 187 © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Nutrient induced changes in the species composition of epiphytes on Cladophora glomerata Kütz. (Chlorophyta) Jane C. Marks1 & Mary E. Power2 1Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, U.S.A. Tel+ 520-523-0918. Fax: +520-523-7500 2Department of Integrative Biology, University of California, Berkeley, CA 94720, U.S.A. Received 2 May 2000; in revised form 26 January 2001; accepted 13 February 2001 Key words: Cladophora glomerata, epiphytes, nutrients, species composition Abstract Cladophora glomerata is a widely distributed filamentous freshwater alga that hosts a complex microalgal epi- phyte assemblage. We manipulated nutrients and epiphyte abundances to access their effects on epiphyte biomass, epiphyte species composition, and C. glomerata growth. C. glomerata did not grow in response to these manip- ulations. Similarly, nutrient and epiphyte removal treatments did not alter epiphyte biovolume. Epiphyte species composition, however, changed dramatically with nutrient enrichment. The epiphyte assemblage on unenriched C. glomerata was dominated by Epithemia sorex and Epithemia adnata, whereas the assemblage on enriched C. glomerata was dominated by Achnanthidium minutissimum, Nitzschia palea and Synedra spp. These results indicate that nutrients strongly structure epiphyte species composition. Interactions between C. glomerata and its epiphytes were not affected by epiphyte species composition in our experiment but may be when C. glomerata is actively growing. Introduction tend to be dominant epiphytes only in habitats with low N:P ratios (some Western North American water- Cladophora glomerata is a widely distributed fila- sheds), whereas the epiphyte assemblages in habitats mentous green alga found in a diversity of aquatic with higher N:P ratios (Great Lakes region) are domin- ecosystems including eutrophic lakes, pristine coastal ated by Cocconeis spp., Diatoma spp., Rhoicosphenia streams, and the marine inter tidal zone (Whitton, curvata and Gomphonema spp. (Table 1 and J. C. 1970; Sheath & Cole, 1992; Dodds & Gudder, 1992). Marks, pers. obs.). C. glomerata grows under a wide range of nutrient C. glomerata and its epiphytes may interact neg- regimes and hosts a taxonomically diverse and archi- atively (e.g. competition for nutrients and light) or tecturally complex micro-algal epiphyte assemblage positively (e.g. C. glomerata provides substrate for that varies among locations (Table 1 and references epiphytes, epiphytes may leak nutrients benefiting C. therein). Epiphyte species composition is known to glomerata). These interactions are likely to vary with respond to gradients of nutrients, light, current, con- nutrient concentrations, epiphyte density and epiphyte ductivity and disturbance regimes within a habitat species composition. Furthermore, because the epi- (Stevenson & Stoermer, 1982b; Luttenton & Rada, phytes on C. glomerata are an important food resource 1986; Jonsson, 1987; Hardwick et al., 1992; Bergey for grazers (Kupferberg et al., 1994), the nature of the et al., 1995; O’Connell et al., 1997). Therefore, vari- interactions between C. glomerata and its epiphytes ation in physical factors (e.g. water chemistry) across will likely depend on characteristics of the grazer habitats is likely to contribute to geographic vari- assemblage. For example, Dudley (1992) and Kupfer- ation in epiphyte assemblages on C. glomerata despite berg (1997) found that C. glomerata growth increased the common substrate. For example, Epithemia spp. when herbivores significantly reduced epiphyte bio- 188 Table 1. A global survey of Cladophora epiphytes across freshwater habitats that have a wide range of nutrient concentrationsa Site & source Nutrient levels Major epiphytes Western United States Watersheds 1 Eel River, CA, U.S.A. NO3 N: 7.9 µg L− Cocconeis pediculus Ehr.; C. placentula Ehr.; Epithemia adnata − 1 This Study NH4-N: 6.3 µg L− (Kütz.) Grun.; E. sorex Kütz.; Rhoicosphenia curvata b 1 S.R.P. : 1.1 µg L− (Kütz.) Grun. 1 Sycamore Creek, Arizona, U.S.A. NO3-N: 20 µg L− Cocconeis; Epithemia; Gomphonema; Navicula; Nitzschia; Synedra 1 (Busch & Fisher, 1981 PO4:-P 50 µg L− Dudley, T. pers. comm.) 1 Madison River, Montana, U.S.A. NO3 N: 6 µg L− Epithemia; Nitzschia fonticola Grun. − 1 (Dodds, 1991a,c) NH4-N: 12 µg L− 1 S.R.P.: 25 µg L− 1 Rattlesnake Creek, NO3-N: 26.6 µg L− Cocconeis; Epithemia; Gomphonema; Melosira; Mougeotia; Navicula; 1 California, U.S.A. PO4-P: 10.84 µg L− Rhoicosphenia curvata; Syndera ulna (Nitz.) Ehr. (Dudley, 1992) Colorado River, Not reported Amphora ovalis var. pediculus Kütz.; Cocconeis pediculus; Cymbella Glen and Grand Canyons, affinis Kütz.; Diatoma vulgare Bory; D. tenue Ag.; Fragilaria Arizona, U.S.A. leptostauron var Dubia Hust.; F. ulna Ehr.; Gomphonema olivacium (Hardwick et al., 1992) (Lyngb.) Kütz.; Nitzschia dissipita (Kütz.) Grun.; Rhoicosphenia (Benenati et. al. 1998) curvata Great Lakes, United States and Canada Watersheds Grand Traverse Bay, Not Reported Cocconeis pediculus; Diatoma vulgare; Rhoicosphenia curvata Lake Michigan, U.S.A. (Lowe et al., 1982) 1 Upper Mississippi River, NO3-N: 730-1830 µg L− Cocconeis pediculus; Diatoma vulgare; 1 Wisconsin, U.S.A. NH4-N: 8-239 µg L− Rhoicosphenia curvata c 1 (Luttenton & Rada, 1986) PO4-P: 3-127 µg L− Tippecanoe River, Not Reported Achnanthes; Cocconeis; Fragilaria; Gomphonema; Melosira; Navicula Indiana, U.S.A. (McShaffrey & McCafferty, 1991) Great Lakes and Upper St. Lawrence Not Reported Chamaesiphon; Cocconeis pediculus; Lyngbya diguetii Gomont; Seaway, U.S.A. Lyngbya epiphytica Hieronymus; Rhoicosphenia curvata (Sheath & Morison, 1982) 1 Lake Huron & Lake Michigan, NO3-N: 157-1475 µg L− Amphora perpusilla (Grun.) Grun.; Cocconeis pediculus; Cymbella 1 U.S.A. NH4-N: 7.7-172 µg L− prostrata var auerswaldii (Rabh.) Reim comb.nov.; Fragilaria 1 (Stevenson & Stoermer, S.R.P.: 3–150 µg L− brevistriata Grun.; F. pinnata Ehr.; Rhoicosphenia curvata 1982 a & b) d St. Lawrence River, Canada Not Reported Cocconeis pediculus; Achnanthes minutissima (Kütz.);f Rhoicosphenia (O’Connell et al., 1997)e abbreviata; Gomphonema minutum (Ag.) Agardh Continued on p. 189 189 Table 1. Continued. Site & source Nutrient levels Major epiphytes European Watersheds River Skawie, Poland Not Reported Cocconeis placentula Ehrenb.; Gomphonema olivaceum (Lyngb.) (Chudyba, 1968) Kütz.; Rhoicosphenia curvata 1 Langley Brook, England NH4-N: 10,000 µg L− Cocconeis placentula; Rhoicosphenia curvata; Gomphonema olivaceum (Hawkes, 1964) River Wear, England Not Reported Cocconeis pediculus; Rhoicosphenia curvata (Peabody & Whitton, 1968) Other Watersheds Cape Maclear, Lake Malawi, Not Reported Cocconeis; Cymbella; Epithemia; Navicula; Rhopalodia Malawi, Africa (Haberyan & Mhone, 1991) Kialing River, China Not Reported Cocconeis placentula; Diatoma vulgare; D. elongatum (Lyngb.) (Jao, 1944) C.Agardh; Gomphonema olivaceum; Melosira varians C.Agardh; Synedra ulna Lake Thingvallavatn, Iceland Dissolved inorganic Achnanthes cleveii Grun.; A. lanceolata Bre´b. ex. Kütz.; A. 1 (Jonsson, 1987) N: 0–29.7 µg L− minutissima Kütz.; A. pinnata Hust.; Cocconeis placentula var 1 PO4-P: 8.4-17.1 µg L− lineata (Ehr.) V.H.; Epithemia turgida (Kütz.) Bre´b.; Gomphenema clevei Fricke; Rhoicosphenia curvata aTaxa are listed alphabetically. All taxa listed are diatoms (Bacillariophyta) except for Chamaesiphon incrustens, Fischerella muscicola, Lyngbya diguetii and Lyngbya epiphytica which are blue-greens (Cyanophyta) and Mougeotia, which is a green algae (Chlorophyta). bS.R.P. = Soluble Reactive Phosphorus. cThis study reported 17 epiphyte species. Species included here constitute >95% of the epiphyte assemblage. d These studies reported 245 epiphyte species. Species included here were reported as dominant taxa. eThis study reported 34 epiphyte species. Species included here were reported as dominant taxa. f Achnanthes minutissima has been renamed Achnanthidium minutissimum. mass. In contrast, Dodds (1991a) saw no effect of the ways in which grazers and nutrients interact to epiphyte removal and concluded that C. glomerata influence dynamics of C. glomerata and its epiphyte and epiphytes did not compete with each other in his assemblage. Specifically, we addressed the following system because C. glomerata was limited by nitro- questions: (1) Are epiphyte biovolume and species gen whereas the common epiphytes were limited by composition affected by nutrient availability? (2) Will phosphorus. highly epiphytized C. glomerata respond to nitrogen Despite much speculation about the relationships and phosphorus enrichment? (3) Will partial removal between C. glomerata and its epiphytes, there have of epiphytes alter the response of C. glomerata’ to been no experimental studies that examine how nutri- nutrient enrichment? We predicted that epiphyte spe- ents and substrate affect C. glomerata epiphyte abund- cies composition would shift with changes in resource ances and species composition. Nor have there been conditions. This would indicate that the epiphyte as- any studies that directly manipulate epiphyte dens- semblage as a whole is not limited by any one nutrient ity to determine how epiphyte density influences C. but rather comprises species with different nutrient glomerata’s response to nutrient enrichment. We ma- requirements so that dominance can change when re- nipulated nutrients and epiphyte abundances to assess sources change. We also predicted that high