Estuarine, Coastal and Shelf Science (2000) 50, 415–424 doi:10.1006/ecss.1999.0577, available online at http://www.idealibrary.com on The Abundance of Heterotrophic Protists Associated with Intertidal Seaweeds E. Armstrong a,d , A. Rogerson b and J. W. Leftley c a University Marine Biological Station, Millport, Isle of Cumbrae KA28 0EG, U.K. b Oceanographic Center, Nova Southeastern University, 8000 North Ocean Drive, Dania, FL 33004, U.S.A. c Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, P.O. Box 3, Oban, Argyll PA4 4AD, U.K. d Current Address: Department of Biological Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K. Received 14 May 1999 and accepted in revised form 25 October 1999 Seaweeds have long been known to support abundant populations of bacteria so it is surprising that few studies have considered the abundance of heterotrophic protists on seaweed surfaces. A year-long study was conducted to investigate the numbers of heterotrophic protists on undamaged and damaged tissue of a number of intertidal species of seaweed. Generally, amoebae and flagellates numbered less than 20 cells cm 2 on undamaged seaweed tissue but were found in greater numbers (more than 20 cm 2 ) on damaged tissue. Ciliates were around 1 or 2 cells cm 2 on undamaged seaweeds and between 1 and 5 cm 2 on damaged tissue. Numbers of heterotrophic dinoflagellates and heterotrophic diatoms were similar to those of ciliates in late summer and autumn when seaweeds were producing increased amounts of dissolved organic carbon. By assuming that ‘ true ’ surface protists inhabited the thin surface film of water on seaweeds, comparisons were drawn with the numbers of protists in nearby open waters. After using appropriate conversion factors, both amoebae and flagellates were more abundant in the surface film (per ml) than in the open water. In particular, surface amoebae averaged around 1800 cells ml 1 but only reached around 19 cells ml 1 in open water. The results are discussed in relation to the various sources of nutrition available to heterotrophic protists on macroalgae. The high numbers of amoebae suggest that they are the major micropredators of bacteria on seaweed surfaces.  2000 Academic Press Keywords: protozoa; macroalgae; algae; bacteria; Scotland coast Introduction Aspects of the ecology of heterotrophic protists in coastal regions have been addressed in recent papers dealing with coastal waters (e.g. Vaque ´ et al., 1997), estuarine waters (e.g. Rogerson & Laybourn-Parry, 1992a; Eldridge & Sieracki, 1993), and the benthos (e.g. Patterson et al., 1989; Butler & Rogerson, 1995). These, and other studies have shown that surfaces are important for microbial interactions whether they are the surfaces of sediment particles (Kemp, 1990) or the surfaces of suspended aggregates (Rogerson & Laybourn-Parry, 1992b). Given the large surface area of coastal seaweed stands, with their attendant high bacterial populations and autotrophic diatom popu- lations (Huang & Boney, 1984; Velji & Albright, 1986), it is surprising that few studies have considered the abundance of heterotrophic protists on seaweeds. Seaweeds offer living surfaces that harbour interest- ing microbial systems. These algae are highly produc- tive components of the coastal ecosystem releasing dissolved organic carbon (DOC) into surrounding waters. The magnitude of this release is a matter of debate but may range from 1% to 40% of net fixed carbon. The wide range of estimates reflects the various methods used by different investigators (Sieburth, 1969; Khailov & Burlakova, 1969; Johnston et al., 1977; Hatcher et al., 1977; Pregnall, 1983). Thus a component of this study was to com- pare the heterotrophic protistan community inhabit- ing DOC-rich water close to the seaweed stands with the community at some distance from the seaweeds in open coastal water. The dense populations of bacteria on seaweed sur- faces are thought to consume photosynthate extremely rapidly (Linkins, 1973). However, despite the effi- ciency of capture of released carbon by bacteria, heterotrophic protists on the surface of algae may be able to benefit from algal carbon exudates directly or indirectly by consuming attached surface bacteria. For 0272–7714/00/030415+10 $35.00/0  2000 Academic Press