Ciliophora
Geographic patterns

As with phytoplankton and bacterioplankton, densities of aloricate ciliates decrease from coastal regions towards the open ocean (Ci. table 1). Peak numbers and biomass generally occur in bays and estuaries. But high densities are also found in nutrient-rich upwelling areas and divergences in the open ocean, whereas zones of convergence yield particularly low values (Mamaeva, 1982b, 1986; Porter et al., 1985; Laybourn-Parry, 1992). This suggests that ciliate distributions depend more on factors like food supply or oceanographic conditions than on geographic latitude or ocean basins. This is corroborated by positive correlations between ciliates and chlorophyll concentration and, less frequently, bacterioplankton biomass (for review see Lynn and Montagnes, 1991). It must be noted, however, that differences in numbers between studies may also be caused by differing methods (see Ci. 3 Sample preparation and analysis).

Aloricate ciliates are often distinctly (up to 50 times) more abundant than the loricate tintinnids (e.g., Beers et al., 1980; Smetacek, 1981; Mamaeva, 1982a; Dolan and Marrasé, 1995). It appears that the percentage of aloricates is slightly lower in neritic waters, where tintinnids even dominate occasionally, and increases towards the open ocean (Margalef, 1973; Revelante, 1981; Capriulo and Carpenter, 1983; Paranjape et al., 1985; Leakey et al., 1994b; Sanders, 1995). At least at the generic level, the community composition of aloricate ciliates is rather similar in nearshore and open ocean areas. Oligotrichs like Strombidium, or occasionally Lohmanniella or Tontonia, usually dominate (e.g., Beers et al., 1975; Mamaeva, 1983, 1984a,b, 1986; Revelante and Gilmartin, 1983; Tumantseva and Kopylov, 1985; Stoecker et al., 1994b; Dolan and Marrasé, 1995). For instance, in the Atlantic sector of the Southern Ocean the oligotrich genera Strombidium, Laboea and Strobilidium make up 83-99% of total ciliate carbon (Klaas, 1997). However, species-specific distributional data are very scarce. These would be especially interesting since Borror (1980) doubted that individual marine species have a cosmopolitan distribution.

Data on dominance of single species are rare since only genera or size classes are often differentiated. From the very few studies available it appears that, as in other zooplankton groups, rather few aloricate species are abundant in any given sample. For instance, in the open Baltic Sea 1-3 species contributed >10% to total ciliate abundance, 0-3 taxa between 5-10% and 50-80% of the species contributed less (Leppänen and Bruun, 1986). Small ciliates (<20-30 µm) often dominate numerically, whereas larger species contribute more to biomass (Fig. Ci 2 ; Beers et al., 1980; Revelante et al., 1985; Sherr et al., 1986; Sushin et al., 1986; Revelante and Gilmartin, 1987; Lynn et al., 1991; Strom et al., 1993; Lessard and Murrell, 1996; Tamigneaux et al., 1997).