Benthic ciliate identification and enumeration: an improved methodology and its application

The study of the ecological role played by benthic ciliates is hampered by the lack of a methodology to accurately enumerate benthic ciliates with goo d taxonomic resolution. As a result, a technique was developed that combine d 2 previously published methods developed to identify and count pelagic ci liates and benthic flagellates, respectively. The new method utilizes centr ifugation in a non-linear density gradient to separate ciliates from sedime nt, and the QPS (quantitative protargol stain) silver-staining technique to stain the cilia and nuclei of ciliates which, after centrifugation, are co ncentrated on cellulose nitrate filters. The wide applicability of the meth od was shown by utilizing it to count and identify ciliates in cores taken from intertidal sediment and sampled on a 2 mm depth interval. The intertid al cores had a total of 41 species or morphotypes present, but no more than 21 species or morphotypes in any single 2 mm layer. Total ciliate abundanc e was as high as 2500 cells ml(-1), with the upper layers having higher abu ndance than deeper layers. There was no obvious pattern with depth either f or the number of species found in any one layer, or for ciliate diversity. The method was further utilized in an experiment that explored the interact ions between benthic ciliates and the ostracod Cyrideis trosa. Ostracods re duced the final abundance of ciliates, but this effect was confined to the upper 5 mm of sediment. Two-thirds of the ostracods were found in the 5-10 mm layer at the end of the experiment, but while there was an effect on cil iate diversity in this layer, there was no effect on total ciliate abundanc e. We conclude that the method is suitable for studying both the natural di stribution and diversity of benthic ciliates and their response to experime ntal manipulations.