High numbers of naked amoebae in the planktonic waters of a mangrove standin southern Florida, USA

This is the first study to examine the abundance of naked amoebae in the wa ter column of a mangrove stand. A total of 37 different morphotypes was not ed and at least 13 of these are probably new species. Over a one-year sampl ing interval, amoebae averaged 35,400 cells liter(-1) (range 2,000-104,000) by an indirect enrichment cultivation method. Densities in the upper end o f this range are the highest ever reported for any planktonic habitat. Vari ation between samples was related to the quantity of suspended aggregates ( floes) in the water column emphasizing that amoebae are usually floc-associ ated. The study also showed that it is essential to disrupt flee material p rior to withdrawing sample aliquots for the indirect counting method since several amoebae can occupy the interstices of aggregates. There is concern that indirect enumeration methods that require organisms to be cultured in the laboratory seriously underestimate the true count. A direct counting me thod using acridine orange staining and epifluorescence microscopy was atte mpted to assess the possible magnitude of the error associated with indirec t counting. While this direct method had limitations, notably the difficult y of unambiguously differentiating between small amoebae and nanoflagellate s, the results suggested that the indirect method gave estimates that were close to the true count (within a factor of two). Mangrove waters are rich in heterotrophic protozoa (similar to 3 x 10(6) liter(-1)) and while the he terotrophic flagellates are by far the dominant group, naked amoebae outnum ber ciliates some 20-fold. The ecological consequences of high numbers of a moebae, particularly the common small forms less than 10 mu m in length, ne ed to be examined for these important coastal sites.