The "neutral" community structure of planktonic herbivores, tintinnid ciliates of the microzooplankton, across the SE Tropical Pacific Ocean

Abstract

We assessed the community characteristics of a group of planktonic herbivores across a species-rich area, the SE Pacific Ocean. A series of 22 stations between the Marquise Islands (7° S 142° W) and the coast of Chile (35° S 73° W) was sampled during the BIOSOPE cruise in 2004. We examined the relationships between taxonomic diversity, morphological diversity, patterns of tintinnid species assemblage, and phytoplankton abundance. Tintinnid community characteristics were estimated from large volume (20-601) discrete depth sampling and phytoplankton were characterized based on HPLC pigment signatures. Across the transect, average water column concentrations of tintinnids ranged from 2-40 cells 1-1 or 8-40 ng C 1-1, and were positively related to chlorophyll a concentrations which varied between 0.07-2 μg 1-1. Large numbers of tintinnid taxa were found, 18-41 species per station, yielding a total of 149 species. Among stations, morphological and taxonomic diversity metrics co-varied but were not significantly related to phytoplankton diversity estimated using a pigment-based size-diversity metric. Taxonomic diversity of tintinnids, as H' or Fishers' alpha, was inversely related to chlorophyll concentration and positively to the depth of the chlorophyll maximum layer. Species abundance distributions were compared to geometric, log-series and log-normal distributions. For most stations, the observed distribution most closely matched log-series, coherent with the neutral theory of random colonization from a large species pool. Occurrence rates of species were correlated with average abundance rather than specific characteristics of biomass or lorica oral diameter (mouth) size. Among stations, species richness was correlated with both the variety of mouth sizes (lorica oral diameters) as well as numbers of species per mouth size, also consistent with random colonization.