Short communicationAbundance and diversity of tintinnids (planktonic ciliates) under contrasting levels of productivity in the Argentine Shelf and Drake Passage
Highlights
► We studied tintinnids in adjacent shelf and oceanic waters during autumn and summer. ► Species richness and diversity did not differ significantly under the four conditions. ► Abundance and biomass were up to twelve times higher in the shelf than in the ocean. ► Abundance and richness were significantly related to chlorophyll a and between them. ► Dissimilar relationships were linked to differences in species and size structure.
Introduction
Ciliates play a key role as trophic link between the smallest and the largest components of planktonic food webs in marine environments. Tintinnids are usually a minority group among ciliates, but are ubiquitous in the microzooplankton and can be important consumers of phytoplankton (Lynn, 2008). The lorica that characterizes tintinnids allows for species classification, and its opening width, the Lorica Oral Diameter (LOD), is linked to ecological features such as the maximum and preferred prey size (Dolan, 2010). In addition, tintinnid species display consistent patterns of diversity (Dolan et al., 2006b) and provide reliable indicators of hydrography and biogeography (Alder, 1999).
One of the main factors that explains tintinnid distribution and abundance is chlorophyll a concentration (Chla), while the relationship between this variable and tintinnid diversity has been reported as weak or non-significant in either neritic (Dolan et al., 2006a, Sitran et al., 2007) or oceanic waters (Dolan et al., 2002, Dolan et al., 2007, Thompson, 2004). Indeed, tintinnid diversity has been shown to be linked more closely with prey diversity than with food concentration (Dolan et al., 2002). In contrast, significant correlations are found between species richness and different measures of productivity (e.g., Chla) for other pelagic organisms and benthic communities, although the type of relationship (linear or quadratic) depends on the taxonomic group and the scale of analysis (Gaston and Spicer, 2004, Witman et al., 2008).
We explored the relationship between Chla and both tintinnid abundance and species richness in two adjacent environments, the Argentine Shelf and the Drake Passage, during autumn and summer. These regions are influenced by multiple oceanographic processes and exhibit dissimilar current circulation patterns (Fig. 1A). While the oceanic waters of the Drake Passage show low, uniform Chla, the mid-depth sector (50–100 m) of the Argentine Shelf is characterized by a strong seasonality in primary production (Rivas et al., 2006). These contrasting conditions are reflected in a significant decrease in the abundance and biomass of total ciliates from the shelf to oceanic waters (Santoferrara and Alder, 2009a). In the present study, we first analyzed the general trends of tintinnid abundance, diversity, species structure and LOD size-classes, and then we addressed the following questions: (1) Is Chla significantly related to tintinnid species richness? (2) If tintinnid abundance is linked to Chla, is there a relationship between abundance and species richness? Knowing these relationships under heterogeneous Chla levels will help to understand productivity effects on marine microzooplankton diversity.
Section snippets
Material and methods
Sixty-six oceanographic stations were sampled on two latitudinal transects on board the icebreaker Almirante Irizar during austral autumn (April 2002) and summer (February 2003) (Table 1). For plankton samples, 100 to 200 L of sub-surface (9 m) seawater were collected by a centrifugal pump, concentrated through a sieve (20 μm pore size) and preserved with Bouin's Solution (10% final). In addition, 2 to 4 L of seawater from 52 of the stations were concentrated on GF/F filters and frozen until
General trends
The data analyzed in this study are summarized in Table 1. Chla reached maximum values up to 6.30 and 17.53 μg L− 1 in the shelf during autumn (46.0°S) and summer (51.7°S), respectively (Fig. 1B). In oceanic waters, Chla was below 0.60 μg L− 1 regardless of the season. Tintinnid abundance and biomass showed similar spatial trends in autumn and summer, with values below 0.25 × 103 ind. L− 1 and 1 μg C L− 1, respectively, in almost all stations (Fig. 1C–D). Higher values were found only in shelf waters, and
Acknowledgments
This work was supported by a fellowship granted to L.S. by Consejo Nacional de Investigaciones Científicas y Técnicas — CONICET, and by funding from the same institution (PIP 0246), Agencia Nacional de Promoción Científica y Tecnológica (PICT 7‐9108), and Instituto Antártico Argentino (Proyecto No. 58) granted to V.A.A.
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