Phytoplankton size structure in transitional water ecosystems: a comparative analysis of descriptive tools

Abstract

1. Individual size has important implications for the physiology and ecology of phytoplankton. The analysis of size structure is a common approach for studying structure and function of planktonic ecosystems, with applications for the monitoring and conservation of aquatic ecosystem health. Several ways to describe the size structure of phytoplankton communities are used, including size-abundance relationships (SAR), non-normalized and normalized biomass size spectra (BSS), non-normalized and normalized number size spectra (NSS). 2. The main goal of this study was to test the effectiveness of the different tools proposed for describing and monitoring phytoplankton guilds in transitional waters. Comparative analyses were undertaken to evaluate (a) the consistency of each tool in describing the phytoplankton guilds, in terms of statistical significance and agreement with theoretical expectations; and (b) the effectiveness of each tool to discriminate conditions/cases. 3. Eleven transitional water bodies occurring along the eastern and western coasts of the Adriatic Basin were sampled in autumn 2004 and spring 2005. 4. Results obtained highlight that: (1) NSS globally showed higher statistical goodness of fit than BSS; (2) slopes of BSS and NSS, both normalized and non-normalized, were less negative than expected following the linear biomass hypothesis; and (3) the non-normalized NSS shape descriptors were best able to discriminate environmental heterogeneity both between and within transitional water ecosystems. 5. Parameters of NSS are suggested to be consistent descriptors of phytoplankton size structure and potentially important tools for monitoring and conservation of transitional water ecosystems.