Contents lists available at ScienceDirect Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind Original Articles Sensitivity of phytoplankton metrics to sample-size: A case study on a large transitional water dataset (WISER) F. Cozzoli a, ⁎ , E. Stanca a , G.B. Selmeczy a , J. Francé b , I. Varkitzi c , A. Basset a a Department of Biological and Environmental Sciences and Technologies, University of the Salento, 73100 Lecce, Italy b National Institute of Biology, Marine Biology Station Piran, Fornace 41, 6330 Piran, Slovenia c Institute of Oceanography, Hellenic Centre for Marine Research, 19013 Anavyssos, Attica, Greece ARTICLE INFO Keywords: Phytoplankton Metric Sample size Uncertainty Precision Monitoring ABSTRACT Phytoplankton is a key element for the assessment of ecological and environmental status of aquatic ecosystems, also in the frame of the MSFD. The methods used to analyse phytoplankton communities are mainly based on time consuming cell counts, dictating a trade-off between the number of cells counted and the number of phytoplankton samples in a monitoring plan, whenever the financial budget, the available personnel or the response time are limited. We investigated the optimal sample size effort, intended as number of phytoplankton cells enumerated per sampling station, of many commonly used metrics. To this aim, precision and uncertainty of the metrics as a function of the sample size have been addressed using a case study carried out in the Lesina lagoon (Apulia, Italy), where 12000 cells per sample have been enumerated within five sampling stations. Overall, we show that some of the commonly used indices/metrics for the description of phytoplankton commu- nities are strongly dependent upon the sampling effort (as number of enumerated individuals per sample), while other metrics are relatively independent. Metrics based on the number of taxa only (e.g. Taxonomic richness), on the ratio between number of taxa and number of individuals (e.g. Margalef's diversity index) or on the species evenness (e.g. Pielou's evenness index) strongly depend on the sample size and their uncertainty (in term of sampling variance) is usually large. On the other hand, metrics of taxonomic diversity accounting for the proportional abundance of each taxa (e.g. Shannon-Wiener's diversity index), metrics accounting for the dominance of the most abundant taxa (e.g. Berger-Parker's dominance index) and metrics derived from the individual size distribution (e.g. Index of Size spectra Sensitivity, ISS Phyto) are able to achieve high precision and low uncertainty already at small sample sizes (virtually less than 200 enumerated cells) and thus they minimize the allocated effort vs. the gained information. Accordingly to our observations, we provide recommendations about the metric selection and the optimal effort per sample to be allocated in phytoplankton monitoring plans. 1. Introduction Phytoplankton community structure is a good indicator of the health and functioning of aquatic ecosystems (EEA, 2015). For this reason, phy- toplankton has been considered a key biological element for ecosystem quality assessment in the Marine Strategy Framework Directive (MSFD, 2008/56/EC), in the European Water Framework Directive (WFD 2000/ 60/EC), in other European water conventions and legislations, like Oslo–Paris Convention (OSPAR, 2009), Helsinki Convention (HELCOM, 2009), and many other national environmental legislations, like the USA Clean Water Act (Copeland, 2016) the Canada Water Act (RSC, 1985, c. C- 11) or the South African Water Research Act (Act No 34 of 1971). The concentration of Chlorophyll-a in water is a valuable and widespread proxy of the overall net phytoplankton production, which is basically de- termined by the trophic level of the aquatic environment (Richards and Thompson, 1952; Strickland and Parsons, 1960). However, Chlorophyll-a measurements lack any information on phytoplankton community structure and often do not allow a correct estimation of the phytoplankton biomass (Sandu et al., 2003; Kasprzak et al., 2008), hampering their utility as para- meter to assess the quality of water bodies. There is a large number of metrics based on the phytoplankton com- munity taxonomic and functional traits that can be potentially included in the ecosystem quality assessment process (Spatharis and Tsirtsis, 2010) in addition to Chlorophyll-a measurements. Biomass, composition, abundance and size spectra of the phytoplankton community, as well as frequency and intensity of phytoplankton blooms, have been considered as fundamental http://dx.doi.org/10.1016/j.ecolind.2017.07.022 Received 14 May 2017; Received in revised form 8 July 2017; Accepted 9 July 2017 ⁎ Corresponding author. E-mail address: francesco.cozzoli@unisalento.it (F. Cozzoli). Ecological Indicators 82 (2017) 558–573 1470-160X/ © 2017 Elsevier Ltd. All rights reserved. MARK