Indicator description, calculation principles, and rationale.

<p>Only species/groups that are included consistently in the zooplankton analysis were used for calculations.</p

Variability of zooplankton indicators in the analyzed datasets.

<p>The datasets are indicated as <i>Station</i>. Box-and-whiskers show median, 25 and 75% percentiles, min and max values. Asterisks (*: p< 0.05, **: p < 0.01, and ***: p < 0.001) indicate significant deviations from Gaussian distribution using the Kolmogorov-Smirnov statistics applied to the data sets with ≥18 years of observations. Shaded columns indicate datasets that are <12 years and thus not eligible for normality testing. See Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158326#pone.0158326.t001" target="_blank">1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158326#pone.0158326.t003" target="_blank">3</a> for the details on the indicators and datasets.</p

Details for the data sets provided by national monitoring programs for indicator testing; deviations in sampling methods from the HELCOM guidelines (i.e., WP2, 100-μm mesh size) are indicated.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158326#pone.0158326.s006" target="_blank">S1 Table</a> for details.</p

Summary of CuSum analysis (S3 Fig) for all indicators and datasets with >12 years of observations.

<p>Periods when UCL or LCL (bold) were violated for >3 consecutive years are shown; the first two digits are omitted for simplicity.</p

Map of the Baltic Sea indicating sampling sites for zooplankton datasets used in this study.

<p>Each dataset is represented by a single circle; when several stations contributed to a dataset, the circle shows the approximate middle of the sampled area. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158326#pone.0158326.t001" target="_blank">Table 1</a> for description of sampling sites and sampling frequencies.</p

Conceptual diagram for MSTS, a two-dimensional indicator comprised by total stock (TZA or TZB) and MeanSize.

<p>The green area represents in-GES condition, orange areas represent sub-GES conditions where only one of the two parameters is adequate, and the red area represents sub-GES conditions where both parameters fail.</p

Indicator Properties of Baltic Zooplankton for Classification of Environmental Status within Marine Strategy Framework Directive

<div><p>The European Marine Strategy Framework Directive requires the EU Member States to estimate the level of anthropogenic impacts on their marine systems using 11 Descriptors. Assessing food web response to altered habitats is addressed by Descriptor 4 and its indicators, which are being developed for regional seas. However, the development of simple foodweb indicators able to assess the health of ecologically diverse, spatially variable and complex interactions is challenging. Zooplankton is a key element in marine foodwebs and thus comprise an important part of overall ecosystem health. Here, we review work on zooplankton indicator development using long-term data sets across the Baltic Sea and report the main findings. A suite of zooplankton community metrics were evaluated as putative ecological indicators that track community state in relation to Good Environmental Status (GES) criteria with regard to eutrophication and fish feeding conditions in the Baltic Sea. On the basis of an operational definition of GES, we propose mean body mass of zooplankton in the community in combination with zooplankton stock measured as either abundance or biomass to be applicable as an integrated indicator that could be used within the Descriptor 4 in the Baltic Sea. These metrics performed best in predicting zooplankton being in-GES when considering all datasets evaluated. However, some other metrics, such as copepod biomass, the contribution of copepods to the total zooplankton biomass or biomass-based Cladocera: Copepoda ratio, were equally reliable or even superior in certain basin-specific assessments. Our evaluation suggests that in several basins of the Baltic Sea, zooplankton communities currently appear to be out-of-GES, being comprised by smaller zooplankters and having lower total abundance or biomass compared to the communities during the reference conditions; however, the changes in the taxonomic structure underlying these trends vary widely across the sea basins due to the estuarine character of the Baltic Sea.</p></div

Shifts in zooplankton community structure (year) that were detected for different α levels using the indicator time series (only entire datasets with more than 12 years of observations were considered).

<p>No community shifts were detected for the Bothnian Bay (BoBFI) data. Years that were consistently detected at all levels of α are in bold.</p

Time coverage for zooplankton data and reference periods based on the existing EQR for Chlorophyll <i>a</i> (RefCon_Chl) and fish body condition (RefCon_Fish) for each dataset.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158326#pone.0158326.t001" target="_blank">Table 1</a> for details.</p