Introduction
To detect changes in multispecies communities caused by environmental changes, the communities have traditionally been studied based on taxonomic classification of species, focusing on their abundance and diversity (Sherman et al. 1998). This approach, however, does not provide immediate information on the functioning and the ecological properties of the communities, but only on their composition. The functional traits can be used to provide information on how changes in the community might lead to changes in the functioning of the system (Hébert et al. 2017).
In the Baltic Sea area, like elsewhere in the world, warming and increasing precipitation are both accelerating due to global climate change. Warming of the Baltic Sea has continued since 1860 (MacKenzie & Schiedek 2007; Belkin 2009) with a rate 1°C per decade in all sub-basins (Lehmann et al. 2011; BACC II Author Team 2015). A salinity decrease of ca. 1-2 units is expected by the end of this century due to more frequent rainfall and the subsequent rise in freshwater run-off from the catchment area (Meier et al. 2012; Holopainen et al. 2016). The increased riverine run-off also brings along more nutrients, impacting the eutrophication status of the Baltic Sea (Leppäranta & Myrberg 2009). The Baltic Sea has been affected by severe eutrophication since the 1960s (Cederwall & Elmgren 1980; Andersenet al. 2016). Due to a large increase in the production of organic matter and the subsequent increased oxygen demand during decomposition of the produced biomass, the Baltic Sea is also exposed to large-scale hypoxia (Conley et al. 2007; Andersen et al.2016). An increase in hypoxia due to rising temperatures and accelerated eutrophication in the Baltic Sea is predicted (Karlson et al.2002; Conley et al. 2009; Carstensen et al. 2014). These changes in environmental conditions modify populations and communities by impacting species distribution ranges, abundance and dominance relations, food-web structures and behavioral and physiological functions of species. Through these effects, the structure and functioning of whole ecosystems can be altered (Thomas et al.2004; Doney et al. 2012). Fish larvae and their recruitment to the population can be especially impacted if the quality of their food community i.e. zooplankton decreases substantially (Edwards & Richardson 2004; Arula et al. 2014).
The aim of the current work is to study how the zooplankton community is associated with measured environmental variables in the Gulf of Riga using a hypothesis-driven approach. We expect that decreasing salinity, warming and decreasing oxygen affect zooplankton negatively. We also expect milder winters to reduce functional group richness (seeHypotheses ). The dataset is extensive and can readily be used to generalize patterns occurring in brackish areas as a consequence of environmental change.