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.