FGR and functional trait composition
In the spring, the functional group richness of zooplankton increased overall during the investigated time period 1960-2010; in the coastal area, the increase was nearly twofold. Several environmental factors caused the increase of the FGR in the springtime community. We found that the changes in salinity and temperature mainly impacted the community by increasing the abundance of simpler, smaller organisms. The higher nutrient load in the system in early spring increased the amount of functional groups. In the late summer, functional group richness gradually decreased from the 1970s and 1980s both in the open sea area and coastal area, respectively, until the 1990s where the richness was at its lowest. This most likely reflects the system response to eutrophication that has been extensive in the Baltic Sea since the 1960s (Cederwall & Elmgren 1980; Andersen et al. 2016).
Regarding functional trait composition, the main long-term shift that occurred during 1960-2010 was the shift from a community dominated by filterers in the 1960s towards a mixed- and predator-dominated community in the early 2000. Also the complexity of the organisms changed from more complex groups (Copepoda) to a community dominated by cladocerans and rotifers. Rotifers are known to increase in murky lakes (Leechet al. 2018); they also respond to warming (Daufresne et al. 2009), and to eutrophication (Vehmaa et al. 2018). Gulf of Riga is still known to be one of the most eutrophicated basins in the Baltic Sea (HELCOM 2018), which is a likely reason for the strong increase in rotifers. Vehmaa et al. (2018) showed that also small-bodied cladoceran such as Bosmina (Eubosmina )maritima respond to eutrophication by increasing their abundance.