Zooplankton community structure
The large decrease in the proportion of copepods in both coastal and open sea springtime communities was among the most notable changes that took place in the zooplankton community structure. Supporting evidence has been found in the Baltic Sea (Suikkanen et al. 2013; Kuosaet al. 2017). The reason to the findings have been suggested to be rapid warming that is affecting animal size negatively (Daufresneet al. 2009). Overfishing could be another factor indirectly affecting copepod populations, by causing diminishing of cod populations, and thereby enabling planktivorous fish populations such as Baltic herring Clupea harengus harengus and sprat Sprattus sprattus to flourish (Casini et al. 2008; Ljunggren et al. 2010). Eutrophication can also affect copepod populations negatively via decreased food quality caused by harmful algae blooms, which have become more frequent in the Baltic Sea and globally (O’Neilet al. 2012). The decrease of copepods occurred simultaneously with the increasing proportion of rotifers and more complex organism groups, named ‘others’. Especially in springtime in the coastal area, the peak of ‘others’ since ca. 1995 was likely mostly attributed to the appearance and rapid increase of Marenzelleria spp. larvae that are abundant in the water column from early spring (Kauppi et al.2018). Marenzelleria first appeared in the southern Baltic Sea in 1985 (Bick & Burckhardt 1989), and has since then become a dominant component of the benthic ecosystem (Norkko et al. 2015).