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).