Functional composition and environmental factors
We expected the abundance of small-bodied organisms, i.e. rotifers and
cladocerans, to increase with decreasing salinity. Interestingly, the
abundance of rotifers showed a slight increase in the late summer
communities when salinity increased. The salinity change during the
investigated time period was very small, however, only <1
unit, and the natural salinity variation that the organisms experience
often exceeds this both seasonally, e.g., in spring during the emergence
from sediment, and spatially, e.g., during diel vertical migration. Most
Baltic organisms are euryhaline, and ecosystems such as the Baltic Sea,
where the communities are already exposed to large variability regarding
a range of environmental parameters, are often expected to be less
impacted by future changes as the organisms are adapting to live in such
variable conditions. The influence of indirect factors such as changes
in food quality, quantity and predation pressure can have a large impact
on the community as a result of changes in environmental parameters.
Vehmaa et al. (2018) demonstrated that food quantity (given as
chlorophyll a and total organic carbon) increased following
eutrophication, using sediment core analysis. However, food quality is
also altered by eutrophication (O’Neil et al. 2012), as harmful
algae blooms are known to increase in frequency, magnitude and duration
(Huisman et al. 2018), also in the Gulf of Riga as shown by
Jurgensone et al. (2011). For example, both phytoplankton and
also microzooplankton biomass and community structure can significantly
affect the structure of zooplankton community. Klais et al.(2016) show that especially nitrogen-fixers and mixotrophs have
increased in the current study area, Gulf of Riga, between spring and
late summer.
Also temperature can act directly or indirectly on zooplankton community
structure; directly, because of the impact it has on the metabolism and
reproduction of the organisms, and indirectly via the competition
mechanisms when different zooplankton functional groups compete for
prey. Increasing temperature brings along a larger share of smaller
sized and lower complexity organisms, as is shown e.g. by Vuorinenet al. (1998), Suikkanen et al. (2013) and Mäkinenet al. (2017). Also in the Gulf of Riga, the share of lower
complexity organisms – most often cladocerans and rotifers – increased
simultaneously with temperature rise. The increasing share of low
complexity organisms which leads to its dominance instead of larger
organisms (copepods) has the potential to decrease the energy
availability of higher trophic level organisms, thus causing changes in
the food supply chain (Leech et al. 2018). Also the carbon
transport from the sea surface to the bottom will likely be decreased,
as copepods are responsible for a large part of the global carbon cycle
(Jónasdóttir et al. 2015).
Based on the model estimations, we did not detect hypoxic conditions
during the investigated time period. Overall, hypoxia is a large problem
in the Baltic Sea, and in the future as a result of rising temperatures
and accelerated eutrophication, hypoxic conditions are predicted to be
even more common both in frequency and duration (Diaz & Rosenberg 1995;
Kabel et al. 2012). The declining oxygen concentration near the
seafloor has the potential to reduce the functional diversity
(Vaquer-Sunyer & Duarte 2008). Via the benthic life stage that many
zooplanktic organisms have, the zooplankton is strongly affected by the
conditions near the sea floor. Where oxygen stress occurs, these low
oxygen conditions near the seafloor have the potential to disturb the
eggs and resting stages of organisms (Lutz et al. 1992). Most
species resting stages and eggs are tolerant to low oxygen conditions,
yet, with oxygen conditions declining further, the share of tolerant
species is slowly reduced. E.g. the nauplii of copepods can hatch at
very low oxygen concentrations (0.3 ml l-1), but under
these near-anoxic conditions their development eventually ceases
(Katajisto 2004). Also reduced diel vertical migration due to low oxygen
levels can change the diversity along the oxygen gradient. The oxygen
deficiency stress near the seafloor has the potential to weaken a niche
by decreasing the share of species that inhabit the deeper layers and
thus increasing the share of species that stay on or closer to the
surface.