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.