The combined effects of climate change, habitat loss and degradation has put substantial strain on aquatic ecosystems (Travis, 2003). One specific pressure on both marine and freshwater environments globally includes changing salinity, as a result of direct human activity such as ‘salting’ roads (Schuler et al., 2017; Hintz et al., 2021) as well as climate change; predicted to affect precipitation and ice melt (Helm et al., 2010; Ross et al., 2021). Salinity change can have a profound impact on aquatic organisms, including altering the distribution, phenology, abundance, composition and trophic interactions of plankton (Intergovernmental Panel Climate Change, 2018).

Zooplankton play a key role in the pelagic food web. They link primary producers (i.e. phytoplankton) to higher trophic levels (such as fish), thus their abundance and community structure directly affect the dynamics of fisheries resources (Sommer et al., 2002) as well as the biodiversity of higher trophic levels such as birds and mammals. How zooplankton will be affected by climate warming has been the subject of great debate (Gyllström et al., 2005; Mackas et al., 2007; Lewandowska et al., 2014; Šorf et al., 2015). However, current understanding of how salinity change in aquatic environments will impact zooplankton communities is mostly limited to the salinization of freshwater environments (e.g. Hintz et al., 2017; Lin et al., 2017; Moffett et al., 2020), estuaries (e.g. Gao et al., 2008) or single-species responses (e.g. Cervetto et al., 1999), while community-level changes and consequences for food web interactions in marine environments remain poorly understood. Studies on the effects of salinization of freshwater environments, have reported a range of responses including, dominance shifts and disruption of trophic interactions (Lin et al., 2017; Gutierrez et al., 2018). These results imply that a similar response to salinity change could be expected in marine environments.

Changes in salinity may directly or indirectly influence zooplankton community composition, leading to the local extinction of some species and the appearance of others. Based on existing literature, zooplankton may be directly affected by salinity depending on their physiology and tolerance to salinity change (Nielsen et al., 2003; Schallenberg et al., 2003; Hall, 2004). Direct responses of zooplankton to stressful salinity conditions include disturbances in reproduction, development and growth (Hart et al., 2003; Santangelo et al., 2014; Bashevkin and Pechenik, 2015). Indirect impacts through trophic interactions are expected because salinity can influence the composition and physiology of phytoplankton (Flameling and Kromkamp, 1994; Bisson and Kirst, 1995; Ayadi, 2004; Chakraborty et al., 2011), affecting the nutritional food quality of zooplankton (van de de Waal et al., 2010) and causing or contributing to food shortages (Perumal et al., 2009). Alterations in zooplankton community composition as a response to salinity change are likely to have negative consequences on food availability and growth of planktivorous fish, such as herring and sprat (Möllmann et al., 2004). This in turn could affect higher trophic levels, such as cod, seabirds and mammals (Alvarez-Fernandez et al., 2015).

Trophic interactions in plankton communities often differ between marine and freshwater environments and thus also their response to environmental perturbations. A recent study by Murphy et al. (2020) showed that warming conditions had no effect on zooplankton density in marine environments, whereas it had a negative effect on zooplankton density and increased grazing by herbaceous zooplankton in freshwaters. Stronger predator-herbivore interactions were also found in freshwater plankton compared to marine plankton communities (Shurin et al., 2002). Sommer and Sommer (2006) argue the difference in the strength of trophic cascades between lentic and marine environments is caused by differences in the zooplankton-phytoplankton link. Cladocerans dominate in freshwaters, whereas copepods dominate in marine ecosystems. Contrary to filter feeders such as cladocerans, copepods can select their food particles based on prey size (Tiselius and Jonsson, 1990), enabling copepod-dominated marine communities to be more resilient to disturbances of trophic cascades due to their selective feeding.

Climate change projections indicate that an increase in precipitation in northern Europe coupled with faster rates of ice-melt will lead to a decline in salinity in northern marine environments (HELCOM, 2013). This includes the Baltic Sea, where salinity is predicted to decrease by 1.5 to 2 psu by the end of the century (Meier et al., 2006). These salinity changes have been shown to be a bottleneck for both marine and freshwater species distribution and diversity, which could have negative impacts on marine ecology, monitoring, modelling and fisheries (Vuorinen et al., 2015). However, these salinity predictions are often uncertain with large variations depending on which general circulation models (GCMs) are used for predictions (Saraiva et al., 2019; Blenckner et al., 2021). Differences in observed data can also depend on region and vertical stratification (Lehmann et al., 2021), such as in the Gulf of Finland, where surface salinity increased by 0.5 psu between 1927-2021 (Merkouriadi and Leppäranta, 2014). Other areas of the world, including the Chesapeake Bay and the Baltic Sea, are predicted to show increasing fluctuations in salinity with climate change (Muhling et al., 2017; Blenckner et al., 2021).

Following these predictions, we designed a salinity change experiment using floating mesocosm platform deployed in the Baltic Sea to investigate the direct and indirect effects of salinity change on zooplankton communities. The use of mesocosms is a well-established tool to explore plankton community responses to environmental change owing to the advantage of manipulation and replication (Benton et al., 2007; Woodward et al., 2010). The brackish environment of the Baltic Sea, where freshwater and marine plankton communities can interact (Telesh et al., 2008), is an ideal study location to investigate the effect of salinity change on natural plankton communities. The general aim of the study was to examine interactions of zooplankton communities under different scenarios of salinity change of the Baltic Sea. We hypothesized that:

(1) Zooplankton abundance will be negatively affected by physiological intolerance to changing salinity,

(2) Salinity change will reduce phytoplankton biomass due to oxidative stress leading to food limitation for zooplankton and altered trophic interactions, and

(3) Salinity change will alter community composition and diversity of zooplankton species depending on their tolerance to salinity; marine species will dominate in higher salinities and brackish and freshwater species in lower ones.

Insights from these findings are important to understand how zooplankton communities may differ under climate driven salinity change and the possible effects it may have on trophic transfer efficiency and overall ecosystem functioning.

Our results imply that changing salinity will lead to a re-organisation of the zooplankton community structure (evenness and community composition). Salinity change had a direct impact on zooplankton evenness and caused a dominance shift from rotifers to cladocerans and later to copepods in high salinity treatments ( Figure 3 ). No such shift was observed in low salinity mesocosms, suggesting that rotifers will dominate zooplankton communities in low salinity environments in the future, such as with declining salinity of the Baltic Sea. Rotifers make up a significant proportion of the mesozooplankton community (Telesh and Heerkloss, 2002), are mostly found in freshwater environments and are generally intolerant to salinity increase (Sarma et al., 2006; Medeiros et al., 2010). Jansson et al. (2020) reported that rotifer abundance has been increasing in the Baltic Sea since the 1960s, but this increase was linked to warming and eutrophication and not declining salinity. This may have been due to the relatively narrow salinity range (5.2-6.4 psu) where their study was conducted (Gulf of Riga). In contrast, our experiment, which simulated future salinity change scenarios (Meier et al., 2012), confirms that rotifers are directly affected by salinity change ( Table 1 ) and dominate in low salinities, accounting for 82% of the community composition ( Figure 3 ).

In contrast to the lowest salinity treatment (3.5 psu), where different rotifer species dominated zooplankton community during the whole experimental period, in the higher salinity treatments (7.5 and 9.5 psu) rotifers were outcompeted by cladocerans and copepods (B. longirostris and E. affinis, Figure 3 ). Hence, we see a reduction in zooplankton evenness at the end of the experiment in 9.5 psu salinity treatment when some zooplankton groups are reduced or lost (e.g. K. quadrata, K. testudo and Brachionus) the shift is complete and the majority of the community is dominated by B. longirostris ( Figure 2 ). In the 7.5 psu salinity treatment the same shift in dominance was found as in 9.5 psu, but it took longer to reach a cladoceran-dominated community ( Figure 1 ). B. longirostris has been found to adapt to a wide range of salinities (Jeppesen et al., 1994; Deasley et al., 2012), thus it was able to outcompete rotifers in 7.5 and 9.5 psu treatments. However, as filter feeders, cladocerans can be negatively affected by toxic cyanobacteria blooms expected to proliferate with projected freshening and warming of the Baltic Sea (Meier et al., 2011) promoting rotifer and copepod abundance. Some rotifers were able to persist in high salinities however, namely K. cochlearis. This could be due to K. cochlaris being able to tolerate fluctuations in salinity (Paturej and Gutkowska, 2015).

Zooplankton richness increased directly after salt addition in all experimental units, with dominant rotifers suffering and copepods benefiting from the disturbance ( Figure 3 ). Copepods dominated by A. tonsa and E. affinis were most abundant in the 9.5 psu salinity treatment. Over the course of the experiment population of A. tonsa declined and was replaced by E. affinis ( Figure S1 , supplementary online material), indicating competitive exclusion, as both species have similar feeding strategies and prey size preferences (Richman et al., 1977; Engstrom, 2000). A. tonsa exhibit a broad tolerance to salinity, being able to survive salinities as low as 0.5 psu, but with an optimum salinity of between 10 and 20 psu (Cervetto et al., 1999; Calliari et al., 2006). E. affinis, which is generally regarded as a brackish species (Lee and Petersen, 2003), has salinity optimum of 10 psu (Karlsson et al., 2018), which is close to our highest salinity treatment and therefore may be better adapted to this salinity range than A. tonsa.

An alternative reason for the increased abundance of E. affinis found in high salinities could be a potential hormetic response to salt addition whereby, in rapidly producing more nauplii, the population of E. affinis is able to adapt to stressful conditions by guaranteeing the survival of at least a few individuals. Further evidence of this is given by an increase in juvenile and adult copepods found in the second week of the experiment, suggesting the nauplii produced in the first week were able to survive the high salinity environment. A hormetic response is when an environmental stressor (e.g. toxins, herbicides) stimulates an adaptive response that increases the resistance of the organism to a level of stress (Calabrese et al., 2007). This phenomenon has been studied elsewhere in nature (Calabrese and Baldwin, 1998; Beckers et al., 2009; Alyokhin et al., 2013; Vargas-Hernandez et al., 2017).

In intermediate salinities (5.5 and 7.5 psu) copepods were outcompeted by cladocerans, which reflects their different salinity optima, copepods preferring marine and cladocerans freshwater conditions (Sommer and Sommer, 2006). In the lowest salinity treatment (3.5 psu), rotifer abundance did not affect the population dynamics of cladocerans or copepods ( Figure 1 ), with groups tracking the pattern of food availability without apparent competition. This result has also been found elsewhere: a study by MacIsaac and Gilbert (1989) showed that a high abundance of rotifers can be maintained in the presence of small cladocerans such as B. longirostris. We found that in the 3.5 and 9.5 psu salinity treatments an increase in community evenness over the first two weeks of the experiment ( Figure 2 ) corresponded to a reduction in chlorophyll-a concentration. Conversely, when evenness declined in those treatments in the third and fourth week, we saw a recovery in chlorophyll-a concentration. We suggest this could be due to a more diverse zooplankton community using their resources more efficiently and therefore being able to reduce producer biomass (phytoplankton) more greatly (Duffy, 2002). In contrast, constant grazing from multiple zooplankton groups keeps phytoplankton concentrations low in the second half of the experiment in the intermediate salinity treatments ( Figure 1 ). Rotifer dominated communities have also been found to be less efficient in controlling phytoplankton biomass than other zooplankton groups (Jakobsen et al., 2003), which might be a reason why we observed recovery of phytoplankton biomass (measured as chlorophyll a concentration) in low salinity treatments by the end of the experiment ( Figure 1 ). This suggests with declining salinity of the Baltic Sea a shift to a rotifer dominated community may lead to less efficient grazing of phytoplankton and the increased risk of intense algal blooms in summer months.

In conclusion, we show that salinity change results in the restructuring of the zooplankton community. In low salinities, the dominance of rotifers throughout the experiment suggests they are most well adapted to freshwater environments, though some brackish species that are able to tolerate fluctuations in salinity (e.g. A. tonsa copepods and B. longirostris cladocerans) can persist in low abundance. With environments predicted to decline in salinity, such as the Baltic Sea, shifts from marine to brackish conditions will result in reduced food availability and increased resource competition between copepods and cladocerans, benefitting the latter due to their better adaptation to brackish conditions. This will therefore lead to a shift from a community comprised of complex organisms (copepods) to simpler ones (cladocerans and rotifers), potentially strengthening the negative effects of warming and eutrophication on zooplankton (Daufresne et al., 2009; Jansson et al., 2020), and consequently altering the functioning of pelagic food web. Although this study focusses on salinity change, in nature environmental variables are rarely acting in isolation to one another. Other stressors such as warming can affect the metabolic activity of zooplankton (Mayor et al., 2015) and lead to multistressor effects (Souissi et al., 2021). Trophic interactions may also be affected by salinity change and effects on other trophic levels, especially with regards to toxic cyanobacteria and phytoplankton should be considered in future studies (Viitasalo and Bonsdorff, 2022).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Study conception and design: CH and AL. Data collection: CH. Data analysis and interpretation of results: CH and AL. Draft manuscript preparation: CH and AL. All authors have reviewed and approved the final version of this manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

This research was funded by the Onni Talas Foundation Finland and the University of Helsinki start-up grant of AL. The experimental mesocosm platform is a part of the AQUACOSM (project No 731065) funded by the European Commission EU H2020-INFRAIA.