Investigating the effects of increased salinity on leaf litter decomposition and mortality of an aquatic invertebrate detritivore (Caecidotea sp.)

Introduction

A growing threat for coastal freshwater wetlands is the expected increase in saltwater intrusion events because of sea-level rise and an increase in the predicted number of large storm events. The rate of warming is predicted to accelerate, and sea-level rise is expected to increase 1–2 m by the year 2100 (Vermeer and Rahmstorf 2009). These impacts include the increase in severe storms and tidal amplitudes driving saltwater further inland (Quintino et al. 2009; Yin 2023). Saltwater intrusion into these systems can be accelerated further by anthropogenic coastal landscape modification, such as those performed on barrier islands. One example is the construction of artificial channels almost 200 years ago on Sapelo Island, Georgia, which has led to extensive surface-level saltwater movement to the interior of the island (Chalmers 1997; Kidder 2023). These at-risk habitats provide ecosystem services, including flood control and storm-surge protection for coastal areas (Costanza et al. 1997; Barbier 2013). The importance of detrital processing in these systems additionally provides services related to carbon sequestration, supplying organic matter to other ecosystems, and supporting other trophic guilds and higher-order taxa including birds, mammals, reptiles, etc. (Vannote et al. 1980; Polis et al. 1997).

Increases in salinity concentrations within otherwise non-saline aquatic habitats lead to the removal of salt-intolerant biota both directly through physiological stress (Peterson and Meador 1994; Tyree et al. 2016) and indirectly through competition with and predation by more salt-tolerant macroinvertebrate taxa (Peterson and Meador 1994; Hart et al. 2003). This can result in trophic cascades and community replacement coupled with diversity simplification by decreases in species richness and evenness (Ley et al. 1994; Nielsen and Brock 2009), loss of plant-influenced critical habitat (Hogue and Breon 2022) and alterations to the ionic chemistry and biogeochemical cycles (Herbert et al. 2015). These losses have potential positive feedback effects on climate-change impacts because of the loss in carbon sequestration by biota, such as in the case of ghost forests left behind by the salinization of freshwater systems (Chen et al. 2023).

Unlike large-order rivers or other exposed bodies of water, small, shaded streams and wetlands cannot rely on autochthonous inputs of carbon from primary producers such as algae and phytoplankton. Instead, these systems rely on allochthonous inputs of carbon, often mainly in the form of abscised leaf material from riparian areas (Vannote et al. 1980; Feio et al. 2021). Once entering the aquatic environment, organic and inorganic compounds start leaching into the water rapidly on the first day (Webster and Benfield 1986). A few days later, hyphomycete fungi and bacteria colonize and begin decomposing organic carbohydrates within the leaf material, softening it up; this process can also increase the nutritional content of leaf material (France 2011). Large detrital material is further broken down by aquatic macroinvertebrate ‘shredders’ that specialize in feeding by physical fragmentation and ingesting leaf material. This entire process creates a fine fraction of detrital material, which can then be further utilized by non-specialist macroinvertebrate consumers such as filterers or collectors (Cummins and Klug 1979). This multi-taxon decomposition pathway makes detrital processing a useful investigatory tool when considering the health of wetlands and small streams (Cummins and Klug 1979; Quintino et al. 2009).

An in-lab microcosm approach was used, following the methods of Connolly et al. (2014), to quantify increased salinity impacts on leaf detritus decomposition rates and the mortality of a common macroinvertebrate shredder (isopods, genus Caecidotea). Connolly et al. (2014) found that a species of this genus experience mortality at as low as 5 PSU, but many individuals can survive to at least 15 PSU. Kidder (2023) found that a species belonging to this genus predictably decreases in abundance along an increasing salinity gradient on Sapelo Island, Georgia. On the basis of their findings, it is expected that these isopods will not tolerate higher salinity and show a predictable decline in survival as a result of physiological stress.

Materials and methods

Results

Distinct salinity regimes were maintained across treatments for the duration of the study (Fig. 1). The average salinity for the four treatments with standard deviations were as follows: 0 ± 0, 5.81 ± 0.94, 10.87 ± 1.38 and 15.87 ± 1.34 PSU. Tanks without isopods had averages of 0 ± 0, 5.86 ± 0.95, 10.89 ± 1.53 and 15.82 ± 1.50 PSU; tanks with isopods had averages of 0 ± 0, 5.86 ± 0.95, 10.89 ± 1.23 and 15.91 ± 1.18 PSU. There were no significant differences in salinity (P > 0.05) between tanks with and without isopods. Overall, the salinities remained in separate regimes, with more variation at the start of the experiment. There was only one instance of overlap that occurred in one of the 10-PSU no isopod replicates that dropped to 5 PSU on Day 33 of the study because too much freshwater was added. The mean water temperature throughout the study was 17.64°C ± 0.99.

Fig. 1.

Change in salinity since isopod addition (Day 0 = experiment Day 14, Day 23 = experiment Day 37) for each replicate. Salinity was measured as practical salinity units (PSU). Treatments included 0 (freshwater), 5, 10 and 15 PSU. Gray lines indicate treatments where no isopods were added to the tanks, and black lines indicate treatments where isopods were added to the tanks.

Isopod mortality increased with an increasing salinity (Fig. 2). The lowest average number of deceased isopods was from the 0-PSU (i.e. freshwater) treatments, with 43% mortality. At 5 PSU, average mortality slightly increased to 46.67%. At 10 PSU, the average mortality increased to 68.33%. However, no isopods survived the 15-PSU treatment, which resulted in an average mortality of 100%. Average isopod mortality differed among salinity treatments (one-way ANOVA, F_3,11 = 25.1197, P = 0.0002), with the 0- and 10-PSU treatments differing significantly from each other (Tukey HSD, P = 0.0381), but not from the 5-PSU treatment (Tukey HSD, 0–5, P = 0.9672; 5–10 P = 0.0820). The highest salinity treatment (15 PSU) was significantly different from all other three salinity treatments (Tukey HSD, 0–15, P = 0.0003; 5–15, P = 0.0004; 10–15, P = 0.0112).

Fig. 2.

Mean percentage ± s.e. isopod mortality by salinity treatment (0, 5, 10, 15 PSU). Different letters denote a significant pairwise difference from Tukey post hoc.

Decomposition of leaf material by isopods was not found to be significantly different among salinities between Day 14 and Day 37 (ANCOVA, F_3,11 = 0.5451, P = 0.5946). Decomposition was also not significantly different across salinity in no isopod tanks (one-way ANOVA, F_3,8 = 0.5963, P = 0.6350; Fig. 3).

Fig. 3.

Leaf litter decomposition as mean percentage original mass remaining ± s.d. by salinity treatment (0, 5, 10, 15 PSU) in tanks with and without isopods.

Discussion

Our study demonstrated that increased salinity has a negative effect on freshwater macroinvertebrate shredders in a laboratory microcosm setting, but we did not find a measurable effect of salinity on decomposition. This has been corroborated by several studies that find increased mortality and decreased activity in freshwater shredders (e.g. Blasius and Merritt 2002; Connolly et al. 2014). Salinities of 15 PSU were lethal to isopod survival with 100% mortality. Similar responses were seen in Caecidotea sp. in a study by Connolly et al. (2014) where 15 PSU caused significantly higher mortality than did 10 PSU. Unfortunately, no tolerance studies aside from Connolly et al. (2014) have been performed on a Caecidotea species found on the eastern coast of the United States. A Caecidotea species found on Sapelo Island, Georgia, has been found to predictably decline across an artificial channel experiencing saltwater intrusion (Kidder 2023; Gordon 2025). This species dominates freshwater locations that do not receive salt input but decline as the frequency and intensity of salt intrusion increase. They are replaced by more tolerant taxa (i.e. Hemiptera, Odonata) in moderate salt conditions (5–10 PSU), extremely tolerant taxa (i.e. Chironomidae, Oligochaeta) in strong conditions (15–20 PSU), and estuarine taxa (i.e. Polychaeta, Sphaeromatidae) in tidal conditions (20–30 PSU). However, in this study isopods had a 43% mortality in 0 PSU, which seems high for the treatment closest to their natural freshwater environment. Higher than expected isopod mortality in freshwater may be associated with water quality decline. A build-up of ammonia for instance could have occurred because of extended days of trial without full water changes. However, this is speculative as nutrients were not measured in this study. Starvation because lack of food is unlikely because of similar leaf disc mass loss across salinity treatments (Fig. 3).

The possible absence of microbes in this experiment is a potential reason why decomposition was not significantly different in the no isopod tanks. Although the support for our hypothesis was absent in this experiment, it has been corroborated by several studies on the impacts of increased salinity on microbial decomposers. (e.g. Connolly et al. 2014; Tyree et al. 2016). Other limitations for this study include the small sample size of replicates; variation was very high in many of the results, and more replicates could have helped reduce this. Additionally, tanks were filled with 500 mL of water, aerated and lids had holes. All of these were potential problems for different reasons. The volume of water used was much greater than the amount needed for 60 1-cm² leaf discs and 20 3.51 ± 0.86-mm isopods. Connolly et al. (2014) performed a similar study on isopod response with 20 mL of water with four isopods plus detritus. Therefore, only one-fifth of what we used would be needed to replicate their methods. The excess water makes accurate salinity measurement more difficult, because saltwater is denser than freshwater; water was not taken from the bottom to prevent any disturbances to decomposition and isopods. Although aeration was necessary for decomposition and isopods, it is also possible that any direct airflow onto leaf discs increased physical fragmentation of the leaves. This is also true in the addition of water to tanks, which was required because of evaporation through the holes in the lids. A closed system with aeration would have been preferable to prevent changes in salinity.

As salinity increases in freshwater wetlands because of sea-level rise, tidal intrusion and intensified storm events, the abundance and diversity of freshwater macroinvertebrates are expected to decline (Hart et al. 2003). Shredder macroinvertebrates and their microbial food sources play a central role in detrital-based food webs, supporting broader trophic dynamics (Cummins and Klug 1979; Marks 2019). Salt-tolerant taxa may replace freshwater communities (Hart et al. 2003; Kidder 2023), but whether key ecosystem functions persist remains uncertain. Over time, these wetlands may transition towards estuarine conditions (Feio et al. 2021; Chen et al. 2023), posing conservation risks for taxa without terrestrial life stages and for isolated habitats such as barrier islands (Ross et al. 2009; Woodward et al. 2010). Coastal development and climate change further exacerbate salinization and physiological stress (Peterson and Meador 1994; Hart et al. 2003).