Subfossil Cladocera as indicators of pH, trophic state and conductivity: Separate and combined effects in hemi boreal freshwater lakes

Identifying lake reference conditions is a crucial task for successful lake restoration. A common method is the paleolimnological approach. However, lakes can be influenced by multiple stressors over time, making it necessary to consider possible combined effects.In this study, we aimed to untangle the interpretation of subfossil Cladocera in relation to different environmental parameters in lakes of Baltic states. To determine indicator species that could be used for reconstructing past environmental conditions, we developed a 78-lake surface sediment training set. Lakes were selected to cover gradients of different lake sizes, depths, electrical conductivity (EC), pH, and trophic states. Redundancy analysis (RDA) identified pH, trophic state and EC as significant parameters influencing subfossil Cladocera species composition. Using IndVal.g analysis, we distinguished species that are indicative of a combination of parameters such as pH<6 and EC<100 µS/cm (Alonella excisa, Alona rustica, Polyphemus pediculus, Holopedium gibberum, Drepanothrix dentata), pH>6 and EC>100 µS/cm (Disparalona rostrata, Leydigia leydigi, Pleuroxus uncinatus, Pleuroxus trigonellus), pH>6 and oligotrophic/mesotrophic conditions(Monospilus dispar). We identified species that can be indicative of several parameters that do not necessarily combine (for example, Alonopsis elongata for pH<6 or oligotrophic/mesotrophic conditions or EC<200 µS/cm), or were indicative of only one parameter (for example Paralona pigra, Ophryoxus gracilis – oligotrophy; Bosmina longispina- mesotrophy; Bosmina longirostris – eutrophy; Chidorus sphaericus, Oxyurella tenuicaudis–hypereutrophy; Leptodora kindtii pH>6; Pleuroxus aduncus EC>100 µS/cm). These findings add to the understanding of subfossil Cladocera species interpretation in paleolimnological samples, enabling better assessment of anthropogenic influence and reference conditions of lake ecosystems.