1. Introduction
The water cycle, or hydrological cycle, refers to the movement of water molecules throughout the globes geological, biological and ecological compartments. Major constituents of this cycle are water evaporating from the oceans, transportation of vapor to continental realms, re-condensation to rain droplets, and flow back via groundwater and surface flow towards the ocean. The varying physical properties, specifically the different weights of stable oxygen and hydrogen isotopes of the water molecule (16O,17O, 18O, 1H,2H), lead to isotope fractionation during all these processes (H. Craig, 1961; Dansgaard, 1964). As a consequence, the isotopic signature of water in both the liquid and the vapor phase is characterized by spatial and temporal variability, and in principal is depleted of the heavier isotopes in water vapor and continental freshwater, compared to the ocean water (Gat, Gonfiantini, & (Eds), 1981). For this reason, mixing processes in the transitional zone between the riverine/freshwater and marine/saline realm lead to linear relationships between salinity and water isotopes in those systems, as observed in multiple river estuaries around the globe (Barrie, Worden, Barrie, & Boyce, 2015; Chamberlayne, Tyler, & Gillanders, 2021; Ingram, Conrad, & Ingle, 1996; Mohan & Walther, 2015; Price, Skrzypek, Grierson, Swart, & Fourqurean, 2012; Swart & Price, 2002).
In precipitation, isotopes exhibit a seasonal signal, with lower / higher values in the cold and warm season, respectively (Bowen & Revenaugh, 2003). This seasonal partitioning of isotopes provides an opportunity to model spatial dynamics of isotopes across a landscape, riverscape, or seascape (Aron et al., 2021; Brennan, Cline, & Schindler, 2019; Orlowski, Kraft, Pferdmenges, & Breuer, 2016). Riverine and lacustrine systems reflect this signal (B. Aichner et al., 2021; Dutton, Wilkinson, Welker, Bowen, & Lohmann, 2005; Halder, Terzer, Wassenaar, Araguas-Araguas, & Aggarwal, 2015; Ogrinc, Kanduc, Stichler, & Vreca, 2008; Reckerth, Stichler, Schmidt, & Stumpp, 2017), but the seasonal amplitude is attenuated and timing of the signal delayed by 1-3 months (Bittar et al., 2016; Jasechko, Kirchner, Welker, & McDonnell, 2016; Reckerth et al., 2017; Rodgers, Soulsby, Waldron, & Tetzlaff, 2005) . The reasons for the time delay between precipitation and river/lake water isotopes, and the smaller seasonal amplitude of the latter, can be attributed to multiple catchment characteristics and processes. Crucial influencing factors on how fast a precipitation isotope signal is transferred into fluvial systems are the flow regime of rivers, and the area, topography and geology of their catchments (Maloszewski, Rauert, Trimborn, Herrmann, & Rau, 1992; McGuire et al., 2005; Rodgers et al., 2005; Sklash, Farvolden, & Fritz, 1976) .
In water bodies with high residence time of the water, such as large and/or voluminous lakes or in the marine realm, the seasonal δ18O and δ2H variability decreases. Instead, mixing processes of water from different sources with variable isotopic signatures, become more dominant on the actual isotopic signature (Benetti, Reverdin, Aloisi, & Sveinbjornsdottir, 2017; Harmon Craig & Gordon, 1965; Frew, Dennis, Heywood, Meredith, & Boswell, 2000). The North Sea, for example, is influenced by both North Atlantic water with high salinity, entering the North Sea basin from the northeast, and inflow of brackish water and freshwater, derived from the Baltic Sea and from rivers, respectively (Harwood, Dennis, Marca, Pilling, & Millner, 2008). The Baltic Sea, in turn experiences occasional inflow intrusion from the North Sea (higher δ18O and δ2H values), but also constantly receives freshwater discharge from rivers (lower δ-values). For these reasons water isotopes show a strong positive correlation with salinity in both the Baltic Sea and North Sea (Ehhalt, 1969; Frohlich, Grabczak, Rô, & bDski, 1988; Jefanova et al., 2020; Richter & Kowski, 1990; Torniainen et al., 2017) .
Isotope signatures of the ambient water are mirrored in the local fauna and flora, e.g. by plants or fish which depend on estuarine water. For example, fish incorporate elements from ambient water into their body structures, for example (ear bones) (Zanden, Soto, Bowen, & Hobson, 2016). These structures form through precipitation from the water the fish currently lives in, and as such, mirror to some extent the current isotopic profile of the water (Patterson, Smith, & Lohmann, 1993). In recent years, oxygen isotopic ratios of otoliths have been commonly used in studies of migration and geolocation of fish, by deriving predictive surfaces, i.e. isoscapes of large-scale water isotope data, in order to retrospectively predict the whereabouts of migrating fish (Torniainen et al., 2017; Trueman, Mackenzie, & Palmer, 2012), and to assign fish to discrete, geographically segregated stocks (Matta, Black, & Wilderbuer, 2010). With respect to aquatic plants or algae, their cellular lipid compounds have been shown to track the hydrogen isotopic signature of the ambient water, but with a potential additional influence of varying salinity (Bernhard Aichner, Hilt, Périllon, Gillefalk, & Sachse, 2017; Häggi, Chiessi, & Schefuß, 2015; He, Nemiah Ladd, Saunders, Mead, & Jaffé, 2020; Ladd & Sachs, 2015, 2017; Sachs & Schwab, 2011; Schouten et al., 2006). These dependencies have frequently been applied in paleoclimatic studies, i.e. for reconstruction of past hydrological conditions and salinities (e.g. (Bernhard Aichner et al., 2019; Leduc, Sachs, Kawka, & Schneider, 2011; Meer et al., 2007)).
Knowledge and understanding about water isotopic gradients has great potential to facilitate application and interpretation of oxygen and hydrogen isotopic compositions in biogenic carbonates and plant lipids. In this study we analysed water isotope dynamics in estuarine river and lagoon systems along the northern German Baltic Sea coast. Main questions of this study are: 1) how far are seasonal isotope signals in rivers transmitted into Baltic Sea estuarine systems? 2) what are the major processes behind the observed gradients? 3) are water isotopes a predictor for salinity in the riverine-marine mixing zone? The major aim is to understand the underlying control mechanisms on seasonal and spatial water isotope variability in this study area, which is essential when fish and plant isotope data are interpreted in ecological and biogeochemical studies.