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.