Spatial and temporal dynamics of water isotopes in the riverine‐marine mixing zone along the German Baltic Sea coast

River estuaries are characterized by mixing processes between freshwater discharge and marine water masses. Since the first are depleted in heavier stable isotopes compared with the marine realm, estuaries often show a linear correlation between salinity and water stable isotopes (δ18O and δ2H values). In this study, we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects, located at the northern German Baltic Sea coast. The data show strong seasonality of isotope values, even at locations located furthest from the river mouths. They further reveal a positive and linear salinity‐isotope correlation in spring, but ‐in two of the three studied transects‐ hyperbolic and partially reverse correlations in summers. We conclude that additional hydrological processes partially overprint the two‐phase mixing correlation during summers: aside from the isotope seasonality of the riverine inflows, the shallow inner lagoons in the studied estuaries are influenced by evaporation processes. In contrast the estuarine outflow regions are under impact of significant salinity and isotope fluctuations of the Baltic Sea. Deciphering those processes is crucial for the understanding of water isotope and salinity dynamics. This is also of relevance in context of ecological studies, for example, when interpreting oxygen and hydrogen isotope data in aquatic organisms that depend on ambient estuarine waters.Spatial and seasonal water isotope dynamics were evaluated along three estuarine lagoon transects at the German Baltic Sea coast. Data reveal a positive and linear salinity‐isotope correlation in spring, but partially reverse correlations in summers. The results show that evaporation processes in the shallow inner lagoons and varying Baltic Sea influence in the outer estuary regions are able to overprint the two‐phase mixing correlation between riverine and marine water masses.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659https://doi.org/10.1594/PANGAEA.93799

Spatial and temporal dynamics of water isotopes in the riverine‐marine mixing zone along the German Baltic Sea coast

Abstract River estuaries are characterized by mixing processes between freshwater discharge and marine water masses. Since the first are depleted in heavier stable isotopes compared with the marine realm, estuaries often show a linear correlation between salinity and water stable isotopes (δ18O and δ2H values). In this study, we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects, located at the northern German Baltic Sea coast. The data show strong seasonality of isotope values, even at locations located furthest from the river mouths. They further reveal a positive and linear salinity‐isotope correlation in spring, but ‐in two of the three studied transects‐ hyperbolic and partially reverse correlations in summers. We conclude that additional hydrological processes partially overprint the two‐phase mixing correlation during summers: aside from the isotope seasonality of the riverine inflows, the shallow inner lagoons in the studied estuaries are influenced by evaporation processes. In contrast the estuarine outflow regions are under impact of significant salinity and isotope fluctuations of the Baltic Sea. Deciphering those processes is crucial for the understanding of water isotope and salinity dynamics. This is also of relevance in context of ecological studies, for example, when interpreting oxygen and hydrogen isotope data in aquatic organisms that depend on ambient estuarine waters

Corroborating otolith age using oxygen isotopes and comparing outcomes to scale age: Consequences for estimation of growth and reference points in northern pike (Esox lucius)

Accurate age estimates are crucial for assessing the life-histories of fish and providing management advice, but validation studies are rare for many species. We corroborated age estimates with annual cycles of oxygen isotopes (δ 18O) in otoliths of 86 northern pike (Esox lucius) from the southern Baltic Sea, compared results with visual age estimates from scales and otoliths, and assessed bias introduced by different age-estimation structures on von Bertalanffy growth models and age-structured population models. Age estimates from otoliths were accurate, while age estimates from scales significantly underestimated the age of pike older than 6 years compared to the corroborated reference age. Asymptotic length ((Formula presented.)) was larger, and the growth coefficient (Formula presented.) was lower for scale ages than for corroborated age and otolith age estimates. Consequentially, scale-informed population models overestimated maximum sustainable yield ((Formula presented.)), biomass at (Formula presented.) ((Formula presented.)), relative frequency of trophy fish ((Formula presented.)), and optimal minimum length limit but underestimated fishing mortality at (Formula presented.) ((Formula presented.)). Using scale-based ages to inform management regulations for pike may therefore result in conservative management and lost yield. The overestimated asymptotic length may instill unrealistic expectations of trophy potential in recreational anglers targeting large pike, while the overestimation in MSY would cause unrealistic expectations of yield potential in commercial fishers.</p

Corroborating age with oxygen isotope profiles in otoliths: consequences for estimation of growth, productivity and management reference points in northern pike (<i>Esox lucius</i>) in the southern Baltic Sea

Accurate and precise age estimates are crucial for assessing the life-history of fish and providing management advice for fisheries, but age validation studies remain rare or absent in many species. Aging from scales is common, as it is non-lethal, but potential for underaging old fish exists. Using 85 northern pike (Esox lucius) collected from the southern Baltic Sea in Germany as a model, we corroborated age readings based on annual cycles of oxygen isotopes (δ18O) in otoliths to infer the timing and validity of growth, so as to compare results with visual age estimations from scales and otoliths. Otoliths were accurate and precise, while age readings from scales systematically underestimated the age of old pike. Fitting population-level von Bertalanffy growth models to the size-at-age data estimated via δ18O-profiling, otoliths or scales revealed a larger terminal length (L∞) and a lower body growth coefficient k in scale-aged fish compared to otolith and corroborated age data. Populating an age-structured model with structure-specific growth model parameters demonstrated that the maximum sustainable yield (MSY) was estimated to be about 37% lower using scale-informed growth models relative to growth models fitted to corroborated and otolith-based size-at-age data. Thus, pike populations assessed and modeled based on scale age readings might appear less productive than they really are. Using scale-based ages to inform management regulations may therefore result in too conservative management and lost biomass yield, while instilling unrealistic angler expectations as to the trophy potential of the fishery

Water isotope values from estuarine systems along the German Baltic Sea coast

Stable isotopes (δ²H and δ¹⁸O) were analyzed in water samples collected at the German Baltic Sea Coast. Transect samples were taken in June 2019, March 2020, and July 2021 at 68 spots along the Schlei, the Zingster Bodden chain, and the Rügener and Greifswalder Boddens. Additionally, at selected spots time series were sampled in biweekly to monthly intervals between March 2020 and March 2021. At shores, water was sampled with a pipette from 0.3 to 0.6 m below water surface and directly transferred into a measurement vial. In deeper parts of the boddens a Limnos sampler was used to obtain water from 0.5 to 1 m below surface. Isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer. The measurement uncertainty was quantified by error propagation, including the parameters a) uncertainties of lab standards; b) errors of standard calibration; c) average standard deviation of replicate measurements. Based on this, measurement uncertainty was estimated to account 0.16 ‰ for δ¹⁸O and 0.57 ‰ for δ²H. Water chemical parameters were measured with WTW measurement devices. The data give information about a) the seasonal isotope amplitude at the sampled locations; b) spatial variability along the transects, and c) the correlation between isotopes and water chemical parameters

Water isotope time series (March 2020 - March 2021) of samples collected at A. P. Møller Skolen, Kleine Breite, Schlei

Time series of stable isotopes (δ2H and δ18O) were analyzed in water samples collected near the A. P. Møller Skolen, Schleswig (Kleine Breite, Schlei), in biweekly to monthly intervals between March 2020 and March 2021. Water was sampled with a pipette from ca. 0.5 m below water surface and directly transferred into a measurement vial. Isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer. Water chemical parameters were measured in-situ with a modular WTW 3440 multiparameter devices and regularly calibrated conductivity cells (MPP 930 IDS, TetraCon® 925-P). The data give information about the seasonal isotope amplitude at the sampled locations and about spatial variability along the transects

Water isotope time series (March 2020 - March 2021) of samples collected in Kloster, Hiddensee

Time series of stable isotopes (δ2H and δ18O) were analyzed in water samples taken near the harbour of Kloster (Hiddensee), in biweekly to monthly intervals between March 2020 and March 2021. Water was sampled with a pipette from ca. 0.5 m below water surface and directly transferred into a measurement vial. Isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer. Water chemical parameters were measured in-situ with WTW multiparameter measurement devices. The data give information about the seasonal isotope amplitude at the sampled locations and about spatial variability along the transects

Water isotope values in samples collected June 2019, March 2020 and July 2020 along estuarine systems at the Baltic Sea coast

Stable isotopes (δ2H and δ18O) were analyzed in water samples collected at 68 spots along the Schlei, the Zingster Bodden chain, and the Rügener and Greifswalder Boddens in June 2019, March 2020, and July 2021. Transect samples were taken at shores from 0.3 to 0.6 m below water surface, using a pipette, and directly transferred into measurement vials. Isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer. Water chemical parameters were measured in-situ with a WTW multiparameter device (Multi 3630 IDS), equipped with a WTW TetraCon 925 electrical conductivity measuring cell. The data give information about the spatial isotope variability along the transects

Water isotope time series (March 2020 - March 2021) of samples collected along the Zingster Bodden chain

Time series of stable isotopes (δ2H and δ18O) were analyzed in water samples collected at the Zingster Bodden chain in biweekly to monthly intervals between March 2020 and March 2021. Herefore, a Limnos sampler was used to obtain water from 0.5 to 1 m below surface. In the laboratory, a WTW 1970i conductivity meter and TetraCon 325 measuring cell were used to analyse electrical conductivity. Sub-samples were transferred into measurement vials before isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer. Water chemical parameters were measured with regularly calibrated WTW multiparameter devices. The data give information about the seasonal isotope amplitude at the sampled locations and about spatial variability along the transects

Length-based assessment of an exploited coastal pike (Esox lucius) stock (Rügen, southern Baltic Sea) underscores the crucial relevance of growth and natural mortality for assessment outcomes

Many capture fisheries are considered data-poor, including the northern pike (Esox lucius) fishery in brackish lagoons in the southern Baltic Sea of Germany. The objective of our work was to assess the exploitation status of this stock, which is perceived by stakeholders to be in decline. Size structure data collected via rod-and-reelangling, fyke nets, and gill nets, and empirical estimates of growth, maturation, and fecundity from the lagoon stock were used to fit the Length-Based Spawning Potential Ratio (LB-SPR) model. Parameter uncertainty in von Bertalanffy growth estimates and natural mortality in the Baltic Sea pike stock was considered in sensitivity analyses. Assessment outcomes were sensitive to estimates of growth rate, particularly asymptotic length L infinity, instantaneous natural mortality M, and gear selectivity. Under-aging of old fish in scale-based age estimates overestimated terminal length and generated negative bias in the estimated stock status. Despite the sensitivity of assessment outcomes to life-history parameter choice, the stock status for the Baltic Sea consistently indicated a fully exploited situation with SPRs robustly above 0.4 and current fishing mortality rates between 0.2 and 0.4 yr-1. This result agreed with previous assessments using catch-only models. Our work serves as a reminder, that when using length-based methods, unbiased growth, and natural mortality estimates are critical for robust assessment outcomes