Radium isotope ratios as a tool to characterise nutrient dynamics in a variably stratified temperate fjord
Graphical abstract
Introduction
Estuaries are key components of the land-ocean interface. They regulate watershed nutrient fluxes to the sea and contribute to the atmospheric CO2 budget (Regnier et al., 2013), providing critical benefits to human society (Barbier et al., 2011; Costanza et al., 2014). Human activities have changed both the quantity and nature of terrestrial carbon and nutrient flowing into estuaries and the coastal ocean with likely consequences for global biogeochemical cycles and climate (Rabouille et al., 2001; Regnier et al., 2013; Ver et al., 1999). However, the role of estuaries as regulators of land-ocean nutrient fluxes is still poorly constrained (e.g. Borges, 2005; Mackenzie et al., 2005; Regnier and Steefel, 1999). This results mainly from the large spatial and temporal variability of the estuarine physicochemical environment.
Sediment-water fluxes and the reaction rates of elements within estuaries are significantly modified by the dynamics of stratification and flushing (e.g. Borges, 2005; Regnier and Steefel, 1999). Spatiotemporal changes of stratification in estuaries affect the transfer of solutes and nutrients between surface water, deep water and sediments. Furthermore, they impact the initiation of algal blooms, including harmful algal blooms (HABs) (Berdalet et al., 2017; Leming and Stuntz, 1984; Wyatt, 2014) and influence the makeup of phytoplankton communities in the water column (Frenette et al., 1996; Mena et al., 2019). Changes to flushing also affect chemical reactivity and solute concentrations (e.g. Andrews and Muller, 1983; Eshleman and Hemond, 1988; Boynton et al., 1995) with knock-on effects on the development of phytoplankton blooms (Tomasky-Holmes et al., 2013) or even on ecological succession, including the relative dominance of phytoplankton over macroalgae (Valiela et al., 1997).
In estuaries, timescales of water exchange with the sea can vary from a few to several hundred days, depending mainly on freshwater discharge (Alber and Sheldon, 1999). Moreover, the spatial variability of water exchange times in systems that are not well mixed, either laterally or vertically, can also vary across two orders of magnitude (Monsen et al., 2002; Webb and Marr, 2016). The complexity of real systems with regard to water renewal timescales is therefore not adequately represented by equations that assume complete mixing such as the flushing time (Geyer et al., 2000) or the tidal prism model (Dyer and Taylor, 1973). To consider the spatial variability occurring in real-world systems, two timescales are necessary: the residence time, i.e. the time any water parcel takes to leave a coastal water body through its outlet to the sea from a given location (Dronkers and Zimmerman, 1982; Monsen et al., 2002), and the water age, i.e. the time a water parcel has spent within a water body since entering it (Zimmerman, 1988). Spatially variable water ages and residence times can be estimated with hydrodynamic models (Chen, 2007; Webb and Marr, 2016), or measured using radioactive tracers (Moore et al., 2006; Tomasky-Holmes et al., 2013). Estuarine circulation models still require a parametrisation of vertical mixing processes, on which questions of accuracy pend (Geyer and Ralston, 2011), and radioactive tracer methods were originally developed for coastal areas that were sufficiently far from the coast to have only one, dominant, radioisotope source (e.g. Moore, 2000). Estimating the spatial variability of water ages is more challenging in areas with multiple potential tracer sources such as estuaries, unless a unique source of constant radioisotope activity throughout the system is assumed (e.g. Moore et al., 2006). Nevertheless, radiotracers can help constrain sediment-water solute fluxes and the spatiotemporal variability of flushing in estuaries (e.g. Moore, 2000; Moore et al., 2006; Tomasky-Holmes et al., 2013). Thus, these techniques can validate modelling forecasts of residence time or water ages where other methods require the expensive collection of high-resolution temporal and spatial data (e.g. 400 drifters and 6000 driftcards in Pawlowicz et al., 2019). Ra isotope activity ratios determine both residence time and water age across a system (Moore, 2000; Moore et al., 2006). The four isotopes of radium (223Ra, 224Ra, 226 Ra, 228Ra) have different half-lives, and the choice of isotope pairs to apply is determined by the timescale of interest (Moore, 2000).
Most importantly, radium isotope activities in a water sample integrate the short-term variability of stratification, sediment inputs, water mixing and renewal over the half-life of the isotope. For this reason, they provide time-averaged estimates of the magnitude of stratification, framing the conditions under which sediment-water fluxes are (or not) mixed throughout the water column. So far, however, very few studies have looked at the spatiotemporal variability of radium in estuaries in this way. As the natural processes leading to radium release in estuaries also lead to the release of key nutrients such as phosphorus, grasping the dynamics of radium availability and mixing in estuaries is likely to improve the understanding of the estuarine biogeochemical and ecological functions.
Here we test the hypothesis whereby radium isotopes can be used to assess the spatiotemporal variability of estuarine flushing, and subsequently, the effect of water column stratification on the fluxes from bay floor sediments and on the nutrient resource-ratio availability in a variably stratified fjord. To test this hypothesis, we use six longitudinal profiles of 224Ra/223Ra ages, nutrient concentration (N, P) and salinity, sampled at different times of the year in surface and deep waters from Killary Harbour in Western Ireland. We illustrate the combined effects of stratification, spatiotemporal changes to water ages and seasonal variability on elemental nitrogen: phosphorus (N:P) resource- ratio availability to the water column, demonstrating how this approach can provide insights into the biogeochemical functioning of estuarine ecosystems.
Section snippets
Study site - Killary Harbour
Killary Harbour is a fjord situated on the west coast of Ireland (Fig. 1). On average, from 2015 to 2018, aquaculture activity in the bay produced 1035 t of salmon (Salmo salar) and 866 t of rope mussel (Mytilus edulis) annually - 7% and 10% respectively of the national production for these species (BIM, 2018a, BIM, 2018b). The fjord is 11 km long, 700 m wide and has a mean tide average depth of 12 m, with a maximum depth of 42 m near the mouth. The mean tide volume is 1.1 × 108 m3, as
Effect of discharge on salinity structure and distribution of Ra ratios
River discharge strongly modifies the salinity structure of Killary Harbour. During the lowest discharge periods, most of the bay had salinities close to or above 30 in surface and deep waters (e.g. zones 2, 3, 4 in Fig. 2a), with a range of salinity and distribution similar to prior observations by Keegan and Mercer (1986). During high discharge periods, the majority of the bay had reduced surface salinities between 0 and 25 (zone 1, 2 and 3 in Fig. 2b). Deep water salinity generally remained
Note on SPM values in this study
SPM values used here were assessed from turbidity measurements in Killary Harbour, converted using SPM/NTU ratios observed by Jafar-Sidik et al. (2017) and validated using both a historical dataset of POC in Killary Harbour from McMahon and Patching (1984) and a winter POC/SPM relationship from Winogradow et al. (2019). SPM/NTU relationships may be site dependent. SPM and POC loads from a catchment may also vary in time as a result of rainfall and changes of land use. Mussels can also amplify
Conclusions
The measurements of 224Ra/223Ra activity ratios, salinity and nutrient concentrations along Killary Harbour show that the magnitude of stratification can be assessed using both the vertical gradients of salinity and Ra ratios. Once the different Ra sources are characterised, the effect of bay sediments as a source of solutes and a first-order estimate of the seasonality of the spatial distribution of water ages can also be determined. During well mixed periods, Ra ratios in surface waters
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Declaration of Competing Interest
None.
Acknowledgment
This publication is a result of research supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and co-funded under the European Regional Development Fund and industry partners of the Irish Centre for Research in Applied Geosciences. Contributions were also made from an Irish Research Council and Environment Protection Agency funded project. Maxime Savatier and Maria Teresa Guerra were supported through an iCRAG PhD fellowship (ref TP GW3.2PhD5
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