4.1 Nutrients distribution along Irish coastline
Nitrogen is generally considered the primary limiting nutrient in coastal ecosystems, meaning that the concentration of this nutrient can limit the growth of algae and aquatic plants (Kant et al. 2011). During summer, the increases in nitrogen can enhance the elevated growth of phytoplankton and/or macroalgae, leading to eutrophication processes. In winter the DIN is monitored, because the levels are expected to be at their seasonal maximum (Hogan 2019). The delivery of nutrients is often associated with the delivery of Dissolver Organic Matter (DOM), and Particulate Organic Matter (POM) which can stimulate the respiration processes (Sandberg et al. 2004; Nydahl, Panigrahi and Wikner 2013; Soares and Berggren 2019; Nakamoto et al. 2020). It is likely that in areas with high loading of nutrients during winter, and/or DOM and POM, also the DIC increases as a consequence. Unfortunately, DOM and POM data for the Irish coastline were not available. The nutrients enrichment to the coast can also promote ocean acidification (Wallace et al. 2014; Cai et al. 2011; O’Boyle et al. 2019), since organic processes which depend on nutrient availability could affect TA and DIC relation, and consequently net ecosystem production.
Most of the analyzed coastal areas in Ireland showed negative correlation between nutrients and salinity, confirming that the freshwater is the principal source. The areas with higher nitrogen inputs towards the coast are the west of Ireland (likely due to the SGD) and the south and southeast probably due to the intense agriculture and use of fertilizers (O’Boyle et al. 2016; 2019). The increase of nitrogen emission is usually due to cattle and/or nitrogen used for fertilizers. However, it is not only about the increase of agriculture but also the national population in Ireland that increased by a quarter of a million since 2013, which led to more wastewater. Even if the enrichment of nutrients is common in many estuarine waters, the eutrophication occurs in certain conditions, when there is insufficient retention time or inadequate light availability (O’Boyle et al. 2015). The major reduction of nutrients loading (mainly phosphate and ammonia) was observed in the Celtic Sea and Irish Sea (O’Boyle, Quinn et al. 2016). However, the disproportion of reducing phosphate over nitrogen creates an imbalance of the N:P ratio, especially in down streams and coastal waters (O’Boyle, Quinn et al. 2016). In the study of O’Boyle et al. (2015), phytoplankton data from EPA have been analyzed, a strong correlation between dissolved oxygen supersaturation and phytoplankton growth was found in the south west of the country. This suggested there are not external inputs of organic matter (allochthonous inputs) but instead in situ phytoplankton growth. The nutrients’ enrichment during the summer period might be enhanced by the slow vertical mixing (Savatier et al. 2021), leading to harmful algae blooms (HAB) to occur (O’Boyle et al. 2016). Contrastingly, during winter it is likely that because of the decrease of primary production (O’Boyle et al. 2015), the delivery of: nitrogen, phosphates, organic carbon, dissolved and particulate organic matter, lead to an increase of DIC in estuarine and coastal waters (Nydahl, Panigrahi and Wikner 2013; Soares and Berggren 2019), creating an imbalance to TA:DIC ratio, eventually affecting the coastal carbonate chemistry.
The nutrients’ level delivered from the freshwater was also high in the west coast. Specifically, in Galway Bay a large negative Pearson’s correlation between nitrogen and silicate with salinity values is shown. The salinity level into the area is controlled by the freshwater in the form of SGD coming from the small bays (Kinvarra and Aughinish and Bell Harbor) which dilute its water (Gregory et al. 2020). High levels of nitrogen species were also recorded in previous studies (Cave and Henry 2011; Smith and Cave 2012; McCormack et al. 2014; Rocha et al. 2015), especially during the winter period. It follows that in cooler months, when the nutrients’ consumption decreases, these can be exported from the bays fed by SGD towards Galway Bay and coastal areas. In addition to SGD, the Corrib river is known to be a source of Phosphate and Nitrogen (EPA 2013; Donnelly 2018) which mainly originates from pasture (Mockler et al. 2017).
From the Pearson correlation it was observed a negative correlation of PO43− and SiO4− with salinity in the North Channel, and a positive correlation of the ToxN with salinity in the Ards Peninsula. From the NIEA database the selected rivers in Northern Ireland showed an average of 12 µmol/l of ToxN; the highest values were between 30 and 40 µmol/l, in Bush River. From the NIEA website most of the rivers in Northern Ireland were classified between good and moderate status in terms of water quality, following WFD 2018. The upper Belfast Harbor was classified in moderate status, which means that the nitrogen loading can be further reduced to reach the good status. From the available sampling stations in Lagan River, it was observed an average value for ToxN of 48 µmol/l, with values higher than 100 µmol/l recorded in the cooler months (from October to March) probably due to the decrease of consumption from primary producers. This might explain the positive correlation found in the Ards Penninsula between ToxN and salinity. The freshwater composition of Lagan River in Belfast could affect the nitrogen level in the adjacent seas, as shown for Ards Peninsula transect.
The east coast of Ireland, in particular Dundalk Bay and Dublin Bay which are the location considered in this study, are showing intermediate trophic status (Devaney et al. 2013). Knowing that Nitrogen and Phosphate are limiting factors, they are constantly monitored. The DIN concentration has remained stable between 2007 and 2016, and decreased in the last years (Hogan 2019). Lastly, the whole Northwest Ireland did not show large delivery of: NO3−, NO2−, SiO4− and PO4−, probably because the area is not significantly affected by the pasture and the use of fertilizers. The rivers feeding the area are almost oligotrophic (AQUAFACT 2013; EPA 2018), and, considering shale/sandstone and basalt catchment geology characterize them, the TA and DIC could be probably low. However, there is no available literature on this.
4.2 TA and DIC distribution along Irish coastline
The chemical erosion of inorganic materials originated by rocks weathering leads to an increase of TA delivery to the coastal zone. The type of weathering reaction is related with the bedrock geology (Hannigan and Kelly-Quinn 2013). The chemical reactions require CO2 and release bicarbonate, such as calcite dissolution or albite hydrolysis (Suchet et al. 2003), the ocean alkalinity naturally increases because of these reactions and transport from rivers (Renforth and Henderson, 2017). Therefore, the TA values observed along the coast can be explained by looking at the watershed geology and the main freshwater source feeding the areas. Previous literature studies tried to understand the relation between carbonate coastal systems and watershed geology, like the study of McGrath et al. (2019) where areas with different watershed geology have been analyzed: Kinvarra Bay, Bantry Bay, Slaney estuary (Wexford Harbor) and Suir (Waterford Harbor). McGrath et al. 2019 showed that spatial differences in TA and DIC between these coastal systems are largely correlated to the high DIC and TA freshwater inputs to the bays in limestone areas, compared with the much lower inputs in granitic/sandstone bedrock catchments (McGrath, et al. 2016). The differences in the TA and DIC composition along the analyzed coastal areas, emphasized that the observed values are highly correlated with the type of catchment geology, but also with the freshwater composition which changes by anthropogenic factors.
The Northern Ireland coastal areas showed low TA and DIC levels compared to the open ocean. TA data from NIEA in Northern Ireland’s rivers, showed values that ranged from 60 to 1000 µmol/l, except for the Dunseverick River that showed values from 1000 to 1800 µmol/l. In Lagan River, which feeds Belfast Harbor the TA varies from 600 to 1600 µmol/l as well as in Enler River that feeds Strangford Lough. The basalts’ weathering leads to the dissolution of elements, such as silicates, calcium, magnesium, sodium, potassium and sulphates (Dessert et al. 2003). The dissolution processes are known to consume acidity, to absorb CO2 from the atmosphere (the rivers become CO2 important sinks (Dessert et al. 2003; Li et al. 2016)) and to increase the water alkalinity (Tole and Lasaga 1984; Brady 1991; Sherlock et al. 1995). Basalt weathering is also temperature dependent, therefore will change in the different periods of the year (Li et al., 2016). Looking at these data from a climate change perspective, the basalt weathering in this area might change due to the relations between the increase of runoff and temperature (Dessert et al. 2003).
On the East coast, the rivers coming from a mixed geology showed moderate TA values (Hannigan and Kelly-Quinn, 2013) e.g., Creggan River and Flurry River feeding Dundalk Bay. Even if the Dublin Bay area has a similar catchment geology to Galway Bay (west of Ireland), the TA and DIC delivery to the coast was found lower. Differently from the groundwater systems where the water can accumulate CO2 because there is no interaction with the atmosphere, the river's CO2 equilibrates with the atmosphere.
On the south coast, River Lee which feeds Cork Harbor is characterized by high DOM with reduced oxygen concentration because of the oxygen consumption of polluting organic matter. River Lee was found with less than 50% of oxygen saturation (O’Boyle et al. 2019), considering that the analyzed data represent winter time, these findings suggest that respiration enhances DIC in the system. In Waterford Harbor a large input of pCO2 comes from Suir Estuary and the river Barrow, both inserted into karstic catchment areas. Intense rainfall which might intensify due to climate change will increase flooding events and bicarbonate weathering (McGrath et al. 2019). In addition, the high TA and DIC values recorded in this area might also derive from SGD sources and not only from the rivers. Satellite imagery study showed temperature anomaly along the west and south of Ireland was observed and linked with SGD plumes (Wilson and Rocha 2012). Further studies to localized freshwater springs to quantify their TA and DIC contribution to the coast are necessary (Wilson and Rocha 2012).
For what concerns the west coast, the data focused on Galway Bay and Dingle Bay. The Dingle Bay data were collected during the oceanographic cruise in October 2017, and because no winter data were available, this location was not considered for the PCA analysis. However, a few observations can be made looking at the TA and DIC distribution from the head to the mouth of the bay. Dingle Bay is chiefly fed by river discharge and River Maine is the main source. The valleys of the Rivers Maine are underlain by a thick cover of Quaternary deposits resting on Carboniferous limestone (Pracht 1997). The ODV representation Fig. 2, shows an increase of DIC from the head to the mouth of Dingle Bay, highlighting the typology of freshwater source enriching the coast in inorganic carbon. However, no previous data were available for the carbonate system parameters in this area. It is important to note that these data were collected after the Ophelia Hurricane, which led to important floods in the area (Guisado-Pintado and Jackson 2019, 2018), as well as the influence from other rivers which might dilute the TA. Further data should be collected in this bay to better understand the drivers of carbonate chemistry and possibly seasonal variations.
The Galway Bay area is widely studied because a large part of it is inserted into a karstic catchment, carboniferous limestone. A few rivers are draining into Galway Bay in the north site characterized by granite and Ordovician igneous volcanic rocks, e.g., Corrib River. Likely, most of TA and DIC sources into Galway Bay area come from the numerous small bays feeding the area, e.g. Kinvarra Bay, Aughinish Bay, Bell Harbor which are in turn fed by water coming from SGD (Drew 2008). In this study two transects were compared, TA and DIC show linear relations with salinity in both surveys. However, TA-salinity and DIC-salinity slope is steeper in the survey of October 2017. In a few literature studies the SGD composition in Kinvarra Bay was characterized (McCormack et al. 2017; Kelly 2018), and it is also monitored from the EPA: TA and DIC values are higher than in the open ocean. TA and DIC were measured in Kinvarra Castle Spring, one of the main spring feeding the area (Schubert et al. 2015), and the average TA and DIC values were 4700 µmol/L− 1 and 4500 µmol/L− 1 respectively (Kelly 2018). The SGD was quantified in previous studies (Schubert et al., 2015; Rocha et al., 2015) and estimated using models (Gill et al. 2013). The estimated water flux in Kinvarra Bay was 8.7 m3 s− 1 by McCormack et al. (2014), and, using radon, a value of 10.4 ± 6.3 x 104 m3 d− 1 (Rocha et al., 2015) was found in summer and in winter (January 2019) when groundwater level was high, a value of approximately 20 x 105 m3 d− 1 (Savatier and Rocha 2021). Considering the freshwater TA and DIC composition in the SGD and the amount of freshwater discharge, it follows that it is likely that this source can be a potential driver of carbonate chemistry composition in Galway Bay. The TA-DIC slope in February 2016 was 0.69 ± 0.02 and in October 2017 was 0.78 ± 0.06. Just a few days before the survey conducted in October 2017, Ophelia hurricane impacted the western coast of Ireland: floods were consistent in the Galway Bay area (Guisado-Pintado and Jackson 2019, 2018). Because Galway is a karstic area, HCO3− dissolution might have increased the TA delivery to the coast, and this could explain the increase in the TA:DIC ratio in October 2017. In the areas where the salinity ranges between 33–34, it is interesting to observe higher TA and DIC values in February 2016 than October 2017. February 2016 has been identified as a month of high discharge in Ireland, the SGD measured in Bell Harbor (one of the small bays inserted in Galway Bay area) was around 3 m3/s (Schuler et al. 2018) which was an extraordinary peak for the location. Considering that SGD in Kinvarra Bay is generally 3 times greater than Bell Harbor, (McCormack et al. 2014; Schuler et al., 2018, 2020) it is likely that the SGD from the other sources was higher as well. Groundwater flooding and associated storage of water was significantly higher than October 2017 (Schuler et al. 2018). The results obtained from the comparison of the TA and DIC in two different seasons in Galway Bay, legitimate the hypothesis that SGD can change TA:DIC ratio, the same hypothesis that also arose from previous studies (Hall and Tank, 2005; Johnson and Wiegner, 2014).