Scottish Marine and Freshwater Science Volume 5 Number 1: UK Ocean Acidification Coastal Monitoring Network - Expanding the Network - Defra Contract C5801/ME5309

Results and Discussion

Feasibility Study

Nineteen samples collected in duplicate were collected in the North Channel using a McLane RAS (Remote Access Sampler) suspended below a subsurface buoy at depth two metres was deployed in the North Channel. Samples were analysed for DIC using an automated Apollo 9000 TOC analyser.

During analytical runs, which were typically 10 hours, problems were encountered with drift in instrument response. It was uncertain if the drift was caused by a faulty sensor or temperature fluctuations in the laboratory. The problem was highlighted when differences were observed when comparing pre and post calibration data. The analysis was repeated several times until a batch of data demonstrated no drift with good agreement between pre and post calibrations. Data from pre and post calibration data were combined ( Figure 4) and used to determine the DIC concentration in each sample.

DIC mean concentrations from duplicate samples taken by moored and discrete water samplers are reported in Table 1. The difference between the two data sets ( Table 1) was generally small with a maximum difference of 5.2%. However, the concentration of DIC in samples collected using the rosette Seabird sampler was higher in 16 (84%) of the 19 pairs of samples. The bias can be seen in Figure 5.

The Lins's concordance coefficient statistic (Lin ^[24] ) was used to determine the level of agreement between corresponding pairs of DIC concentrations from the two sets of samples. The concordance correlation coefficient ( ρ _c ) 0.71 (ρ _c <0.9) indicates that there were significant differences between the concentration of DIC measured in the water samples collected by the RAS sampler and those collected using the Sea-Bird sampler.

Despite repeated attempts, some questions remain over the quality of the analytical data because of the drift encountered with the instrument response. In addition, other method errors such as the possibility of air being introduced during removal and filtration of water samples from the Seabird rosette sampler and discrepancy in depth and sample timing, in relation to the moored sampler, when deploying the Seabird water sampler are likely to have contributed to the lack of agreement between DIC concentrations in samples collected by the two sampling techniques. The purpose of this study was to undertake a preliminary assessment of whether automated water samplers deployed on instrumented moorings could be used to collect samples for DIC. At the present time it is not possible to answer this question and a further, more detailed comparison should be undertaken to include a moored sampler with sample preservation, the introduction of a procedure to minimise aeration of samples during processing and filtration, a more robust analytical procedure with regular quality control and precise judgement of depth and sample timing.

Discrete Samples

Discrete Samples Collected at Stonehaven

Water samples have been collected at the MSS long term coastal monitoring site at Stonehaven ( Figure 6) for TA and DIC analysis since November 2008. Samples were collected at the surface (1 m) and just above the seabed (45 m). Water samples collected between January 2009 and August 2011 were collected and analysed, by the NOC, as part of the Defra PH project and UK Ocean Acidification project. Samples collected since Sept 2011 have been analysed, by NOC, as part of a Marine Scotland Science ROAME.

An initial assessment of the entire TA/ DIC data collected at Stonehaven (2008-2013) was made. DIC concentrations ranged from 2002 - 2134 µM/kg (mean 2092 µM/kg, n = 375). TA concentrations ranged from 2170 - 2309 µM/kg (mean of 2275 µM/kg, n = 134). As a consequence of nitrate uptake by phytoplankton cells during an algal bloom TA concentrations will increase. Therefore, it would be expected that TA concentrations will follow an annual cycle around the algal bloom ( Figure 7). A strong TA seasonal cycle was observed at Stonehaven between 2009 and 2011, inclusive, however no seasonal cycle has been observed since 2012 ( Figure 7). DIC concentrations would also be expected to follow an annual seasonal cycle, mirroring that of nitrate where concentrations decrease during the algal bloom. Stonehaven DIC concentrations increase over the winter months to a maximum in March before decreasing to minimise around July ( Figure 8). The Defra pH study reported that this maximum could potentially be attributed to calcite dissolution in the area. MSS are investigating calcifying organisms (cocolithopohres) at Stonehaven as part of a schedule of service programme, the results of which will be reported as part of the MSS ROAME. Similar to TA, where the seasonal cycle was lost at Stonehaven in 2012, the DIC seasonal decrease at the time of the algal bloom was not observed. In 2013 the DIC seasonal cycle appears to have returned, minimising in May ( Figure 8), however, analysis of samples collected since July have yet to be undertaken any assumption should be treated with caution at this stage.

The pH, calcite and aragonite saturation states were calculated using CO ₂SYS (version 2.1). At Stonehaven the CaCO ₃ saturation state (Ω) was >1 for both calcite and aragonite at both depths (1 and 45 m) and all years, indicating that the waters are supersaturated and organisms should be able to calicify. The pH has remained consistent at both sampling depths since 2009 ( Figure 9) following a seasonal cycle maximising during the summer months. The calculated pH at the site ranged from 7.8 to 8.4 mol/kg (seawater scale, mean = 8.1 mol/kg, n = 331).

SGOA and GOA-ON have both identified the need for a commitment to long-term monitoring at sites in coastal and inshore waters to distinguish long-term anthropogenic signals from short-term spatial and temporal variability. The long-term monitoring at Stonehaven highlights this, with the seasonal cycle in TA and DIC breaking down at the site during 2012. To understand the changes that occurred during 2012 it is clear there is a need for integrated monitoring, which includes measurement physical, chemical and biological parameters such as phytoplankton and temperature.

Discrete Samples Collected in the Minch North (West of Scotland)

Fifty-eight discrete water samples were collected on the MV Isle of Lewis between January and July from the Minch North and analysed for TA and DIC by NOC. Salinity, chlorophyll and nutrient analysis was undertaken by SEPA. A number of sample bottles broke during transit from the SEPA office to the NOC. In total 28 samples were analysed for DIC and 27 for TA. Salinity and nutrient corrected TA and DIC concentrations are shown in Figure 10. Where duplicate results are available, the maximum and the minimum values are displayed additionally to the mean (Figure 10). DIC concentrations ranged from 2057 µM/kg to 2194 µM/kg (Mean = 2106 µM/kg, SD = 29.3, n = 28) while TA concentrations ranged from 2169 µM/kg to 2345 µM/kg (Mean = 2289 µM/kg, SD = 31.7, n = 27). As a consequence of the limited project duration there is insufficient observations to determine the seasonal cycle. However, both DIC and TA concentrations drop at the end of May. SEPA measured the chlorophyll-a concentrations for the same period (January-July) in the Minch North and are shown in Figure 10 along with the TA and DIC concentrations. The reduction in the TA and DIC concentrations tie in with an algal bloom at the end of May to early June. A decrease in DIC concentrations during the spring bloom is to be expected. However, TA concentrations would be expected to increase around an algal bloom and it is unclear why the concentration decreased at this time.

The initial six month sampling period financed by UK- IMON does not allow for observation of seasonal trends, but informs the design of future long-term monitoring at this location and has therefore been very useful.

Discrete Samples Collected at Buoys

Discrete water samples were collected at the three sites (Celtic Deep SmartBuoy, Liverpool Bay SmartBuoy and AFBI's mooring in offshore waters of the western Irish Sea.) in March (except Celtic Deep SmartBuoy), May and August 2013 by AFBI. Samples were collected just below the surface to a depth of approximately 4 m ( AFBI mooring only). Water samples were stored at room temperature prior to analysis for TA and DIC by NOC. Mean concentrations of TA and DIC in water samples collected at the offshore sites (Celtic Deep Buoy and AFBI mooring) were 2313 µM/kg (n=15) and 2080 µM/kg (n = 15), respectively ( Figure 11).

Mean TA and DIC concentrations at the coastal site of Liverpool Bay were 2283 µM/kg (n=9) and 2076 µM/kg (n=9), respectively ( Figure 11). This is similar to the surface TA and DIC mean concentrations at the Stonehaven coastal site during the same period (March-August 2013) of 2260 µM/kg (n=18) and 2104 µM/kg (n=18), respectively.

As a consequence of the limited sampling (March, May and August) at the sites it was not possible to observe a seasonal cycle for TA or DIC. A seasonal cycle for TA and DIC in Liverpool Bay has previously been observed with TA concentrations reaching a maximum around the spring algal bloom in May and June ^[25] .