DISCLAIMER

All views and opinions expressed on this page are those of the individual and not necessarily those of the University of Southampton.

Plymouth Field Course 2019

Group 7

Dissolved Oxygen

General trends from the data show that there is a higher oxygen concentration at the surface, with it decreasing with depth. An issue with the analysis is that oxygen was only measured from the Niskin bottles, sampled at specific depths for purposes other than looking at the percentage of dissolved O2. This means that there is a maximum of three data points for each sample site, with the majority only having two. This makes it difficult to draw meaningful conclusions backed up with evidence.

However, as previously mentioned, all sites show the expected trend of having greater concentrations of dissolved oxygen closer to the surface. This is because more photosynthesis occurs in this layer, thus adding more O2 to the waters.

Equilibration with the atmosphere is also a reason for greater levels of dissolved oxygen at the surface than at depth. Remineralisation of detritus and other particulate matter occurs further down the water column, resulting in a further reduction in the oxygen concentration.

The behaviour of oxygen in the water column cannot be fully deduced due to the lack of data, but vague trends can be recorded and they fit with predictions.

Nutrients

The concentration of all the nutrients measured (phosphate, silicon, nitrite and nitrate) all increase with depth, reflecting the uptake in the surface waters by phytoplankton and the remineralisation of particulate organic matter below the thermocline leading to an increase in the concentrations in deeper waters (Burkhardt et al (2014)). Stations further from the tidal mixing front generally exhibited higher concentrations in deep waters below the thermocline. For example, station 23 (the furthest offshore station) has the highest nutrient concentrations of all the stations at depth. Whilst stations closer to the shore were not sampled at the increased depth, we would expect these stations to go through similar increases as the offshore stations such as station 23, but perhaps less pronounced as the thermocline at these stations is less developed and some mixing may occur.

The increase in Silicate concentration in stratified Stations 22 and 23 was particularly pronounced (figure 2d). This may be due to the high abundances of siliceous phytoplankton such as Chaetoceros that utilise silicon in the surface waters to form the frustules (Chen et al., 2014).  Station 23 has 524 Chaetoceros cells/ml whilst station 18 furthest inshore has 0 of these diatoms and only 5 cells/ml of other diatoms.

Nitrite exhibits the same trends as the other nutrients with an increase in concentration with depth, though the maximum concentrations observed at depth are an order of magnitude lower than nitrate. More strongly stratified stations exhibit highest nitrite concentrations at depth due to increased accumulation of nutrients just below the thermocline, and through the limitation of photoautotrophy at these depths allowing nitrification rates to peak at these depths (Zakem et al. (2018)).

Figure 1 - Bottle data of dissolved oxygen vs depth for each station

Figure 2a showing phosphate depth profiles for stations 18, 19, 20, 22 and 23 collected in Niskin bottles off the coast of Plymouth in the Western English Channel

Figure 2b showing nitrate depth profiles for stations 18, 19, 20, 22 and 23 collected in Niskin bottles off the coast of Plymouth in the Western English Channel

Figure 2c showing nitrite depth profiles for stations 18, 19, 20, 22 and 23 collected in Niskin bottles off the coast of Plymouth in the Western English Channel

Figure 2d showing silicate depth profiles for stations 18, 19, 20, 22 and 23 collected in Niskin bottles off the coast of Plymouth in the Western English Channel

Figure 3 showing chlorophyll depth profiles for stations 18, 19, 20, 22 and 23 collected in Niskin bottles off the coast of Plymouth in the Western English Channel

Chlorophyll

Methods

Three filters were prepared from each Niskin bottle at each station by using a pump to vacuum 50ml of sample water through. The filters were then preserved in acetone until lab analysis with the fluorometer.

Findings

The chlorophyll profile from the bottles shows a deep chlorophyll maximum (DCM) at Stations 22 and 23 (Figure 3). These appear at sample depths of 24.1m and 26.2m respectively. Station 18 and 19 both had only two Niskin bottles taken, so it cannot be speculated whether a chlorophyll maximum may have existed deeper. Station 20 has been sampled at 2.8m, 19.2m and 44.3m and appears to only slightly increase with depth rather than increase then decrease again as in Stations 22 and 23. However, this may be because the DCM could exist at a depth between 19.2m and 44.3m that hadn’t been sampled rather than not exist at all. In order to verify this, further bottles could have been taken. However, looking at the fluorescence profile taken by the CTD (click here to view offshore physics; figure 10), there doesn’t appear to be a significant peak at depth; there appears to be a gradual increase between 0m and 20m then a high fluorescence with a lot of noise but fairly constant until a depth of 60m. This suggests there may not have been a deep chlorophyll maximum but just a minimum at the surface where the phytoplankton are grazed by zooplankton.