Offshore: Chemical

Falmouth, 2014

Due to calm waters and fair weather, a time series was constructed from RV Callista to compare levels of nutrients, oxygen saturation and chlorophyll throughout the water column ranging in temperature and salinity. RV Callista was positioned approximately 9 miles from Falmouth where the water flow would be relatively independent of coastal riverine output. The water body here was highly stratified with a strong thermocline.

Note: all time is measured in UTC

Figure 1 shows greater oxygen saturation observed in surface waters earlier in the day, such as 08:30 UTC and 10:30 UTC which exhibited oxygen saturation between 105 and 110 %, whereas the 16:30 UTC sample was undersaturated in oxygen. This difference could be due to greater phytoplankton productivity occurring earlier in the day, as supported by Harding et al (1981).

Oxygen saturation maximum occurred around 25 m throughout the day, coinciding with the chlorophyll maxima observed on the CTD data (figure 5). After this point oxygen saturation decreased with depth at all times sampled, which could be due to heterotrophic respiration below the thermocline.

Nitrate

Most stations exhibited nitrate minima around the chlorophyll maxima, at approximately 25 m depth (figure 2), which could be due to uptake by phytoplankton. At approximately 09:00 the ADCP data demonstrated tidal-dominated shear velocity, resulting in increased internal mixing relative to surface mixing; this could explain the greater nitrate concentrations measured during the 09:30 and 16:30 sampling. The extreme surface nitrate concentration recorded at 16:30 has been omitted from the data to increase the resolution of remaining stations, as it may be considered as anomalous.

Silicon

Generally the morning stations had low surface silicon concentrations (figure 3). From 08:30 till 10:30 the concentrations measured ranged between 0 and 1 μmol/L, which could also be explained by the increased morning phytoplankton activity. Samples from 13:30 demonstrated much greater surface silicon concentrations. However the samples taken at 15:30 did not follow this trend, possibly due to sustained rainfall during this period diluting surface silicon concentration.

Phosphate

The phosphate surface concentrations measured in the afternoon were significantly greater than those in the morning, for example sampling at 15:30 gave a surface phosphate concentration of around 0.58 µmol/L, compared to the 0-0.1 μmol/L concentrations recorded in the 08:30-14:30 samples (figure 4). Again this can be explained by greater phytoplankton productivity earlier in the days (Harding et al., 1983). The phosphate minimum zone observed was visibly lower than either silicon or nitrate at around 30 m depth. However a maximum could be seen at approximately 50 m, which could be explained by remineralisation of nutrients below the thermocline (Pingree et al., 1977). This is supported by the low oxygen saturation levels recorded at similar depths; indicating heterotrophic bacterial respiration (see figure 1). However the 30 m sample taken at 10:30 contained a very high phosphate concentration, potentially due to a decrease in productivity measured at this depth as indicated by the chlorophyll depth profile (see figure 5).

Chlorophyll

Double chlorophyll maxima were observed from 08:30 until 10:30, occurring at approximately 20 and 45 m; with the 45 m sample appearing more pronounced. At 11:30 there was only a single chlorophyll maximum around 25 m depth, shown again at 13:30. However at 14:30 chlorophyll concentration appeared to be vertically homogenous.

References

Fernand L., Weton, K., Morris, T., Greenwood, N., Brown, J., Jickells, T., 2013, ‘The contribution of the deep chlorophyll maximum to primary production in a seasonally stratified sea, the North Sea’, Biogeochemistry, 113(1-3), pp. 153-166.

Harding Jr L.W., Meeson B.W.; Prezelin B. B.; Sweeny B.M., 1981, ‘Diel periodicity of photosynthesis in marine phytoplankton’, Marine Biology, 61(2-3), pp. 95-105.

Pingree, R.D., Maddock, L., Butler, E.I., 1977, ‘The influence of biological activity and physical stability in determining the chemical distributions of inorganic phosphate, silicate and nitrate’, Journal of the Marine Biological Association of the United Kingdom, 57(4), pp. 1065-1073.

Oxygen Saturation

Figure 1: Oxygen saturation depth profiles taken at hourly intervals from 08:30. Click to enlarge.

Figure 3: Silicon concentration depth profiles taken at hourly intervals from 08:30. Click to enlarge.

Figure 4: Phosphate concentration depth profiles taken at hourly intervals from 08:30. Click to enlarge.

Figure 5: Chlorophyll concentration depth profiles taken at hourly intervals from 08:30, (a) measured from a CTD-mounted fluorometer, and (b) samples with Niskin bottles from discrete depths. Click to enlarge.

Figure 2: Nitrate concentration depth profiles taken at hourly intervals from 08:30. Click to enlarge.

a

b

This page looks at:

• Background
• Oxygen Saturation
• Nitrate
• Silicon
• Phosphate
• Chlorophyll

Background