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Results: Biology

Results: Chemistry

Aim

The aim was to produce a continuous time series at a stationary location, measuring deep chlorophyll maximum, phytoplankton and zooplankton, whilst taking multiple CTD readings allowing us to measure variables such as temperature, salinity and turbidity in a temporal environment.

Methods  

Three main methods of survey were conducted on board the ‘Terramare’. The first was a hand deployed CTD (YSI probe). This probe measures the physical aspects of the water column such as temperature, salinity, turbidity, dissolved oxygen saturation, depth and chlorophyll A. We encountered an issue with the chlorophyll measurement, however as there was little sediment in the water column we used the SPM (suspended particulate matter) as a proxy for phytoplankton.

The final method of survey was using the bongo net. The Bongo net was attached to the crane and lifted by a winch over the side of the boat and lowered to the sampling depth. The Boat then travelled at 1 Knot for 2 minutes to collect the sample. Once collected, the net was raised out of the water, hosed down and brought back on board. The sample was collected in a sock, which was attached to the end of the net and held over a bucket until the excess water filtered out. The sample was then poured into a bottle and processed back at the laboratory.

The second method of survey was a Niskin bottle that was deployed by a mechanically operated winch off the side of the vessel. We deployed the Niskin bottle to the depth where the deep chlorophyll maximum was observed and other depths of interest.

Table 2. Location and times of 5 stations collected by MTS. Terremare on 08/07/17

Results: Physical

Figure 1. Stacked bar graph showing percentage (%) composition of phytoplankton assemblage by number of individual cells per group. Samples taken from stations offshore Falmouth on MTS Terramare at various depths. See separate map for exact coordinates of locations. Phytoplankton groups contributing to < 3.5% of the assemblage at any one station have been removed for clarity.  Any genus under 3.5% composition was excluded for clarity.

Figure 2. Stacked bar graph showing percentage (%) composition of zooplankton assemblage by number of individual cells per group. Samples taken offshore Falmouth in transects (23, 24, 25, 26, 27) on MTS Terramare at various depths. Zooplankton sampled with tow net. See separate map for exact coordinates of transects. Zooplankton groups contributing to < 2.25% of the assemblage at any one station have been removed for clarity.

Figure 11. Depth profiles (m) of temperature at time stations each with an hour interval between stations. The samples were taken from a predetermined point offshore. All stations show a prominent thermocline as surface temperature has a stepped negative correlation with depth. Another observation is the increase in surface temperature throughout the time series.

Temperature

Figure 12. Depth profiles (m) of salinity at time stations each with an hour interval between stations. The samples were taken from a predetermined point offshore. The salinity at stations 23, 24 and 27 remains relatively constant at depth with little overall variability. Station 26 has a surface salinity 0.6 lower than the other 3 time stations but after 10 metres depth return’s to a salinity of the other 3 stations. Stations 27 and 24 show outliers as the salinity spikes at 10 metres.

Salinity

Figure 13. Depth profiles (m) of oxygen saturation at time stations each with an hour interval between stations. The samples were taken from a predetermined point offshore. Stations 23 and 24 show an increase in oxygen saturation with depth up to 25 metres. At this depth the oxygen saturation peaks and rapidly decreases with depth. Stations 26 and 27 deviates from this trend as it remains relatively constant in the surface 20 metres then rapidly decreases with depth.

Dissolved Oxygen

Figure 14Depth profiles (m) of turbidity at time stations each with an hour interval between stations. The samples were taken from a predetermined point offshore. All stations remain relatively constant in the surface 20 minutes with small oscillations of turbidity observed in each station. Past 20 metres the turbidity gradually increases at all stations. Station 24 deviates from this trend as the turbidity at this station slightly decreases past 30 metres then rapidly increases up to 50 metres. All stations show a pea in turbidity between 45-50 metres, past this depth the turbidity gradually decreases.

Turbidity

Table 1. Metadata collected on MTS. Terrmare on 08/07/17

Figure 4. Depth profile (m) of silicon, phosphate and nitrate in (µmol/litre) at the first station T23 (08:38 UTC) with their own corresponding axis. Silicon decreases in concentration to 20 m which then increases to 50 m. Both phosphate and nitrate lines increase with depth then rate of increase rises 20 m on their respective axis with a positive relationship.  

Figure 5. Depth profile (m) of silicon, phosphate and nitrate concentrations (µmol/litre) at the last station T27 (14:13 UTC) with their own corresponding axis. Silicon has linear increase with depth only differing by a small amount. Both phosphate and nitrate have a constant decrease in with depth showing they are negatively correlated yet there is a greater change in the nitrate concentration.

Figure 8. Time series of silicon concentration (µmol/litre) against depth (m) with each time station between 08:38 UTC (T23) and 14:13 UTC (T27) varying by about an hour between stations. Across all stations there are different depths and number of samples taken giving a variation in profiles. All stations show an increase in concentration from the surface values as depth increase. One exception is T23 which has a trough at 20 m then further increasing to the highest silicon value at 50 m. Over time (T23-T26) rate of this increase generally decrease to only small changes. T27 shows much higher values of silicon from surface and stays consistently high down to 49 m.

Figure 7. Time series of phosphate concentration (µmol/litre) with depth (m) at different time stations from 08:38 UTC (T23) to 14:13 UTC (T27), ~one hour between station. Across all stations there are different depths and number of samples taken giving a variation in profiles. As time passes phosphate concentration varies with T23, T26 and T27 all increasing at different rates from their surface values. Whereas, the other time station shows a decrease in concentration with depth.

Phytoplankton

Zooplankton

Pseudo-nitzschia is dominant in high-nutrient conditions, and relies particularly on silicate as it is a diatom. It is exclusively marine and as such not found near estuaries. Its success can be seen through algal blooms (NOAA) that are frequent during the spring months. The conditions offshore are high in nutrients and as such allows for the success of Pseudo-nitzschia.

Rhizosolenia is also a diatom and relies on high nutrients and silicate for growth. The abundance of various diatom sub-groups depend on their adaptation to different conditions [1](Leventer and Dunbar, 1996).

Rhizosolenia is more abundant at the surface and Pseudo-nitzschia at 20-30m. This suggests that Rhizosolenia depends more on light and temperature than nutrients compared to Pseudo-nitzschia.

At 50m depth there was a great diversity of phytoplankton. This could be due to a range of factors such as light attenuation of the water, how deep the nutricline was and migration of species. Some of the more abundant groups such as Proboscia and Leptocylindrus are diatoms and thrive in high nutrients; Laboea has mixotrophic capabilities [2](Stoecker et al., 1988) and can therefore survive by heterotrophic means at depth.

Copepods thrive in a range of environmental conditions, like salinity, temperature and nutrient loading [3](Lawrence et al., 2004). There are many species of the subclass which are adapted to different conditions, which is likely why copepods are dominant at all depths offshore. Copepod success relies mostly of food availability and quality [3](Lawrence et al., 2004), something which is abundant in summer.
Copepod reproduction is hindered by diatom blooms [4](Miralto et al., 1999). As blooms usually occur in spring, reproduction (and nauplii presence) is not very affected in the middle of summer.

Although there are few marine species of Cladocera, these are good competitors, and can avoid interspecific competition by eating different size food [5](Sommer and Stibor, 2002). Summer months typically have plenty of food available, which aids the success of the Cladocera.

Discussion

T23 composite figure of silicon, phosphate and nitrate represents the ratios of the nutrients in comparison with one another. In the surface 20 metres, the increase of nitrate concentration from accumulation is greatly reduced due to uptake of photosynthetic activity. Past the deep chlorophyll maximum, the constraint of biological uptake on nitrate accumulation is lost. This allows the rate of nitrate to rapidly increase past this point. By considering the Redfield ratio of 16 N :1 Phosphate, it can be observed in the surface 20 metres that nitrate is being removed at an increased rate over phosphate. This rate will be equal to the rate predicted by the Redfield ratio.

T23 is a time station earlier in the time series than T27 therefore we should expect to see an overall depletion of nutrients in the water column throughout the time series. Notice in figure 2 the composition of nutrients in the water column has altered from the ratio observed in figure 1. Throughout the time series, the biological uptake of nutrients will have been increasing resulting in a fall of all nutrient concentrations from the concentrations recorded earlier in the time series.

Figure 6. Time series of nitrate concentration (µmol/litre) against depth (m) with each time station differing about an hour from stations starting 08:38 UTC (T23) to 14:13 UTC (T27). Across all stations there are different depths and number of samples taken giving a variation in profiles. T26 surface is an outlier in the data giving an irregularly high number. In general sites show an increase (at varying rates) in concentration except from T27 which starts with a higher surface nitrate than others giving a decrease in nitrate with water depth.

The thermocline observed in this graph is at the expected depth as this time series was sampled in midsummer (July) so the thermocline has developed but is around 10-20 metres. Past this depth the temperature rapidly decreases with depth. The second observation is explained as the later time station will have been exposed to solar energy for longer through the day and will experience the more intense solar radiation around midday. This causes the later time stations to have a higher surface temperature.

The salinity would remain relatively constant with depth as the time series was in a coastal marine dominated system. Therefore, it was unlikely that there would be any other water masses with varying salinities which would cause mixing. The salinity spike at stations 24 and 27 maybe due to anomalous readings caused by the CTD.

The trend observed at stations 23 and 24 is explained by the presence of a chlorophyll maximum which is 20 metres deep at station 23 but as the time series progresses to station 24 the chlorophyll maximum extends to 30 metres depth which may be due to increased penetration of solar energy or migration of phytoplankton. Past this maximum the dissolved oxygen decrease due to reduced or absent phytoplankton activity.

The constant turbidity observed in the surface 20 metres maybe due to reduced phytoplankton activity in these layers as its midsummer and the nutrients would be depleted nearer the surface. Past this the steady increase in turbidity maybe explained by the chlorophyll maximum observed at this depth in figure 3. The turbidity continues to increase past the depth of the supposed chlorophyll maximum observed in figure 3, this may be explained by the ‘raining’ of organic material associated with phytoplankton blooms.

In a similar process to the accumulation of silicon and phosphate, nitrate accumulates overnight facilitated by riverine inputs. Nitrate is then assimilated during photosynthetic activity which increases throughout the time series. T26 shows an outlier as the surface concentration is unlikely to increase to increase to that concentration due to fore mentioned processes.

Silicon shows an overall trend of increasing with depth, due to accumulation of silicon from riverine inputs. However, diatoms assimilate silicon within their tests during periods of photosynthetic activity. This depletes silicon in the surface photic waters mediated by the availability of sunlight. This can be observed in figure 3 as there is a low surface concentration throughout the time series due to photosynthetic activity of diatoms. At night there is little photosynthetic potential due to the poor solar intensity. This prevents uptake of silicon by diatoms allowing it to accumulate in the surface ocean. As solar irradiance increases throughout the day, the concentration of silicon in the surface ocean will decreases. Observe T23 on the graph has a higher surface concentration than the following time stations. T23 shows an original decrease of silicon concentration from the surface, with a trough at 20 metres. This is due to the presence of a deep chlorophyll maximum.

Figure 9. Dissolved oxygen saturation percentage against depth over a time series between 08:38 UTC (T23) and 14:13 UTC (T27). All sites have different number of samples and depth these were taken to illustrate the water column. Overall the percentage saturation decreases with depth at varying rates. Over time the surface saturation increases peaking at T25 where the percentage starts to fall. T23 shows a peak at 20 m depth, ~ 10% greater than surface. At T26 dissolved oxygen stays high down to ~20 m which then sharply decreases in the next 10 m of water.

References

[1] http://onlinelibrary.wiley.com/doi/10.1029/96JC00204/abstract
Leventer, Dunbar, 1996, Journal of Geophysical Research.

[2] https://link.springer.com/article/10.1007%2FBF02112135?LI=true
Stoecker et al, 1988, vol 99, 415-423.

[3] http://www.sciencedirect.com/science/article/pii/S0272771404001635     Lawrence et al 2004, Estuarine, Coastal, shelf science vol 61 547-557. Seasonality, environment, nutrients.

[4] http://www.nature.com/nature/journal/v402/n6758/abs/402173a0.html
Miralto et al 1999, Nature, vol 402, 173-176. On copepod reproduction.

[5] Sommer & Stibor, (2002), Ecological Research vol 17 issue:2 p161-174

Figure 3. Images of the top 4 species of Phytoplankton found in the collected samples on board the MTS. Terramare on 08/07/17

Nitrate

Phosphate

Silicon

The oxygen saturation at the surface is high due to the interaction of the air sea interface. Throughout the time series, the oxygen saturation decreases from the surface due to: net respiration from organics, increased distance from the air sea interface and a stratification of the water column. A peak can be observed at time station T23, this is explained by the presence of a deep chlorophyll maximum facilitating photosynthesis as a rate greater than the gross respiration.  

Chlorophyll has a low surface concentration which is to be expected during mid-summer (period of sample). Nutrients become depleted in the surface layers during the spring bloom resulting in little photosynthetic activity at the surface past this time of year. At stations 26 and 25 an increase in chlorophyll is observed with increasing depth due to a shoaling of nutricline forming a deep chlorophyll maximum.

Dissolved Oxygen

Chlorophyll

Figure 10.Time series of chlorophyll concentration (µg/L) with depth at each time station (~ hour apart) between 08:38 UTC (T23) and 14:13 UTC (T27). At each station there are a different number of samples taken at different depths to give a full profile. All stations show similar values at 2 m and have a positive correlation yet the rate and range in values vary over the time series. Highest value in chlorophyll was at 30 m at T25. Lower peak around this depth was also seen in T26 which also had another high value just below 20 m.

Surface phosphate concentrations are relatively low compared to silicate and nitrate levels. Observe in figure 4 that all stations show an increasing phosphate concentration with depth due to accumulation and reduced uptake compared to surface depths. Small variation of phosphate concentration is observed throughout the time series. However, there is a noticeable trend of decreasing phosphate concentration past 20 metres depth for T25-T27. This would be explained by the presence of a deep chlorophyll past this depth which remove phosphate from the water column past this. The deep chlorophyll maximum would develop later in the day explaining why T23 and T24 do not follow this trend.

OFFSHORE