GROUP
Plymouth Field Course 2019
4
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OFF-SHORE
Nutrient Depth Profiles
C25:
At C25, there was an increase in nutrient and chlorophyll concentrations with depth between 5m to 18m. Beyond this point, however, chlorophyll concentrations began to decrease from ~1.3µg/L down to ~0.7µg/L, whilst phosphate increased at a lower rate. Nitrate increased at a greater rate, so this depth was likely to be the nutricline. The concentration of chlorophyll started to decrease beyond this point as the availability of light is too low for photosynthesis, so only larger plankton species capable of vertical migration are located below the nutricline. Nitrate continued to increase at a greater rate beyond this point due to the presence of urea and potentially the decomposition of plankton, which remineralised the nitrate.
C26:
At C26, the nutrient and chlorophyll concentrations increased with depth until ~16m. Here, the nutrient concentrations started to decrease until ~24m while chlorophyll concentration increased very slightly at the 16m mark and began decreasing at the 24m mark. At this point, the nutrient concentrations began to increase once again. This likely describes the presence of a double thermocline, where the nutrient concentrations would have changed rapidly beyond 16m and 24m. The double thermocline was likely to have occurred either from a period of rapid heating from solar radiation at the surface, or long term tidal mixing causing the variation along the water column to occur.
C27:
At Station C27, a deep chlorophyll maximum (DCM) was observed at 23m using the CTD profile, explaining the increase in chlorophyll at that depth. Correspondingly, nitrate, phosphate and silicate concentrations were all appreciably lower than at all other stations, with nitrate being depleted in the surface waters. However, a slight increase in nitrate and silicate concentration was actually observed towards the DCM, perhaps because of remineralisation of sinking particulate organic matter. Unfortunately, the deepest bottle sample for Station C27 was contaminated and so nutrient concentrations weren’t recorded, meaning any further patterns with depth cannot be observed from these data.
At Station C28, a large increase in nitrate concentration was observed, particularly at greater depths. For this reason, nitrate concentration was moved to the lower X axis, as it was greater than chlorophyll and silicate concentration at 65m. Chlorophyll showed a lesser deep chlorophyll maximum at 25m than observed at Station C27. Concentrations of all 3 nutrients recorded noticeable increases below the thermocline at 29m, but only nitrate increased further again by a comparably large amount with depth. However, it is possible that this extremely high value for nitrate concentration (4.57 μmol l-1) was an outlier produced by sampling errors as it is considerably outside the expected value (0.597 +/- 1.215 μmol l-1).
C28:
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