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Silicon

Figure 2.2.1 illustrates the chemical data collected for dissolved silicon () throughout the water column. There is a general increase in concentration following a slight decrease between the surface waters and 39 meters. The surface water concentrations decrease due to diatoms stripping the dissolved silicon to build frustules. The gradual increase of dissolved silicon with increasing depth, shown in stations 5-7, reflects the deep nutrient rich, colder waters.

The highest concentrations of , reaching 1.11 µmol L-1 (see figure 2.2.1), were measured at station 9. This station was closest to the coastline and measurements were taken in shallower depths in comparison to stations 5, 6 and 7. The lowest concentrations of dissolved silicon were measured at station 6. This is due to the extra coastal inputs of nutrients from sources such as rivers and rocks through the estuary. Therefore concentrations of and other macronutrients will be expected to be greater at station 9 compared to station 5, 6 and 7.

Chemistry

Figure 2.2.1

Figure 2.2.2

Phosphate

High concentrations of 0.11 µmol L-1 of phosphate () are found in the coastal waters around station 9 (see figure 2.2.3). This is due to phosphate being transported into the oceans via freshwater inputs and weathering of rocks. Furthermore, lower phosphate concentrations in the deeper, nutrient rich waters of station 6, are depleted from 0.05 µmol L-1 to 0.00 µmol L-1, within the top 18 meters. However below 18 meters, due to processes such as regeneration and upwelling, concentrations begin increase from 0.00 µmol L-1 to 0.11 µmol L-1 with the increasing depth. With the exception of station 7, similar patterns are shown, illustrating the concentrations of phosphate with varying depths – depletion, regeneration and sometimes further depletion. Station 9 demonstrates this but within shallower depths. Station 7 shows no depletion of phosphate and only begins to increase with depth at 23 meters.

Chlorophyll and Oxygen

At stations 5-7 Oxygen saturation (see figure 2.2.5) is correlated with fluorescence (which indicates chlorophyll) – as chlorophyll indicates the presence of photosynthesising phytoplankton which produce oxygen. Below the peak of oxygen the saturation decreases with depth as there is more respiration than photosynthesis. The oxygen peak is a few meters higher in the water column than the chlorophyll peak, possibly because the phytoplankton are also respiring and using up a little bit of the oxygen at that depth, as well as this mixing processes may be mixing the produced oxygen up. At station 6 oxygen saturation is highest at the surface, this is likely due to diffusion of gasses into the water from the atmosphere.

The only station that does not fit the same pattern is station 9, where chlorophyll and oxygen saturation mirror each other – as chlorophyll increases oxygen decreases. Chlorophyll at station 9 increases with depth reaching a maximum very close to the bottom.

The of fluorescence peak due to higher chlorophyll occur slightly deeper at stations 7 and 8 compared to stations 5 and 6. As the stations get closer to shore the fluorescence peaks become less prominent (see figure 2.2.4).

Figure 2.2.4

Figure 2.2.5

Figure 2.2.3

Nitrate

Nitrate () is depleted by phytoplankton between depths of 18 - 28 meters (see figure 2.2.2), which is inversely proportional to the peak in chlorophyll at similar depths (see figure 2.2.4). This trend is illustrated at station 5 and 6. However, nitrate is completed depleted at station 7 until 23 meters, where it then begins to increase slightly to a concentration of 1.78 µmol L-1 at a depth of 39 meters. then decreases slightly, with increasing depth. This pattern correlates with the chlorophyll concentrations measured at station 7.

Nitrate is depleted rapidly within the top 10 meters of station 9. This could be due to nitrate being taken up by phytoplankton blooms and becoming a limiting nutrient. Whereas, higher concentrations of 10.31µmol L-1 at a depth of 10 meters, are found at station 6, in the deeper, offshore waters. This is due to the regeneration and upwelling of nutrients.