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TEMPORAL PONTOON SURVEY:

Discussion and Conclusions:

The temperature shows a maximum value at the shallowest depths of the estuary due to the heating of surface waters by the Sun.

The salinity increases as depth increases as the more saline, denser seawater of the estuary sinks below the fresh, less dense river water.

No correlation was observed in the profile of pH against depth which was expected, as a change in pH is only likely to be detected at depth with the input of a different body of water.

The concentration of dissolved oxygen decreases as depth increases due to the oxygenation of water as a result of photosynthetic processes by phytoplankton which are able to photosynthesise at a depth where there is photosynthetically active radiation (PAR). Furthermore, an increased salinity which causes the solubility of oxygen to decrease, microbial decomposition and the lack of contact with the atmosphere for the diffusion of oxygen means that dissolved oxygen concentration is lower at depth.

The flow decreased at a depth of between 3-4m due to the turbulence caused by the mixing between the two layers, and the flow was slowest at the bed of the estuary at 0851 UTC, 0926 UTC and 1007 UTC. At the bed of the estuary the flow is 0ms-1 due to friction, which therefore produces a decreased flow in the overlying water column as can be seen in the depth profile.

Light availability decreases at an exponential rate as depth increases; this is due to the attenuation of light in water. Chlorophyll-a pigments, found in phytoplankton absorb visible light (most strongly in the shorter wavelengths; blue and violet) in order to raise the energy level of an electron as part of photosynthesis. Furthermore, the increased turbidity of estuaries due to flocculation means that light is not able to penetrate further into the water column due to the absorption of light by these particles, which has negative implications for predators that rely on sight to hunt.

The chlorophyll concentration (mgL-1) peaked at the bed of the estuary due to the resuspension of benthic algae (Irigoien & Castel, 1997) caused by the anthropogenic activity in the area, including the deployment of sampling equipment, specifically the Niskin bottle; the constant movement of the ferry and the mooring of other boats to the pontoon.

Irigoien, X & Castel, J (1997). Light Limitation and Distribution of Chlorophyll Pigments in a Highly Turbid Estuary: the Gironde (SW France). Estuarine, Coastal and Shelf Science. 44. 507 - 517