Offshore

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Offshore

Nutrients Plankton Water Column DAPI

Introduction

As the estuary of Plymouth sound transitions into the coastal zone and eventually open shelf sea, shear and mixing decrease while in the spring and summer heat input from the sun increases. This should lead to stratification of the water column forming and a phytoplankton bloom with a chlorophyll maximum forming around the thermocline, where there is enough light and nutrients for phytoplankton growth. This stratification should be more prominent offshore due to the decreased mixing and is the subject of this investigation.

Aims

The aim of the offshore survey was to observe any stratification in the water column at L4 and sample changes in the phytoplankton and zooplankton population in response to this. Then we are able to compare this with a simultaneous survey being taken at E1 further out to sea where stratification should be increased in order to see the change of the water columns with time and as the tide moved out moving the water from L4 towards E1.

Methodology

The Falcon Spirit was used to get to the L4 sample site where CTD profiles were taken every 30 minutes for 5 and a half hours to a maximum depth of 50m and minimum of 45m, along with NISKIN bottle samples every hour which targeted the bottom, surface and any features of note such as a thermocline or a chlorophyll maximum. Additionally, an ADCP was running constantly providing a picture of flow speeds and aiding visualisation of the water column. A zooplankton net was used at the surface allowing the zooplankton population to be sampled and any changes in this over time quantified. The CTD data was plotted into graphs allowing the water columns temperature (°C), salinity, fluorescence, turbidity and density to be regular measured in situ. The water samples from the NISKIN bottles were filtered if needed and split into bottles for measuring oxygen, chlorophyll and nutrients.

We made sure there were no bubbles in the 100mL Oxygen samples and then they were transferred into a bottle with 1mL of Manganese Chloride and 1mL of alkaline Iodide and incubated. For the chlorophyll samples, 50mL of water was filtered through a vacuum filtration. Additionally, in a lab the next day the nutrients and zooplankton were analysed showing the distribution of zooplankton and nutrients through-out the water column and how they changed over time.