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Offshore

The aim of the offshore survey was to investigate the physical properties of the water column, such as stratification and light attenuation. This study also seeks to establish the influence of the physical structure on coastal plankton communities and the distribution of nutrients.

Due to the increase of water column depth with distance offshore, the physical nature of the water column and therefore biological and chemical components can be expected to change correspondingly. To investigate this change, the offshore survey was carried out at five stations perpendicular to Falmouth Estuary (Figure 1). The study was undertaken using an Acoustic Doppler Current Profiler (ADCP), a plankton net and a CTD rosette carrying a CTD, six Niskin bottles and various sensors including a fluorometer and a transmissometer.

Methodology

The CTD was lowered through the water column to a depth close to the bottom and interesting features, such as peaks in chlorophyll, were identified for sampling. Once these sites had been decided the Niskin bottles were individually fired as the rosette was brought to the surface, collecting water samples at each depth of interest. At each station, a plankton net was lowered starting and finishing at a range of depths e.g. 50 to 20 meters to sample the zooplankton species growing at these depths. Chlorophyll, phytoplankton, dissolved oxygen, nitrate, phosphate and silicate were processed from the Niskin bottle water samples. The water samples were then stored for the laboratory session. The onboard methodology was undertaken as follows:

 Dissolved Oxygen: The sample was added to a glass bottle with manganese sulfate and alkali iodide azide. These reagents, through a chemical reaction, formed a precipitate. Replicates were taken at various points and the samples were then stored in a bucket of water.

 Phytoplankton samples: The samples were added to brown glass bottles containing Lugols Iodine.

 Zooplankton samples: The water collected using the plankton net were added to 1 litre plastic bottles. Formalin was added to preserve the samples.

 Chlorophyll: 50ml of water from the Niskin bottles was filtered and the filter paper was placed in a test tube of acetone. Three replicates were taken for each Niskin bottle. The test tubes were then stored in a dark fridge.

 Nitrate and Phosphate: The Niskin bottle sample was filtered then stored in brown glass bottles.

 Silicate: When processing the silicate sample, the Niskin bottle sample is filtered and added to a plastic bottle in an attempt to accurately preserve the sample.

The samples were then taken to the lab & were analysed using standard methodology.

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