Falmouth is located on the south coast of Cornwall, England. It is situated next to the Fal estuary, and boasts the third deepest natural harbour in the world. The Fal estuary is a ria, or drowned river valley which flooded in the last ice age. The river Fal is split into six main tributaries and 28 minor creek. At the entrance of the Fal estuary is Carrick Roads, a tidal basin with a deep channel. The Fal estuary is a designated special area of conservation (SAC) due to the presence of live maerl beds and various eel grass species (Zostera sp.). The Fal is a macrotidal estuary with a spring tidal range of 5.3m 1.

We spent two weeks in Falmouth, from 25th June to 5th July, collecting a variety of data at a number of sites around the estuary. These included a survey of the estuary itself, an offshore time-series at the mouth of the estuary, a geophysical survey and another time series recorded from the King Harry pontoon. From these, a general picture of the physics, chemistry and biology of the estuary has been constructed and represented here graphically - all of which can be clicked on to display full size. This website is best viewed in Mozilla Firefox or Google Chrome.

Summary of findings

The homogeneity of water properties and high salinities throughout the estuary reflected the overall dominance of sea water over the area surveyed. The temperature and salinity profiles over the fortnight suggest the estuary is, in general, very well mixed – the salinity did not show variation of more than 1 nor the temperature more than 2°C from the surface to the sea bed. The Richardsons numbers calculated for the ‘offshore’ survey mostly stood below 0.25, suggesting turbulent flow. These indicators of a lack of stratification show the Fal estuary to be a well-mixed one, further supported by tidal ranges of above 4m supporting its macrotidal classification. These effects may have been exacerbated by the spring tide period over which the surveys were carried out.

Nutrients were analysed in a time series offshore and on the pontoon. Although nutrients all appear to vary greatly over time and space, all show a clear peak at mid tide. This was particularly evident in the ‘offshore’ time series with chlorophyll, nitrate, phosphate and silicon all peaking at 3 hours before high water, the same time in the tidal cycle that the estuarine survey nutrients peak. Similarly, the pontoon data shows a trend of chlorophyll peaking 3 hours after high water. This coincides with the maximum water velocity recorded at mid tide in the offshore as well as the estuarine survey.

The large variations in chemical data are due primarily to restricted time and bottle availability. In addition, low levels of nitrate in the ‘offshore’ survey produced a lack of useable data and probable high scavenging of phosphate in the estuary have left this, also, at low levels2.

On the whole, dissolved oxygen in the estuary was high – oxygen values never fell below 100%. However, at the offshore station the values were lower and did not exceed 95% over the half tidal cycle. This suggests that more saline waters have a higher productivity and hence the oxygen is more rapidly depleted than in the less saline waters further upstream.

Within the estuary, SAC restrictions limited biological sampling methods to phytoplankton bottles and zooplankton nets. Unfortunately, the data gathered from these did not appear to follow any distinct patterns although chlorophyll data strongly indicates higher biological activity at lower salinities, or where the water is more influenced by river water, as would be expected. A more in-depth analysis of the biology around Falmouth was provided by the habitat mapping, which was carried out further away from the estuary where conservation restrictions did not limit sampling method.

The information of this website is personal interpretation and does not necessarily affect the views of the University of Southampton or the National Oceanography Centre.

Home Offshore Estuary Habitat Mapping Pontoon

All of our group’s photos, throughout the field course, can be seen on our Group 9 Instagram!