Introduction

Home Offshore Time Series Geophysical Estuary

University of Southampton Falmouth Field Course 2013 26th June - 6th July 2013 Group 9

26th June 2013 - Offshore Sampling
Falmouth Tides (UTC): HW 07:20 5.1m
LW 13:50 0.3m
Cloud cover: 100% - 0%
Sea State: Flat - Slight
Air Temperature: 14.9°C

Introduction

Tidal fronts are determined by water depth and tidal currents. A front exists between tidally mixed waters (well mixed and close to shore) and stratified waters (water columns with increased depth).The position of the front is dependent on the tidal state and season. For example during spring tides, fronts move further offshore due to stronger turbulent mixing, enabling mixing to greater depths (Brown et al. 1999). The group chose to sample and collect data along a transect crossing the front in the shelf seas of Falmouth, in order to determine how vertical mixing processes effect the structure and functional properties of plankton and zooplankton communities both directly and indirectly.

Four stations were chosen between Blackrock and ‘Station P2’, where it was known the front existed. ‘Station P2’ was chosen as the first station due to it having been sampled by University of Southampton Masters students in the previous weeks. This gave the option of using time series data at this particular location. Other stations over the front were sampled on the ebb tide to prevent sampling of the same body of water. On route to station 1, the on board CTD was monitored to locate the front. This information was used to determine the other sites chosen for sampling. The aim was to sample ‘Station P2’, the stratified side of the front, the front and the well mixed side of the front. Due to time restrictions water samples were not collected at stations 3 and 4; therefore to create a complete transect we collaborated data with Group 11. On route to station one group 9 (G9), the front was located at 50°07.326N, 004°57.750W.

Station 1
Location: 50°05.763N, 004°52.027W
Time: 08:23 09:59 UTC
Comments: CTD profile taken from sea surface to 67.7m depth, water samples collected at 41m, 27m, 15m and 1.2m . Zooplankton mesh net deployed and closed between depths of 40-30m, 30-16m, and 16m-surface. With minimal wind effects, the ship drifted approximately one nautical mile North East during sampling. It was assumed that the RV Callista moved with the sampled water body. Cloud cover8/8

Station 2
Location: 50°07.990N, 004°54.800W
Time: 10:33 - 11:00 UTC
Comments: Station appeared on the cusp of the front as chlorophyll increased and temperature decreased. CTD profile taken from sea surface to 50m, water samples collected at 32.5m, 16.4m and 5.8m. Zooplankton mesh net deployed and closed between 20-10m covering the chlorophyll maximum. Nsikin bottle misfired at depth 32.5m, sample used in lab analysis to help compare results Cloud cover greatly reduced to 0/8.

Station 3
Location: 50°08.507N, 004°58.185W
Time: 11:30 - 11:40 UTC
Comments: CTD rosette deployed to ensure the front had not been missed, and RV Callista was still on route to pass over the front. CTD deployed to 23m. Maximum depth was 24m. No zooplankton measurements or water samples taken. Cloud cover 0/8.

Station 4
Location: 50°08.037N, 005°00.752W
Time: 11:55 - 11:59
Comments: The front was not located. It was suggested that pockets of eddies, that had broken up the front, were detected. CTD deployed to 11.7 m. Maximum depth was 13.1m. No zooplankton measurements or water samples were taken. Cloud cover 0/8

Station 1 (G11)
Location: 50°08.691N, 005°001.472W
Time: 14:18
Comments: Water samples collected at 21.8m, 14.1m, 15m and 1.3m . Zooplankton mesh net deployed and closed between depths of 25-18m, and 18m-10m. Maximum depth was 30.9m