Water Column Characterization Using a C3 Submersible Fluorometer

Introduction
The C3 Submersible Fluorometer's performance was evaluated by Dr. Jason G. Smith from Moss Landing Marine Laboratories, CA. Evaluation consisted of vertical profiling at Monterey Wharf II, which is located within Monterey Bay, CA (Figure 1). The main interest was testing the C3's capacity for characterization of local environments by describing vertical distributions of fluorescent properties and turbidity (side scatter).

Figure 1: Monterey Wharf II sampling station located within Monterey Bay, CA.

C3 Submersible Fluorometer Configuration
The C3 Submersible Fluorometer was configured with 3 sensors (chlorophyll, phycocyanin, and turbidity), a temperature probe, a pressure sensor for measuring depth, and a submersible battery pack used for depth profiling eliminating the need for extended cables. The C3 Submersible Fluorometer is rated for a depth of 600meters and can operate at temperatures from -2 to 50 degrees Celsius. It is able to store 480,000 data points and has a maximum sampling rate of 1 second.

Monterey Wharf II Profiling
Profiles were made in September on the 4th, 11th, 18th, and 25th in 2008. The last profile was taken on December 18th, 2008, when the water conditions were observed as being as close to blue as possible for the coastal environment. Each profile was taken between 9:30 and 10:30 am (PST), by hand, while trying to control the drop and raise rates to a few inches per second. Fixed 5 second holds were made to characterize layers at 0.25 - 0.5 meter depth intervals. Average maximum depth for this location is 8 meters and temperatures ranged from about 13 - 17 degrees Celsius in September to 12 degrees Celsius in December.

Profiling Data
The two physical water parameters measured during profiling were turbidity, recorded as NTU, and Temperature. Relative Fluorescence Units (RFU) were recorded for Chlorophyll (Chl) and Phycocyanin (PC). All data were binned per 0.5 meter intervals, averaged, and plotted vs. depth.

Turbidity, which is a measure of light scatter, primarily from suspended solids, ranged from 6-15 NTU for water column depths <7 meters indicating a relatively uniform distribution of suspended solids through most of the water column. From 7 - 8 meters depth, turbidity values ranged from 12 - 90 NTU. The largest turbidity concentrations were measured near the bottom during ebb tides and may have been due to soft sediment disruption during flood tides.

Temperature data provided information on water column stratification and changing thermoclines. Thermoclines are an indication of a stratified water column (i.e. layered water masses). Changes in temperature during profiling are used to identify thermoclines which may help researchers determine mixing rates between layers, look at settling rates, provide information related to localization of biological activity, etc. The C3 Submersible Fluorometer recorded two sharp thermoclines for the month of September, the first at 2 meters on 9/4/08 and the other at 5 meters on 9/25/08. As the month progresses there is an observed breakdown and gradual return of a thermocline at 5 meters. The C3's highly sensitive temperature probe was useful in tracking temperature changes (thermoclines) through the water column, providing evidence for the possible movement of water masses and tidal influence for this location.

Figure 2: Temperature profiles taken at Monterey Wharf II.

Relative Fluorescence Measurements for Chl and PC proved useful in tracking bloom activity and determining algal groups. PC:Chl ratios were calculated from fluorescence responses to determine if PC-containing algae were present within the algal community. Lab analysis indicated the presence of the dinoflagellate (Akashiwo sanguinae) and diatoms as dominant groups in the population.

The largest PC:Chl ratios coincided with a small dinoflagellate (A. sanguinae) bloom localized at the 2 meter thermocline on 9/4/08. This might suggest the presence of some PC-containing algae, such as cryptophytes, comingling with dinoflagellates. As the thermocline was disrupted the following week, the Chl signal from 0 - 5 meters increased. Diatoms were the dominant group during this profile and there was a noted decrease in PC:Chl ratios near the surface. The C3 recorded a very large Chl signal on 9/18/08 corresponding to a dinoflagellate bloom that must have occurred around this date. The lowest PC:Chl ratios were calculated during this bloom event. As the dinoflagellate bloom faded, the PC:Chl ratios remained low. A final profile taken in December showed little fluorescence response from both Chl and PC.

Figure 3: Profiles at MWII for chlorophyll (left) and PC:Chl ratios (right) showing progression and declination of a dinoflagellate (Akashiwo sanguinae) bloom.

Conclusion
The C3 Submersible Fluorometer was used for profiling at MWII during the month of September and December. Data retrieved from C3 profiles showed it's ability to detect changes in algal abundance, track an algal bloom, and characterize the water column. Calculating PC:Chl ratios from C3 recorded fluorescence responses throughout the water column provided information on the distribution of different algal groups within the algal community. The C3's fast sampling rate and robust memory allow users to collect detailed information to help characterize their aquatic system or local environment. Turner Designs thanks Dr. Jason G. Smith for taking the time to rigorously test the C3 Submersible Fluorometer and collect these data profiles.