Derwent Estuary water sampling (KSA224 assessment 2025)

Brief description

The aim of this trip was to investigate the salt wedge of the Derwent estuary, looking at the different properties along and across the estuary. This was achieved by examining; temperature, salinity, chlorophyll-a concentration, nutrients (including nitrate, ammonia, phosphate and silicate), pressure, turbidity, fluorescence, dissolved oxygen and pH. Sampling took place in the morning, in sunny, relatively calm conditions with the low tide on the sampling day at approximately 11:04 am. Two transects were used, with three different locations tested along the estuary and four tested across, at the approximate locations: (-42.94014, 147.38385), (-42.96321, 147.57525), (-42.98388, 147.37083), (-42.9795, 147.336), (-42.9744, 147.3585), (-42.97594, 147.36394) and (-42.97616, 147.38914). Sampling consisted of CTD sensor measurements, a plankton net tow, Secchi disk measurements, and a collection of water samples for analysis.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: CTD Rosette A CTD rosette was deployed at each location to create a depth profile. Water was also collected at three different depths. At each location a log sheet with time, bottom depth, depth of firing, date and location was completed. At each location the CTD was programmed via the event log on a laptop to fire on the upcast and at specific depths based off input pressures. These pressures corresponded to three depths: one approximately 2m from the bottom, one at the middle and one at 5m. The CTD was turned on prior to deployment to start recording data. After being deployed it was held stationary at 2m depth for calibration before continuing with the descent. Once reaching the bottom it was brought up and fired at the three pre-ordered pressures. After returning to the surface the CTD was turned off and samples of each depth were placed into previously labelled sample bottles for further analysis. Oxygen (125mL BOD bottle), pH (100mL container), nutrients (2 x 10mL tubes) and chlorophyll (500 mL amber bottle) were taken in this order. All sample bottles were rinsed three times with the sample water before the final sample was added. Water Measuring The water samples from the CTD at each location and depth were measured for oxygen and pH. The oxygen and pH probes were prepared by rinsing with deionised water. The oxygen was recorded straight after receiving the sample, with measurements taken after the probe stabilised. Two replicates for each sample were taken. The pH was recorded afterwards, with three replicates for each sample being taken. All measurements were recorded on a pre-prepared log sheet. Filtration and chlorophyll samples A filter rig was prepared with filters for three samples. 500mL chlorophyll samples from the CTD Niskin bottles were filtered. The filter was then placed into previously labelled extraction vials using forceps. These vials were immediately wrapped in aluminium foil and placed on ice for analysis later. This was repeated for each location’s new chlorophyll samples. Later the chlorophyll was extracted from the filters through adding 90% acetone and leaving it for 24 hours. The sample was then analysed by a fluorometer. Secchi disk The Secchi disk was lowered off the sunny side of the boat with two designated observers watching. Once the disk could no longer be seen by either observer it was slowly pulled up. The distance of the rope, from the ocean’s surface to the Secchi disk, when each observer could see the disk again was recorded. The measurements along with time, observer and conditions were recorded on a log sheet. This was then repeated at each location. Plankton tow At each location the flow meter reading was recorded prior to deploying the plankton net off the back of the boat. It was then towed for a couple of minutes before being bought back in where the flow meter reading was recorded again. The net was rinsed down and the displaced volume was recorded (volume of whole sample – volume of water only) after filtering the sample. The sample was homogenised, and one drop was placed on a counting cell where it was observed under a microscope with notes taken if Gymmnodinium catenatum was found. All measurements and mesh size of plankton net were recorded on a log sheet. Nutrient Analysis Nutrients were analysed by the lab at CSIRO Hobart. This included using a AA3 segmented flow analyser. Prior to running the samples through the machine, the analyser was calibrated using six standards at the start of the run before running a quality control. Methods were synchronous to Go-Ship Hydro Manual recommendations for nutrient analysis. Note: Nitrite samples were corrected using CSRIO Hobart's standard programming for a baseline shift in the instrument, some results had a shift in baseline which exceeded the expanded uncertainty, so these should not be used in publications, this was shown by data quality flags. Data Analysis Data quality flags were created for the CTD data, chlorophyll data and nutrient data. The flags followed guidelines by the ODV generic quality flags scheme in which; 0=good quality, 1=unknown quality, 4=questionable quality and 8=bad quality.

Notes

Credit
KSA224/324 Oceanographic Methods class 2025 (IMAS)