Project aim

The overall objective of the project is to describe the physical structure of the Santiago river plume, chlorophyll and nutrients concentration, large phytoplankton groups, zooplankton biomass, and fish larvae abundance and community structure. The hydrographic and biological data from this oceanographic campaign will be used in undergrad and grad thesis, as well as scientific publications.

The first objective of this campaign was prospection and recognition of physical and biological variables in the plume area of Santiago river. The second objective was to adjust sampling logistics and board work of scientific crew for subsequent cruises of the project ‘Physical-biological processes in river plume fronts of the Mexican Pacific’.

Report objective

The aim of this report is to present the study area of the project and the vertical structure of the Santigo’s river plume during January 2016, including temperature, salinity, density anomaly, dissolved oxygen, fluorescence and turbidity. As well as the geographic location of continuous measurements in surface transects of temperature, salinity, density anomaly, dissolved oxygen, fluorescence and turbidity.

In order to use the information in this report, a written permission from project responsible Dr. Emilio A. Inda-Díaz is required (eindad@uan.edu.mx).

Study area

The reported data were obtained in the area of Santiago’s river mouth in the Nayarit continental shelf region, from the coastline to approximately 25 km (Figure 1), between 21º42’00” N-20º30’00’’ N and 105º30’00’’ W-104º15’00’’ W. Table 1 summarizes the geographic location, depth, date and time of occupation of the hydrographic stations. Table 2 summarizes date, time, initial position, final position and length of continuous surface transects.

[Figure ID: f1] Figure 1.

Study area and geographic position of stations and transects.

Instruments and Calibration

This section describes the calibration and data acquisition of the SBE-911plus CTD (SCPlusV2_ RS232) manufactured by Sea Bird Electronics, Inc., serial number (01907465), which consists of a submari ne unit (SBE-911 plus). The SBE-911 unit consists of a pressure box (up to 6800 m deep capacity), containing power sources and electronics for data acquisition and telemetry, in addition to the pressure sensor. Outside there are modular sensors for temperature (SBE 3plus premium), pressure (Digiquartz® Pressure sensor), and conductivity (SBE 4C), which are fed by a constant spe ed pump controlled seawater flow (30 mls-1). The unit provides up to eight input channels to connect optional sensors; during PROFRENTES-00 campaign extra sen sors were used for dissolved oxygen (SBE 43), fluores cence (Fluorometer WET Labs WETstar), and turbidity (Turbidity Meter Seapoint).

CTD Calibration

The processes for pressure, temperature, conductivity and dissolved oxygen sensors calibration in laboratory have been described by Godínez et al., (1995). Table III presents coefficients of the manufacturer’s calibration for temperature (T), salinity (S), dissolved oxygen (DO), fluorescence (Fl) and turbidity (Tz) used sensors.

CTD Data Acquisition

The obtained data with temperature, salinity, dissolved oxygen, fluorescence and turbidity sensors were stored in the internal memory of the unit, both in the vertical profiles and transects. Additionally, in the surface transects, they were stored in real time in a computer on deck connected to a load data cable (Sea- Bird Wet-pluggable Load-Bearing data cable MCIL-4FS to DB-9S, 100 m). The technical specifications given by the manufacturer for each sensor are shown in Table 4.

Vertical profiles were measured by manually lowering the CTD at ~0.5 meters per second. For continuous surface transects, the CTD was placed in an ad hoc wooden box (1 m x 0.5 m x 0.5 m) of 250 L, which was fed with seawater pumped at 1 m depth (750 L/min), the residence time of seawater was 20 seconds, and navigation speed 6 knots.

Data processing

The 7.26.7 version of SBE Data Processing© (Sea-Bird Electronics) was used for data processing. This software consists of modules to convert, edit, process and graph oceanographic data obtained with the Sea-Bird 911plus CTD, and is part of the Seasoft V2 software suite 2017 (http://www.seabird.com/software/sbe-data-processing).

Data were downloaded and processed to obtain profiles of measured parameters (temperature, salinity, dissolved oxygen, fluorescence and turbidity) as well as density (derived). During the data processing the ‘noise’ was reduced, errors eliminated, and calculations were made to finally obtain values at every 0.1 m depth. All “raw” data recorded by the CTD during the cast were processed and converted to conventional units using DATCNV module. Due to the high vertical (profiles) and horizontal (transects) variation in coastal shallow data, the process suggested by the manufacturer (Sea-Bird Electronics, 2017a) was modified on an empirical basis and the WILDEDIT module was not used.

Due to differences in response time of each sensor, the first step for data processing was to advance measurements using the ALIGNCTD module in 0.05 s temperature and conductivity, 5 s dissolved oxygen, and 0.11 s fluorescence and turbidity. This delay is independent of that due to sensors’ conduit position (0.073 s; Sea-Bird INC, 1992), which was automatically adjusted when configuring the SBE-911plus control unit.

To eliminate depth inconsistencies due to boat pitching and manual descent/ascent of the CTD, the data were adjusted in the LOOPEDIT module. All data were maintained (from the surface). Data obtained with a CTD speed of less than 0.1 m were eliminated.

The next step was reduction of undesirable ‘noise’ (due to different sensor responses) through the FILTER module, using a single pole low-pass filter with a constant of 10 s for temperature, conductivity, fluorescence, dissolved oxygen and turbidity. This filtering is based on the criterion of visually minimizing peak salinity profiles (Morrison et al., 1994). Finally, data were averaged in centered 0.1 m blocks using the BINA VG module.

The reported temperature used to derive variables is ITS-90, the salinity is UPS, density is calculated from the seawater state equation according to the UNESCO proposal (1991) and density anomaly is reported γθ (kg•m³). The used algorithm to calculate dissolved oxygen concentration uses the Owens and Millard equation (1985). All these algorithms are performed internally by software provided by (Sea-Bird Electronics, 2017^b; http://www.seabird.com/software/sbe-data-processing).

To locate errors that have not been eliminated using the procedures described above, a visual inspection was carried out after the data had been processed. Most of the errors are due to communication failure between the CTD and computer, sometimes they could be due to lack of time to sensors’ stabilization at surface, however, for this campaign a special care has been taken to stabilize sensors due to the high vertical and horizontal variation in the physical properties measured in river plumes. These errors are eliminated by editing the original files, reviewing the process, and if necessary is repeated completely.