Overview
Monitoring the yearly evolution of temperature stratification is a critical component in many lake management and research programs due to its pronounced effects on aquatic chemistry and biology. Fisheries management, hydroelectric plants, selective withdrawal dams, and numerous aspects of aquatic and sediment research often depend on having temperature profile data for a water body.
Lake stratification develops seasonally as increasing solar radiation in the spring and summer heats up surface waters. Heat quickly dissipates in the upper layers of water, and deeper waters remain cool. Because warmer water is significantly less dense than colder water, temperature-dependent density differences develop that prevent mixing and result in the formation of isolated layers of water. Due to the absence of water circulation between the layers, each strata of water develops its own distinct chemistry.
Among the more common concerns is the depletion of oxygen in the deeper layers of stratified lakes. Dissolved oxygen declines sharply at the thermocline, or region of greatest temperature change. Below 7m depths, dissolved oxygen is insufficient to support most aquatic life.
In the fall, the temperature and density gradient weakens as surface waters cool and sink. Mixing by wind and the sinking of cooler waters will eventually destroy the gradient, a process known as fall turnover. The resulting isothermal conditions restore water circulation and oxygen to the deeper layers of the lake.
T-Node Temperature String
Temperature stratification is measurable with temperature string technology, such as the NexSens T-Node connectorized water temperature sensor. The NexSens system features an integral digital sensor secured in a protective housing for underwater deployments down to 200 meters in fresh, brackish, or seawater. The sensors can be connected in-series using UW underwater cables and suspended vertically in a water column or horizontally along a stream or riverbed.
Temperature data is transmitted on a 1-wire temp string bus. In addition to the 1-wire bus, the T-Node offers pass-through signals for SDI-12 and RS-485, allowing the user to connect Nodes and other environmental measurement sensors along the string. This sensorBUS architecture has been designed so that researchers can easily build and customize multi-point temperature strings or more powerful environmental monitoring networks.
The T-Node sensors feature a plug-and-play interface to NexSens SDL500 submersible data loggers and offshore data buoys. Data collection options include standalone, radio, and cellular telemetry. NexSens iChart Software is a Windows-based program for interfacing both locally (direct-connect) and remotely (through telemetry) to an SDL500 data logger or network of data loggers. For connection to external data collection platforms, the T-Node sensor string can integrate with a NexSens controller for data output in SDI-12 or Modbus RTU.
sensorBUS Architecture
Developed to replace, expand, and enhance centralized parallel wiring for prevailing analog and SDI-12 signal transmissions, sensorBUS technology simplifies the creation of environmental monitoring networks by combining several popular sensor interface types on a single 8-wire bus. sensorBUS is mainly used at the field level with interfacing capabilities to smart sensors that incorporate 1-wire temp string, RS-485 multi-drop or SDI-12 hardware/software connection. sensorBUS is the solution for linking multiple sensors using open, standardized technologies.
sensorBUS includes three industry standard digital interfaces (1-wire temp string, RS-485 multi-drop and SDI-12) along with both 12 and 5VDC power. 1-wire temp string is a multi-point communications network capable of supporting 128 temperature measurement nodes (sensors) over 200 meters. Data is transmitted and received on a single wire. RS-485 is a multi-point communications network capable of supporting hundreds of nodes (sensors) over a few thousand feet. The balanced 2-wire system is constructed with a twisted pair of conductors surrounded by a shield. SDI-12 is a serial data interface capable of supporting up to 10 sensors. The 3-wire system includes 12VDC, Ground and SDI data signals.
Data Logging and Telemetry
Buoys, such as NexSens Temperature Profiling Buoys, are ideal platforms from which to monitor temperature information. They consist of a cross-linked polyethylene foam hull with a tough polymer skin coating. The round center housing accommodates the NexSens SDL500 submersible data loggers. Three NexSens 5-watt solar power packs are designed to mount to the top of the buoy to provide continuous power to the data logger and communications module.
Top- and bottom-mounted stainless steel eye-nuts accommodate moorings and lifting rigs for quick and easy deployment. The buoy is moored to the bottom via anchors, chain, and shackles. NexSens application engineers offer recommendations for deployment based on site conditions to ensure the systems will remain in location.
The SDL500 submersible data logging system consists of an iSIC data logger and communications module housed in a fully-submersible, 5 diameter round enclosure. The SDL500 can be outfitted with either a spread spectrum radio or cellular modem for real-time communication to shore.
Managing Data
Once the temperature profiling system is set up, data is logged at a user-defined interval (minimum one minute). The user also sets the interval at which data is transmitted via radio or cellular telemetry. A typical monitoring system logs temperature data every 10 minutes and transmits data every 30 minutes.
Data is transmitted to a nearby (or remote) computer running iChart Software, which serves as the centralized interface and database for all incoming data. All data and sensor configuration settings are also stored in a single iChart database.
The software offers a unique historical report creation tool that can generate customized reports with data from all sensors in an iChart database. When creating a report, users can include specific information about the monitoring site, location, sensors, and project. After creation, reports can be converted to PDF, exported to Microsoft Excel, sent to interested parties via e-mail, uploaded to a web server, and more. The report template can also be saved and automatically generated, further automating the reporting process.
The NexSens WQData web datacenter is an optional service that automatically generates an online graphical interface for viewing, analyzing, and downloading data in real-time. This datacenter allows project members and stakeholders to experience the lake stratification information remotely as it occurs.