Abstract
Estuaries have the particularity of being in constant change concerning the salinity of the water, since the river carries fresh water to the mouth, while the force of the tide pushes salty water upstream, reaching several kilometers upstream given the right conditions. These bodies of water do not mix instantly, allowing the appearance of a distinct border between the two, known as the front. This document presents a monitoring system composed of four-probe salinity sensors, which acquire conductivity, temperature, and pressure data to calculate salinity, arranged in such a way as to make measurements along the water column, to detect the shape of the front, as well as monitoring the estuary. The sensors, and the monitoring system in general, are cost-effective, low-power, and accurate in long-term deployment, even when installed in adverse conditions such as at sea. The sensors consume $26 \mu \mathrm{A}$ in sleep mode and 10 mA during measurement, and the measurement time is less than 100 ms. The choice of a four-point configuration allows overcoming the inevitable decay of the electrodes and possible measurement errors, contributing to autonomous long-term operation.

Abstract
The present work shows the development of a linear electromagnetic generator (LEG) for applications in marine and fluvial sensors. The LEG was submitted to tests with small oscillating frequencies of 0.1-0.4 Hz, and the output power obtained for these frequencies has allowed to supply energy to at least 6 sensors with a total consumed power of 5.76mW or to charge the batteries when the sensors are in a sleep mode. As a result, it is possible to extend the operation time and reduce the logistic costs for sea or river sensors applications, by using the proposed micro underwater generator.

Abstract
The authors wish to make the following erratum to this paper [1]: Equations (1), (7), and (9) are incorrect and must be replaced by the following equations: [Formula presented] The authors apologize for this literal mistake, but emphasize that the content of the article is still correct, since all calculations were performed with the correct equations. The manuscript will be updated and the original will remain online on the article webpage, with a reference to this Erratum.

Abstract
The lack of penetration of light and electromagnetic radiation beyond a few meters in the ocean makes acoustics the technique of choice for data transmission, target detection and ocean sensing in general. Acoustic transducers are typically based on piezoelectric materials due to the good response at high frequencies. Depending on the application it can be built using ceramics, polymers and composite materials. In the hydrostatic mode PZT ceramics hydrophones have low performance due to the low hydrostatic piezoelectric stress value. On the other hand, PVDF have shown relatively high hydrostatic mode response. This work presents the development of a PVDF hydrophone for deep sea applications. The hydrophone was subjected to a pressure test up to 25 MPa to evaluate the response variation under high hydrostatic pressure. The results show an increase up to 6 dB sensitivity under 15 MPa pressure.

Abstract
Energy harvesting devices can increase autonomy of submersible marine sensors. However, only the water movements can be used as energy source, since neither solar or temperature gradients are available bellow surface waters. A Linear Electromagnetic Generator (LEG), in a milliwatt energy harvester, is presented. Any moving parts are in contact with water, thus avoiding biofouling problems in the harvester. In this work, a 100mm length, 60mm diameter, cylindrical LEG was designed to maximize output power, and analyzed the effects of magnets size and geometry as well as coils position, at several working conditions. Two coils were used, with an internal resistance of 130 ? in 1500 turns, together with N38-N42 magnets. A mean electrical power of 25 mW (100 mW peak) was experimental measured in the optimized configuration, in realistic conditions, which is enough to power almost any electronic low-power sensor.

Abstract
Salinity measurement in water is typically performed with conductivity sensors. However, for long-term marine deployments, loss of precision is observed, mainly due to electrode drift (oxidation and degradation occurs in the presence of water, salts and bio-fouling), which results in inaccuracy of measurements. A cost-effective, low-power, four-probe salinity sensor is presented, to accurately measure long-term deployments in oceans, rivers and lakes. The four-probe methodology overcomes many of the drift problems, and the use of low-cost stainless-steel electrodes (avoiding platinum or titanium materials) can still achieve good long-term stability, in the practical salinity scale range from 2 to 42 PSU. Low-power electronics (200 µA in sleep-mode and 1 mA in active-mode) based on a ratiometric ADC conversion, and a low-power microcontroller with non-volatile memory, complements the proposed sensor, to achieve an autonomous salinity sensor for long-term marine deployments, with autonomy above 1 year with a 1 min-1 sample rate, using a common 2400 mA x 3.7 V lithium battery.