Application of in-situ and on-line measurements to environmental studies in Lake Müritz, northern Germany

A new system, MINIBAT, for in-situ measurements and continuous collection of water samples was used in Lake Müritz (Mecklenburg-Vorpommern) to make detailed and simultaneous measurements of temperature, conductivity, pressure, particle density, H2S, O2, pH, nutrients and heavy metals. The underwater measuring unit consists of a multisensor unit, a micro-processor and an underwater pump (pumping rate up to 0.8 l/min) carried in a small, towed frame. The onboard unit (IBM compatible computer) is able to record sensor and location data transmitted from a GPS system and echosounder. Onboard, the water samples are split for introduction into a H2S flow-through sensor and for aliquots which are filtered through standard membrane filters and preserved either for trace metal determination or nutrient analysis. All data are stored in a data bank which can be used as a base for mapping the different parameters within the water body. This compact and portable system represents a breakthrough for hydrochemical investigations of lakes and rivers and was used to map the hydrochemical parameters and anthropogenic influences in Lake Müritz. In-situ measurements made during the summer stagnation showed that a 10 m-thick layer enriched in H2S built up in the bottom water. This anoxic hypolimnion was characterized by a steadily increasing H2S content approaching the sediment surface and with an associated increase in the contents of the nutrients NH4+ and PO43− and a decrease in the heavy metal contents.