We derive from a dynamic model that light availability, phytoplankton density, and the carbon : nutrient ratio of phytoplankton biomass should all be negatively related to mixed surface layer depth, whereas the areal standing stock of phytoplankton should show a unimodal, and total and dissolved nutrients a horizontal or increasing, relationship to mixing depth. These predictions agree closely with data from 65 central European lakes during summer stratification. In addition, zooplankton biomass was strongly negatively related to mixing depth in a subset of lakes. A decrease in mixing depth is thus a form of enrichment with light of the mixed surface layer, the effects of which could propagate to higher trophic levels.

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A method for determining enzymatically hydrolyzable phosphate (EHP) in natural waters is described, based on the determination of released inorganic phosphate after the hydrolysis of organophosphoric esters by free, dissolved phosphohydrolases (mainly phosphatase) produced by the biota. The method gives higher values in highly eutrophic waters than the classical procedure of Strickland and Parsons; in samples from less eutrophic habitats the two methods give similar results. The new method is simple, accurate, and can be used in both freshwater and marine studies. It is particularly recommended for determination in hypereutrophic waters, when Pi concentrations exceed 25 µg P liter−1. Representative field data from the euphotic zone of seven lakes and two marine habitats are presented.

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Temperate lake ecosystems are generally characterized by a strong annual cycle, and the relationships between observations of such ecosystems and external forcing variables can exhibit a complex structure. Furthermore, the observational data record is often short. This makes it difficult to assess the relationships between external forcing factors and their impact on the biological succession. Cyclo‐stationary maximum cross‐covariance analysis (MCCA) allows the effects of seasonality to be modeled in a flexible way, and we describe this statistical technique in detail. MCCA offers an objective method to approximate the high‐dimensional total cross‐covariance structure by defining “weighting” patterns. With a predictor set of reduced dimension, a suitable regression between forcing variables and ecological response variables can be set up. Cyclo‐stationary MCCA is used here to analyze the influence of meteorological variables (air temperature, wind speed, global radiation, humidity, and precipitation) on 13 biological and biogeochemical indicator variables of Plußsee, a small lake in northern Germany. The main weather influence on the indicator variables was found to be connected to winter temperature. From the covariance structure the following major signals were detected to be related to higher winter temperature: a more intense spring algal maximum, a higher zooplankton biomass during the algal maximum, a less intense loss of nutrients to the hypolimnion, a higher summer bloom together with changes in the nutrient concentrations, and stronger oxygen consumption in autumn.

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The Ruhr, with an average flow of 80.5 m3/s at its mouth, is a comparatively small tributary to the Rhine River that has to perform an important task: to secure the water supply of more than 5 million people and of the industry in the densely populated region north of the river. The complex water management system and network applied by the Ruhrverband in the natural Ruhr River Basin has been developed step by step, over decades since 1913. And from the beginning, its major goal has been to achieve optimal conditions for the people living in the region. For this purpose, a functional water supply and wastewater disposal infrastructure has been built up. The development of these structures required and still requires multi-dimensional planning and performance. Since the river serves as receiving water and at the same time as a source of drinking water, the above-standard efforts of Ruhrverband for cleaner water also help to conserve nature and wildlife. Ruhrverband has summed up its environmental awareness in the slogan: "For the people and for the environment". This basic water philosophy, successfully applied to the Ruhr for more than 80 years, will be continued in accordance with the new European Water Framework Directive, enacted in 2000, which demands integrated water resources management in natural river basins, by including the good ecological status of surface waterbodies as an additional goal.

A series of impoundments occur on the final 46 km stretch of the Ruhr river where phosphorus, nitrogen, and organic carbon are present abundantly, causing several heavy algal blooms during the year. This is detrimental for the treatment capacity of the water works. Also, an oxygen shortage is always recorded in the river after algal decay at low flows, thus requiring artificial aeration. By balancing all oxygen supply and consumption it is shown that about 2/3 of the oxygen demand is due to algal decomposition. When evaluating the water quality data of the past 30 years and by conducting special laboratory tests, results show that phosphates initiate algal growth. Therefore, a phosphate model has been developed for the river which predicts the phosphate content for 1988 and 1998. It is expected that by reduction of this phosphate content maximum algal growth can be cut by about 50 %.