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Eutrophication remains one of the foremost environmental issues threatening the quality of surface waters yet comparatively little is known of the timing, magnitude and characteristics of nutrient-related changes in highly calcareous (marl) lakes. This review focuses on marl lake ecology and chemistry, their known responses to eutrophication, and also highlights questions that remain unanswered.
In good condition, marl lakes support a diversity of macrophytes, especially Characeae and Potamogetonaceae, which can grow to considerable depth. High water transparency and low phosphorus and phytoplankton concentrations are facilitated by the coprecipitation of marl and phosphorus. Although large amounts of phosphorus can be thus removed, buffering against eutrophication, macrophyte communities can undergo significant change under rather low nutrient concentrations. Maximum colonisation depth declines and tolerant species replace sensitive species, with losses particularly among charophytes. Marl lakes are therefore ecologically highly sensitive.
The effects of coprecipitation on long-term burial of phosphorus are contested. Several palaeolimnological studies have identified iron complexes as more important than calcite, as chemical conditions in the sediment may promote either calcite dissolution or calcite-bound phosphorus exchange, or possibly both. Some marl lakes have been shown to have phosphorus concentrations which, compared with other lake types, are higher than expected in winter and lower in summer. The phosphorus binding capacity of marl sediment has not to our knowledge been adequately researched.
Marl precipitation may be inhibited by high phosphate or organic matter concentrations in the water, or when biological communities effecting precipitation (picoplankton, charophytes, epiphytes) are disturbed. Highly impacted marl lakes having low species diversity and lacking precipitation may be misidentified as eutrophic, high-alkalinity lakes. More studies addressing the interaction between external loading, phosphorus cycling and marl precipitation in relation to biological communities are required to assess to what extent marl lakes can buffer eutrophication, and what factors contribute to disturbed marl precipitation.
The development of modelling in aquatic ecology has focused on mechanistic biogeochemical models. However, such models have substantial data requirements for inputs and also for proper validation, which hinders their use for less studied systems. Another significant problem with complex models is their structural and computational difficulty, and thus often an associated absence of proper uncertainty analysis for the model results. This makes the use of the outputs for public policy making (e.g. in lake management) rather questionable. We see no compelling reason (other than lack of awareness of choices) why all lakes and all questions should necessarily be studied using the same high-profile models. Here we review two alternative statistical approaches, Linear Mixed Modelling and Structural Equation Modelling, and the different ways they have been used to extract maximum information from existing data. These methods offer promise for tackling the problems highlighted above, although our aim is not to promote any one method over the others. Rather, we want to stimulate debate about the remaining unknown factors in lake modelling as well as about the balance between data and models, and the still too uncritical way in which model outputs are interpreted and used for decision making.
Lobo/Broa reservoir research was established in 1971 as a model of limnological and ecological research in aquatic ecosystems in Brazil. The present paper describes the ecological dynamics of the reservoir, the climatological and hydrological interactions, limnological factors and the responses of the aquatic biota to the forcing functions mainly of precipitation and wind. It also emphasises management issues and technologies, and processes developed to control eutrophication. Ecosystem services are described.
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