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Limnetic aggregates from a turbid delta lake with low dissolved nutrient availability were studied in relation to light and dissolved nutrient availability. Quick light‐attenuation restricts the euphotic zone to the top surface layer of the water column, whereas mineralization processes in the sediment specifically provide dissolved nutrients near the lakebed. This suggests neither the pelagic nor the benthic zone provides the combination of resources required for microalgal growth. Nutrient mineralization in aggregates could bridge this apparent spatial gap in light and nutrients by providing dissolved nutrients in the euphotic zone, promoting microalgal growth. To explore this, aggregates obtained from turbid and phosphate‐limited lake Markermeer (The Netherlands) were exposed in the laboratory to phosphate‐replete and phosphate depleted conditions, at high‐light and low‐light availability. Confocal microscopy revealed that aggregates exhibited alkaline phosphatase activity and contained microalgae, other microbes, and extracellular polymeric substances. The spatial distribution of the phosphatase activity in aggregates largely matched that of chlorophyll a (Chl a)‐lacking microbes, suggesting that these microbes were responsible for the activity. Colorimetric quantification revealed that aggregates exhibited over 1.9‐fold higher phosphatase activity than surrounding water. Two‐day exposure to different light and phosphate availabilities affected aggregate composition. Phosphate depleted conditions resulted in more Chl a‐lacking microbes and more phosphatase activity than phosphate‐replete conditions. Low‐light intensity resulted in higher abundance of extracellular polymeric substances than high‐light intensity. In contrast to aggregates from deep stratified systems, Markermeer aggregates were not enriched with dissolved phosphorus. These results suggest that P‐cycling in aggregates differs between shallow turbid and deep stratified ecosystems.

Lake Markermeer, a large shallow lake in The Netherlands, suffers from turbidity and ecology problems. As part of a study aiming to mitigate these problems, we study flocculation processes in the lake; in particular, the possible mutual flocculation between algae and re-suspended bed sediments. We show that sediment re-suspended from the bed of the lake can flocculate, forming flocs for which size is a function of the turbulence level in the water column. Moreover, we also demonstrate that algae and re-suspended bed sediments can mutually flocculate, yielding organicinorganic aggregates. These aggregates have different features to those of their individual components, some of which have been measured and characterized in this paper. Furthermore, the characteristics of the resulting organic-inorganic flocs are strongly influenced by the type of algae in the aggregate. We found that, in the case of flocs consisting of bed sediments and filamentous algae, flocculation yields smaller flocs than for bed sediments only, resulting in an increased turbidity in the water column. In the case of flocs consisting of bed sediments and colonial algae, flocs grow faster and become larger than bed sediment flocs, which may result in the depletion of most colonies from the water column.