Benthic hotspots in the pelagic zone: Light and phosphate availability alter aggregates of microalgae and suspended particles in a shallow turbid lake: Benthic hotspots in the pelagic zone

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.