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

Abstract

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.

Novel ecosystems in urbanized areas under multiple stressors: Using ecological history to detect and understand ecological processes of an engineered ecosystem (lake Markermeer)

“…the Since 2006, changes in plankton species dominances have been observed (Noordhuis 2010). The currently dominant phytoplankton species may clog in a different way to silt particles in the water than the previously dominant species (Noordhuis et al 2014, Brinkmann et al 2019. If their aggregates with silt result in larger flocs (De Lucas Pardo 2014), these could lower uptake by filter-feeding Dreissena and negatively affect the mussel banks in lake Markermeer and turbidity of the water column.…”

“…In the last decade, changes in flocculation are observed. These are ascribed mainly to alterations in phytoplankton composition (Lucas de Pardo 2014, Noordhuis et al 2014) with microalgae species adapted to live on aggregates (Brinkmann et al 2019).…”

“…Sedimentation and resuspension processes determined phytoplankton occurrence and made phytoplankton switch from benthic to pelagic environments for shorter of longer periods (Jagtman and Urk 1988, Mur et al 1990, Van Duin 1992 as both resuspended silt and phytoplankton sediment in equal proportions (Mur et al 1990). This mechanism of co-sedimentation may determine the behaviour, composition, and development of phytoplankton in lake Markermeer (Brinkmann et al 2019), suppresses the development of cyanobacteria (Mur et al 1990, Ibelings 1997, and probably determines the differences between lake Markermeer and lake IJsselmeer phytoplankton assemblages (Jagtman and Van Urk 1988). During their stay in the sediment, light conditions are very unfavourable for phytoplankton, especially for species like the cyanobacteria Microcystis that have much more difficulties coping with rapidly changing light conditions and longer periods in darkness than the green algae Scenedesmus (Mur et al 1990).…”

“…De Lucas Pardo (2014) tested the effects of colonial algae and filamentous algae on flocculation, in particular floc size and structure, and found that with the changes in species composition, flocculation has changed towards the formation of larger aggregated flocks. In a detailed study of individual aggregates, Brinkmann et al (2019) showed that micro-algae could obtain phosphorus from the aggregates.…”

“…Resuspension, turbidity, silt and phytoplankton cannot be regarded as separate factors in the complex co-resuspension dynamics (e.g. Brinkmann et al 2019). Silt and phytoplankton are most directly subjected to these processes with turbidity and light regime restrictions as output which in turn limit the possibilities for ecology and nutrient availability.…”