Freshwater Ecology of Cyanobacteria

Whether cyanobacteria thrive in a body of water depends less on whether they are present — they almost always are — than on whether conditions let them outgrow their competitors and outlast their losses. Warm temperatures, calm and stratified water, long water residence times and ample light favor the planktonic bloom-formers; buoyancy, efficient light harvesting, phosphorus storage and nitrogen fixation are the traits that let them exploit those conditions [1, 2, 3]. Warm summers can tip the balance strongly: modeling and observations from a European lake showed that a heatwave summer promoted cyanobacterial dominance [4].

Streams and rivers work differently. Flow flushes plankton downstream, so in small streams cyanobacteria are mostly benthic: films, mats and colonies attached to rocks, wood and sediment, sharing the surface with diatoms, green algae, bacteria and fungi [5, 6]. Large, slow rivers sit in between; cyanobacteria are routine members of their phytoplankton even when they do not dominate [7]. The contrast between a shaded, flowing stream such as Allen Branch and an open, sunlit farm pond is therefore a contrast between two different ecological games.

Planned sections

3.1 Planktonic versus benthic cyanobacteria

Life in the water column versus life on surfaces; buoyancy regulation by gas vesicles in plankton [8]; mat formation, detachment and downstream dispersal in rivers [9, 6].

3.2 Lakes, reservoirs, ponds, wetlands, streams and rivers

How depth, mixing, flow and residence time differ among habitats, and which cyanobacteria each tends to support. Reservoirs combine river inflow with lake-like stratification.

3.3 Light availability and shading

Light as a control on both planktonic and benthic producers; efficient light harvesting by phycobilisomes in turbid water [2]; canopy shading in small streams.

3.4 Water temperature

Higher optimal growth temperatures of many bloom-forming cyanobacteria; heatwaves and climate warming [1, 4, 10].

3.5 Mixing and stratification

Thermal stratification, buoyant species at the surface, and why artificial mixing can suppress cyanobacteria in some deep lakes but not shallow ones [11].

3.6 Water residence time

Slow-growing populations need time; flushing in streams and at high flows removes plankton. Low, slow flows set the stage for river blooms such as those on the Ohio River [12].

3.7 Seasonal succession

The seasonal sequence of phytoplankton and zooplankton in lakes and the mechanisms behind it [13]; overwintering akinetes and colonies on sediments [14].

3.8 Competition with other phytoplankton

Nutrient affinity, storage and nitrogen fixation as competitive traits [2]; ratios of nitrogen to phosphorus [15].

3.9 Grazing by zooplankton

How grazing pressure shapes plankton and why cyanobacteria often escape it (developed in Chapter 6) [16].

3.10 Microbial interactions

Bacteria associated with colonies and mats, viruses and parasites, and the microbial community of stream biofilms.

Connection to local research

Key terms

Planktonic

Suspended in the water column.

Benthic

Attached to or living on the bottom.

Biofilm

Microbial community in a slime matrix on a surface.

Stratification

Layering of a lake by temperature and density.

Residence time

Average time water spends in a water body or reach.

Succession

Predictable seasonal change in community composition.