Filamentous algae on the bottom of a small pond.

Filamentous algae bloom (FAB) in a small pond. This one is formed by Spirogyra spp. Photo: Angela Shambaugh.

For many years, lake lovers around Vermont have asked me about filamentous algae on the bottom of lakes. These algae can be found in almost all waterbodies – lakes, ponds, rivers, streams, wetlands. They grow among aquatic vegetation, on rocks and logs, and on the sediment. Sometimes, these algae are almost invisible, barely growing above the sediment or rock. They can also be very abundant and lush, floating in soft clouds above the sediment or tangled in aquatic vegetation.

Filamentous benthic algae need good light and nutrients, just like phytoplankton (algae floating in the water column). There isn’t a lot of competition between phytoplankton and benthic algae – those algae living on the bottom or attached to something – for nutrients. Phytoplankton get theirs from the water column. In contrast, the benthic algae get theirs right where they grow. However, sunlight for photosynthesis is a resource both need and both get from the same place. Since phytoplankton grow in the water, they get the first shot at available light. Whatever they don’t use or block may eventually reach the benthic algae. Turbidity and the depth of the water body also influence how much light makes it down to the benthic algae.

Scientists have begun to notice that benthic algae, especially filamentous green algae are appearing in clearwater lakes where they previously were not common. This has been happening in Vermont as well. Recently, I came across a journal article that talked about this phenomenon and the possible causes. The article by Yvonne Vadeboncoeur and many other scientists is very interesting (see the full title and link below if you’d like to read it).

Why are filamentous algae blooming in clear water lakes?

Vadeboncoeur et al. outline three possible explanations for these new filamentous algae blooms (FABs). The first is a change in nutrient loading. More nutrients in the water column will grow more phytoplankton, which in turn will limit how much light reaches the bottom. FABs need lots of light, so increased loading from the watershed is likely to cut off the light they need for growth. Groundwater, however, is delivered right to the benthic zone. It doesn’t support phytoplankton growth, so the water column remains clear and FABs have all the light they need. Nutrient concentration in shallow groundwater is influenced by the local watershed, climate, and geology. Development, agriculture, atmospheric deposition, and septic systems all may contribute nutrients to shallow groundwater networks. The article shares the story of FABs growth in Lake Tahoe CA due to groundwater pollution in the basin.

The second explanation considers how changes in lake stratification, waves and water level may encourage FABs. Climate change is increasing the amount of time lakes remain stratified during the summer and decreasing ice cover during the winter. In a clear water lake, Vadeboncoeur et al. propose this change increases the growing season for FABs. Longer stratification may reduce the amount of phytoplankton in the water column, allowing more light to reach the bottom. The nearshore waters may also warm more than offshore waters. Like the temperature-related density difference that creates stratification, this temperature difference can prevent nearshore and offshore water from mixing, making for happy FABs.

Wave action and water levels have mixed effects on FABs. They can break up FABs and prevent them becoming too abundant. They can interfere with the temperature dynamics noted above. However, wave action can create more substrate by washing away fine sediments and leaving coarser materials that FABs attach to. Changing lake levels can increase shore erosion, but also provide new space for FAB growth. At significantly higher or lower lake levels, there may be newly available nutrients or structures that promote FAB growth. Groundwater delivery also may change with lake level, again changing how nutrients are delivered to the benthic zone.

Spirogyra spp in rivers and under the microscope

Several views of Spirogyra spp, a common FAB. Photo: Proyecto Aqua,

The last explanation proposed by Vadeboncoeur et al. considers how changing biotic interactions – relationships between organisms – might affect FABs. Zebra mussels, for example, eat phytoplankton and detritus, which improves water clarity. Their waste is released in the benthic zone where FABs live. That combination resulted in a burst of filamentous green FABs in the Great Lakes (see the example in Vadeboncoeur et al.). Insect larvae and other grazers living in the benthic zone do a good job of controlling algae growth. If something reduces their number or otherwise changes their eating habits, green algae FABs can quickly increase.

Aquatic ecosystems, like all ecosystems on the planet, respond to the physical, geological, and biological relationships occurring in them. Change is always happening. Often, it is small and we don’t really notice. The increased reporting of FABs in clear water lakes suggests that there are big changes happening in these aquatic ecosystems. We know a lot about phytoplankton and nutrient cycling in the open waters of our lakes, but very little about benthic algae and nutrient cycling in the benthic zone. Vadeboncoeur et al. suggest that the concept of eutrophication will need to expand to include these lake bottom interactions so we can better protect our clearwater lakes.

Read the original journal article: Blue waters, green bottoms: benthic filamentous algal blooms are an emerging threat to clear lakes worldwide.