Sources of Plankton Blooms

For many years it was generally thought that planktonic algae blooms involved a few resting stages germinating from the bottom or a few cells hanging on from some previous time in the water column encountering adequate nutrients under sufficient light, resulting in enough growth to discolor the water and be called a “bloom” (Note: there is no technical definition of a bloom; it is in the eye of the beholder, although there is general agreement that there should be reduced clarity and increased color involved). This mode of bloom formation is certainly possible and does occur, but for the harmful algal blooms currently getting so much media attention, there are two other modes of bloom formation that seem to be dominant.

One mode is metalimnetic accumulation, which means that algae accumulate in the boundary layer between upper, well-lit, warmer waters (the epilimnion) and lower, darker, colder waters (the hypolimnion). This can only happen where the lake is deep enough to develop a thermal gradient, usually more than 20 feet in New England, but thermal stratification can develop in shallower lakes under the right circumstances. Algae in this zone get enough light from above and enough nutrients from below to grow into a dense layer which may be disrupted by wind or may synchronously rise in response to lower light availability or changes in temperature that signal breakdown of thermal stratification. These blooms are most often evident in late summer or early fall, but can be moved into upper waters by summer storms. Several types of golden algae that produce taste and odor use this mode of growth, and water supplies with intakes in this boundary layer have to deal with them. Several blue-green algae known for toxicity also utilize this mode of growth and are a threat to both human health and lake ecology.

The other mode involves growth at the sediment-water interface, much like algae mats, with an eventual rise in the water column, again like the mats. But we think of these algae as planktonic forms, and never really considered their origin. It appears that these algae, mostly blue-greens and often potentially toxic forms, gain most nutrients from decay or other releases from sediment, accumulate excess amounts in cells, grow to full “maturity” on the sediment, then produce gas pockets in cells and rise over just a few days to form a ready-made bloom at the surface.

These bottom-formed blooms are particularly problematic in lakes known for being “clean” for many decades. The inputs from the watershed over many years were assimilated into the bottom sediments and at some point the balance tipped, such that the accumulated nutrients, especially phosphorus, became available for uptake at the sediment-water interface. It appears that this is related to oxygen loss in the surficial sediment, largely a function of organic build up. The water column may still have low nutrient levels, but the rising blue-greens already have enough extra nutrients to survive for days to a few weeks, and the resulting blooms seems to come out of nowhere to impair lake use and threaten ecological functions. Worse yet, this appears to bring nutrients into surface waters, allowing follow on blooms when the initial algae die off. This mode of bloom formation is both a harbinger of eutrophication and a vector of it.

Planktothix sp., a metalimnetic bloom former

Dolichospermum lemmermannii, a bottom bloom former

NALMS 2017 International Symposium Registration Open

The 2017 NALMS conference will be held in Westminster, CO, just outside Denver. This annual gathering showcases the best in lake management through presentations, exhibits and networking. This is a gathering of researchers, consultants, governmental representatives and lake enthusiasts. If you work on lakes, you really can’t afford to miss it. But if you just love your lake and want to know more about how to keep it in its best condition, this is a golden opportunity. The best experts in lake management are readily accessible and enjoy interacting at this gathering. It is always a great event, starting with workshops and progressing through 3 days of interactive sessions. This year it all starts on Monday, November 6th and ends on Thursday, November 9th. Check out details at the NALMS website at https://www.nalms.org/.

Your Lake and You Publication Available

For many years NALMS has had this newspaper-style publication available, one that has a lot of basic info on lakes and can be customized for any particular lake, association, or state. Your Lake and You has recently been updated and is provided as an electronic file that may be very useful to your lake group. As explained on the NALMS website, the 2017 online edition of the Your Lake & You! booklet is an updated version of the 8-page newspaper and helps explain to homeowners the steps they can take to protect the lakes they live on and love. This wonderful resource is loaded with basic lake information, strategies for taking better care of lakes, and descriptions of resource publications. Find it online at https://www.nalms.org/nalms-publications/.

Lakes Appreciation Month Wraps Up

In case you missed it, July was officially “Appreciate Your Lake” month. For members of NALMS, July represents a time to reflect on how we value lakes, usually with a focus on individual favorites. A number of people were interviewed on National Public Radio’s program Here and Now, segments of which can be found online hereOf course, you can appreciate your lake any time, and some of us like our lakes even better at other times of year besides summer. But summer is the key season to most, and July marks the annual kick off of the Secchi Dip In, an annual collaborative data collection effort that has resulted in a magnificent data base that allows us to know what the distribution of water clarity is for all regions of the USA. If you measured Secchi transparency in July 2017, please submit your data to this valuable data base. Check out the NALMS website for more on Lakes Appreciation Month and what you can do for your lake!

 

Algae Mats

Two groups of algae form almost all the mats: green and blue-green algae. These mats mostly form on the bottom, utilizing nutrients at the sediment-water interface, then move upward as they trap their own photosynthetic gases or accumulate gas released from the sediment under thick tangles of filaments. These mats may continue to grow for a time at or near the surface as a function of stored nutrients from their time on the bottom or from additional nutrients in the water column. Yet ultimately they tend to wind up on the surface, often blown to the edges by wind, in large decaying masses that turn various colors from yellow to blue and may be quite malodorous. In great quantities, they can really detract from the lake experience.

One partial exception includes the “cotton candy” or “cloud” growths of certain filamentous greens, mostly in the Spirogyra group. These algae do get their start at the bottom, but grow upward in a loose, slimy affiliation that looks like a mass of light green cotton candy or a cloud in the water. When you try to grab it, there is not much to grab, but your hand feels slimy. These algae produce a lot of mucilage, hence the slimy feel, and have enough structural strength to expand into these underwater, microscopic, “tinker-toy” conglomerations.

Sometimes a green mat will remain anchored to the sediment while part of it floats upward, creating a pillar in the water. Blue-green mats of Plectonema are brown to black and don’t rise in New England lakes until late summer, if at all, but from about Maryland south they can be a major impediment to lake use from early summer on. In New England, blue-green surface mats are most often chunks of Oscillatoria that break free of the bottom; these are very dark blue to black, often with brown sediment on the underside (still attached from the bottom) and they often have a distinct and unpleasant odor.

Algae mats are a clear indication that nutrients have accumulated in the sediment in water shallow enough for light to penetrate to the bottom. Once formed, algae mats are very hard to kill, as the outer filaments protect the inner filaments to a large degree. Removing the sediment is the most effective approach, but is very expensive and involves an often tedious permitting process. Treating the sediment with a phosphorus inactivator or algaecide before dense mats form is often effective, but results are not permanent.

Green algal mats   

Blue-green algal mats