The Concept of Limiting Factor for Algae

The concept of a limiting factor is very old, summarized in Leibig’s Law of the Minimum, which used a bucket with staves of different lengths to show that the shortest stave controlled the depth water could attain in the bucket. The theory is simple, perhaps too simple to capture the variation induced by having multiple species present with varying optimal nutrient ratios, light requirements, and maximum growth rates. In reality, multiple factors control the growth of algae, with some species more impacted by one factor than others. There is therefore constant competition going on, with each species increasing in abundance to the extent possible before some species-specific limit is reached based on the relation of the individual species’ needs and available resources.

There are very few generalizations that apply to all algae with regard to limiting factors. Light is very important to photosynthesis, but some algae survive under very low light and some are “facultatively heterotrophic”, consuming organic compounds, bacteria, other algae, and even zooplankton to gain energy rather than depending on photosynthesis. Nitrogen is a key nutrient, but cyanobacteria with specialized cells called heterocytes can convert dissolved nitrogen gas into ammonium. Since our atmosphere is 78% nitrogen, it is virtually impossible to prevent these algae from acquiring some nitrogen. Phosphorus is the closest thing to a sure limiting factor, being relatively rare in the earth’s crust but critical to energy transfer. There is no substitute for phosphorus, and it can be controlled by multiple means in active management practices, so it is the logical target of choice in algae control programs that seek to prevent algae growth to bloom proportions.

Approaching algae management with multiple limiting factors is a good idea. In many cases, best management practices that control phosphorus also limit nitrogen and other nutrients. Increased flushing can reduce detention time such that growth rates are insufficient to generate blooms. Adjusting the fish community to encourage more and larger herbivorous zooplankton can increase algae consumption rates and funnel energy into desirable fish while limiting algae biomass. Yet control over planktonic algae will lead to deeper light penetration, which may provide benthic algae enough light to grow using nutrients available at the sediment-water interface during decomposition or other release mechanisms. Dredging can remove algae resting stages and nutrient reserves, limiting benthic production, but the cost is usually extreme.  

Using the concept of limiting factor(s) for algae is appropriate but more than a little complicated. The situation is far more complex that the simple bucket analogy suggests, and the more we can learn about the lake we want to manage the more successful we are likely to be in achieving our goals.

NECNALMS Leaders Meet

Representatives of all six New England states met in Concord in early December to review programs and get updates on lake issues in each state. The spread of invasive plants and increased frequency of cyanobacteria blooms continue to be the primary biological threats. Retirements and reduced staffing in state agencies represent the primary administrative issue. Funding cutbacks, especially for federal “pass-through” monies, constitute the greatest economic disincentive for lake management. Yet the demand for lake management remains high, and many lake associations and towns have been addressing issues on their own. NECNALMS continues to seek ways to support efforts by New England states to foster effective and sound lake management.

Lake leaders discussed past and future conferences. New Hampshire is due to host NECNALMS in 2018, but with the normal organizers very involved in national efforts by NALMS and changes in policies at the typical conference venues, it is not certain that there will be a 2018 NECNALMS conference. It is possible that NALMS will come to New England in 2019, in which case the NECNALMS leadership will be very involved in planning that conference starting in mid-2018. A decision will be made soon.

The 2017 NALMS symposium was held in early November just outside Denver, CO, and was both well-attended and well-run. The program was diverse and opportunities for interactions were plentiful. The Colorado Lake and Reservoir Management Association was the local host, and did a fine job on the arrangements. NALMS has been experiencing some financial stress relating to federal freezes on programs and funds, but is managing carefully to avoid shortfalls. The 2018 conference will be held in Cincinnati, OH and a decision on the 2019 location should be made this coming spring.

NECNALMS leaders are also very active on the national and international levels through NALMS. Perry Thomas of VT serves as the Region I director, while Amy Smagula of NH is the NALMS Secretary. Jeff Schloss of NH is the conference planner, while Ken Wagner of MA returns to duty at the start of 2018 as Editor-in-Chief of the NALMS peer-reviewed scientific journal, Lake and Reservoir Management.

Forms of Nitrogen

Nitrogen comes in multiple forms but, much like phosphorus, not all are available to all algae and plants. Total nitrogen is akin to total phosphorus, providing a maximum estimate of nitrogen that might be utilized for plant and algae growth, but different species make better use of some forms than others, and the range of nitrogen forms is wider than for phosphorus, so relationships are more complicated.

Nitrogen gas makes up about 78% of the earth’s atmosphere and reaches equilibrium with the aquatic environment, but only some specialized organisms can use nitrogen gas directly. Included are certain cyanobacteria, or blue-green algae, which is why a low ratio of non-gaseous nitrogen to phosphorus favors cyanobacteria; they have access to a nitrogen source that other algae can’t use.

One key set of nitrogen compounds is the sequence from ammonium to nitrite to nitrate, with conversion of ammonium to nitrite and nitrite to nitrate in the presence of oxygen and specific bacteria. The conversion is fairly fast, especially for nitrite, so nitrate should be the most abundant of these three nitrogen forms when oxygen is abundant. When oxygen is used up, as can occur in the deep zone of lakes over the summer during stratification when decomposition uses up oxygen and atmospheric replenishment is minimal, ammonium accumulates. Ammonium and nitrate are differentially preferred by different species of algae and higher plants, and so can affect aquatic biology. Ammonia, which has one less hydrogen molecule than ammonium and is toxic to aquatic animal life, is present as a fraction of ammonium depending on temperature, pH and other water quality factors, upping the stakes for which forms of nitrogen are present at what concentration.

Total Kjeldahl nitrogen (TKN) is determined by a digestion that turns organic nitrogen into ammonium, so this test measures all but nitrate and nitrite nitrogen. Addition of nitrate/nitrite to TKN is functionally equivalent to total nitrogen. The organic fraction (TKN minus ammonium) is not directly available to support plant growth, but decay processes will make some of that organic fraction available over time.

When assessing nitrogen in lakes and tributaries, the minimum testing to gain reasonable understanding of nitrogen influence includes TKN and nitrate/nitrite, although it is useful to have ammonium as well to separate the organic fraction from TKN.