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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.4. Causes of Eutrophication

1.4.1. Assessment Approaches

Following factors determine the relative role of phosphorus or nitrogen in limiting the abundance or productivity of phytoplankton:

• The ratio of nitrogen to phosphorus in hydrologic and meteorologic inputs to a lake and in the vertical fluxes of dissolved nutrients within the lake.
• Preferential losses from the euphotic zone by processes, such as denitrification, sorption of phosphorus to particles and differential settling of particles with different nitrogen to phosphorus ratios.
• The relative magnitude of external supply to internal recycling and redistribution.
• The contribution from nitrogen fixation. Unfortunately these processes have been measured in a coordinated manner in only very few lakes. Instead, inferences from several indicators of nutrient limitation must be made.

The ambient concentrations of nutrients sometimes can provide an indication of the likelihood of limitation. Often the limiting nutrient is reduced to very low concentrations, while nutrients less in demand have higher concentrations. However, nutrients are present in different forms, which vary in their relevance to assessing limitation. In most studies of rivers and standing waters, the forms of phosphorus and nitrogen are operationally defined based on available analytical methods. The distinction between particulate and dissolved forms depends on the porosity of the filter used to separate the two fractions; filters with porosities approximately 0.5 ƒÊm are commonly used. Total dissolved phosphorus is often divided into soluble reactive phosphorus, which can sometimes be considered dissolved inorganic phosphorus, and dissolved organic phosphorus. Similarly, total dissolved nitrogen includes dissolved inorganic ammonium, nitrate, and sometimes nitrite and urea, and dissolved organic nitrogen. Total particulate phosphorus and nitrogen are determined as particulate inorganic phosphorus and nitrogen and particulate organic phosphorus and nitrogen. In some cases, concentrations of total phosphorus or nitrogen are measured; these include all the dissolved and particulate forms. However, only a portion of the total phosphorus or nitrogen is biologically available, and special analytical techniques are required to determine that fraction.

The nitrogen to phosphorus ratio in particulate matter suspended in lakes is a potentially valuable index of the nutritional status of the phytoplankton, if contamination from terrestrial detritus can be discounted. Healthy algae contain approximately 16 atoms of nitrogen for every atom of phosphorus. Ratios of nitrogen to phosphorus less than 10 often indicate nitrogen deficiency and ratios greater than 20 can indicate phosphorus deficiency.

The rate of uptake of radioactive phosphate by particulate matter suspended in lakes is a widely used index of phosphorus demand by the plankton. Turnover times are typically rapid when phosphorus is in short supply and are slow when supply is adequate.

Nutrient limitation can be assessed by experimental manipulation of nutrient levels. Experiments can be carried out on scales ranging from small flasks to enclosures containing many liters to whole lakes. Large volume experiments provide more realistic conditions than small containers. Enclosures with volumes ranging from 10s to 100s of liters can be replicated with multifactorial experimental designs that permit discrimination of interacting factors leading to changes in water quality.

Phytoplankton species composition changes in response to eutrophication. Although general trends in the development of certain assemblages of phytoplankton are associated with trophic status, particular phyla or class cannot be assigned exclusively to one level of eutrophication. While cyanobacteria are commonly observed under eutrophic conditions, other species can be important instead, and it is not fully understood why the cyanobacteria sometimes outcompete other algal groups. Several characteristics of cyanobacteria have been invoked to explain their success. Bloom-forming cyanobacteria are known to tolerate low light and can shade other algae by forming surface scums; their filamentous and gelatinous forms may retard grazers; some species have high affinity for dissolved nitrogen and carbon dioxide. Cyanobacteria grow rapidly during conditions with stable stratification, and some have the ability to assimilate atmospheric nitrogen.

Often nitrogen to phosphorus ratios are low in eutrophic lakes and high in mesotrophic and oligotrophic ones, and blooms of nitrogen-fixing cyanobacteria have been induced experimentally in lakes after reducing nitrogen to phosphorus ratios in inputs (Figure 1.15.). For example, dissolved nitrogen concentrations in highly eutrophic lakes in western Canada prior to the onset of Aphanizomenon flos-aquae blooms are at their minimum with nitrogen to phosphorus ratios as low as 1 for approximately one week. These short-lasting spring minimum of nitrogen to phosphorus ratios trigger the onset of nitrogen-fixing cyanophyte blooms. However, the seasonal mean values of the nitrogen to phosphorus ratios in these lakes are as high as 20 to 30, and, therefore, misleading in assessing nitrogen limitation, as nitrogen fixation rapidly restored the nitrogen to phosphorus ratios to levels of 15 and higher. Nitrogen deficit may trigger the appearance of nitrogen fixing species, which can alter the nitrogen to phosphorus ratio by increasing the value of the numerator by higher input of atmospheric nitrogen and rectify the overall nutrient balance.

Figure 1.15. Relationship of the cyanophyte abundance (percent) to nitrogen to phosphorus ratios in lake water of different trophic states.

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