Confianza horizontal: la (poco) mejor valoración de la ciudadanía

Plankton ecologists have been observing and studying seasonal variation of phytoplankton for many years (Pearsall, 1930, 1932; Hutchinson, 1967; Sommer et al., 1986; Interlandi et al, 1999; Huszar et al., 2003; Dupuis & Hann, 2009). Phytoplankton populations in lakes and reservoirs are composed of different species. Variation in

phytoplankton species composition in a lake follows a similar seasonal pattern from year to year and among lakes of similar trophic status (Tilman et al., 1982). This seasonal

periodicity of regular substitution (replacement, sequence) of species is called seasonal phytoplankton succession (Wetzel, 2001; Reynolds, 2006; Mitsch & Gosselink, 2007). Such successional patterns in phytoplankton composition could be expressed as seasonal changes in total biomass, species richness, and diversity.

Factors that influence phytoplankton succession can be allogenic or autogenic. Succession caused by the organisms themselves is called autogenic (life cycle, competition, predation, parasitism, allelopathy, and other factors under biological control), when distribution of species is governed by its response to the environment (temperature, light, turbulence, water chemistry, and other external factors) it is called allogenic succession (Smayda, 1980; Tilman et al., 1982; Sommer, 1987; Lampert, 2007). Allogenic and

autogenic, however, are not mutually exclusive between each other. For example, annually temperature changes, stratification, and water movement are among the important

allogenic factors influencing development of the phytoplankton community, whereas light could have both autogenic (light attenuation by phytoplankton and detritus) and allogenic (daylight, mixing depth) influences. Higher the phytoplankton density results in steeper the light gradient. This will affect not only the distribution of phytoplankton through the water column, but also will increase the selective advantage for motile phytoplankton (Lampert,

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2007). Since phytoplankton succession depends on environmental factors, generalization for different lakes is difficult to make. The phytoplankton succession varies regionally and may vary from year to year and from lake to lake in response to change to local conditions.

One attempt to explain seasonal succession of phytoplankton in correlation with physical and biological factors is the Plankton Ecology Group (PEG) model. The PEGroup, which consists of 30 plankton ecologists, developed a conceptual model based on a

comparative study of phytoplankton and zooplankton succession in 24 temperate lakes (Sommer et al., 1986). The model correlates seasonal changes in phytoplankton biomass and species variation to changes in environmental factors including light, temperature, nutrient availability, and mixing of the water column. The model summarizes the seasonal variations in phytoplankton and zooplankton in 24 sequential events (patterns) (Sommer et al., 1986).

General trends in phytoplankton and zooplankton variations are shown in Figure 3, where intensity of controlling environmental factors are indicated by the thickness of the black horizontal bars beneath the biomass graph. Phytoplankton succession in eutrophic lakes typically has a spring peak (spring maximum), which happens after the spring turnover. In this period, temperatures and daylight period are increasing, and nutrients become more abundant due to mixing. The rapidly increased phytoplankton population consists of fast growing species. Growing zooplankton are then grazing on phytoplankton. Because of grazing phytoplankton population decreases, which leads to food limitation for zooplankton. Decreasing of both phytoplankton and zooplankton results in a period called ‘clear-water’ phase. In this phase, nutrients may accumulate. The lack of grazing pressure and increased nutrient concentrations in early summer gives an opportunity to

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species richness is high, consisting of small sized species, which are susceptible to grazing, and slow growing larger sized colonies, which is resistant to zooplankton pressure. As phytoplankton grow, the nutrients are consumed and become a limiting factor for growth, causing a rapid reduction in biomass. Upon fall as temperatures and day-length decrease, phytoplankton population also decreases. However, during fall turnover nutrient

concentrations may increase due to mixing which in turn may result in a slight increase of phytoplankton. After the fall, peak phytoplankton continues to decrease in general trend (Sommer, 1986).

Figure 3. The original PEG model.

Seasonal (winter through autumn) biomass patterns in eutrophic water bodies. Focus on phytoplankton (blue solid line) (dark shading, inedible for zooplankton; light shading, edible for zooplankton). The thickness of the horizontal bars indicates the seasonal change in relative importance of physical factors, grazing, nutrient limitation, fish predation, and food limitation (adopted from Sommer et al., 2012)

The PEG model (Sommer et al., 2012) is a good starting point to illustrate the seasonal variation in the main growth factors for phytoplankton. In the PEG model the seasonal patterns of major phytoplankton species are described based on the observed succession of phytoplankton in Lake Constance (an N-limited lake) and was compared with 23 other lakes (Sommer et al., 1986). Briefly, in spring phytoplankton population consists of

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fast growing species such as small diatoms and in some lakes large diatoms,

Cryptophyceae, and small green algae (Sommer et al., 1986). According to nutrient kinetic parameters included in Table 1, diatoms have relatively higher maximum growth rates (m) and higher specific nutrient uptake rates (Vm) for N and P than the rest of the

phytoplankton groups. Their higher growth rates make diatoms good nutrient competitors and grow fast when nutrients concentrations are higher. Mixing of the water column is beneficial to keep diatoms suspended (Reynolds, 2006). In addition to N and P, diatoms require dissolved silica (Si) to build their cells. After “clear water”, Cryptophyceae, and inedible green algae become dominant and deplete P concentrations. In addition, most of the large colonies of green algae, similar to diatoms, require mixing to retain suspension in the water column. Green algae usually have higher requirements for P (Tilman & Kielsing, 1984) and decrease rapidly under a P-limited condition (Sommer et al., 1986). That

condition gives opportunity for growth of large diatoms such as Asterionella and Fragilaria.

Asterionella and Fragilaria may grow well at low P and high Si concentrations. However, Si

usually becomes exhausted after a higher spring development of diatoms (Sommer, 1991). In addition, these algae can use their advantages only if kept in suspension due to

turbulence. A summer phytoplankton population consists of large dinoflagellates

(Ceratium), which could co-dominate with Cyanobacteria. Both dinoflagellates and

Cyanobacteria have relatively lower m and Vm values for both N and P, which implies slower growth rates compared to other phytoplankton groups. Dinoflagellates and Cyanobacteria also have a high resistance to grazing due to the bigger sized cells. In addition, dinoflagellates are able to migrate vertically, while Cyanobacteria can regulate buoyancy. Vertical migrations make them able to adjust their position in the water column and to exploit vertical gradients of light and nutrient sources in stratified lakes.

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Nitrogen depletion, found in one of 24 lakes during a stratified period, favors a shift to nitrogen-fixing species of filamentous blue-green algae (Anabaena, Aphanizomenon). These Cyanobacteria have the ability to fix N2 from the atmosphere, which makes them well adapted to a nitrogen-limited condition. However, Cyanobacteria are also “poor

competitors” for P (Fogg et al., 1973, Smith, 1983), which makes them less competitive in P- limited condition. Towards fall, increased mixing and nutrient concentrations may result in growth of filamentous and large algae (diatoms, Ceratium, green algae). However, a

decrease of underwater light and temperatures results in a general reduction of phytoplankton population (Sommer et al., 1986).

Although the PEG model might not fit to all phytoplankton community changes, the model provides a conceptual framework for interpretation of phytoplankton succession regarding factors such as temperature, light, mixing, nutrient availability, competition, and loss processes. Phytoplankton are very sensitive to the changes in factors such as

temperature and nutrients, and any changes in these factors usually results in deviation from typical phytoplankton succession. Therefore, changes in natural phytoplankton

succession and community structure are an essential feature in lakes and reservoirs trophic status assessment.

A long-term study on Lake 227 in the Experimental Lake Area, Canada showed that manipulation in nutrient status resulted in change phytoplankton community (Schindler et al, 2008). The phytoplankton population shifted from green algae and non-fixing

Cyanobacteria species to N2-fixing Cyanobacteria species. In recent decades, eutrophication of the water bodies have resulted in increased phytoplankton growth, decreased

phytoplankton diversity and shifts in typical phytoplankton population structure (Schindler, 2008). Eutrophic lakes have been more often associated with an increase of

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intensity and frequent blooms of Cyanobacteria (Oliver & Ganf, 2000; Paerl & Huisman, 2008; Schindler et al, 2008; Smith & Schindler, 2009).