Embedded Files
Historical spatial cover of Totora (emmergent aquatic macrophytes) from 1979 to present based on satellite remote-sensing (Landsat)
by Jhasmin Duarte Tejerina
& Mishel Justiniano Ayllón (IIGEO/UMSA)
Evolution of the area of Totora reedbeds over the last 4 decades
To interpret the behavior of the Totora reed beds, we prepared Table 1, comparing the Totora surface areas (coverages) between the dry and wet seasons, according to the lake levels in the different years of study, selected as either presenting high or low water levels.
Table 1 - Area of Totora reed beds (ha) in the Bolivian sector of Lago Menor and average water levels (m) as a function of years and seasons (wet vs. dry). Computed from Landsat-2, 5 and 8 satellite images.
To better understand the behavior of the cattail with lake levels, the graph in Fig. 1 shows that in the years when the lake level rose (1986, 1997, 2004 and 2013), the Totora surface area was low during wet periods - with the exception of 2013 - and increased during the dry months. This behavior could be interpreted as follows: as the lake level rose, the Totora became progressively submerged, and consequently, the coverage (surface area) of Totora was lower, with the smallest surface area (3,876 ha) of the last 4 decades in 1986 (wet season). When Totora are submerged, the water inhibits their respiration and reduces the supply of energy for their growth and photosynthetic activity.
Figure 1 - Totora reed surface areas in the Bolivian sector of Lago Menor, in dry and wet periods between 1979 and 2019. Own elaboration. Traductions: 'Superficies de totorales del Lago Menor (ha)' = Totora surface areas of Minor Lake (ha); 'Nivel del lago (m s.n.m.)' = Lake water level (m a.s.l.); 'Superficies en epoca húmeda' = Surface areas in wet season; 'Superficies en epoca seca' = Surface areas in dry season; 'Nivel promedio en epoca húmeda' = Average level in wet season; 'Nivel promedio en epoca seca' = Average level in dry season.
Figure 2 - Evolution of the amplitude of annual water level, between maximum and minimum (in cm), of the elevation of Lake Titicaca between 1979 and 2020. Source: SENAMHI-BO monthly elevations for the Huatajata limnimetric station (Duarte Tejerina et al. 2021).
On the contrary, in the years with low lake level (1979, 1996, 2010, 2015, 2018 and 2019), the Totora surface area was greater in the wet season than in the dry season -- with the exception of 2010, 2018 and 2019 --; the years 2018 and 2019 were not considered as extreme. For 1979 it was not possible to work with dry season data due to the presence of clouds in the image, nor was it possible to work with the 1983 images since they were not available.
Figure 3- Monthly evolution of the elevation of Lake Titicaca during the years of the study. To obtain the corresponding altitude, the following formula is applied: Altitude (m) = [elevation (cm) / 100] + 3,804.41 (m). Source: SENAMHI-BO elevations for the Huatajata limnigraph (Duarte Tejerina et al. 2021).
Thus, two parameters combine themselves and are important for determining Totora surface areas between dry and wet years and seasons: a) the annual variation in the lake's elevation between minimum and maximum levels, and b) the rate at which the elevation rises in wet seasons and falls in dry seasons.
It is important to mention that 2015 was considered in our study, due to the phenomenon that occurred that year. The 2015 wet season was exceptionally long and lasted until April-May. Consequently, the excess of nutrients transported to the lake by the rivers of the Katari basin (organic matter of human origin) as well as by runoff in the crop areas (fertilizers) along the shoreline, caused a phytoplankton bloom (or 'bloom'), constituted by a single genus of unicellular green microalgae of diameter ≤ 10 µm, Carteria sp. (Chlorophyta) (Achá et al. 2018). This bloom covered almost the entire water mirror of the northeastern and central regions (up to the north of the Taraco peninsula) of Lago Menor. Apparently, massive nutrient inputs also benefited Totora growth. In fact, Totora cover was the highest (8,551 ha) of all those observed during wet periods, excluding 1979 (11,690 ha).
Comparison of zones in the study area
Figure 4 - Delimitation of the three regions (A, B, C) in the Bolivian sector of Lago Menor, our study area. Own elaboration.
Table 2 - Percentages of Totora cover between wet (left) and dry (right) seasons for the 3 zones (A, B, C) for the years 1986 to 2019. Traductions: 'Año' = Year; 'Epoca húmeda' = Wet season; 'Epoca seca' = Dry season; 'Zona' = Zone.
Evolution of Totora cover - Dry season
The map in Fig. 4 shows 3 zones (A = North, B = Central, and C = South) distributed in the study area according to Freddy Loza's thesis in 2004. Through this separation, we compared the percentages of Totora cover (extension) in the different years of interest. The highest percentages of Totora cover are found in zone B corresponding to the central area of Lago Menor, which is distinguished by being shallower.
Figure 5 - Comparative evolution of Totora cover in the dry seasons of the years 1986, 1996, 1997, 2004, 2010 and 2013.
Regions A and C have lower coverage percentages, probably due to the presence of nearby communities, the use of motorized boats, and greater depths. Figs. 5-6 present the comparative maps of Totora cover in dry and wet seasons, respectively, during the studied years.
Evolution of Totora cover - Wet season
Figure 6 - Comparative evolution of Totora cover during the wet seasons of 1979, 1986, 1996, 1997, 2004 and 2010.
Totora phenology
In the 2019 wet season (November-April), 18% (1,016 ha) of Totora was in good condition (young), and 82% (4,767 ha) in poor condition (senescent). In the 2019 dry season, 4% (250 ha) of cattail were estimated to be in good condition (young), and 96% (6,698 ha) in poor condition (senescent) (Table 3).
Table 3 - Comparative covers of Totora between young and senescent stages in 2019, in relation to wet and dry seasons.
Stem densities of Totora
The effects of seasonal variability and climatic events have an important impact on Totora phenology and the density of their stems. Previous studies had lower values in the different density categories studied (dense, semi-dense and sparse).
Figure 7 - Stem densities/m2 of Totora in 2019. Average of total data, and minimum and maximum values, obtained from Totora counts at both times of the year. Traductions: 'Húmeda' = Wet; 'Seca' = Dry; 'Densidades tallos' = Stem densities; 'Mínimos' = Minimums; 'Promedio' = Average; 'Máximos' = Maximums.
In April 2019 (end of rainy season), the average Totora stem density was 316 stems/m2, with a minimum of 170 stems/m2 and a maximum of 680 stems/m2 . In July 2019 (mid-dry season), the average Totora stem density was 287 stems/m2 with a minimum density of 123 stems/m2 and a maximum of 470 stems/m2 (Fig. 7). For the sparse density category (see Table 4):
-In wet season, the range of Totora stem density was 0-226 stems/m2, with an area of 3,376 ha distributed mainly in the northern and southern part of Lago Menor.
-In the dry season, the range of Totora stem density of was-156 stems/m2, with an area of 4,142 ha distributed mainly on the shores of the lake (litoral).
Table 4 - Totora stem stems/m2 and cover areas (ha) in 2019, according to the density category and season.
Figure 8 - Distribution of Totora densities in both wet and dry seasons, in 2019. Traductions: 'Época húmeda' = Wet season; 'Época seca' = Dry season; 'Denso' = Dense; 'Semi-denso' = Dense; 'Ralo' = Sparse.
Page updated
Google Sites
Report abuse