Phytoplankton communities

Communities of phytoplankton microalgae

Prepared by Viviana Cruz Hernández (IE/UMSA)

We evaluated 139 phytoplankton samples, by inverted microscopy, identified based on the traditional Taxonomic Classification and by Functional Groups according to their morphology (Kruk et al. 2010), to estimate density (org/mL) and biovolume (mm3/L) represented by bar graphs, by season, zone, and sampling time. 

We identified 59 genera among the phyla: Bacilliarophyta (20), Chlorophyta (20), Charophyta (7), Euglenozoa (= Euglenophyta) (3), Cyanobacteria (4), Myozoa (= Dinophyta) (2), Cryptophyta (2) and Ochrophyta (1). We divided the stations into 4 sampling zones (North-Western, North-Eastern, Central, and Katari river water system; see the sub-page <Material and Methods/Lake campaigns - Periodic monitoring>). In the following, we present the composition of the microalgae. 

Northwest zone 

We evaluated 10 samples in the Chúa Trench (LC). During 2020, between wet season (H) and dry-wet transition (DWT), green algae (Chlorophyta and Charophyta) predominated, mainly by the genera Klebsorbidium (48 and 74 org/mL), Oocystis (2, 14 and 15 org/mL). During 2021, in wet, wet-dry transition (THS) and dry seasons, diatoms (Bacillariophyta) dominated, in particular Fragilaria (90, 156, 118, 58 and 47 org/mL) and Oocystis (21, 18, 18,15 and 20 org/mL). Cryptomonas had low density (27 and 26 org/mL) during the 2021 wet season.

Northeast zone 

Central zone

Katari River Water System zone 

The Katari water system could only be sampled during the H and THS periods. Spirogyra (675 org/mL) dominated in SK1. During H 2021, Nitzchia (half 123 org/mL) dominated in SK2, SK3, SK5, SK7 and SK10. During THS, Anabaena (127 org/mL) dominated in SK7, and Spirogyra (44 org/mL) during SK10. Scenedesmus (204 org/mL) dominated in SK4. Epithemia (60 org/mL) and Nitzchia (60 org/mL) dominated in SK9.

Phytoplankton classification on the basis of its functional morphology (MBFG) 

The MBFG classification by Kruk et al. (2010) groups microalgae according to their functional morphology, considering physiological, morphological and phenological characteristics related to the environment. For example, the uptake of resources and light depends on the dimensions of an organism, its volume and surface/volume ratio. This classification takes into account volume, maximum linear dimension (MLD), area, and the presence of mucilage, flagellum, gas vesicles, and silicon structures. Thus, microalgae are differentiated into 7 functional groups. We performed an Analysis of Similarity (ANOSIM) to identify differences in the composition of functional groups between zones, and representative groups in each zone. 

The analysis showed significant differences between the different zones (p-value ≤ 0.05); with the exception of the Northwest (NO) vs Sistema Katari (SK) zones, (p-value = 0.44), suggesting a high similarity between them. Group VI dominated the NW and SK zones, despite their different levels of eutrophication, so it is necessary to combine with information on physicochemical conditions. The Chúa Trench (LC), the only one in the NW zone, mostly presented alkaline conditions, highly oxygenated and with low nutrient levels. In contrast, near the mouth of the Katari River (SK), conditions were extreme, with slightly acidic water, high turbidity, low oxygenation and strong currents. The dominance of Group VI in the Katari System agrees with Izaguirre et al. (2012), who similarly, recorded this group in poorly illuminated waters and turbid shallow lake systems, as described by Allende et al. (2019). Turbulence generated by water flow could be beneficial for the resuspension of diatoms (Group VI) as opposed to cyanobacteria and green algae. 

Group IV was common between the EC and NE zones. It is characterized by medium-sized organisms without special characters (e.g. flagella) (Kruk et al. 2010) that dominate environments with good quality, abundant zooplankton, low light attenuation and few nutrients (Pacheco et al. 2010; Izaguirre et al. 2012). Group V dominated in the EC and NE zones, represented by medium-sized, flagellate, mixotrophic organisms. They can tolerate mixing zones, with low light and characteristics of meso- to eutrophic environments. They exhibit high to moderate tolerance to light limitation and current conditions (Kruk et al. 2010). Group III was present in the Central Zone (CE) and the Katari System (SK), dominating ≥ 80% of the microalgal community. It includes filamentous organisms with the presence of aerotopes (gaseous vacuoles that regulate the depth in the water column). They are tolerant to light limiting conditions, presenting optimal development in eutrophic environments with low light penetration (Rangel et al. 2016, Magalhães et al. 2020). These conditions are representative of the Cohana area and the Katari system. Group II was present in the NE and EC zones, made up of flagellate organisms with silica structures. They predominate in oligotrophic to mesotrophic waters with aquatic vegetation (Kruk et al. 2010). These algae were present in up to 50% of the biovolume in NE5, BH, LH, CE1-4, CE12-14 and CE18-19 (except BH), in the presence of submerged macrophytes (Charophyta). 

Group VII predominated in LC (up to 42% of biovolume), also in BH, LH and CE4 (up to 35% in BH, ≤ 20% in the other stations). Group VII predominates in waters with high or low nutrient content (Kruk & Segura 2012); also in clear waters with vegetation (Izaguirre et al. 2012), being sensitive to mixing processes and low resource conditions (Kruk & Segura 2012; Kruk et al. 2010). Kruk et al. (2010) concluded that their classification best captures the ecological function of phytoplankton so that functional groups based on morphology may be best suited to predict community composition. Understanding and predicting the phytoplankton communities of the Titicaca Lago Menor is important because it is vital for the proper functioning of the lake. Paradoxically, they have been little studied. The study of the functional groups in relation to the conditions of Lake Titicaca still needs to be deepened, as this classification was originally proposed for other regions.

Distribution maps of some important microalgae

We made distribution maps with Surfer 16 software. We considered the biovolume (mm3/L) of genera of importance, such as: Carteria, causing the bloom recorded in 2015 (Achá et al. 2018); Spirogyra, being that some species can form blooms (Gubelit 2019); Oscillatoria, as well as other cyanobacteria, as they present a high surface/volume ratio can tolerate low light conditions (Naselli-Flores et al. 2007, Reynolds et al. 2014), as well as turbid environments with a high trophic degree (González & Roldán 2020).