Phytoplankton response to pH rise in a N-limited floodplain lake: relevance of N2-fixing heterocystous cyanobacteria

• Published in: Aquatic sciences Vol. 72; no. 2; pp. 179 - 190
• Main Authors: Unrein, Fernando, O’Farrell, Inés, Izaguirre, Irina, Sinistro, Rodrigo, dos Santos Afonso, María, Tell, Guillermo
• Format: Journal Article
• Language: English
• Published: Basel SP Birkhäuser Verlag Basel 01-03-2010; Springer; Springer Nature B.V
• Subjects: Algae; Animal and plant ecology; Animal, plant and microbial ecology; Aquatic plants; Biological and medical sciences; Biomedical and Life Sciences; Cyanobacteria; Ecology; Floodplains; Fresh water ecosystems; Freshwater & Marine Ecology; Fundamental and applied biological sciences. Psychology; General aspects; Lakes; Life Sciences; Marine & Freshwater Sciences; Nitrogen; Nitrogen fixation; Nutrient requirements; Oceanography; Phytoplankton; Plankton; Research Article; River basins; Summer; Synecology; Argentina; pH; Phytoplankton; N; fixing cyanobacteria; Aquatic environment; Flood plain; Cyanobacteria; Nitrogen fixation; Lakes; Bacteria
• ISSN: 1015-1621; 1420-9055
• DOI: 10.1007/s00027-009-0115-1

Phosphorus has been traditionally regarded as the controlling nutrient for phytoplankton growth, however, N-limitation is likely to occur in several environments. For example, nitrogen is considered the main nutrient limiting phytoplankton in floodplain lakes of the Paraná River basin. However, N
2
-fixing heterocystous cyanobacteria (N
2
-cyano) are usually absent in these water bodies. The low pH values frequently found may limit the development of these algae. We hypothesise that long-term lake isolation allows conspicuous phytoplankton growth during summer, resulting in high photosynthetic rates and pH. This scenario combined with low DIN (dissolved inorganic nitrogen) would favour the development of N
2
-cyano. Phytoplankton composition was studied during 16 months in a vegetated and isolated floodplain lake in the Paraná basin. Additionally, pH was artificially increased in in situ mesocosms to test effects on phytoplankton structure. Lake phytoplankton was dominated by flagellates (cryptophytes and euglenophytes) and small coccoid algae (chlorophytes and colonial cyanobacteria). Algal biomass was highest during warm periods. Although pH increased up to 8.8 during the second summer period, N
2
-cyano remained rare, most likely because of the high DIN concentration recorded. The alkalophilic diatom
Cyclotella meneghiniana
dominated and was positively correlated with pH. Conversely, PO
4
=
concentrations in the mesocosm experiment were high and DIN remained relatively low. pH enhancement in the treated mesocosms (avg. pH = 8.2) promoted the development of N
2
-cyano (
Anabaena
spp.) and
C. meneghiniana,
which after 1 month of incubation accounted together for 50% of the biomass in contrast to less than 1.6% in control containers. Our results support the hypothesis that during the warm season and under low DIN concentration, high pH favours N
2
-cyano growth in these lakes. We provide new evidence supporting the idea that even under optimal nutrient conditions, N
2
-cyano do not thrive unless other requirements are satisfied.