Autotrophic picoplankton assemblages in subtropical reservoir: temporal and vertical dynamics in abundance and biomass

Abstract

The composition, biomass and dynamics of autotrophic picoplankton (APP, size range of 0.2–2 μm) in a subtropical reservoir (Cerrillos, Puerto Rico) were examined from January to December 2013. Samples were collected monthly in the limnetic zone. Because of their small size, APP were identified and quantified using flow cytometry and epifluorescence microscopy. The warm monomictic nature of this reservoir resulted in thermal stratification during most of the year. Near surface dissolved oxygen was relatively high (7.61 mg/L). However, hypoxic and anoxic conditions were present in the hypolimnion during the whole period of study. Flow cytometry analyses allowed the identification of two populations of picoplanktonic organisms. Picocyanobacteria groups (phycoerythrin-rich Synechococcus type) were dominant throughout the study, with maximum summer abundances of 6.6 × 104 cells/mL, although picoeukaryotes were temporally important. Annual mean total chlorophyll-a (Chl-a) in the euphotic zone was 7.54 ± 3.41 μg/L. Fractionated Chl-a exhibited a relatively constant pattern: ranging from 0.24 to 1.99 μg/L. The average contribution of APP to total Chl-a concentration was 11%. Chl-a concentration reflected the annual variation in picoplankton, being higher during summer when solar radiation was also higher. The picoplanktonic community was structured in accordance to thermal stratification, nutrients and light availability. Understanding the dynamics of picoplankton and their response to environmental change are important because they are a good indicator of the trophic status of reservoirs, especially drinking water reservoirs, and can guide management decisions.

Keywords

• Chlorophyll
• flow cytometry
• picoplankton
• picocyanobacteria
• Synechococcus
• nutrients