HUMIC SUBSTANCES ENHANCE ESTUARINE PHYTOPLANKTON PRODUCTION, BUT ALSO PROMOTE COMMUNITY CHANGES

PAN, JERÓNIMO; FLORES, FABIO LUCAS

Mar del Plata

Congreso; XVIII Congreso Latinoamericano de Ciencias del Mar; 2019

ALICMAR

Humic substances enter estuarine systems from soil and sediment reservoirs, through rainfall-leaching and flooding and may impact planktonic community composition and productivity, favoring certain phytoplankton groups over others. In order to test the hypothesis that an increment in humic substances alters phytoplankton production, as they provide an additional source of nutrition for auto- and mixotrophic plankton, experiments were carried out with the plankton community from a temperate coastal lagoon in Argentina (Mar Chiquita; ~60 km2; tributary basin ~10,000 km2). First, a lixiviate of humic substances was prepared incubating topsoil (2 kg from saltmarsh berm + 2 kg from grassland) in 40-L ambient estuarine water for 7 days (prior to experimental setup), and filtered (0.2-μm). Two experimental treatments were considered: (1) a mixture of 33% v/v lixiviate+66% v/v estuarine water; and (2) ambient estuarine water diluted with 33% v/v filtered water [control]. Experiments looked into phytoplankton community changes, and consisted on incubations of control and lixiviate-enriched estuarine water in 9-L mesocosms (n=3) for 7 days, with daily addition of 10µM NO3 and 1µM PO4 for nutrient-replete growth; they were replicated in Dec 2017 and Oct 2018 (Austral summer and spring, respectively). The parameters considered were spectrophotometrically-determined chl-a, and microplanktonic community composition from microscopy counts. Humic acids were estimated spectrophotometrically. Phytoplankton growth was somehow coupled with temperature changes (with a few h lag) in both seasons. In the summer, the humic lixiviate consistently presented higher chl-a content than the control (ranging 8-51%; Fig. 1), thus the addition of humic substances significantly increased phytoplankton standing stocks. Conversely, both treatments presented rapid exponential growth in the first 3-d in the spring, followed by a decline and plateau, but there were no significant differences in chl-a between treatments (Fig. 2). In the summer experiment, the community changed from being initially dominated by diatoms and planktonic filamentous cyanobacteria, to a dominance of Anabaena spiroides (which presented significantly higher densities in the humic lixiviate than in controls from d-5 onwards). Also, the floristic composition changed divergently in the two treatments, not only in relation to A. spiroides, but also with significantly higher densities of phytoflagellates and aloricate ciliates in the humic lixiviate than in the control. These findings open new avenues for the study of future scenarios with increased terrestrial leaching into estuaries, the fate of primary production and its translocation into upper trophic levels.