Abstract Picoplankton are central global carbon (C) cycling players and often dominate the ocean plankton communities, especially in low latitudes. Therefore, evaluating picoplankton temporal dynamics is critical to understanding microbial stocks and C fluxes in tropical oceans. However, the lack of studies on low-latitude picoplankton communities translates into a common conception that there is an absence of seasonality. Herein, we studied the temporal variation in abundance (measured by flow cytometry), and carbon flux (taking bacterial production and respiration as proxies) of the picoplanktonic community for the first time, as well as their environmental drivers in a low-latitude (05° 59’ 20.7″S 035° 05’ 14.6″W) Atlantic coastal station. We performed monthly samplings between February 2013 and August 2016 in a novel microbial observatory – hereafter called the Equatorial Atlantic Microbial Observatory – established on the northeastern Brazilian Atlantic coast. Our results revealed stability in temporal dynamics of picoplankton, despite a considerable inter-annual variation, with some related to the El Niño (ENSO) event in 2015. However, weak environmental relationships found were not enough to explain the variation in picoplankton’s abundance, which suggests that other factors such as biological interactions may lead to picoplankton abundance variation over time. Heterotrophic bacteria dominated picoplankton during the entire study period and between photosynthetic counterparts, and Synechococcus showed greater relative importance than picoeukaryotes. These results bring a novel perspective that picoplankton may exhibit more pronounced fluctuations in the tropical region when considering inter-annual intervals, and is increasing prokaryotic contribution to carbon cycling towards the equator. (C latitudes Therefore oceans However lowlatitude latitude seasonality Herein measured cytometry, cytometry , cytometry) taking proxies time 05° 05 (05 59 207S S 20 7 035 14.6″W 146W W 14 6 station 201 coast interannual inter annual ENSO (ENSO 2015 picoplanktons s counterparts picoeukaryotes intervals equator 0 (0 5 2 03 1 (

Laguna Chascomús is an eutrophic, turbid, shallow lake typical of the Pampa region of Argentina. This shallow lake is permanently mixed and displays a high degree of spatial homogeneity. The cyclical periods of excess and shortage of rain, characteristic of this area, result in periodic drying and flooding events. According to newspaper articles published during the first half of the XX century, the lake was originally turbid. The first patches of rooted vegetation appeared after the 1913 y 1914 floods. During most part of the XX century the lake remained in a "clear", vegetated state. During this period the dominant fish species was the silverside (pejerrey), Odontesthes bonariensis, which accounted for over 60% of the total fish catches (in biomass). Instead, by the end of the latest century the lake shifted to a turbid state that persisted until present. Currently, the phytoplankton primary production is limited by light and its values are among the highest ones reported for natural aquatic systems. In light limited systems, the transparency may be expected to be controlled by the amount of incident light, through a negative feedback loop with primary production. These predictions have been confirmed, both in mesocosm experiments and "in situ". The phytoplankton biomass, dominated by nanoplanktonic cyanobacteria accounts for 75% of the total amount of carbon in the water column. The equilibrium predicted by the light limitation theory could not hold if the model assumptions were violated. This situation could happen if the phytoplankton biomass were controlled by herbivore zooplankton. However, such a control of primary producer by herbivores seems to be precluded due to the absence of large-sized zooplankton grazers, presumably due to the composition of the fish assemblage. In contrast to earlier reports of the 1960, the proportion of pejerrey biomass is presently quite low (0.04%) and the community is dominated by omnivore microphagous, such as the sabalito (Cyphocharax voga). The dominance of sabalito could not only explain the composition of the zooplankton community, but it also could contribute to prevent or delay the consolidation of sediments.

La laguna Chascomús es un típico lago somero, eutrófico y turbio de la Pampa Deprimida. Se encuentra permanentemente mezclada y presenta un grado elevado de homogeneidad espacial. La alternancia entre períodos de déficit y de exceso de agua, característica de la región, determina que la laguna sufra ciclos periódicos de sequía e inundación. Las primeras crónicas indican que a principios del siglo XX la laguna era turbia y que las primeras matas de vegetación habrían aparecido después de las inundaciones de 1913 y 1914. Hacia mediados del siglo XX, Chascomús se encontraba en un estado de aguas claras y colonizada de manera profusa por macrófitas. En ese momento, aproximadamente 60% de la biomasa de peces correspondía al pejerrey (Odontesthes bonariensis). En las últimas décadas, esta condición se modificó y en la actualidad la laguna está estabilizada en un estado turbio en el que la producción primaria fitoplanctónica es muy alta y se encuentra limitada por luz. La biomasa de fitoplancton representa el 75% del carbono de la columna de agua y predominan las cianobacterias nanoplanctónicas. Estudios de campo y experimentos en mesocosmos indican que la transparencia del agua está controlada por la cantidad de radiación incidente a través de una retroalimentación negativa con la producción primaria. La biomasa fitoplanctónica elevada se mantiene debido a la ausencia de un control efectivo por parte del zooplancton herbívoro. Como resultado de la presión de depredación que ejerce la comunidad actual de peces, la composición del zooplancton no presenta filtradores eficientes (e.g., Daphnia y otros cladóceros de gran tamaño). En la actualidad el pejerrey representa un porcentaje muy bajo de la biomasa total de peces (0.04%), mientras que los micrófagos omnívoros, como el sabalito (Cyphocharax voga), son dominantes. La dominancia del sabalito no sólo explicaría la baja abundancia del zooplancton, sino que al mismo tiempo contribuiría a evitar o retardar la consolidación del material sedimentado.