Seasonal Abundance, Biomass and Morphological Diversity of Picoplankton in Lake Superior: Importance of Water Column Mixing

Pico and nano-sized plankton play an important role in regulating the trophic dynamics and key biogeochemical cycles in most pelagic environments, such changes in their abundance and population dynamics can alter ecosystem-level changes [1,2]. Both heterotrophic (Hpico) and phototrophic (Ppico) picoplankton are particularly important in oligotrophic lakes, with deep water column depths and long residence times [3,4]. In ecosystems such as these, Ppico contribute significantly or dominate phytoplankton biomass and primary production [5], while Hpico often dominate heterotrophic metabolism and augment secondary production [6-8]. Despite this, our knowledge of how plankton influence biogeochemical budgets in large ecosystems remains fragmented, because the analysis of picoplankton requires specific methods that often are not routinely used in long term monitoring programs [9,10]. Given this, estimates of picoplankton abundance and biomass do not often exist, and if they do, most are very limited in time and space (few measurements at few locations made per year, e.g., Ivanikova et al. [11]) and rarely are include both heterotrophic and phototrophic components of the assemblage [12]. Lake Superior is one of the largest lakes world-wide, where biogeochemical misbalances have been identified, and initial estimates of the plankton assemblage suggest dominance by small plankton [11]. Significant discrepancies have been identified in the carbon balance for this lake, whereby attempts to reconcile the carbon mass balance have not been successful [13,14]. One possible explanation for the apparent misbalance might be attributed to the turnover rates of small heterotrophic and phototrophic plankton that has not been adequately incorporated into the modeling efforts needed to close the Lake Superior’s carbon cycle [15]. With this in mind, the first step is to establish reliable, first-order estimates of picoplankton abundance and biomass. Thus, we present some of the only seasonal estimates of abundance, biomass, and morphological diversity for the entire picoplankton assemblage in Lake Superior (both heterotrophic and phototrophic components). There were three specific objectives in the study. First, we censused both heterotrophic and phototrophic picoplankton abundance in Lake Superior over the May to October period (three seasons) which captured a broad range of environmental conditions in the lake (warm monomictic lake); Second, we assessed the morphological variation in the picoplankton assemblage. Third, measured we biomass of the entire picoplankton assemblage relative to other measurement so plankton biomass.