Behavioral response as a predictor of seasonal depth distribution and vertical niche separation in freshwater phytoplanktonic flagellates

Abstract

The distribution of phytoplanktonic flagellates in aquatic ecosystems has been widely attributed to a number of driving factors. In this study, we evaluated the influence of behavior on the daytime, seasonal depth distribution and vertical niche separation of five phylogenetically contrasting species of freshwater flagellates. A model predicting distribution was formulated using the dominant behavioral preferences for light, oxygen, and carbon dioxide, previously quantified in laboratory experiments, and was subsequently applied to the physical and chemical conditions measured in a small, strongly stratifying, hypertrophic lake. This model predicted the daytime depth distributions of natural populations of flagellates well, with an average areal fit of above 56% for all species; above 74% for Ceratium furcoides, Chlamydomonas sp., and Dinobryon sertularia; and of up to 93% during stratification. Regression analyses showed no significant variation from a 1 : 1 relationship between the predicted and observed average depths of species in the water column. The model also predicted the constriction into discrete vertical niches upon stratification and delineated the progression from Plagioselmis nannoplanctica in surface waters, through C. furcoides and D. sertularia, to Chlamydomonas sp. and Euglena gracilis deeper in the water column. Changes in observed distributions could not be directly correlated with other members of the plankton community or explained by the segregation of nutrients. The model's wider, transferable applicability suggested that behavioral response to environmental gradients may predict many patterns of distribution, particularly during stratification. In addition to known physiological and biochemical influences, this investigation emphasized the importance of behavioral response in the functional ecology of phytoplanktonic flagellates.Phytoplanktonic flagellates are a diverse group of unicellular or colonial eukaryotic microorganisms characterized by controlled motility combined with an ability to photosynthesize. Commonly occurring in both freshwater and marine habitats, they form an important functional component of the phytoplankton community (Salonen et al. 1984;Stone et al. 1993) and are frequently the principal contributor to harmful algal blooms and red tides (Smayda 1997). The spatial and temporal distributions of flagellates in aquatic ecosystems have been attributed to a large number of driving factors such as physiological response (e.g., Harris et al. 1979), competitive interactions (Riegman et al. 1996), succession (Gasol et al. 1992), the changing distribution of resources (Klausmeier and Litchman 2001), differential predation (Elser and Carpenter 1988), and regimes of turbulent mixing propagated by wind or wave action (Kamykowski 1995). In much the same way, cell motility is also widely considered to be an important influence on distribution; however, to date and due to its complexity, the full ecological significance of active behavioral response still remains uncertai...

Spatial overlap in lake phytoplankton: Relations with environmental factors and consequences for diversity

“…Lower SO when water columns were wellstratified suggests that motile phytoplankton can actively spatially separate where stable habitat heterogeneity occurs, as has been noted by others in detailed single-lake studies (Gervais 1998;Olli and Seppälä 2001;Clegg et al 2007). However, it remains possible that under highly stratified conditions (high Ddens), that competitive exclusion of different phytoplankton groups in the zone of overlap could also be driving these patterns of lower SO.…”

“…Species packing occurred because nutrient consumption closer to the thermocline (coincident with the nutricline) by a good light competitor forced a better nutrient competitor to grow more deeply, but still shallower and spatially very close to the first (i.e., packing species in). Empirical observations also support a prediction of phytoplankton species co-existence within very thin layers (Gervais 1998;Olli and Seppälä 2001;Clegg et al 2007). Presumably trade-offs in the use of resources (functional traits) are at work, enabling co-existence (Troost et al 2005;Schwaderer et al 2011).…”

“…We provide generalized support across a wide variety of north temperate lakes and natural phytoplankton communities for what, until now, have been either mainly theoretical explorations (Weissing and Huisman 1994;Klausmeier and Litchman 2001;Mellard et al 2011) or detailed empirical observations within isolated lakes or in mesocosms (Gervais 1998;Olli and Seppälä 2001;Clegg et al 2007). However, our study provides new insight into how lake factors might influence different diversity components (richness vs. evenness; taxonomic vs. functional) in different ways.…”

“…In colored lakes, light attenuation is even more severe and thermocline depths are also shallow (Snucins and Gunn 2000). In response to such gradients, positive consequences for biodiversity are predicted where minimal mixing permits phytoplankton to utilize behavioral and physiological mechanisms to lessen spatial overlap between species (Weissing and Huisman 1994;Clegg et al 2007;Yoshiyama et al 2009). In previous work, we demonstrated that indeed, individual phytoplankton spectral groups (identified by major pigments) have peak biomass at different depths, likely owing to varied responses to gradients of light and nutrients across lake types (Longhi and Beisner 2009).…”

Untitled

“…In particular, motile taxa may benefit from weak mixing intensity because low turbulence enables them to move to depths with optimal growth conditions (Klausmeier and Litchman 2001). Such habitat choice could accentuate vertical niche separation of different taxa (Elliott et al 2002;Clegg et al 2007). Thus, if weak mixing leads to the development of multiple gradients of growthenhancing and growth-inhibiting environmental parameters, this could enable substantial vertical niche partitioning and enhance diversity compared to a well-mixed system of the same overall depth.…”